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PUBLICATIONS

5

NIST SPECIAL PUBLICATION 260-116

U.S. DEPARTMENT OF COMMERCE/Technology Administration

National Institute of Standards and Technology

Standard Reference Materials:

Glass Filters as a Standard Reference

Material for Spectrophotometry

Selection, Preparation, Certification,

and Use of SRM 930 and SRM 1930

R. Mavrodineanu, R. W. Burke, J. R. Baldwin, M. V. Smith,

QC J. D. Messman, J. C. Travis, and J. C. Colbert

100

. U57

NO. 260-1 16

1994
The National Institute of Standards and Technology was established in 1988 by Congress to "assist

industry in the development of technology . . . needed to improve product quality, to modernize

manufacturing processes, to ensure product reliability . . . and to facilitate rapid commercialization . . . of products based on new scientific discoveries."

NIST, originally founded as the National Bureau of Standards in 1901, works to strengthen U.S.

industry's competitiveness; advance science and engineering; and improve public health, safety, and the

environment. One of the agency's basic functions is to develop, maintain, and retain custody of the

national standards of measurement, and provide the means and methods for comparing standards used

in science, engineering, manufacturing, commerce, industry, and education with the standards adopted

or recognized by the Federal Government.

As an agency of the U.S. Commerce Department's Technology Administration, NIST conducts basic

and applied research in the physical sciences and engineering and performs related services. The Institute

does generic and precompetitive work on new and advanced technologies. NIST's research facilities are located at Gaithersburg, MD 20899, and at Boulder, CO 80303. Major technical operating units and their

principal activities are listed below. For more information contact the Public Inquiries Desk, 301-975-3058.

Technology Services

• Manufacturing Technology Centers Program

• Standards Services

• Technology Commercialization

• Measurement Services

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• Information Services

Electronics and Electrical Engineering

Laboratory

• Microelectronics

• Law Enforcement Standards

• Electricity

• Semiconductor Electronics

• Electromagnetic Fields 1

• Electromagnetic Technology1

Chemical Science and Technology

Laboratory

• Biotechnology

• Chemical Engineering 1

• Chemical Kinetics and Thermodynamics

• Inorganic Analytical Research

• Organic Analytical Research

• Process Measurements

• Surface and Microanalysis Science

• Thermophysics2

Physics Laboratory

• Electron and Optical Physics

• Atomic Physics

• Molecular Physics

• Radiometric Physics

• Quantum Metrology

• Ionizing Radiation

• Time and Frequency1

• Quantum Physics1

Manufacturing Engineering Laboratory

• Precision Engineering

• Automated Production Technology

• Robot Systems

• Factory Automation

• Fabrication Technology

Materials Science and Engineering

Laboratory

• Intelligent Processing of Materials

• Ceramics

• Materials Reliability1

• Polymers

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Building and Fire Research Laboratory

• Structures

• Building Materials

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Computer Systems Laboratory

• Information Systems Engineering

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Computing and Applied Mathematics

Laboratory

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1 At Boulder, CO 80303.

2 Some elements at Boulder, CO 80303.
NIST Special Publication 260-116

Standard Reference Materials:

Glass Filters as a Standard Reference

Material for Spectrophotometry

Selection, Preparation, Certification,

and Use of SRM 930 and SRM 1930

R. Mavrodineanu

R. W. Burke

J. R. Baldwin

M. V. Smith

J. D. Messman

J. C. Travis

J. C. Colbert

Office of Measurement Services

Technology Services

National Institute of Standards and Technology

Gaithersburg, MD 20899

U.S. DEPARTMENT OF COMMERCE, Ronald H. Brown, Secretary

TECHNOLOGY ADMINISTRATION, Mary L. Good, Under Secretary for Technology

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY, Arati Prabhakar, Director

Issued March 1994
National Institute of Standards and Technology Special Publication 260-116

Natl. Inst. Stand. Technol. Spec. Publ. 260-116, 198 pages (March 1994)

CODEN: NSPUE2

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1994

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402-9325
Preface

Standard Reference Materials (SRMs), as defined by the National Institute of Standards

and Technology (NIST), are well-characterized materials produced in quantity and certified for

one or more physical or chemical properties. They are used to assure the accuracy and

compatibility of measurements throughout the Nation. SRMs are widely used as standards in

many diverse fields in science, industry, and technology, both within the United States and

throughout the world. They are also used extensively in the fields of environmental and clinical

analysis. In many applications, traceability of quality control and measurement processes to the

national measurement system is carried out though the use of SRMs. For many of the Nation's

scientists and technologists, it is therefore of more than passing interest to know the details of

the measurements made at NIST in arriving at the certified values of the SRMs produced. The

NIST Special Publication 260 Series is a collection of publications reserved for this purpose.

The 260 Series is dedicated to the dissemination of information on different phases of the

preparation, measurement, certification, and use of NIST SRMs. In general, much more detail

will be found in these publications than is generally included in scientific journal articles. This

enables the user to assess the validity and accuracy of the measurement processes employed, to

judge the statistical analysis, and to learn details of techniques and methods utilized for work

entailing the greatest care and accuracy. These publications also should provide sufficient

additional information so SRMs can be utilized in new applications in diverse fields not foreseen

at the time the SRM was originally issued.

Inquiries concerning the technical content of this publication should be directed to the

author(s). Other questions concerned with the availability, delivery, price, and so forth, of

NIST SRMs will receive prompt attention from:

Standard Reference Materials Program

Building 202, Room 204

National Institute of Standards and Technology

Gaithersburg, MD 20899

Thomas E. Gills, Acting Chief

Standard Reference Materials Program

iii
Abstract

This publication describes the various factors that can affect the proper functioning of a

spectrophotometer and suggests procedures to assess and control these factors. Particular

consideration is given to the long- and short-term stability of a spectrophotometer, wavelength

accuracy, spectral bandpass, stray radiation, and the accuracy of the transmittance or absorbance

scale. A description is given of the Standard Reference Materials (SRMs) that can be used to

control these factors. The methods for the preparation, certification, and use of two such

materials (SRM 930 and SRM 1930) are also presented. The results obtained in the actual use

of these SRMs are examined in some detail. An Appendix contains the reproduction of several

publications and Certificates relevant to the subject discussed in this publication.

Keywords: Accuracy in absorption spectrophotometry; glass filters; spectral bandpass;

spectrophotometry; standard reference materials; stray radiation; testing of spectrophotometers;

transmittance (absorbance) accuracy; wavelength accuracy.

Disclaimer

This publication is a contribution from the U.S. Government and hence is not subject to

copyright. The identification of specific commercial instruments and products is given only to

permit reproduction of the work described in this paper. In no instance does such identification

imply recommendation or endorsement by the National Institute of Standards and Technology,

nor does it imply that the particular equipment or product is necessarily the best available for

the purpose.

iv
Foreword

Since its inauguration in 1901, the National Institute of Standards and Technology

(NIST), formerly the National Bureau of Standards (NBS), has issued nearly 2000 different

Standard Samples or Standard Reference Materials (SRMs). Many of these have been renewed

several times; others have been replaced or discontinued as technology changed. Today, over

1000 SRMs are available, together with a large number of scientific publications related to the

fundamental and applied characteristics of these materials. Each material is certified for

chemical composition, chemical properties, or its physical or mechanical characteristics. Each

SRM is provided with a Certificate or a Certificate of Analysis that contains the essential data

concerning it properties or characteristics. The SRMs currently available cover a wide range

of chemical, physical, and mechanical properties, and a corresponding wide range of

measurement interests in practically all aspects of fundamental and applied science. These SRMs

constitute a unique and invaluable means of transferring to the user accurate data obtained at

NIST, and provide essential tools that can be used to improve accuracy in practically all areas where measurements are performed.

In addition to SRMs, the National Institute of Standards and Technology issues a variety

of Reference Materials (RMs) which are sold, but not certified by NIST. They meet the ISO

Guide 30-1981 (E) definition for RMs, and many meet the definition for CRMs. The

documentation issued with these materials is either a: (1) "Report of Investigation," the sole

authority being the author of the report. RMs are intended to further scientific or technical

research on particular materials. The principal consideration in issuing RMs is to provide a

homogeneous material so that investigators in different laboratories are assured that they are

investigating the same material. (2) "Certificate," issued by the certifying agency (other than

NIST), e.g., other national laboratories, other government agencies, other standardizing bodies,

or other non-profit organizations. When deemed to be in the public interest and when alternate

means of national distribution do not exist, NIST acts as the distributor for such materials.

Further information on the reference materials available from NIST may be obtained

from the Standard Reference Materials Program, National Institute of Standards and Technology.

Information on other NIST services may be obtained from Technology Services, National

Institute of Standards and Technology, Gaithersburg, MD 20899.

In addition to these types of materials, NIST provides a number of additional services.

These include: Calibration and Related Measurement Services, National Standard Reference

Data System, Accreditation of Testing Laboratories, National Center for Standards and

Certification Information, Weights and Measures Program and Proficiency Sample Programs.

v
OTHER NIST PUBLICATIONS IN THIS SERIES

Trahey, N.M., ed., NIST Standard Reference Materials

Catalog 1992-93, NIST Spec. Publ. 260 (February

1992). SN003-003-03 146-1*

Michaelis, R.E., and Wyman, L.L., Standard Reference

Materials: Preparation of White Cast Iron Spectrochemical Standards, NBS Misc. Publ. 260-1 (June

1964). COM74-11061**

Michaelis, R.E., Wyman, L.L., and Flitsch, R.,

Standard Reference Materials: Preparation of NBS

Copper-Base Spectrochemical Standards, NBS Misc.

Publ. 260-2 (October 1964). COM74-11063**

Michaelis, R.E., Yakowitz, H., and Moore, G.A.,

Standard Reference Materials: Metallographic Characterization of an NBS Spectrometric Low-Alloy

Steel Standard, NBS Misc. Publ. 260-3 (October

1964) . COM74-11060**

Hague, J.L., Mears, T.W., and Michaelis, R.E.,

Standard Reference Materials: Sources of

Information, Publ. 260-4 (February 1965). COM74-

11059**

Alvarez, R., and Flitsch, R., Standard Reference

Materials: Accuracy of Solution X-Ray Spectrometric

Analysis of Copper-Base Alloys, NBS Misc. Publ.

260-5 (February 1965). PB 168068**

Shultz, J.I., Standard Reference Materials: Methods for

the Chemical Analysis of White Cast Iron Standards,

NBS Misc. Publ. 260-6 (July 1965). COM74-

11068**

Bell, R.K., Standard Reference Materials: Methods for

the Chemical Analysis of NBS Copper-Base Spectrochemical Standards, NBS Misc. Publ. 260-7 (October

1965) . COM74-11067**

Richmond, M.S., Standard Reference Materials: Analysis

of Uranium Concentrates at the National Bureau of

Standards, NBS Misc. Publ. 260-8 (December 1965).

COM74-11066**

Anspach, S.C., Cavallo, L.M., Garfinkel, S.B., et al.,

Standard Reference Materials: Half Lives of Materials Used in the Preparation of Standard Reference

Materials of Nineteen Radioactive Nuclides Issued by

the National Bureau of Standards, NBS Misc. Publ.

260-9 (November 1965). COM74-11065**

Yakowitz, H., Vieth, D.L., Heinrich, K.F.J. , et al.,

Standard Reference Materials: Homogeneity Characterization of NBS Spectrometric Standards II: Cartridge Brass and Low-Alloy Steel, NBS Misc.

Publ. 260-10 (December 1965). COM74-11064**

Napolitano, A., and Hawkins, E.G., Standard Reference

Materials: Viscosity of Standard Lead-Silica Glass,

NBS Misc. Publ. 260-11** (November 1966).

Yakowitz, H., Vieth, D.L., and Michaelis, R.E.,

Standard Reference Materials: Homogeneity Characterization of NBS Spectrometric Standards III: White

Cast Iron and Stainless Steel Powder Compact, NBS

Misc. Publ. 260-12 (September 1966).

Spijkerman, J.J., Snediker, D.K., Ruegg, F.C., et al.,

Standard Reference Materials: Mossbauer

Spectroscopy Standard for the Chemical Shift of Iron

Compounds, NBS Misc. Publ. 260-13** (July 1967).

Menis, O., and Sterling, J.T., Standard Reference

Materials: Determination of Oxygen in Ferrous Materials (SRMs 1090, 1091, 1092), NBS Misc. Publ.

260-14** (September 1966).

Passaglia, E. and Shouse, P.J., Standard Reference

Materials: Recommended Method of Use of Standard

Ligh-Sensitive Paper for Calibrating Carbon Arcs

Used in Testing Testiles for Colorfastness to Light,

NBS Spec. Publ. 260-15 (July 1967). Superseded by

SP 260-41.

Yakowitz, H., Michaelis, R.E., and Vieth, D.L.,

Standard Reference Materials: Homogeneity Characterization of NBS Spectrometric Standards IV: Preparation and Microprobe Characterization of W-20%

Mo Alloy Fabricated by Powder Metallurgical

Methods, NBS Spec. Publ. 260-16 (January 1969).

COM74-11062**

Catanzaro, E.J., Champion, C.E., Garner, E.L., et al.,

Standard Reference Materials: Boric Acid; Isotopic,

and Assay Standard Reference Materials, NBS Spec.

Publ. 260-17 (February 1970). PB 189457**

Geller, S.B., Mantek, P. A., and Cleveland, N.G.,

Calibration of NBS Secondary Standards Magnetic

Tape Computer Amplitude Reference Amplitude

Measurement "Process A", NBS Spec. Publ. 260-18

(November 1969). Superseded by SP 260-29.

Paule, R.C., and Mandel, J., Standard Reference

Materials: Analysis of Interlaboratory Measurements

on the Vapor Pressure of Gold (Certification of SRM

745). NBS Spec. Publ. 260-19 (January 1970).

PB190071**

260-20: Unassigned

Paule, R.C., and Mandel, J., Standard Reference

Materials: Analysis of Interlaboratory Measurements on the Vapor Pressures of Cadmium and Silver, NBS

Spec. Publ. 260-21 (January 1971). COM74-11359**

Yakowitz, H., Fiori, C.E., and Michaelis, R.E.,

Standard Reference Materials: Homogeneity Characterization of Fe-3 Si Alloy, NBS Spec. Publ. 260-22

(February 1971). COM74-11357**

vi
Napolitano, A., and Hawkins, E.G., Standard Reference

Materials: Viscosity of a Standard Borosilicate Glass,

NBS Spec. Publ. 260-23 (December 1970).

COM71-00157**

Sappenfield, K.M., Marinenko, G., and Hague, J.L.,

Standard Reference Materials: Comparison of Redox

Standards, NBS Spec. Publ. 260-24 (January 1972).

COM72-50058**

Hicho, G.E., Yakowitz, H., Rasberry, S.D., et al.,

Standard Reference Materials: A Standard Reference

Material Containing Nominally Four Percent

Austenite, NBS Spec. Publ. 260-25 (February 1971).

COM74-11356**

Martin, J.F., Standard Reference Materials: NBS-U.S.

Steel Corp. Joint Program for Determining Oxygen

and Nitrogen in Steel, NBS Spec. Publ. 260-26

(February 1971). PB 81176620**

Garner, E.L., Machlan, L.A., and Shields, W.R.,

Standard Reference Materials: Uranium Isotopic

Standard Reference Materials, NBS Spec. Publ.

260-27 (April 1971). COM74-1 1358**

Heinrich, K.F.J. , Myklebust, R.L., Rasberry, S.D., et

al., Standard Reference Materials: Preparation and

Evaluation of SRMs 481 and 482 Gold-Silver and

Gold-Copper Alloys for Microanalysis, NBS Spec.

Publ. 260-28 (August 1971). COM71-50365**

Geller, S.B., Standard Reference Materials: Calibration

of NBS Secondary Standard Magnetic Tape (Computer Amplitude Reference) Using the Reference Tape

Amplitude Measurement "Process A-Model 2," NBS

Spec. Publ. 260-29 (June 1971). COM7 1-50282**

Supersedes Measurement System in SP 260-18.

Gorozhanina, R.S., Freedman, A.Y., and Shaievitch,

A.B., (translated by M.C. Selby), Standard

Reference Materials: Standard Samples Issued in the

USSR (A Translation from the Russian), NBS Spec.

Publ. 260-30 (June 1971). COM71-50283**

Hust, J.G., and Sparks, L.L., Standard Reference

Materials: Thermal Conductivity of Electrolytic Iron

SRM 734 from 4 to 300 K, NBS Spec. Publ. 260-31

(November 1971). COM71-50563**

Mavrodineanu, R., and Lazar, J.W., Standard Reference

Materials: Standard Quartz Cuvettes for High Accuracy Spectrophotometry, NBS Spec. Publ. 260-32

(December 1973). COM74-50018**

Wagner, H.L., Standard Reference Materials: Comparison of Original and Supplemental SRM 705, Narrow

Molecular Weight Distribution Polystyrene, NBS

Spec. Publ. 260-33 (May 1972). COM72-50526**

Sparks, L.L., and Hust, J.G., Standard Reference

Material: Thermoelectric Voltage of Silver-28 Atomic

Percent Gold Thermocouple Wire, SRM 733, Verses

Common Thermocouple Materials (Between Liquid

Helium and Ice Fixed Points), NBS Spec. Publ. 260-

34 (April 1972). COM72-50371**

Sparks, L.L., and Hust, J.G., Standard Reference

Materials: Thermal Conductivity of Austenitic

Stainless Steel, SRM 735 from 5 to 280 K, NBS

Spec. Publ. 260-35 (April 1972). COM72-50368**

Cali, J. P., Mandel, J., Moore, L.J., et al., Standard

Reference Materials: A Reference Method for the

Determination of Calcium in Serum NBS SRM 915,

NBS Spec. Publ. 260-36 (May 1972).

COM72-50527**

Shultz, J.I., Bell, R.K., Rains, T.C., et al., Standard

Reference Materials: Methods of Analysis of NBS

Clay Standards, NBS Spec. Publ. 260-37 (June

1972). COM72-50692**

Richard, J.C., and Hsia, J.J., Standard Reference

Materials: Preparation and Calibration of Standards

of Spectral Specular Reflectance, NBS Spec. Publ.

260-38 (May 1972). COM72-50528**

Clark, A.F., Denson, V.A., Hust, J.G., et al., Standard

Reference Materials: The Eddy Current Decay

Method for Resistivity Characterization of

High-Purity Metals, NBS Spec. Publ. 260-39 (May

1972). COM72-50529**

McAdie, H.G., Garn, P.D., and Menis, O., Standard

Reference Materials: Selection of Differential

Thermal Analysis Temperature Standards Through a Cooperative Study (SRMs 758, 759, 760), NBS Spec.

Publ. 260-40 (August 1972) COM72-50776**

Wood. L.A., and Shouse, P.J., Standard Reference

Materials: Use of Standard Light-Sensitive Paper for

Calibrating Carbon Arcs Used in Testing Textiles for

Colorfastness to Light, NBS Spec. Publ. 260-41

(August 1972). COM72-50775**

Wagner, H.L., and Verdier, P.H., eds., Standard

Reference Materials: The Characterization of Linear

Polyethylene, SRM 1475, NBS Spec. Publ. 260-42

(September 1972). COM72-50944**

Yakowitz, H., Ruff, A.W., and Michaelis, R.E.,

Standard Reference Materials: Preparation and

Homogeneity Characterization of an Austenitic

Iron-Chromium-Nickel Alloy, NBS Spec. Publ.

260-43 (November 1972). COM73-50760**

Schooley, J.F., Soulen, R.J., Jr., and Evans, G.A., Jr., Standard Reference Materials: Preparation and Use of

Superconductive Fixed Point Devices, SRM 767,

NBS Spec. Publ. 260-44 (December 1972).

COM73-50037**

vii
Greifer, B., Maienthal, E.J., Rains, T.C., et al.,

Standard Reference Materials: Development of NBS

SRM 1579 Powdered Lead-Based Paint, NBS Spec.

Publ. 260-45 (March 1973). COM73-50226**

Hust, J.G., and Giarratano, P.J., Standard Reference

Materials: Thermal Conductivity and Electrical

Resistivity Standard Reference Materials: Austenitic

Stainless Steel, SRMs 735 and 798, from 4 to 1200

K, NBS Spec. Publ. 260-46 (March 1975).

COM75-10339**

Hust, J.G., Standard Reference Materials: Electrical

Resistivity of Electrolytic Iron, SRM 797, and

Austenitic Stainless Steel, SRM 798, from 5 to 280

K, NBS Spec. Publ. 260-47 (February 1974).

COM74-50176**

Mangum, B.W., and Wise, J. A., Standard Reference

Materials: Description and Use of Precision Thermometers for the Clinical Laboratory, SRM 933 and

SRM 934, NBS Spec. Publ. 260-48 (May 1974).

Superseded by NIST Spec. Publ. 260-113. COM74-

50533**

Carpenter, B.S., and Reimer, G.M., Standard Reference

Materials: Calibrated Glass Standards for Fission

Track Use, NBS Spec. Publ. 260-49 (November

1974). COM74-51185**

Hust, J.G., and Giarratano, P.J., Standard Reference

Materials: Thermal Conductivity and Electrical

Resistivity Standard Reference Materials: Electrolytic Iron, SRMs 734 and 797 from 4 to 1000 K,

NBS Spec. Publ. 260-50 (June 1975).

COM75-10698**

Mavrodineanu, R., and Baldwin, J.R., Standard

Reference Materials: Glass Filters As a SRM for

Spectrophotometry -Select ion, Preparation,

Certification, and Use-SRM 930 NBS Spec. Publ.

260-51 (November 1975). COM75-10339**

Hust, J.G., and Giarratano, P.J., Standard Reference

Materials: Thermal Conductivity and Electrical

Resistivity SRMs 730 and 799, from 4 to 3000 K,

NBS Spec. Publ. 260-52 (September 1975).

COM75-11193**

Durst, R.A., Standard Reference Materials: Standardization of pH Measurements, NBS Spec. Publ. 260-

53 (December 1978). Superseded by SP 260-53 Rev.

1988 Edition. PB88217427**

Burke, R.W., and Mavrodineanu, R., Standard Reference

Materials: Certification and Use of Acidic Potassium

Dichromate Solutions as an Ultraviolet Absorbance

Standard, NBS Spec. Publ. 260-54 (August 1977).

PB272168**

Ditmars, D.A., Cezairliyan, A., Ishihara, S., et al.,

Standard Reference Materials: Enthalpy and Heat

Capacity; Molybdenum SRM 781, from 273 to 2800

K, NBS Spec. Publ. 260-55 (September 1977).

PB272127**

Powell, R.L., Sparks, L.L., and Hust, J.G., Standard

Reference Materials: Standard Thermocouple

Material, Pt-67: SRM 1967, NBS Spec. Publ. 260-56

(February 1978). PB277172**

Cali, J. P., and Plebanski, T., Standard Reference

Materials: Guide to United States Reference

Materials, NBS Spec. Publ. 260-57 (February 1978).

PB277173**

Barnes, J.D., and Martin, G.M., Standard Reference

Materials: Polyester Film for Oxygen Gas Transmission Measurements SRM 1470, NBS Spec. Publ.

260-58 (June 1979). PB297098**

Chang, T., and Kahn, A.H., Standard Reference

Materials: Electron Paramagnetic Resonance Intensity

Standard: SRM 2601; Description and Use, NBS

Spec. Publ. 260-59 (August 1978). PB292097**

Velapoldi, R.A., Paule, R.C., Schaffer, R., et al.,

Standard Reference Materials: A Reference Method

for the Determination of Sodium in Serum, NBS

Spec. Publ. 260-60 (August 1978). PB286944**

Verdier, P.H., and Wagner, H.L., Standard Reference

Materials: The Characterization of Linear Polyethylene (SRMs 1482, 1483, 1484), NBS Spec. Publ.

260-61 (December 1978). PB289899**

Soulen, R.J., and Dove, R.B., Standard Reference

Materials: Temperature Reference Standard for Use

Below 0.5 K (SRM 768), NBS Spec. Publ. 260-62

(April 1979). PB294245**

Velapoldi, R.A., Paule, R.C., Schaffer, R., et al.,

Standard Reference Materials: A Reference Method

for the Determination of Potassium in Serum, NBS

Spec. Publ. 260-63 (May 1979). PB297207**

Velapoldi, R.A., and Mielenz, K.D., Standard Reference

Materials: A Fluorescence SRM Quinine Sulfate

Dihydrate (SRM 936), NBS Spec. Publ. 260-64

(January 1980). PB80132046**

Marinenko, R.B., Heinrich, K.F.J. , and Ruegg, F.C.,

Standard Reference Materials: Micro-Homogeneity

Studies of NBS SRM, NBS Research Materials, and

Other Related Samples, NBS Spec. Publ. 260-65

(September 1979). PB300461**

Venable, W.H., Jr., and Eckerle, K.L., Standard

Reference Materials: Didymium Glass Filters for

Calibrating the Wavelength Scale of Spectrophotometers (SRMs 2009, 2010, 2013, 2014). NBS

Spec. Publ. 260-66 (October 1979). PB80104961**

viii
Velapoldi, R.A., Paule, R.C., Schaffer, R., et al.,

Standard Reference Materials: A Reference Method

for the Determination of Chloride in Serum, NBS

Spec. Publ. 260-67 (November 1979). PB801 101 17**

Mavrodineanu, R., and Baldwin, J.R., Standard

Reference Materials: Metal-On-Quartz Filters as a SRM for Spectrophotometry SRM 2031, NBS Spec.

Publ. 260-68 (April 1980). PB80197486**

Velapoldi, R.A., Paule, R.C., Schaffer, R., et al.,

Standard Reference Materials: A Reference Method

for the Determination of Lithium in Serum, NBS

Spec. Publ. 260-69 (July 1980). PB80209117**

Marinenko, R.B., Biancaniello, F., Boyer, P. A., et al.,

Standard Reference Materials: Preparation and Characterization of an Iron-Chromium-Nickel Alloy for

Microanalysis: SRM 479a, NBS Spec. Publ. 260-70

(May 1981). SN003-003-02328-1*

Seward, R.W., and Mavrodineanu, R., Standard

Reference Materials: Summary of the Clinical

Laboratory Standards Issued by the National Bureau

of Standards, NBS Spec. Publ. 260-71 (November

1981). PB82135161**

Reeder, D.J., Coxon, B., Enagonio, D., et al., Standard

Reference Materials: SRM 900, Anti-epilepsy Drug

Level Assay Standard, NBS Spec. Publ. 260-72 (June

1981). PB81220758

Interrante, C.G., and Hicho, G.E., Standard Reference

Materials: A Standard Reference Material Containing

Nominally Fifteen Percent Austenite (SRM 486),

NBS Spec. Publ. 260-73 (January 1982).

PB82215559**

Marinenko, R.B., Standard Reference Materials: Preparation and Characterization of K-411 and K-412

Mineral Glasses for Microanalysis: SRM 470, NBS

Spec. Publ. 260-74 (April 1982). PB82221300**

Weidner, V.R., and Hsia, J.J., Standard Reference

Materials: Preparation and Calibration of First

Surface Aluminum Mirror Specular Reflectance

Standards (SRM 2003a), NBS Spec. Publ. 260-75

(May 1982). PB82221367**

Hicho, G.E., and Eaton, E.E., Standard Reference

Materials: A Standard Reference Material Containing

Nominally Five Percent Austenite (SRM 485a), NBS

Spec. Publ. 260-76 (August 1982). PB83 115568**

Furukawa, G.T., Riddle, J.L., Bigge, W.G., et al.,

Standard Reference Materials: Application of Some

Metal SRMs as Thermometric Fixed Points, NBS

Spec. Publ. 260-77 (August 1982). PB831 17325**

Hicho, G.E., and Eaton, E.E., Standard Reference

Materials: Standard Reference Material Containing

Nominally Thirty Percent Austenite (SRM 487), NBS

Spec. Publ. 260-78 (September 1982). PB83 1 15576**

Richmond, J.C., Hsia, J.J., Weidner, V.R., et al.,

Standard Reference Materials: Second Surface Mirror

Standards of Specular Spectral Reflectance (SRMs

2023, 2024, 2025), NBS Spec. Publ. 260-79 (October

1982). PB84203447**

Schaffer, R., Mandel, J., Sun, T., et al., Standard

Reference Materials: Evaluation by an ID/MS

Method of the AACC Reference Method for Serum

Glucose, NBS Spec. Publ. 260-80 (October 1982).

PB84216894**

Burke, R.W., and Mavrodineanu, R., Standard Reference

Materials: Accuracy in Analytical Spectrophotometry, NBS Spec. Publ. 260-81 (April 1983).

PB83214536**

Weidner, V.R., Standard Reference Materials: White

Opal Glass Diffuse Spectral Reflectance Standards for

the Visible Spectrum (SRMs 2015 and 2016), NBS

Spec. Publ. 260-82 (April 1983). PB83220723**

Bowers, G.N., Jr., Alvarez, R., Cali, J. P., et al.,

Standard Reference Materials: The Measurement of

the Catalytic (Activity) Concentration of Seven

Enzymes in NBS Human Serum (SRM 909), NBS

Spec. Publ. 260-83 (June 1983). PB83239509**

Gills, T.E., Seward, R.W., Collins, R.J., et al.,

Standard Reference Materials: Sampling, Materials

Handling, Processing, and Packaging of NBS Sulfur

in Coal SRMs 2682, 2683, 2684, and 2685, NBS

Spec. Publ. 260-84 (August 1983). PB84109552**

Swyt, D.A., Standard Reference Materials: A Look at Techniques for the Dimensional Calibration of

Standard Microscopic Particles, NBS Spec. Publ.

260-85 (September 1983). PB841 12648**

Hicho, G.E., and Eaton, E.E., Standard Reference

Materials: A SRM Containing Two and One-Half

Percent Austenite, SRM 488, NBS Spec. Publ.

260-86 (December 1983). PB84143296**

Mangum, B.W., Standard Reference Materials: SRM

1969: Rubidium Triple-Point - A Temperature

Reference Standard Near 39.30° C, NBS Spec. Publ.

260-87 (December 1983). PB84149996**

Gladney, E.S., Burns, C.E., Perrin, D.R., et al.,

Standard Reference Materials: 1982 Compilation of

Elemental Concentration Data for NBS Biological,

Geological, and Environmental Standard Reference

Materials, NBS Spec. Publ. 260-88 (March 1984).

PB84218338**

Hust, J.G., Standard Reference Materials: A FineGrained, Isotropic Graphite for Use as NBS

Thermophysical Property RMs from 5 to 2500 K,

NBS Spec. Publ. 260-89 (September 1984).

PB85 112886**

ix
Hust, J.G., and Lankford, A.B., Standard Reference

Materials: Update of Thermal Conductivity and

Electrical Resistivity of Electrolytic Iron, Tungsten,

and Stainless Steel, NBS Spec. Publ. 260-90

(September 1984). PB851 15814**

Goodrich, L.F., Vecchia, D.F., Pittman, E.S., et al.,

Standard Reference Materials: Critical Current

Measurements on an NbTi Superconducting Wire

SRM, NBS Spec. Publ. 260-91 (September 1984).

PB85118594**

Carpenter, B.S., Standard Reference Materials:

Calibrated Glass Standards for Fission Track Use

(Supplement to NBS Spec. Publ. 260-49), NBS Spec.

Publ. 260-92 (September 1984). PB851 13025**

Ehrstein, J.R., Standard Reference Materials: Preparation

and Certification of SRM for Calibration of

Spreading Resistance Probes, NBS Spec. Publ.

260-93 (January 1985). PB85177921**

Gills, T.E., Koch, W.F., Stolz, J.W., et al., Standard

Reference Materials: Methods and Procedures Used

at the National Bureau of Standards to Certify Sulfur

in Coal SRMs for Sulfur Content, Calorific Value,

Ash Content, NBS Spec. Publ. 260-94 (December

1984). PB85 165900**

Mulholland, G.W., Hartman, A.W., Hembree, G.G., et

al., Standard Reference Materials: Development of a 1mm Diameter Particle Size Standard, SRM 1690,

NBS Spec. Publ. 260-95 (May 1985). PB861 13693**

Carpenter, B.S., Gramlich, J.W., Greenberg, R.R., et

al., Standard Reference Materials: Uranium-235

Isotopic Abundance Standard Reference Materials for

Gamma Spectrometry Measurements, NBS Spec.

Publ. 260-96 (September 1986). PB87 108544**

Mavrodineanu, R., and Gills, T.E., Standard Reference

Materials: Summary of the Coal, Ore, Mineral,

Rock, and Refactory Standards Issued by the National

Bureau of Standards, NBS Spec. Publ. 260-97

(September 1985). PB86 110830**

Hust, J.G., Standard Reference Materials: Glass

Fiberboard SRM for Thermal Resistance, NBS Spec.

Publ. 260-98 (August 1985). SN003-003-02674-3*

Callanan, J.E., Sullivan, S.A., and Vecchia, D.F.,

Standard Reference Materials: Feasibility Study for

the Development of Standards Using Differential

Scanning Calorimetry, NBS Spec. Publ. 260-99

(August 1985). PB86 106747**

Taylor, J.K., Trahey, N.M., ed., Standard Reference

Materials: Handbook for SRM Users, NBS Spec.

Publ. 260-100 (February 1993). PB93183796**

Mangum, B.W., Standard Reference Materials: SRM

1970, Succinonitrile Triple-Point Standard: A

Temperature Reference Standard Near 58.08° C,

NBS Spec. Publ. 260-101 (March 1986).

PB86197100**

Weidner, V.R., Mavrodineanu, R., Mielenz, K.D., et

al., Standard Reference Materials: Holmium Oxide

Solution Wavelength Standard from 240 to 640 nm - SRM 2034, NBS Spec. Publ. 260-102 (July 1986).

PB86245727**

Hust, J.G., Standard Reference Materials: Glass

Fiberblanket SRM for Thermal Resistance, NBS

Spec. Publ. 260-103 (September 1985).

PB86 109949**

Mavrodineanu, R., and Alvarez, R., Standard Reference

Materials: Summary of the Biological and Botanical

Standards Issued by the National Bureau of

Standards, NBS Spec. Publ. 260-104 (November

1985). PB86155561**

Mavrodineanu, R., and Rasberry, S.D., Standard

Reference Materials: Summary of the Environmental

Research, Analysis, and Control Standards Issued by

the National Bureau of Standards, NBS Spec. Publ.

260-105 (March 1986). PB86204005**

Koch, W.F., ed., Standard Reference Materials: Methods

and Procedures Used at the National Bureau of

Standards to Prepare, Analyze, and Certify SRM

2694, Simulated Rainwater, and Recommendations

for Use, NBS Spec. Publ. 260-106 (July 1986).

PB86247483**

Hartman, A.W., and McKenzie, R.L., Standard

Reference Materials: SRM 1965, Microsphere Slide

(10 fxm Polystyrene Spheres), NBS Spec. Publ.

260-107 (November 1988). PB89 153704**

Mavrodineanu, R., and Gills, T.E., Standard Reference

Materials: Summary of Gas Cylinder and Permeation

Tube Standard Reference Materials Issued by the

National Bureau of Standards, NBS Spec. Publ.

260-108 (May 1987). PB87209953**

Candela, G.A., Chandler-Horowitz, D., Novotny, D.B.,

et al., Standard Reference Materials: Preparation and

Certification of an Ellipsometrically Derived

Thickness and Refractive Index Standard of a Silicon

Dioxide Film (SRM 2530), NIST Spec. Publ.

260-109 (October 1988). PB89 133573**

Kirby, R.K., and Kanare, H.M., Standard Reference

Materials: Portland Cement Chemical Composition

Standards (Blending, Packaging, and Testing), NBS

Spec. Publ. 260-110 (February 1988).

PB88193347**

Gladney, E.S., O'Malley, B.T., Roelandts, I., et al.,

Standard Reference Materials: Compilation of

Elemental Concentration Data for NBS Clinical,

Biological, Geological, and Environmental Standard

Reference Materials, NBS Spec. Publ. 260-111

(November 1987). PB88 156708**
Marinenko, R.B., Blackburn, D.H., and Bodkin, J.B.,

Standard Reference Materials: Glasses for Microanalysis: SRMs 1871-1875, NIST Spec. Publ.

260-112 (February 1990). PB90215807**

Mangum, B.W., and Wise, J. A., Standard Reference

Materials: Description and Use of a Precision

Thermometer for the Clinical Laboratory, SRM 934,

NIST Spec. Publ. 260-113 (June 1990).

PB90257643**

Vezzetti, C.F., Varner, R.N., and Potzick, J.E.,

Standard Reference Materials: Bright-Chromium

Linewidth Standard, SRM 476, for Calibration of

Optical Microscope Linewidth Measuring Systems,

NIST Spec. Publ. 260-114 (January 1991).

PB91 167163**

Williamson, M.P., Willman, N.E., and Grubb, D.S.,

Standard Reference Materials: Calibration of NIST

SRM 3201 for 0.5 in. (12.65 mm) Serial Serpentine

Magnetic Tape Cartridge, NIST Spec. Publ. 260-115

(February 1991). PB9 11 87542**

Mavrodineanu, R., Burke, R.W., Baldwin, J.R., et al.,

Standard Reference Materials: Glass Filters as a Standard Reference Material for Spectrophotometry -

Selection, Preparation, Certification and Use of SRM

930 and SRM 1930, NIST Spec. Publ. 260-116

(February 1994).

Vezzetti, C.F., Varner, R.N., and Potzick, J.E.,

Standard Reference Materials: Antireflecting-Chromium Linewidth Standard, SRM 475,

for Calibration of Optical Microscope Linewidth

Measuring Systems, NIST Spec. Publ. 260-117

(January 1992). PB92-149798**

Williamson, M.P., Standard Reference Materials:

Calibration of NIST Standard Reference Material

3202 for 18-Track, Parallel, and 36-Track, Parallel

Serpentine, 12.65 mm (0.5 in), 1491 cpmm (37871

cpi), Magnetic Tape Cartridge, NIST Spec. Publ.

260-118 (July 1992). PB92-226281**

*Send order with remittance to: Superintendent of

Documents, U.S. Government Printing Office,

Washington, DC 20102. Remittance from foreign

countries should include an additional one fourth of

the purchase price for postage.

**May be ordered from: National Technical Information

Services (NTIS), Springfield, VA 22161.

For information phone (703-487-4650)

To Place an Order with PB# phone (800-553-6847)

Table of Contents

Preface iii Abstract iv

Disclaimer iv

Foreword v

Other Publications in the SP260 Series vi

PART ONE

1. General 1

1.1 Description of the High-Accuracy Spectrophotometer 2

1.2 Standard Reference Materials 9

PART TWO

2. Factors Affecting the Proper Functioning of a Spectrophotometer 14

2.1 Short- and Long-Term Stability 14

2.2 Wavelength Accuracy 14

2.3 Spectral Bandpass 14

2.4 Stray Radiations 14

2.5 Accuracy of the Transmittance or Absorbance Scales 15

2.6 Conditions Associated with the Stability of the Analytical Sample

(Color Reaction) and its Homogeneity 15

3. Procedures to Assess and Control Some of the Factors that

Affect the Proper Functioning and Accuracy of a Spectrophotometer 15

3.1 Short- and Long-Term Stability 15

3.2 Wavelength Accuracy 18

3.2.1 SRM 2009a, Didymium Glass 19

3.2.2 SRM 2034, Holmium Oxide Solution 19

3.3 Spectral Bandpass 22

3.4 Stray Radiations 24

xiii
Table of Contents (Continued)

3.4.1 SRMs 2032 and 2033 26

3.4.2 A New Method for Measurement of Stray Radiant Energy 27

3.5 Accuracy of the Transmittance or Absorbance Scales 28

4. Standard Reference Materials for Transmittance in Spectrophotometry 29

4.1 Description and Selection of Materials for Use as Standard Reference

Materials in Spectrophotometry 29

4.2 Preparation of the Glass Filters 31

4.2.1 Flatness and Parallelism 32

4.2.2 Optical Homogeneity 33

4.3 Cleaning of the Glass Filters 36

4.4 Filter Holders 38

5. Stability of SRM 930 42

5.1 Reproducibility of Transmittance Measurements 42

5.2 Temperature Dependence 42

5.3 Accelerated Visible Radiation Exposure Test 44

5.4 Transmittance as a Function of Time 45

5.5 Interlaboratory Transmittance Measurements 50

6. Scatter, Homogeneity, Polarization, and Positioning of Schott NG

Glass Filters 54

6. 1 Influence of Scatter, Homogeneity, and Polarization on Transmittance 54

6.2 Influence of Positioning of the Filters on Transmittance 56

7. Example of Using SRM 930 for Transmittance Scale Verification

and Calibration 56

8. Transmittance Transfer Standards from the National Physical Laboratory

in Great Britain and the Physics and Engineering Laboratory, DSIR, in

New Zealand 57

xiv
Table of Contents (Continued)

9. Summarizing Remarks 58

10. Uncertainty Estimation for SRM 930D 60

10.1 Overall Estimated Uncertainty 62

Acknowledgments 63

Note 63

References 64

xv
List of Figures

Figure 1. Schematic illustration of the IARD single-beam, high-accuracy

spectrophotometer 2

Figure 2. Arrangement of the two radiation sources used with the IARD

high-accuracy spectrophotometer 4

Figure 3. Circular platform holding seven cuvette or filter holders 5

Figure 4. Illustration of the relation between radiation fluxes F and

corresponding photocurrents I in an ideal case 6

Figure 5. View of the double-aperture unit for linearity testing 7

Figure 6. Control chart for a spectrophotometer 17

Figure 7. Transmittance of a didymium glass filter 20

Figure 8. Transmittance of a holmium oxide glass filter 21

Figure 9. Spectral characteristics for five transparent materials from 200 nm

to700 nm 23

Figure 10. Influence of spectral bandpass on the transmittance of a Chance

ON-10 glass filter measured at 400.0, 445.0, 490.0 and 590.0 nm 25

Figure 1 1 . Influence of spectral bandpass on the transmittance of a Schott

NG-4 glass filter measured at 440.0, 465.0, 590.0 and 635.0 nm 26

Figure 12. Instrumentation to examine, clean, and mount the glass filters used

to produce SRMs 930 and 1930 31

Figure 13. Conventional instruments for observing the figure patterns of a glass

filter and for determining the thickness of a glass filter 32

Figure 14a. Schematic of densitometer for the determination of transmittance

homogeneity of glass filters 33

Figure 14b. Densitometer placed in a light-tight enclosure 34

Figure 14c. Light source for densitometer 35

xvi
List of Figures (Continued)

Figure 15. Stainless steel holders for six 50x50 mm, and twelve 30x10 mm

glass filters 36

Figure 16. Soxhlet extractors used for the final cleaning of the 50x50 mm

and 30x10 mm glass filters with electronic-grade isopropyl alcohol .... 37

Figure 17. Filter holder with removable shutters 38

Figure 18. Aluminum alloy filter holder 39

Figure 19. Details of the aluminum alloy filter holder body 39

Figure 20. Details of the retaining spring used with the filter holder unit

from Figure 19 40

Figure 21. Details of the front and rear shutters used with the filter holder

unit from Figure 19 40

Figure 22. Illustration of SRM 930, Glass Filters for Spectrophotometry 41

xvii
List of Tables

Table 1 . Summary of the functioning characteristics of the IARD

high-accuracy spectrophotometer 8

Table 2. Standard Reference Materials for spectrophotometry provided

by the National Institute of Standards and Technology 10

Table 3. Homogeneity of a glass filter with a nominal 20 percent

transmittance (Filter Number 20-1245) 36

Table 4. Reproducibility of transmittance measurements on seven Schott NG

glass filters 42

Table 5. Influence of temperature on the transmittance of seven Schott NG

filters at five wavelengths 43

Table 6. Percent transmittance of three Schott NG glass filters before

and after exposure to accelerated visible radiation test 44

Table 7. Stability of transmittance measured on SRM 930 as a function

of time 45

Table 8. Comparison of transmittance measurements performed on a set of

three Schott NG glass filters over a period of two years 46

Table 9. Comparison of transmittance measured on three Schott NG glass

filters at NPL and NIST 47

Table 10a. Comparison of percent transmittance values obtained from 1974

to 1992 for SRM 930, Set No. 100, filter with a nominal

10% transmittance 48

Table 10b. Comparison of percent transmittance values obtained from 1974 to

1992 for SRM 930, Set No. 100, filter with a nominal 20% transmittance 49

Table 10c. Comparison of percent transmittance values obtained from 1974 to

1992 for SRM 930, Set No. 100, filter with a nominal 30% transmittance 50

Table 1 1 . Comparison of transmittance measured on three Schott NG

glass filters at NPL and NIST 51

Table 12. Comparison of transmittance measurements performed by four

laboratories on three Schott NG glass filters 52

xviii
List of Tables (Continued)

Table 13. Measurement of scatter, homogeneity, and polarization on three

Schott NG glass filters 54

Table 14. Transmittance of a set of SRM 930 at two wavelengths obtained

at NIST using the IARD and the RPD high-accuracy

spectrophotometers 55

Table 15. Percent transmittance measured at 590.0 nm on a Schott NG glass

filter, 2 mm thick, having a nominal 30% transmittance for

normal incidence and for angles of 3°, 6°, and 9° 56

Table 16. Comparison of percent transmittance measurements performed on

a Schott NG filter using the NIST Inorganic Analytical Research

Division instrument and a conventional spectrophotometer 57

Table 17. Estimation of Overall Uncertainty for SRM 930d 60

xix
List of Appendices

Appendix 1. Reprint, Comments on spectrometry nomenclature

Appendix 2. Reprint, An accurate spectrophotometer for measuring the

transmittance of solid and liquid materials

Appendix 3. Reprint, Standards for checking the calibration of

spectrophotometers (200 to 1000 nm)

Appendix 4. Reprint, Spectral transmittance characteristics of holmium oxide

in perchloric acid solution

Appendix 5. Reprint, Heterochromatic stray light in UV absorption

spectrometry: a new test method

Appendix 6. Reprint, Reflection correction for high-accuracy transmittance

measurements on filter glasses

Appendix 7. Reprint, Adaptation of a high-accuracy spectrophotometer for

ultraviolet work

Appendix 8. Reprint, Sintered mixtures of phosphors in

polytetrafluoroethylene resin for fluorescence standards

Appendix 9. Certificate, SRM 930d, Glass filters for spectrophotometry . . .

Appendix 10. Certificate, SRM 1930, Glass filters for spectrophotometry . . .

Appendix 11. Certificate, SRM 2031, Metal-on-quartz filters for

spectrophotometry

Appendix 12. Certificate, SRM 93 le, Liquid absorbance standards for

ultraviolet and visible spectrophotometry

Appendix 13. Certificate, SRM 935a, Crystalline potassium dichromate for

use as an ultraviolet absorbance standard

Appendix 14. Certificate, SRM 2032, Crystalline potassium iodide,

heterochromatic stray radiant energy standard for ultraviolet

absorption spectrophotometry

xx
List of Appendices (Continued)

Appendix 15. Certificate, SRM 2033, Crystalline potassium iodide with

attenuator, heterochromatic stray radiant energy standard for

ultraviolet absorption spectrophotometry

Appendix 16. Certificate, SRM 2009a, Didymium glass filter for checking

the wavelength scale of spectrophotometers

Appendix 17. Certificate, SRM 2034, Holmium oxide solution wavelength

standard from 240 to 650 nm

Appendix 18. Certificate, SRM 936, Quinine sulfate dihydrate

Appendix 19. Certificate, SRM 1931, Fluorescence emission standards for

the visible region

xxi

Part One

1. GENERAL

The need for reference materials to verify the accuracy of measurements in molecular

absorption spectrometry (or spectrophotometry*) has been associated with the early stages of

development of this analytical technique since its beginning. It is only in the recent years,

however, that accurately calibrated Standard Reference Materials (SRMs) have become available

to verify the accuracy of the transmittance and wavelength scales of conventional

spectrophotometers and to assess the amount of stray radiant energy in those instruments.

To underline the importance of this factor, it is perhaps useful to remember that in the

field of clinical chemistry alone, more than 1,000,000 spectrophotometric tests were performed

daily in the United States in the late 1960's and early 1970' s. Today, at an estimated growth

rate of 15 percent per year, the number of analytical measurements performed in the clinical

laboratories in this country is on the order of several billion per year [2]

.

Solid materials in the form of glasses with adequate optical transmittance characteristics,

such as Chance ON- 10, Schott NG-4, etc., or liquid compounds such as pyrene or p-nitrophenol,

and solutions of inorganic compounds such as potassium dichromate, cobalt ammonium sulfate, or holmium oxide were used extensively in intralaboratory tests to assess the precision of

spectrophotometers used within a particular laboratory. When these materials were used to

verify the proper functioning of such instruments on an interlaboratory basis, however, the

results indicated that rather poor accuracy was obtained for both wavelength and transmittance

values. This situation was confirmed by numerous and extensive interlaboratory tests, and the

detailed results were described in a number of publications. The critical paper by Rand in 1969

[3] , the extensive monograph edited by Burgess and Knowles [4] , the papers cited in references

5-9, and the more recent work edited by Burgess and Mielenz [10] are relevant.

All determinations that use spectrophotometry are based on the measurement of the

optical transmittance of the analytical samples. The accurate measurement of this parameter is

an essential requirement, especially when used to establish the optical characteristics of the

material, such as the molar absorptivity, and also in enzyme studies and dosimetry.

Transmittance accuracy is also indispensable for interlaboratory exchange and comparison of

analytical data. As a result of this situation and the numerous requests from the users of

spectrophotometers, and from clinical chemists in particular, the National Bureau of Standards

(NBS) initiated in 1969 a project to study the various factors that affect the accuracy of

spectrophotometric measurements, and to provide means to identify, assess, monitor, and, if

possible, decrease the uncertainty of such measurements [11]. In fact, this project was an

extension of the pioneering activity in the general field of spectrophotometry that existed at the

National Bureau of Standards since the early 1920's [12] and has continued uninterrupted to the

present time.

* The nomenclature used in this paper is defined in reference 1

.

1
The sources of variance that are responsible for the uncertainty of spectrophotometric

measurements were identified as wavelength accuracy, spectral bandpass, radiation pathlength,

stray radiant energy, transmittance scale accuracy, and polarization effects of the instrument.

Since no adequate instrumentation was available to study these parameters individually in a

quantitative manner and with a proven accuracy, it was decided to design and construct a special

high-accuracy spectrophotometer for this purpose. This research instrument was completed and

tested in 1970, and it has been used since then for the certification of all the SRMs for

transmittance and stray radiant energy produced to date at NIST. A condensed description of

this instrument is given in the following section.

1.1 Description of the High-Accuracy Spectrophotometer

The high-accuracy spectrophotometer [13-16], located in the Inorganic Analytical

Research Division (IARD), is a single-beam instrument and is illustrated in figure 1. Its major

components, described only briefly here, are: (a) a radiation source; (b) a monochromator; (c) a sample holder; (d) a system to verify the accuracy of the transmittance measurements; (e) an

integrating sphere-photomultiplier unit connected to a digital voltmeter; and (f) a data acquisition

and display system. A circular neutral wedge, located between the radiation source and the

entrance slit of the monochromator, is used to select various levels of radiation flux required by

the measurements. Quartz-lithium fluoride achromats are used as objectives to produce the

necessary beam geometries.

Precision Optical Bench

us iiiiii mi

Itll-ltClllCT incTiiriiTiiiTii

Mirror

OrculargjNeutral Wedge

NPL Aperture for Linearity Control

(After F.J J. Clarke)

I Predisperser BCD

U

<A)4!B)=(A+B>

Lens i 1m Grating Monochromator (Czerny -Turner)

Carriage

Precision Optical Bench in Lightproof enclosure

Sample

1Blank

Light Constant Power

Source Supply

Integrating Sphere

Shutter

Figure 1. Schematic illustration of the IARD single-beam, high-accuracy spectrophotometer.

The double-aperture unit is placed on the optical bench only when linearity

measurements are performed.

2
The radiation source for the spectral range of 400-1000 nm is a ribbon tungsten filament

lamp powered by a constant current source. The stability of this current is monitored with a

potentiometer capable of detecting changes of 2-3 parts in 106

. For the ultraviolet region from

200 to 400 nm, a deuterium discharge lamp is used. These two light sources are interchangeable, and their images can be directed onto the entrance slit of the monochromator

using a mirror at 45°.

The monochromator is a 1-m Czerny-Turner grating instrument provided with a quartz

prism predisperser, functioning as a double monochromator to reduce stray radiant energy.

These parts are illustrated in figure 2.

The sample-carrying system provides for manual measurements of one sample and its

blank, or of seven samples and eight blanks in a sequential order. They can be operated

manually or automatically using a microcomputer. The integrating sphere, placed at the end of

the optical path, is made from a hollow aluminum sphere provided with a target coated on the

inside with BaS04. The image of the exit slit of the monochromator is projected onto the target

of the integrating sphere (see fig. 3) to which an end-on photomultiplier (PM) tube detector is

attached.

The data acquisition and presentation system consists of a digital voltmeter capable of

taking 40 readings per second. The radiation fluxes obtained for the sample I, and for the

reference beam I0 passing through air (I0 air) for solid materials or the blank sample (I0 bi^)

for solutions, are used by the microcomputer to calculate transmittance T,

/

*o air

, or internal

transmittance Tj,

blank

and the corresponding values of transmittance density,

(-log 10T), or absorbance, (-log 10Tj), respectively. The microcomputer also controls all of the

measuring sequences including wavelength settings, neutral wedge adjustments, stepping of the

sample carousel, and transmittance measurements.

An essential element of this instrumentation is the system for verifying the accuracy of

the transmittance measurements. As mentioned previously, the radiation fluxes passing through

the sample and reference channels generate corresponding photocurrents i and i0 , respectively,

at the photomultiplier. These photocurrents are substituted for I and IG

in the above

transmittance expression, and their ratio is the optical transmittance value for the material

measured. If these photocurrents can be measured accurately, then the optical transmittance

value will be accurate, provided wavelength accuracy, radiation pathlength accuracy, adequate

spectral bandpass, and freedom of stray radiant energy have been achieved.

A necessary relationship, which is implicit for the accurate measurement of photocurrent,

is that the detector photocurrent be a linear function of the radiation flux. This is illustrated in

3
Figure 2. Arrangement of the two radiation sources used with the IARD high-accuracy

spectrophotometer. A low power laser beam is used to verify the alignment. At

left: the tungsten filament lamp for measurements in the visible. On the optical

bench: deuterium low pressure discharge source for measurements in the

ultraviolet. Also on the optical bench: the 45° mirror and the circular neutral

wedge. At right: the 1-m Czerny-Turner spectrometer provided with the

predisperser and stepping motor shaft encoder. The sample holder, integrating

sphere-photomultiplier unit, and the two achromat objectives are placed on optical

bench located in the light-tight enclosure visible at the rear. Also at right: the

console containing the electronic and pneumatic controls.

4
Figure 3. Circular platform holding seven cuvettes or filter holders. An improved unit (not

illustrated) is provided with eight additional cuvettes or filter holders for the blank

samples. At right: one of the two lithium fluoride-quartz achromats and the

integrating sphere-photomultiplier unit.

the simple graph in figure 4. The abscissa represents arbitrary radiation fluxes F, and the

ordinate represents the relative photocurrent signals I. In the ideal case, when F=0, then 1=0.

For a certain value of F, called here arbitrarily 100 percent, there corresponds a maximum

value of the photocurrent I. These determine two fixed points on the graph, and any

measurement of radiant flux will be accurate if it falls on the straight line which connects the

origin 0 with the 100-percent point.

Various methods may be used to measure the linearity of these photocurrents. A

technique based on the radiation-addition principle using two apertures was selected for its

simplicity and freedom from errors [17]. The two-aperture system consists of a metal plate

provided with two rectangular windows A and B (see fig. 5). Each window can be closed by

shutters operated remotely through pneumatic controls. This aperture system is placed past the

5
F

o

F

Radiation Flux

Figure 4. Illustration of the relation between radiation fluxes F and corresponding

photocurrents I in an ideal case.

exit slit of the monochromator and always within the optical solid angle of the instrument. The

image of the apertures is then produced on the target of the integrating sphere.

The linearity verification consists of measuring the photocurrent produced with aperture

A open and B closed; then B open and A closed; and finally with both apertures A and B open.

If the system is linear, the sum of the photocurrents obtained with apertures A and B open

separately should be equal to the photocurrent obtained when both apertures A and B are open

at the same time. If this is not the case, the system shows a nonlinearity which is proportional

to the amount by which the sum of (A) + (B) differs from (A + B). This difference is then

used to correct the transmittance measurements. This operation is repeated over the range of

transmittance levels needed. During the individual measurement of (A), (B), and the combined

(A + B), experimental conditions must not change (see Appendix 2). A critical study by K.D.

Mielenz and K.L. Eckerle [18] has concluded that the possible causes of errors generated by

optical diffraction and interferences occurring at the apertures of the double-aperture unit will

not affect the accuracy of the measurements at 1 part in 105.
View of the double-aperture unit for linearity testing. The two pneumaticallycontrolled shutters are located on the plate. One aperture is open, the other is

closed.
The stability of the various electronic and optical parts of the IARD high-accuracy

spectrophotometer was determined and the results are summarized in table 1 . From these data

it can be concluded that the transmittance of a stable glass filter, which has a nominal percent

transmittance ranging from 10 %T to 50 %T, can be determined with an uncertainty of 1 part in

104. Since the same uncertainty is obtained when the linearity of the photocurrent is measured

as described above, the accuracy of the transmittance is also assessed with an uncertainty of 1 part in 104

over this same transmittance range.

Table 1. Summary of the Functioning Characteristics of the IARD

High-Accuracy Spectrophotometer

Stability of the electronic system 1 to 2 parts in 105

Stability of the electronics and PM tube (dark) 4 parts in 105

Stability of electronics, PM tube, and radiofluorescent source 7 parts in 105

Stability of electronics, PM tube, and tungsten ribbon filament 2.2 parts in 104

Reproducibility of transmittance measurements 1 part in 104

A similar high-accuracy spectrophotometer, designed by K.D. Mielenz et al. [19], was

constructed and is used in the NIST Radiometric Physics Division (RPD) for the study of various

physical parameters in spectrophotometry [20]. This RPD instrument differs from the IARD

spectrophotometer described here in that it uses reflecting rather than refracting optics. Some

of the parameters studied on the RPD research instrument are: interreflection phenomena,

polarization, homogeneity of the photomultiplier sensitive surface, effectiveness of integrating

spheres and diffusers, scatter and homogeneity of transparent solid optical filters, calibration of

glass wavelength filters, etc. The RPD reference instrument is also used from time to time to

verify the transmittance measurements performed with the IARD high-accuracy

spectrophotometer

.

8
1.2 Standard Reference Materials

The high-accuracy spectrophotometer housed in the Inorganic Analytical Research

Division is described in this publication because it is a primary transmittance standard. The only

way known to us by which the accurate transmittance values determined with this instrument can

be transferred to the users of spectrophotometers is with the help of Standard Reference

Materials, or SRMs. These SRMs are defined as "reference materials, one or more of whose

property values are certified by a technically valid procedure, accompanied by or traceable to a certificate or other documentation which is issued by a certifying body," and their objective

is to assure the accuracy and compatibility of measurements by transferring to the user the

accurate values obtained at NIST.

A number of liquid and solid optical SRMs for the spectral range from 200-800 nm have

been prepared and certified at NIST for the verification of the accuracy of the wavelength and

transmittance scales of conventional spectrophotometers, and for assessing the stray radiant

energy. Standard Reference Materials for fluorescence spectrometry are also available. A

summary of the major characteristics of these optical filter SRMs, together with information on

the publications prepared at NIST that describe in detail the selection, characteristics,

preparation, calibration procedures, and uses of these materials, is given in table 2. This

publication gives a detailed description of the materials and methods used to produce and certify

SRM 930 and SRM 1930, Glass Filters for Spectrophotometry.

The reader who is interested in the more fundamental problems associated with

high-accuracy spectrophotometry and luminescence measurements is advised to consult the

publications listed in references 21 and 22, including the papers mentioned in those publications.

Reprints from several papers closely related to the subject discussed in this publication are reproduced here in the Appendices as a convenience to the reader. The Certificates for the

optical SRMs described in table 2 are also reproduced in the Appendices.

9
Table 2. Standard Reference Materials for Spectrophotometry Provided by the

National Institute of Standards and Technology

SRM Material Composition

Transmittance (Solids)

SRM 930

Glass Filters for Spectrophotometry

i Solid filters made of Schott NG-4 and NG-5

| optically neutral glass. Obtained from the

Schott-Jenaer Glaswerk, Mainz, Germany

i

i

Three filters with nominal percent

transmittances of 10, 20 or 30%. Each filter

is mounted in a black-anodized aluminum

alloy holder provided with front and rear

shutters. The filters are stored in a

cylindrical aluminum alloy container.

SRM 1930

Glass Filters for Spectrophotometry

-+

i Solid filters made of Schott NG-3 and NG-

| 1 1 optically neutral glass. For further information, see above. +i

Three filters with nominal percent

transmittances of 1 , 3 or 50%. For further information, see above.

SRM 2031

Metal-on-Quartz Filters for Spectrophotometry

Solid filters made of semi-transparent,

evaporated chromium-on-fused silica (nonfluorescent) plates.

Three filters with nominal percent

transmittances of 10, 30 or 90%. Each filter

is made from two fused-silica plates

assembled by optical contact. The 90%T

filter is made from one clear plate. The

10%T and 30%T filters are made by

assembling a fused-silica plate which

carries a film of semi-transparent chromium

to produce the desired transmittance, and a

clear plate. Each filter is placed in a metal

alloy holder provided with front and rear

shutters.

Absorbance (Liquids)

SRM 931

Liquid Absorbance

Standards for Ultraviolet and Visible Spectrophometry

Liquid filters made from a solution of Co

and Ni metals dissolved in a mixture of

nitric-perchloric acids. The pH of the

solution is about 1 . Prepared in the NIST

Inorganic Analytical Research Division.

Three solutions and a blank are issued in sealed ampoules and have a nominal

absorbance of 0.3, 0.6, or 0.9.

SRM 935

Crystalline Potassium

Dichromate for Use as an

Ultraviolet Absorbance

Standard

Crystalline potassium dichromate of established purity obtained from the J.T. Baker Chemical Co., Phillipsburg, NJThe crystalline material of reagent grade

purity is offered in glass bottles provided

with plastic screw caps.

Stray Radiant

Energy

SRM 2032

Potassium Iodide for Use as

a Stray Light Standard

Crystalline potassium iodide of established

purity obtained from the J. T. Baker

Chemical Co., Phillipsburg, NJ.

The reagent-grade, crystalline Kl is 99.8%

pure as indicated by the manufacturer.

Moisture is 0.007%. Homogeneity was

determined by absorbance measurements

at wavelengths 265, 267 and 270 nm, and was found adequate.

10
Use Rsraiairks

The transmittance of each filter is measured with the high-accuracy

spectrophotometer at wavelengths

440, 465, 546.1, 590 and 635 nm

using spectral bandpasses not exceeding 2.2, 2.7, 6.5, 5.4 and 6.0 nm, respectively.

This SRM is intended as a reference source for the verification of the

transmittance or absorbance scales of conventional spectrophotometers in the visible spectral region.

The use of this SRM is limited to the

visible spectrum and requires narrow

spectral bandpasses. For further

details, consult the Certificate included in Appendix 9.

i -+-

For further information, see above. i For further information, see above,

i

i -4 — -+

For further information, see above;

also, consult the Certificate included

in Appendix 10.

The transmittance of each filter is measured with the high-accuracy

spectrophotometer at wavelengths

250, 280, 340, 360, 400, 465, 500,

546.1, 590 and 635 nm.

This SRM is intended as a reference source for the verification of the

transmittance and absorbance scales

of conventional spectrophotometers in the ultraviolet and visible spectral

regions.

SRM 2031 has good optical neutrality

and can be used to calibrate spectrophometers with wide spectral

bandpasses from 250 to 635 nm (with

possible extension to about 3 ym).

For further details, consult the

Certificate included in Appendix 1 1

;

also, see NBS Special Publication

260-68.

The absorbance of each solution was

measured at wavelengths 302, 395,

512 and 678 nm with the highaccuracy spectrophotometer using

spectral bandpasses not exceeding

1 .5, 2.0, 3.3 and 8.5 nm, respectively.

This SRM is intended for the

verification of the accuracy of transmittance or absorbance scales of conventional spectrophotometers in the ultraviolet and visible spectral

regions. 4

SRM 931 requires the use of narrow

spectral bandpasses, and provides

only one certified value in the

ultraviolet spectral region. For further

details, consult the Certificate in Appendix 12.

The apparent specific absorbance is

certified for ten concentrations at wavelengths 235, 257, 313, 345 and

350 nm using spectral bandpasses not exceeding 1.6 nm.

This SRM is intended to be used as a reference standard for the verification of the accuracy and linearity of the

absorbance scale of conventional

spectrophotometers in the ultraviolet spectral region.

SRM 935 requires accurate

preparation of solutions of potassium

dichromate in 0.001 N perchloric acid. SRM 935 requires the use of narrow

spectral bandpasses, and provides

certified values only in the ultraviolet spectral region. For further details,

consult the Certificate in Appendix 13;

also, see NBS Special Publication

260-54.

The specific absorbances were measured with the high-accuracy

spectrophotometer at wavelengths

240, 245, 250, 255, 260, 265, 270 and

275 nm at 23.5°C using a spectral

bandpass of 0.2 nm.

SRM 2032 is to be used to assess

heterochromatic stray light in the

ultraviolet region below 260 nm in absorption spectrophotometers.

SRM 2032 should be stored in the

original low-actinic glass bottle and

cardboard container, protected from

exposure to light and humidity. The

estimated stability is 3 years. For

further details, consult the Certificate

in Appendix 14.

11
Table 2. Standard Reference Materials for Spectrophotometry Provided by the

National Institute of Standards and Technology (Continued)

SRM Material Composition

Stray Radiant Energy (Continued)

SRM 2033

Potassium Iodide for Use as

a Stray Light Standard with Radiation Attenuator

Crystalline potassium iodide of established

purity, and two semi-transparent

evaporated metal-on-fused silica filters contained in a metal alloy holder provided

with two shutters. The Kl was obtained

from the J. T. Baker Chemical Co.,

Phillipsburg, NJ. The filters were made in the NIST Fabrication Technology Division.

Potassium iodide as for SRM 2032. Two

semi-transparent chromium-on-fused silica (non-fluorescent) filters of optical quality.

Wavelength

SRM 2009a

Didymium Glass Filter for Checking the Wavelength

Scale of Spectrophotometers

The didymium glass was prepared by

Corning Glass Works, Corning, NY, as

Corning 5120 Filter Glass.

Rare earth oxides in a glass matrix. This

filter is 1 cm wide, 3 cm high and 3 mm

thick. Each filter is placed in a metal alloy

holder which fits into the cuvette holder of the spectrophotometer.

SRM 2034

Holmium Oxide for Use as Wavelength Standard in Spectrophotometry

Holmium oxide of established purity

dissolved in perchloric acid.

Holmium oxide is offered as solutions

sealed in 10-mm fused-silica, nonfluorescent cuvettes of optical quality.

Fluorescence

SRM 936

Quinine Sulfate Dihydrate

SRM 1931

Fluorescence Corrected

Emission Spectra

The quinine sulfate dihydrate was a special

lot of material obtained from J. T. Baker

Chemical Co., Phillipsburg, NJ.

Solid, sintered mixtures of inorganic

phosphors in polytetrafluoroethylene resin.

SRM 936 contains 1 .7% impurities

determined by liquid chromatography and

believed to be dihydroquinine sulfate dihydrate. The water content is 4.74 ± 0.05% determined by the Karl Fisher

method, and 4.57 ± 0.04% by weight loss. The theoretical value is 4.60%.

The four inorganic phosphors are: ZnS:Ag;

Zn 2Si04Mn; ZnCdS:Ag; and CaSi03:Pb,Mn.

Each sample is mounted in black-anodized

aluminum alloy holders stored in a red

anodized cylindrical aluminum alloy

container. A blank sample is also provided.

12
Certification Use

Same as for SRM 2032. The

transmittance of the evaporated

metal-on-fused silica filters was

measured at 255 nm with the highaccuracy spectrophotometer. The

nominal value is 1 0%T for each filter, and a combined value of 1 %T.

SRM 2033 is to be used to assess

heterochromatic and isochromatic

stray light in absorption

spectrophotometers.

Same as for SRM 2032. The two semitransparent, evaporated metal-on-fused

silica filters in the metal holder should

be stored in the plastic container

provided with SRM 2033. Consult the

Certificate in Appendix 15.

The wavelengths of maximum

absorption were determined with a

high-precision spectrophotometer for bandwidths in the range 1.5 to 10.5

nm and for 14 to 24 wavelengths in the range 400 to 760 nm. The

instrument has a wavelength

accuracy of 0.04 nm.

The filters are to be used in calibrating

the wavelength scale in the visible spectral region for spectrophotometers

having nominal bandwidths in the

range 1.5 to 10.5 nm.

SRM 2009a was not measured

individually. It is recommended for most

applications. For further details, consult

NBS Special Publication 260-66, and

the Certificate in Appendix 16.

The wavelengths of maximum

absorption of holmium oxide

solutions in perchloric acid is determined for various spectral

bandpasses at the nominal

wavelengths: 241 , 249, 278, 287,

333, 345, 361, 385, 416, 451, 467,

485, 536, and 640 nm.

This SRM is to be used as a reference

standard for the verification of the

accuracy of the wavelength scale of absorption spectrometers, in the

ultraviolet and visible spectral regions.

Holmium oxide solutions in perchloric

acid were selected as a wavelength

standard because the absorption bands

are narrower than those of the holmium

oxide glass. Consult the Certificate in Appendix 17; also, see Appendix 4 and

NBS Special Publication 260-102.

The material is certified for the

relative molecular emission spectrum

E(A), in radiometric units for a

solution of 1.28 x 10"6 mol/L in 0.105

mol/L perchloric acid, using an

excitation wavelength of 347.5 nm. The certified values of the molecular

emission spectrum at 5-nm intervals from 375 to 675 nm are given. This

certification was made with the NIST

reference fluorescence spectrometer.

This SRM is to be used for evaluation

of methods and calibration of fluorescence spectrometers. A

solution of 0.1 mg/mL in 1000 mL

0.105 mol/L perchloric acid is recommended. It should be stored in the dark in a well-stoppered glass

bottle. This solution is stable for three

months. SRM 936 is for "in vitro" diagnostic use only.

The material should be kept in its orginal bottle and stored in the dark at 30°C or less. Under these conditions

SRM 936 is stable for three years. See

NBS Special Publication 260-64, and

Certificate in Appendix 18.

The nominal maximum emission

spectra are 450, 530, 580 and 620

nm. All measurements are made

with the NIST reference fluorescence

spectrometer.

These materials are intended for the

evaluation of methods and calibration of fluorescence spectrometers in the

spectral range from about 420 to about 675 nm.

A NIST Special Publication in the

SP 260 series describing the production

and certification of this SRM is in preparation. For further information,

consult the reprint in Appendix B, and

the Certificate in Appendix 19.

13
Part Two

2. FACTORS AFFECTING THE PROPER FUNCTIONING OF A

SPECTROPHOTOMETER

There are a number of factors that can affect the proper functioning of a

spectrophotometer and prevent the acquisition of meaningful data. Some of these factors are described below.

2.1 Short- and Long-Term Stability

Short- and long-term stability is related to the instrument's mechanical, optical, and

radiation measuring design and to the quality of the materials and workmanship used to

implement the design. The conditions under which the instrument is operated, such as vibration,

humidity and temperature, dust and corrosion, will also affect its stability. Other critical factors are the short- and long-term stability of the radiation source, the reproducibility of the

positioning of the sample holder and carriage, and the stability of the read-out system.

2.2 Wavelength Accuracy

Wavelength accuracy is a function of the stability of the optical system and of the

accuracy of the wavelength scale, or wavelength cam, and its reproducibility.

2.3 Spectral Bandpass

The spectral bandpass of a spectrophotometer is the spectral interval AX in wavelength

that emerges through the exit slit of width w of the instrument: AX = w — where — is the

dx dx

reciprocal linear dispersion of the spectrophotometer. Spectral bandpass is a parameter that

depends on the optical design and of the associated elements: grating, prism, or filters used in

the instrument. The magnitude of the spectral bandpass for a specific spectrophotometer over

the whole spectral range of the instrument is important information that is provided by the

manufacturers of the particular instrument.

2.4 Stray Radiations

Stray radiations can originate in the spectrometer and in the sample compartment. The

stray radiation produced in the spectrometer is the radiant flux at wavelengths different from

those of the nominal spectral bandpass transmitted through the instrument at a particular

wavelength. The stray radiation produced in the sample compartment is the radiant flux that

14
reaches the photosensitive detector without passing through the absorbing sample. The former

is the heterochromatic stray radiation, while the latter is the isochromatic stray radiation.

2.5 Accuracy of the Transmittance or Absorbance Scales

Accuracy of the transmittance or absorbance scales refers to the capability of a

spectrophotometer to measure ratios of radiant fluxes with a known accuracy and to produce true

transmittance (T) or absorbance (A) measurements. This fundamental parameter is determined

ultimately by the linearity of the detection system when the other parameters such as wavelength

accuracy, adequate spectral bandpass, and photometric precision are in control.

2.6 Conditions Associated with the Stability of the Analytical Sample (Color Reaction)

and its Homogeneity

Conditions associated with the stability of the analytical sample (color reaction) and its

homogeneity include dissociation and association reactions, radiation scatter inside the sample,

polarization, fluorescence, temperature, particulate matter, and surface conditions. These

conditions are characteristics of the analytical sample and of the chemical reactions involved in

the spectrophotometric process, and are to a large extent independent of the spectrophotometer

used for the measurements. Since these factors can affect in a significant manner the precision

and accuracy of the measurement, however, they are mentioned here to remind the analyst that

he must be aware of their existence, and that he must evaluate their magnitude in the particular

analytical process considered before proceeding with the actual measurement. As a result of this

knowledge, the analyst must be able to select the most adequate analytical procedure for the

matrix and chemical species under consideration, and to establish ways to eliminate, miiiimize,

or compensate for these causes of errors. From the factors mentioned above, those which are relevant to the subject discussed in this publication will be examined in more detail.

3. PROCEDURES TO ASSESS AND CONTROL SOME OF THE FACTORS THAT

AFFECT THE PROPER FUNCTIONING AND ACCURACY OF A

SPECTROPHOTOMETER

3.1 Short- and Long-Term Stability

The short- and long-term stability of a spectrophotometer can be verified and monitored

by using glass filters specially selected for this purpose. Stable radiation sources, such as tritium-activated luminescent materials, could be considered for the same purpose [16]. The use

of such a source, however, is not always possible due mainly to problems associated with the

design and operation of the instrument.

15
Of the several kinds of glass filters that can be used, those constituting SRM 930 and

SRM 1930 are most suitable. The optical characteristics of these glass filters are discussed in

Section 4. Since the stability of a spectrophotometer is the first test that should be carried out,

however, we will describe their use for this purpose now.

As a general rule, it is desirable to place the spectrophotometer in a clean laboratory and

in an area where the temperature is as constant and reproducible as possible, protected from

direct sunlight, sudden changes of temperature, corrosive atmosphere, dust, and vibration. The

tests should be carried out after the instrument has been verified by the manufacturer, and it is

found to perform well and to meet all the required specifications.

After an initial warming-up period, the "0%T" and "100%T" readings on the scale are

verified following the procedure described by the manufacturer of the particular instrument. The

wavelength dial is set at a selected value, usually the one at which the transmittance of the

analytical sample will be determined, e.g., 635.0 nm. One of the SRM 930 glass filters in its

metal holder, e.g., the filter having the nominal percent transmittance of 30 %T, is inserted in

the sample compartment. An empty filter holder is positioned in the reference compartment,

or the compartment is left empty. The transmittance of the glass filter is measured following

the method specified by the manufacturer, and the results are recorded. This procedure is

repeated at all wavelengths at which the analytical samples will be measured. Similar

transmittance measurements are performed using the glass filters having nominal percent

transmittances of 20 %T and 10 %T. The temperature at which these measurements are made

should also be recorded. In this manner, the performance of the instrument is determined over

the working spectral range for a transmittance interval and at a temperature that covers most of

the conditions of analytical determinations under consideration. Similar measurements can also

be made using SRM 1930.

The values obtained are displayed in a graphical form by plotting on the ordinate the

transmittance found for every wavelength, and on the abscissa the date at which these

measurements were made. An example of such a graph is given in figure 6 for two

wavelengths, 635.0 nm and 590.0 nm. Similar graphs should be made for the other wavelengths

of interest for the nominal 10%T, 20%T, and 30%T filters. The results for the measurements

shown in figure 6 were obtained in a laboratory in which the surrounding temperature was

maintained at 24.0 °C + 1°C. This temperature was recorded on the graphs as a common

value for all measurements. When solutions are measured, the temperature of the liquid in the

cuvette should be determined. Where the temperature changes from measurement to

measurement, its value should be recorded and written on the graph under the corresponding

transmittance value so that temperature corrections can be made if necessary. The procedure

described above should be performed every time the spectrophotometer is used: before, and

preferably after, the analytical work is carried out.

The data plotted in figure 6 were obtained with a commercial spectrometer provided with

a quartz prism double spectrometer, and a transmittance scale divided in 1000 units between 0

and 100 percent transmittance. The size of each division, and the stability and reproducibility of

16
AT WAVELENGTH 590.0 nm

Figure 6. Control chart for a spectrophotometer showing the variation of transmittance at

24.0 °C as a function of time for a neutral glass filter at 635.0 nm and 590.0 nm.

the instrument is such that it permits visual interpolation between each division, to fractions of

a division; hence, the ordinate scale on the graphs in figure 6 was selected to reflect this

resolution.

An examination of the data shows that the stability of the instrument, as expressed by

percent transmittance, when operated at 24.0 °C from October 22 to February 5, varied from

32.17 %T to 32.28 %T, with an average of 32.23 %T at 635.0 nm. At 590.0 nm, the

equivalent values were 30.79 %T, and 30.90 %T, respectively, with an average of 30.85 %T.

17
The spread, 0.11 %T for both wavelengths, results from the inherent instability of the instrument

and of the glass filters used. This situation changed markedly on February 8 when

transmittances of 31.94 %T and 30.59 %T were found at 635.0 nm and 590.0 nm, respectively.

Since these values largely exceeded the specified spread established for this particular instrument

by the manufacturer, a field engineer from the manufacturer was called in. An examination of

the spectrophotometer revealed that a potentiometer was malfunctioning. After replacement of

this faulty part the instrument was tested again on February 18, and the transmittance values

measured on the same glass filters were within the established spread of the instrument as originally established. As shown in figure 6, the results indicate that the spectrophotometer was

again performing according to its specifications.

This example illustrates the usefulness of the short- and long-term stability tests as well

as data presentation using control charts. This test procedure permits the detection and diagnosis

of possible malfunctions in an instrument before such malfunctions would affect the validity of

measurements. It is obvious that similar control charts can be prepared using absorbance values

instead of transmittance values.

The periodic verification of the stability of spectrophotometers, as described above and

illustrated in figure 6, is used routinely in the NIST laboratories. This procedure is highly

recommended to all users of such instruments, and it is performed as previously mentioned with

the help of specially selected glass filters. It should be mentioned here that the precision of such

tests, however, cannot exceed the stability of the glass filters, which will be discussed in Section

5 for the materials used in this work.

3.2 Wavelength Accuracy

Although the test discussed in section 3.1. may show that a spectrophotometer exhibits

good instrument stability, large errors in the accuracy of its wavelength scale may produce

unreliable measurements. The wavelength accuracy of a spectrophotometer can be verified by

the use of (a) an emission source capable of producing discrete radiations of suitable intensity

and adequate wavelength spacing throughout the spectral range of interest, e.g., 200 to 800 nm,

or (b) glass filters or solutions with sharp absorption bands.

(a) Adequate sources of discrete radiations at well-known wavelengths, most suitable for

use in the calibration of the wavelength scale of a spectrophotometer, are the low-pressure quartz

discharge tubes containing mercury vapor, helium, or neon. Such tubes are available

commercially from the manufacturers of laboratory instrumentation at reasonable prices. One

type of a low-pressure discharge tube has a cylindrical shape with the total length of 120 mm

from which a 50-mm portion constitutes the quartz discharge tube. This tube should be placed

immediately in front of the entrance slit of the spectrometer.

(b) Wavelength calibrations can also be made by using a glass filter having a number

of strong and narrow absorption bands suitably spaced over the spectral range of interest. Two

18
materials have been used or suggested for this purpose: glasses containing rare-earth oxides

such as didymium glasses (mixture of praeseodymium and neodymium oxides) and holmium

oxide glasses. They have been used for many years at the National Institute of Standards and

Technology. The transmittance characteristics of the didymium and holmium oxide glasses are

illustrated in figures 7 and 8, respectively.

A detailed description of the properties and use of low-pressure discharge tubes and of

glass filters with sharp absorption bands is given in NBS Letter Circular LC-1017, "Standards

for Checking the Calibration of Spectrophotometers," by K.S. Gibson, H.J. Keegan, and J.C.

Schleter, reissued in January 1967. Part of this paper is reproduced in Appendix 3. Further

recommended practice can be found in the Manual on Recommended Practices in

Spectrophotometry, published by the American Society of Testing and Materials, 1916 Race

Street, Philadelphia, PA 19103. A comprehensive chapter by Vinter on wavelength calibration

is found in reference 23.

3.2.1 SRM 2009a, Didymium Glass

As seen in table 2, SRM 2009a is issued by NIST as a wavelength standard. It consists

of a didymium glass filter having the nominal spectral transmittances shown in figure 7. This

filter is produced in a nominal size of 10x30x3 mm thick. It is mounted in a metal holder that

fits into the cuvette holder of the spectrophotometer. The spectral bandpass-wavelength

dependence is described in detail in Special Publication 260-66 and in the Certificate that

accompanies each standard. The 14 to 24 certified wavelengths are in the spectral range

400 - 760 nm and are given for spectral bandpasses from 1.5 - 10.5 nm (see Certificate in

Appendix 16).

3.2.2 SRM 2034, Holmium Oxide Solution

Since SRM 2009a is used to verify the wavelength accuracy in the visible spectral range

only, it was necessary to produce an SRM that could be used in the ultraviolet as well. An

adequate material for use in the ultraviolet and visible spectral range was obtained by dissolving

holmium oxide in dilute perchloric acid [24]. The spectral transmittance of the aqua ion

Ho(H2

0);*

+

[25], obtained when holmium oxide is dissolved in a 10 percent aqueous perchloric

acid solution, is illustrated in figure 2 of the paper reproduced in Appendix 4 [24]. The 14

sharp transmittance minima are evenly distributed throughout the spectrum from 240 to 640 nm,

and provide a means of verifying the wavelength accuracy of conventional spectrophotometers

over that spectral range. The wavelengths corresponding to the 14 transmittance minima for

several bandwidths and temperatures are given in Appendix 4.

A list of the wavelengths of minimum transmittance of the holmium oxide solution

reported by other workers is given in table 6 of the paper in Appendix 4. The details of these

measurements are given in reference 4. The data shown in this table indicate that values

obtained by workers in different laboratories are generally in good agreement, and that the NIST

data also agree well with these workers' previously published data. These workers also

19
<

z

DC

1.00

.90

.80

.70

.60

.50

.40

.30

.20

.10

0

CORNING 5120 DIDYMIUM GLASS. 3.01 mm THICK

350 360 370 380 390 400 420 440 460 480 500

WAVELENGTH. NANOMETERS

600 650

1.00

.90

.80

.70

< .60

.50

40U

.30

.20

.10

8.8 nm

8.4 nm

/8.7 nm

0 J L

0.56 0.60 0.70 0 80

9.7 nm

J I I I I L J L J L

1.00 1.20 1.40 1.60 1.80 2.00

WAVELENGTH. MICROMETERS

2.20 2.40 2.60

Figure 7. Transmittance of a didymium glass filter. The spectral bandpass used is marked

at the corresponding transmittance minimum. Courtesy of Corning Glass Works.

20
CORNING 3130 HOLMIUM GLASS. 2 24 mm THICK

0.38 nm

0.33 nm

1^

II

0.77 nm 410

i i ii

236 240 245 250 255 260 270 280 290 300 320 340 360 380 400 WAVELENGTH. NANOMETERS

350 360 370 380 390 400 420 440 460 480 500

WAVELENGTH. NANOMETERS

550 600 650 750

100

90

.80

.70

.60

.50

.40

.30

.20

10

0

3 5 nm

7.2 nm 3 83 nm

3.8 nm

J! 1_ -I 1_ J I 1- 0 56 0.60 0 70 0.80 1 00 120 1.40 1 60 1.80 2 00

WAVELENGTH. MICROMETER

220 2.40 2.60

Figure 8. Transmittance of a holmium oxide glass filter. The spectral bandpass used is

marked at the corresponding transmittance minimum. Courtesy of Corning Glass

Works.

21
found that the wavelengths of minimum transmittance of holmium oxide in similar solutions of

perchloric acid are not sensitive to variations in temperature and concentration. They also

concluded that the wavelengths of minimum transmittance were least affected by changes in

spectral bandwidth for bandwidths less than 1 nm, but that large shifts can be encountered at

bandwidths exceeding 1 nm.

The reported wavelengths of minimum transmittance of the holmium oxide solutions

appearing in Appendix 4 are estimated to be uncertain by no more than ±0.1 nm at the 95

percent confidence limit. These wavelengths of minimum transmittance were found to be

essentially unaffected by changes in temperature at 25 °C + 5 °C. They were also unaffected

by variations in the concentration for solutions containing 2 percent, 4 percent, and 6 percent

by weight of holmium oxide. The critical parameter affecting the measured values of minimum

transmittance was found to be the spectral bandwidth setting of the spectrophotometer. For

spectral bandwidths less than 1 nm, the wavelength shift is generally less than 0.2 nm.

As a result of this work, SRM 2034, "Holmium Oxide Solution as an Ultraviolet and

Visible Wavelength SRM," was issued in 1985. It consists of an all-fused-silica rectangular

cuvette with a 10-mm nominal lightpath, provided at one end with a fused silica tube for filling

the cuvette with a solution of 4 percent holmium oxide by weight in 10 percent aqueous

perchloric acid. The purity of the holmium oxide is 99.99 percent (as indicated by the

manufacturer) and the perchloric acid and distilled water are pure reagent grade prepared at

NIST by sub-boiling distillation. After filling the cuvette, the tubular end is sealed by fusion.

As far as it is known, the solution is stable under typical laboratory conditions for at least 5

years.

Users of SRM 2034 can most effectively determine the wavelength error associated with

their instrument by using the NIST certified data listed in Special Publication 260-102 (also in

Appendix 4) that are representative of the spectral bandwidth setting ordinarily used with the

instrument (see certificate in Appendix 17). The wavelengths of minimum transmittance of the

holmium oxide solution for spectral bandwidths greater than 3 nm have not been evaluated.

3.3 Spectral Bandpass

As mentioned in Section 2.3, the spectral bandpass over the entire spectral range for

various slit widths is an instrumental parameter which should be provided by the manufacturer.

For spectrophotometers with prisms or gratings, the spectral bandpass (AX) is related to

reciprocal linear dispersion (dX/dx) and slit width (w) by the equation given in Section 2.3. For

example, for an instrument with a reciprocal linear dispersion of 15 nm/mm and a slit width of

0.2 mm, the bandpass is 3.0 nm. For prism instruments, the reciprocal linear dispersion and,

therefore, the bandpass vary with wavelength. The following discussion will be limited to the

influence of this important parameter, which is often neglected, on the measurements of

transmittance for the glass filters discussed in this publication.

22
Three types of glass filters having a transmittance that varies only moderately with

wavelength were used: the Chance ON-10, Corning 8364, and Schott NG-4 optically neutral

glasses. The transmittance characteristics of these glasses as a function of wavelength in the

spectral interval of 350 nm to 700 nm were obtained using a Cary 14 spectrophotometer with

an adequate spectral bandpass. The results are illustrated in figure 9, where it can be seen that

the Corning 8364 and Chance ON-10 glasses exhibit similar characteristics. Figure 9 also shows

that these two glasses have an absorbance, or transmittance, more dependent on wavelength than

the Schott NG-4 glass. Since the Corning filter produced results similar to those found for the

Chance ON-10 glass, these two filters will be discussed collectively.

L±J O

<

CD

QTO

CO

CD

<

CORNING

8364

CHANCE

ON-10

SCHOTT

NG-4

EVAPORATED METAL ON QUARTZ

[INCONEL]

RONCHI RULING ON GLASS

100 LINES/in.

i J L 1 1

200 300 400 500 600

X, NANOMETERS

700

Figure 9. Spectral characteristics for five transparent materials from 200 nm to 700 nm [1 1]

.

23
Using a Cary 16 spectrophotometer, the transmittances of the Chance ON- 10 and Schott

NG-4 glasses were measured, as indicated in figures 10 and 11, respectively, at four

wavelengths using variable slit widths, and consequently, various spectral bandpasses from 0.5

nm to 100.0 nm. Figure 10 illustrates the dependence of the transmittance for the Chance

ON-10 (and Corning 8364) glass filter on the spectral bandpass at 400.0, 445.0, 490.0, and

590.0 nm. It can be seen that the transmittance varies with the bandpass, and that this

dependence is stronger at the wavelengths where the glass has a stronger and sharper absorption

band. For instance, at 400.0 nm a rather narrow spectral bandpass of 0.5 nm will be required

for this type of glass to obtain an accurate transmittance value, with everything else being equal,

in particular the wavelength and photometric scale accuracy. At 490.0 nm, however, a spectral

bandpass of 15 nm would be adequate. The same dependence, but to a smaller extent, is

illustrated in figure 11 for a Schott NG-4 glass. It can be seen here that larger spectral

bandpasses could be used to produce accurate transmittance values.

This transmittance-spectral bandpass dependence is summarized in the graph in the upper

left corner of figure 10, where it can be seen that the use of wider spectral bandpasses will

produce the inaccurate Tl and T2 values when compared with Tj' and T2

', which are the

transmittance values obtained using an infinitesimal spectral bandpass. As a general rule, the

value of the spectral bandpass required to produce accurate transmittance measurements with an

uncertainty not greater than 0.1 percent should be 1/20 of the symmetrical natural spectral

bandwidth, at half intensity, of the material to be analyzed and at the wavelength at which the

measurement is performed. When this information is not available, it can be determined by

measuring the transmittance (or absorbance) of the material at the wavelength of interest, using

various spectral bandpasses (or slit widths) as illustrated in figures 10 and 11. The largest

bandpass or slit width that can be used is given by the value found at the end of the horizontal

portion of the transmittance-spectral bandpass curve. The selection and use of adequate spectral

bandpass is one of the indispensable conditions that must be fulfilled to obtain true transmittance

values.

3.4 Stray Radiations

An instrumental parameter that can measurably affect the accuracy of transmittance

measurements is the stray radiant energy (SRE) or stray light. This can be defined generally as unwanted radiation that is measured by the photodetector. Stray radiant energy contributes to the

degradation of optical transmittance measurements and to the reduction of specificity, sensitivity,

and linearity of absorbance measurements.

As mentioned briefly in Section 2.4., two major types of SRE can be identified:

heterochromatic and isochromatic. The former originates in the spectrophotometer and can be

defined as radiation transmitted by the spectrometer that is of a wavelength different from the

spectral bandpass for which the instrument was set. The latter originates in the sample

compartment and can be defined as radiation of the same wavelength as that which falls on the

sample, but reaches the photodetector of the spectrophotometer without passing through the

analytical sample.

24
~«- I I I I I I I I (==)0.0 10.0 20.0 30.0 40.0

SPECTRAL BANDPASS, NANOMETERS

Figure 10. Influence of spectral bandpass on the transmittance of a Chance ON-10 glass filter

measured at 400.0, 445.0, 490.0 and 590.0 nm. The graph at left, upper corner,

describes in general the relation between spectral transmittance and spectral

bandpass. Measurements made with a smaller bandpass | | provide accurate T

values when compared with those made with a larger spectral bandpass [11].

From these simple definitions it might be concluded that both types of stray light can be

assessed easily. Unfortunately, this is not the case. Actually, the accurate measurement of stray

light is subject to ambiguity and is difficult to define and measure [26]

.

25
SCHOTT GLASS FILTER NG-4

X, NANOMETERS

400 500 600 700

OPTICAL TRANSMITTANCE VERSUS SPECTRAL

BANDPASS FOR A SCHOTT GLASS FILTER NG~4

0.0 5.0 10.0 15.0 20.0 250 300 35.0 nm

~ I l

1 1 1 f— 1

^ \

0.0 200 40.0 600 80.0 100.0

SPECTRAL BANDPASS, NANOMETERS

Figure 1 1 . Influence of spectral bandpass on the transmittance of a Schott NG-4 glass filter

measured at 440.0, 465.0, 590.0 and 635.0 nm [11].

3.4.1 SRMs 2032 and 2033

At the present time, two SRMs (SRMs 2032 and 2033) are available to assess both the

heterochromatic and isochromatic stray light. Heterochromatic stray light can be measured by

using a variety of techniques, including the use of blocking optical filters, and this approach was

selected to produce SRM 2032. This material consists of crystalline potassium iodide of

reagent-grade purity which has sharp cutoff absorption near 260 nm, and can be used to assess the stray light below this wavelength. Solutions from this compound were certified for their

26
specific absorbances at eight wavelengths from 240 - 275 run in increments of 5 rrm with an

uncertainty of 5 percent. The measurement consists of setting the spectrometer at a wavelength

below 260 nm, say 255 nm, and placing in the sample compartment a quartz cuvette containing

the potassium iodide solution. Any appreciable amount of light detected is assumed to be

heterochromatic stray light. See also the Certificate for SRM 2032 in Appendix 14.

Isochromatic stray light can be assessed using SRM 2033. This material consists of two

filters made from a semi-transparent evaporated metal on a fused silica substrate each of which

has a nominal percent transmittance of 10 %T. One filter is mounted in a metal holder that can

be inserted in the sample compartment of the spectrophotometer. This holder is provided with

a front and rear shutter, one of which has a window in which the second filter is mounted. This

system is used to assess the maximum isochromatic stray light in the sample compartment that

results from reflections of the incident radiation at the surfaces of the sample and various optical

components and reaches the photodetector without passing through the sample.

The isochromatic SRE test measurement is performed by placing this unit in the sample

beam of the spectrophotometer with the opaque shutter at the rear of the filter holder. Under

these conditions, if a signal is detected, it is caused by reflections at the surface of the filter

exposed to the incident radiation. This radiation is scattered from the walls and other

components of the sample compartment and reaches the photodetector without passing through

the sample. The signal is the isochromatic stray light. See also the Certificate for SRM 2033

in Appendix 15.

3.4.2 A New Method for Measurement of Stray Radiant Energy

A different approach to the measurement of heterochromatic stray light in ultraviolet

absorption spectrometry was described recently by Mielenz, Weidner, and Burke [27]. The

principle and application of the new method is discussed in detail in the publication reproduced

in Appendix 5, and since it appears to have particular merit, it is briefly summarized below.

The test method uses the same solution filters with sharp UV absorption edges as specified in ASTM Test Method E387, but one measures the apparent absorbance of a 10-mm

pathlength cell in the sample beam relative to a 5-mm pathlength cell in the reference beam.

Scanning toward shorter wavelengths, an apparent absorbance maximum, which is a direct

measurement of the heterochromatic stray light, is recorded. This method was found to be in

satisfactory agreement with the ASTM method in comparative tests of several

spectrophotometers at different wavelengths between 200 and 390 nm, using potassium chloride,

potassium iodide, sodium iodide, acetone, and sodium nitrate solution filters. The new method

proved to be simpler than the ASTM method when small amounts of stray light were measured,

the main advantage being that the apparent absorbance maximum occurs at considerably lower

scale values than the corresponding absorbance plateau measured by the ASTM method. This

reduces the need for successive attenuations of the reference beam every time the

spectrophotometer runs off scale; in many instances, the new method required no attenuation.

27
3.5 Accuracy of the Transmittance or Absorbance Scales

Optical transmittance is due to an intrinsic property of matter and characterizes a

particular transparent material. Since this parameter is not known a priori, it must be

determined by experimental procedures. Photometric accuracy is an essential condition that must

be fulfilled by a spectrophotometer for the production of accurate transmittance or absorbance

values.

As mentioned earlier, true transmittance values can be obtained only by using accurate

measuring techniques and by taking into consideration all factors that can affect and distort the

data. In this discussion the optical transmittance T of a solid material includes the reflection

losses that occur at the air-solid interface. The internal transmittance Tj is defined as the

transmittance of the material corrected for reflection losses. This internal transmittance can be

calculated in principle from the transmittance by using the well-known Fresnel equations as shown in the reprint of Appendix 2. The internal transmittance is obtained experimentally when

the measurements are made using a blank sample in the reference beam of the

spectrophotometer. When a blank is not available, as in the case of SRMs 930 and 1930, the

measurements are made against air in the reference beam. Under these circumstances the

resulting transmittance value includes the reflection losses (which are about 8 percent for a glass

filter having an index of refraction of 1.5). A discussion of this subject by Mielenz is given in

reference 20. The absorbance A of a material is related to the internal transmittance Tj by the

expression A = -log 10Tj while the transmittance density D is related to the transmittance T by

the expression D = -log 10T.

Transmittance is the ratio of two radiant fluxes. It is therefore necessary that the

transmittance scale of the spectrophotometer be accurate. The transmittance of a particular

material is also a function of wavelength; hence the wavelength scale of the monochromator

should also be accurate, and appropriate spectral bandpasses should be used. These conditions

were examined in the previous sections. For highest accuracy, the measurements should be

made using collimated radiation. Such radiations define unambiguously the actual pathlength

through the transmitting medium, the reflection losses, and eliminate the effects of polarized

radiations that are produced at the surface of the sample (see Appendices 2 and 6). Other

important factors, already mentioned, that must be considered are: the homogeneity, flatness,

parallelism, and stability of the sample, radiation scatter inside the sample, interference

phenomena, stray radiation, polarization, fluorescence, temperature, particulate matter, and

surface conditions. Since transmittance measurements depend on a diversity of factors,

meaningful values can be obtained only by defining the experimental conditions for obtaining

transmittance data.

Spectrophotometers are used to perform two types of measurements:

(1) Quantitative determination of chemical species using the relation between optical

transmittance of the material and the concentration as a measuring parameter. Under these

circumstances, the photometric scale of the spectrophotometer is calibrated in meaningful units,

28
using a series of reference solutions having known concentrations of the species to be

determined, rather than values of optical transmittance.

The accuracy of the measurements is related to the accuracy with which the

concentrations of the reference solutions are known and to the precision (stability, sensitivity)

of the spectrophotometric method and instrument used. The accuracy of the transmittance or

absorbance scale per se, is not a critical factor in such measurements.

The precision and stability of the instrument are, however, important parameters that

should be verified before and after each series of measurements. This can be done, as already

discussed, by the careful use of selected solid or liquid reference filters having well established

and stable transmittance values.

(2) Determination of the optical transmittance characteristics of solid or liquid materials,

and the determination of certain physicochemical constants such as molar absorptivity and

equilibrium constants. In all of these cases, the accuracy of the transmittance or absorbance

scales of the measuring instrument, among other things, is essential to provide true values.

Ways to establish and check this important parameter are critically needed. For example,

current interest in molar absorptivity values, as an index of the purity of biological or clinical

materials, requires greater accuracy of measurement. Also, the accuracy of the determination

of equilibrium constants of chemical reactions in solutions and the determination of enzyme

activity in international units is dependent on true values of their molar absorptivities.

4 STANDARD REFERENCE MATERIALS FOR TRANSMITTANCE IN

SPECTROPHOTOMETRY

4.1 Description and Selection of Materials for Use as Standard Reference Materials in

Spectrophotometry

Such materials should fulfill the following conditions: (a) be transparent in the spectral

range of interest, usually between 200 nm and 800 ran; (b) have a transmittance independent of

wavelength (optically neutral); (c) have a spectral transmittance independent of temperature; (d)

have low reflectance and be free of interferences; (e) be non-fluorescent; (f) be stable,

homogeneous, and free of strain; (g) have mechanical stability for the size used (thickness,

length, width) and be easy to fabricate by conventional techniques used in optical shops; (h) be

simple to use in conjunction with the conventional spectrophotometers available today in

analytical laboratories; (i) be readily available and relatively inexpensive.

These conditions are fulfilled to a smaller or larger extent by several different materials

including transparent solids and liquids, grids, rotating sectors, and polarizers. Of these

materials, the neutral optical glass filters mentioned in the first group below, most nearly

satisfied the required conditions and constitute the most acceptable compromise when compared

with the other materials.

29
Several neutral glasses were examined, and from these the Corning 8364, the Chance

ON-10, and the Schott NG-4 glasses were initially selected. As illustrated in figure 9, the

spectral transmittances of these glasses are compared with the transmittances of an evaporated

metal-on-quartz filter (non-fluorescent fused silica of optical quality) and a Ronchi ruling on a

glass substrate. The transmittances of the latter two materials exhibit the least dependence on

wavelength from 200 nm to over 700 nm. The Ronchi ruling transmittance below 350 nm is

limited only by the transmittance of the glass substrate and not by the nature of the ruling itself.

For the evaporated metal-on-quartz filter, the attenuation of radiations is produced by reflection

rather than absorption. This material, therefore, was not selected since there is a possibility that

reflected radiation is generated in conventional spectrophotometers. The Ronchi ruling was

rejected since it is subjected, by its nature, to diffraction phenomena. From the remaining

glasses, the Schott NG glass was finally selected since this material exhibits the best optical

neutrality. Excluding its limited spectral transmittance range to the visible and near-infrared

spectral domain, the Schott NG glass is the best material that satisfies the other conditions

enumerated above.

A number of procedures were developed and used for the preparation of SRM 930 and

SRM 1930, and these were tested under a variety of conditions. Of these procedures, only those

that were finally selected and are used for the preparation of these SRMs will be described here.

The optically neutral NG glasses for the filters were produced by Schott of Mainz,

Germany, and are designated as "Jena Colored and Filter Glass." The following Schott NG

glasses were selected: NG-3, NG-4, NG-5, and NG-11. This choice permits stepwise coverage

of a nominal transmittance range from 1%T, 3%T, 10%T, 20%T, 30%T to 50%T, using

nominal glass thickness of 1.5 to 2.0 mm. All glass material was received from the

manufacturer in the form of 150x150 mm (6x6") square plates. From this raw material the

individual filters were obtained by adequate cutting, grinding, and fine polishing in the optical

shops at NIST.

Neutral Glass Filters

Transparent Solids

Evaporated Metals-on Quartz

Inorganic

Transparent Liquids

Organic

Non-Supported: Metal or Plastic Grids

Grids

Supported: Rochi Ruling on Glass or Quartz

Concentric with the Radiation Beam (e.g., Pool Sector)

Rotating Sectors

Eccentric to the Radiation Beam

Polarizers

30
4.2 Preparation of the Glass Filters

Each glass filter is cleaned manually with distilled water using a piece of optical lens

paper. This is followed by a preliminary examination for visible defects using a stereo-microscope with 12X magnification. This examination is performed in an

all-polypropylene hood provided with a vertical laminar flow of filtered air, illustrated in

figure 12.

Figure 12. Instrumentation to examine, clean, and mount the glass filters used to produce

SRMs 930 and 1930. The entire equipment is placed and used in an

all-polypropylene hood provided with vertical clean air flow.

31
4.2.1 Flatness and Parallelism

During the recent years, additional measurements have been performed to determine the

flatness, parallelism, and optical homogeneity of the glass material. The flatness is assessed by

observing the fringe pattern obtained against a flat quartz plate. This is accomplished by using

a conventional instrument (Type "U" reflex viewer 6" capacity from The Van Keuren Co., 176

Waltham Street, Watertown, MA 02172) illustrated in figure 13 (left side). The parallelism is

measured with a thickness mechanical gauge provided with an electronic sensor capable of a

resolution of 0.1 fim (Model 7008A made by the Federal Products, a unit of Esterline Corp.,

Providence, R.I.) illustrated in figure 13 (right side).

Figure 13. Conventional instruments for observing the figure patterns of a glass filter (left)

and for detennining the thickness of a glass filter.

32
4.2.2 Optical Homogeneity

The homogeneity of the glass is determined by measuring the optical transmittance with

densitometer designed for this purpose. The densitometer, illustrated schematically in figure

14a, and pictorially in figures 14b and 14c, is a single-beam arrangement having as a major

characteristic a constant radiation source S made of a tritium-activated phosphor (12.3 years

half-life) with a maximum emission at 560 nm. A mechanical chopper C is placed after the light

source to produce an ac signal (75 cps). A glass lens L forms a reduced image (1x4 mm) of the

source at the filter F. The filter F under investigation is placed in a holder that can be moved

in the x-y directions by two micrometer screws (0.5 mm/turn) operated by individual stepping

motors (200 steps/revolution). The radiation is received on a Budde diffuser [28] followed by

a photomultiplier tube, conventional digital electronics, and printer.

Figure 14a. Schematic of densitometer for the determination of transmittance homogeneity of

glass filters. S = radiation source made of a tritium-activated phosphor, C =

mechanical light chopper; L = glass lens; F = glass filter under investigation

placed in a holder that can be moved in the x-y directions by two stepping motors

attached to micrometer screws; B = Budde' s diffuser; PM = photomultiplier tube.

Scarining of the glass filter is performed by operating the stepping motors of the x-y

system through a microcomputer programmed to measure the transmittance at nine selected

points on the 10x30 mm filter [29]. The use of this measuring tool provides a means to

individually test each filter and to select the ones that conform to specifications. An example

of this transmittance homogeneity check is illustrated by the data from table 3.

The distribution of the test points on the face of the filter is

33
Figure 14b. Densitometer placed in a light-tight enclosure. From right to left: lens producing

the image of the light source at the filter; filter holder with x-y scanning capability

operated by stepping motors; counter for the x movement; pneumatic shutter

followed by Budde's diffuser and photomultiplier tube. At bottom left: pneumatic

"off" and "on" valve for operating shutter.

34
Figure 14c. Light source for densitometer. From right to left: tritium-activated luminescence

light source in a holder; light chopper.

35
Table 3. Homogeneity of a Glass Filter with a Nominal 20 Percent Transmittance

(Filter Number 20-1245)

Mean

Standard

Deviation

Adjusted

Mean

Delta

Center

Relative

Delta

Test Point [5] 0.94664 0.00026 0.94664 0.00000 0.00000

Test Point r ai

[4] 0.94713 A AA 1 T5 0.00123 A C\A H 1 A 0.94714 0.00050 0.05283

Test A. VOI Point X Villi rnL A 1 0.94718 0 00043 0.94720 0 00056 0 05864

Test Point [2] 0.94661 0.00037 0.94664 0.00000 0.00001

Test Point [3] 0.94728 0.00018 0.94732 0.00068 0.07142

Test Point [6] 0.94689 0.00059 0.94694 0.00030 0.03125

Test Point [9] 0.94643 0.00031 0.94649 -0.00015 -0.01633

Test Point [8] 0.94595 0.00031 0.94602 -0.00062 -0.06592

Test Point [7] 0.94727 0.00035 0.94735 0.00071 0.07503

Test Point [8] 0.94682 0.00053 0.94691 0.00027 0.02818

Test Point [5] 0.94654 0.00021 0.94664 0.00000 0.00000

4.3 Cleaning of the Glass Filters

Each glass filter which passes the inspections is washed manually with a diluted aqueous

solution of a nonionic detergent such as alkyl phenoxy polyethoxy ethanol (Triton X-100, Rohm

and Haas), rinsed with distilled water, and dried in air. The filters are then transferred in a 12-

unit stainless steel holder and placed in a Soxhlet extractor, as illustrated in figures 15 and 16,

respectively.

Figure 15. Stainless steel holders for six 50x50 mm, and twelve 30x10 mm glass filters.

36
Figure 16. Soxhlet extractors used for the final cleaning of the 50x50 mm and 30x10 mm

glass filters with electronic-grade isopropyl alcohol.

An aluminum slug is inserted at the bottom of each extraction flask to reduce the volume

of solvent required for the functioning of the extractor and to shorten the syphoning time. To

avoid bumpy boiling, followed sometimes by an undesirable overflow of the isopropyl alcohol

from the boiling flask into the extraction vessel, a side tube provided at the bottom with a coarse

glass frit is inserted through the side of each flask. A steady stream of clean air, from a compressed gas cylinder equipped with a gas regulator and needle valve, is passed through the

tube to produce a continuous and gentle bubbling. The glass filters are then washed for three

hours, at a rate of three syphonings per hour, using isopropyl alcohol of electronic grade (see

references 30 and 31). After this period the glass filters are dried in air in the laminar flow

hood.

Following the cleaning procedure, the glass filters are placed individually in the special

aluminum alloy holders made at NIST in the instrument shops. This operation is performed by

using the tools illustrated in figure 12, which include an anti-static brush and plastic-tipped

tweezers. The mounted filters are placed in a covered plexiglass box and are aged for at least

37
2 weeks in the laminar flow hood where they are exposed continuously to the radiation of the

fluorescent lamps provided at the top (total 120 watts, arbitrarily chosen). Since the individual

metal holder contributes significantly to the usefulness of the glass filters, it will be described

in the next section.

4.4 Filter Holders

The shape and dimensions of these metal holders are given in figures 17 through 22.

Established in conformance to the dimensions of the rectangular sample compartments of most

conventional spectrophotometers, the holders were designed to hold the glass filters without

producing mechanical strains in the glass material, as determined by a polariscope examination.

The metal material used for the holder is an aluminum alloy 2024 (4.5% Cu, 1.5% Mg, 0.6%

Mn). The two-pronged retaining spring is made of phosphor bronze. The metal holder and

retaining spring are both anodized flat black. The spring is secured by a screw and washer made

of black nylon. The dimensions and shape of the window were selected to avoid vignetting even

for spectrophotometers having a low-pass radiation beam. Each filter comprising SRM 930 and

SRM 1930 is provided with a removable front and rear shutter made from flat black Delrin.

II hllll

Figure 17. Filter holder with removable shutters. From left to right: front and rear view of

the holder body provided with dove-tail grooves; retaining spring with nylon screw

and washer; Schott NG glass filter; two shutters; front view of the filter holder

with front shutter, rear view of the filter with rear shutter; filter holder with both

shutters closed.

38
Figure 18. Aluminum alloy filter holder. The retaining spring and the front and rear shutters

are illustrated. Dimensions are given in mm.

Ma (\-iio) —

*)0 (mo)

(k&i) — \\,\ —

|

f 1

1

1

1

u —(jTn 4-

V---

T 1

NB7) (VVT) C.'5)00> y—l i i—

r

0

£ -VOA

\0.\<o -O.O

t

Figure 19. Details of the aluminum alloy filter holder body. Dimensions in parentheses are

in inches. Dimensions not in parentheses are in mm. (Tolerance +0.2 mm.)

39
1 i :1

^==- r-AA

fofa

1

Figure 20. Details of the retaining spring used with the filter holder unit from figure 19.

Material: phosphor-bronze or beryllium-copper. Finish: black oxide. Tolerance:

+0.1 mm. Dimensions in parentheses are given in inches, all others are in mm.

(sv>>

1 w^/w.L. {te&Jkbf)

oA VAV^ —-

*

2..>^k\^.\|!>v.'. 1^)V.ac:v^ v>\x^n

k. T\NO <v>CSa c^yiN

Figure 21 . Details of the front and rear shutters used with the filter holder unit from figure 19.

Material: black Delrin, two pieces for every filter holder. Dimensions in

parentheses are in inches, all others are in mm. Tolerance: ±0.1 mm except on

38.0 (1.5) R.

40
SRM 930, consisting of three glass filters with nominal percent transmittances of 10, 20,

and 30 %T, is stored in a cylindrical black-anodized container for protection (see fig. 22). SRM

1930, consisting of three glass filters with nominal percent transmittances of 1, 3, and 50%T,

is stored in cylindrical blue-anodized containers identical to those used to store SRM 930. Each

glass filter comprising SRMs 930 and 1930 is placed in an individual metal holder. An empty

metal filter holder, also included in SRMs 930 and 1930, should be placed in the reference beam

of the spectrophotometer to provide identical transmittance measuring conditions.

The anodizing process used to produce the flat black layer, which coats the aluminum

alloy filter holders and the cylindrical containers, was examined by Heslin and Hunkeler [32]

for possible degassingof undesirable chemical species thatcould affect in time the transmittance

properties of the glass filters. The results of this study indicate that no such deleterious effect occurs.

Figure 22. Illustration of SRM 930, Glass Filters for Spectrophotometry, showing the

black-anodized aluminum alloy container for storing three filters in their holders

and an empty filter holder. One of the three glass filters, Schott NG-4, is shown

disassembled with the retaining spring, nylon screw, washer, and the filter holder

with the front and rear shutters.

41
5. STABILITY OF SRM 930

As a rule, before taking measurements with the IARD high-accuracy spectrophotometer,

a warmup period of one hour is required. The room temperature is kept at 21 °C ± 1 °C, and

the relative humidity is about 35 percent. The particulate matter is controlled through special

filters that rates the room in the 100,000 class.

5.1 Reproducibility of Transmittance Measurements

Seven Schott NG glass filters having nominal percent transmittances ranging from 10%T

to 50 %T were placed in the sample-holder carousel of the high-accuracy spectrophotometer and

measured in six replications. The results, given in table 4 (see also Appendix 2), show that

these measurements can be reproduced with a pooled relative standard deviation of 0.010

percent.

Table 4. Reproducibility of Transmittance Measurements on Seven Schott

NG Glass Filters: Numbers 2, 4, 6, 8, 10, 12, and 14

Percent Transmittance

Replication No. 2 4 6 8 10 12 14

1 33.327 21.711 12.236 50.990 33.377 20.906 13.473

2 33.325 21.710 12.237 50.983 33.377 20.903 13.471

3 33.321 21.711 12.241 50.992 33.383 20.900 13.474

4 33.320 21.708 12.240 50.998 33.375 20.901 13.470

5 33.323 21.710 12.239 50.983 33.379 20.901 13.474

6 33.325 21.710 12.238 50.986 33.377 20.904 13.470

Average 33.324 21.710 12.238 50.987 33.378 20.902 13.472

Rel. Percent a 0.0080 0.0051 0.0150 0.0072 0.0083 0.0108 0.0141

Ave. Rel. Percent a 0.010

5.2 Temperature Dependence

The temperature dependence of the transmittance was determined for the same Schott NG

type of glass by measuring the transmittance at a room temperature of 20.5 °C and 25.5 °C and

at 440.0 nm, 465.0 nm, 546.1 nm, 590.0 nm, and 635.0 nm. The results are given in table 5.

From these data, it can be concluded for the type of glass used that the transmittance dependence

on temperature over the tested temperature range, in the spectral interval from 440.0 nm to

635.0 nm, is insignificant. For a variation of ±1 °Cto2 °C, this dependence averaged less

than 0.2 percent of the measured transmittance values. The influence of temperature on the

transmittance of glass filters is discussed by A.T. Young [33].

42
Table 5. Influence of Temperature on the Transmittance of Seven

Schott NG Filters at Five Wavelengths

Percent Transmittance

25.5 °C 20.5 °C

v> aveiciigin Date Date Average

(nm) jLHiierence

1-14 1-15 1-17 1-23 1-22 1-24

Q ")Q Q 1(\ y.JK) y.jZ y.jZ y.jZ +0.02

18.70 18.69 18.70 18.73 18.72 18.73 +0.03

29.08 29.07 29.09 29.11 29.10 29.11 +0.03

440.0 32.95 32.96 32.95 32.96 32.95 32.96 +0.01

9.89 9.89 9.89 9.91 9.91 9.91 +0.02

18.53 18.53 18.53 18.54 18.54 18.54 +0.01

29.15 29.15 29.17 29.20 29.20 29.20 +0.04

1 1 c\i 1 1 .VJ 1 1 C\A 1 1 (\A 1

1

1 1 .vj 1 1 f>^ 1 1 ha +0.01

21.06 21.08 21.08 21.09 21.10 21.09 +0.02

32.20 32.21 32.22 32.24 32.25 32.24 +0.03

465.0 35.62 35.61 35.61 35.61 35.61 35.61 0.00

11.69 11.69 11.69 11.70 11.70 11.70 +0.01

20.92 20.91 20.92 20.93 20.92 20.92 0.00

32.27 32.27 32.30 32.32 32.33 32.33 +0.05

Q Q/1 y.yn Qy.yj Qy.yj Q Q"3 y.yj Q Q1 y.yj O Q1 y.yj -0.02

19.59 19.60 19.61 19.57 19.58 19.57 -0.03

30.69 30.69 30.70 30.67 30.68 30.67 -0.02

546.1 33.76 33.76 33.76 33.70 33.71 33.71 -0.05

10.55 10.57 10.57 10.54 10.53 10.54 -0.02

19.43 19.47 19.48 19.43 19.43 19.44 -0.03

30.74 30.76 30.78 30.75 30.73 30.75 -0.02

O.OZ e o O.OZ 8 81 o.oj 8 TO o. ly o.oU 8 en o.oU -0.02

18.02 18.04 18.04 18.00 18.00 18.00 -0.03

27.62 27.65 27.65 27.62 27.62 27.62 -0.02

590.0 31.20 31.22 31.22 31.14 31.14 31.14 -U.U/

9.37 9.38 9.38 9.34 9.35 9.35 -0.03

17.89 17.90 17.90 17.86 17.86 17.86 -0.04

27.69 27.70 27.71 27.68 27.67 27.68 -0.02

9.85 9.86 9.86 9.83 9.83 9.83 -0.03

19.47 19.48 19.49 19.45 19.44 19.45 -0.03

27.74 27.74 27.76 27.72 27.71 27.73 -0.03

635.0 32.65 32.67 32.66 32.59 32.60 32.60 -0.06

10.44 10.44 10.44 10.40 10.41 10.42 -0.03

19.34 19.35 19.35 19.30 19.31 19.31 -0.04

27.78 27.80 27.80 27.77 27.77 27.78 -0.02

43
5.3 Accelerated Visible Radiation Exposure Test

The stability of the Schott NG glass to visible radiations was tested by exposing three

filters having a nominal percent transmittance of 10, 20, or 30%T to the high intensity radiation

of a source with a spectral distribution similar to the conventional fluorescent lamps used for

the illumination of laboratories. Each high intensity exposure lasted 21 hours and was

equivalent to 21,000 hours of normal exposure. The transmittance measurements listed in table

6 were made before and after each of the two exposures. For each wavelength, the first of the

six columns gives the transmittance of the glass filters before the exposure, and the second

column gives the transmittance measurements after the 21 hours of accelerated exposure. The

percent difference between these two measurements is given in the third column. The same

filters were exposed again for another 21 hours. Column 4 gives the transmittance values

before this exposure, while column 5 gives these values after this second 21 -hour accelerated

exposure. The percent difference between these data (columns 4 and 5) is given in column 6.

Transmittance measurements were made at 440.0 run, 465.0 run, 590.0 nm, and 635.0

nm. From these data it can be concluded that the Schott NG type of glass tested exhibits an

acceptable stability when exposed to the visible radiation experiment described. Similar

stability tests performed on other neutral glasses have indicated that these glasses were less

stable by a factor of four when compared with the Schott NG glass. Consequently, the Schott

NG material was selected for SRMs 930 and 1930.

Table 6. Percent Transmittance of Three Schott NG Glass Filters Before and After

Exposure to Accelerated Visible Radiation Test

440.0 nm 465.0 nm

Filler Before After % Diff. Before After % Diff. Before After % Diff. Before After %

Diff.

1 12.96 12.96 0.0 12.97 12.96 0.08 15.22 15.22 0.0 15.22 15.21 0.07

2 20.53 20.53 0.0 20.52 20.52 0.0 23.23 23.23 0.0 23.23 23.22 0.04

3 34.45 34.43 0.06 34.43 34.42 0.03 37.34 37.32 0.05 37.32 37.31 0.03

Filter

590.0 nm 635.0 nm

Before After % Diff. Before After % Diff. Before After % Diff. Before After %

Diff.

1 12.19 12.19 0.0 12.20 12.17 0.25 13.36 13.36 0.0 13.37 13.35 0.15

2 19.61 19.60 0.05 19.61 19.59 0.10 21.03 21.03 0.0 21.04 21.01 0.14

3 33.44 33.42 0.06 33.43 33.40 0.09 35.02 34.98 0.11 35.01 34.98 0.09

44
5.4 Transmittance as a Function of Time

The variation of transmittance as a function of time for the Schott NG type of glasses

(NG-4 and NG-5) was determined by using three filters, which constitute our own reference set,

and measuring them over a period of four years. Table 7 presents the results obtained at 440.0

nm, 465.0 nm, 590.0 nm, and 635.0 nm.

Table 7. Stability of Transmittance Measured on SRM 930 as a Function of Time

waveiengui Percent Transmittance

(nm) Date 1 2 3

05-18-71 32.87 19.80 11.59

07-12-72 32.88 19.83 11.62

08-30-73 32.91 19.81 11.59

440.0 11-16-73 32.96 19.84 11.61

01-09-74 OQ oZ.yo 1 O OA 1 1 /C)

1 1 .oz 08-01-74 32.94 19.84 11.61

01-13-75 32.95 19.84 11.62

05-18-71 35.53 22.59 13.56

07-12-72 35.54 22.62 13.59

08-30-73 35.54 22.62 13.59

465.0 11-16-73 35.62 22.62 13.58

01-09-74 JJ. Uj 1 J .JO

08-01-74 35.60 22.62 13.57

01-13-75 35.60 22.62 13.58

05-18-71 31.13 19.16 10.37

07-12-72 31.14 19.20 10.41

08-30-73 31.14 19.20 10.41

590.0 11-16-73 31.21 19.19 10.40

01-09-74 31.21 19.20 10.41

08-01-74 31.18 19.20 10.40

01-13-75 31.19 19.20 10.41

05-18-71 32.55 20.60 11.37

07-12-72 32.59 20.65 11.39

08-30-73 32.59 20.65 11.39

635.0 11-16-73 32.66 20.65 11.40

01-09-74 32.68 20.67 11.41

08-01-74 32.63 20.66 11.41

01-13-75 32.64 20.66 11.40

45
The largest difference observed after 4 years of use did not exceed about 3 parts in 1000.

The measurements also seem to indicate a trend toward an increase of the transmittance of these

glass filters. This is in accordance with the known property of glass surfaces to produce a layer

of SiO when exposed to atmospheric conditions. This phenomenon is called "blooming" of the

glass, and the thin SiO deposit which is formed acts as an anti-reflection layer, resulting in an

apparent increase of the transmittance of the glass.

Similar measurements were made on another set of three glass filters over a period of

two years. The results of these transmittance measurements, which are listed in table 8, confirm

the data in table 7.

Table 8. Comparison of Transmittance Measurements Performed on a Set of Three

Schott NG Glass Filters Over a Period of Two Years

Percent Transmittance

Filter Wavelength 1971 1972 1973

Identification (nm) May December April

440.0 11.59 11.62 11.60

10%T 465.0 13.56 13.59 13.57

590.0 10.37 10.40 10.39

635.0 11.36 11.40 11.39

440.0 19.80 19.83 19.81

20%T 465.0 22.59 22.62 22.60

590.0 19.16 19.20 19.19

635.0 20.60 20.65 20.65

440.0 32.87 32.88 32.92

30%T 465.0 35.53 35.54 35.59

590.0 31.13 31.14 31.20

635.0 32.55 32.58 32.64

The transmittances of several sets of SRM 930 were determined, after several years of

use under routine laboratory conditions, to assess the effect of time and handling on this

parameter. Table 9 presents the results obtained for Set No. 106, certified in 1972. The column

marked Tl contains the transmittance values as certified on November 29, 1972. This set was

measured again on February 27, 1974, in an "as received" condition; the transmittance values

obtained are given in the column marked T2

. The difference between these measurements is

given in the column marked ATl

. The transmittance of these glass filters, after about 2 years

of use in a laboratory, is lower than the certified values, suggesting a possible contamination of

the glass surface. This supposition was verified by submitting the filters to the cleaning

46
procedure described previously. The filters were remeasured, and the data obtained are given

in the column marked T3

, which represent the average of four series of measurements carried

out over a period of three days. The difference between the T3 measurements (after cleaning)

and the Tl measurements (original certification) is given in the column marked AT2

. The AT2

results, which indicate a positive difference, confirm the hypothesis of a surface contamination

of the glass during their use and verify the natural tendency of the glass to cover itself with a

layer of SiO as was already discussed. As mentioned above, the results of this phenomenon is

an apparent increase of the transmittance of the material.

Table 9. Percent Transmittance of SRM 930, Set 106, After Two

Years of Use in a Laboratory

T, T2 T3 Filter Wavelength As Certified As Received AT! (Cleaned AT2 Number (nm) (11/29/72) (2/27/74) and (T3

- Tx ) Remeasured)

440.0 9.23 9.23 0.00 9.27 +0.04

1-106 465.0 10.95 10.92 -0.03 10.99 +0.04

590.0 8.77 8.74 -0.03 8.78 +0.01

635.0 9.83 9.80 -0.03 9.84 +0.01

440.0 16.53 16.51 -0.02 16.58 +0.05

2-106 465.0 18.78 18.73 -0.05 18.82 +0.04

590.0 15.00 15.84 -0.06 15.92 +0.02

635.0 17.33 17.25 -0.08 17.33 0.00

440.0 30.02 29.98 -0.04 30.12 +0.10

3-106 465.0 33.37 33.29 -0.08 33.47 +0.10

590.0 29.91 29.81 -0.10 29.94 +0.03

635.0 30.90 30.78 -0.12 30.90 0.00

Additional data concerning the stability of SRM 930 are collected from measurements

performed on filter sets that are returned to NIST for verification and, where needed, for

recalibration. Since their issuance in 1971, about 100 sets of SRM 930 have been sold each year

in the United States and overseas and, to date, over 1700 sets have been produced. A number

of users routinely return the filters for verification on an annual basis, as suggested in the

Certificate. Other users request recalibration less frequently. At the present time at least 50 sets are verified per year.

From the pool of recalibration data, about 85 percent of the sets indicate that the

transmittance values stay within the 0.5 percent relative uncertainty noted in the Certificate. The

remaining 15 percent of the sets show a change larger than 0.5 percent relative; none has shown

a change that exceed 1.5 percent relative.

47
In some cases an excellent transmittance stability is demonstrated. For instance, Set No.

100, issued in 1974, has been measured over a period of 19 years in our laboratory. The

transmittances measured on these glasses from 1974 through 1992 are given in table 10. The

transmittance data in 1992 reproduce the initial values in 1974 with differences that exceed 0.5

percent relative only in one case (nominal 10%T filter at 635 nm, table 10a).

Table 10a. Comparison of Percent Transmittance Values Obtained From 1974 to 1992

for SRM 930, Set No. 100, Filter with a Nominal 10% Transmittance

Wavelength

Date 440 nm 465 nm 546.1 nm 590 nm 635 nm

1974 9.43 11 to 11. lo 1 A 11 10. 12 O C\A 10.00

1975 9.45 11 in 11.19 10.12 O C\A 8.94 10.01

1976 9 45 111.17 1 19 10 12 8 95 10 01

1977 9.44 11.18 10.11 8.93 10.01

1978 9.45 11.19 10.11 8.93 10.00

1979 9.45 11.19 10.12 8.94 10.00

1980 9.45 11.19 10.12 8.94 10.00

1981 9.45 11.19 10.12 8.94 10.01

1982 9.46 11.21 10.14 8.96 10.03

1983 9.45 11.19 10.13 8.94 10.01

1984 9.47 11.21 10.14 8.96 10.02

1985 9.46 11.19 10.13 8.96 10.01

1986 9.48 11.22 10.16 8.99 10.05

1987 9.47 11.21 10.14 8.97 10.03

1988 9.47 11.21 10.15 8.98 10.04

1989 9.48 11.22 10.15 8.98 10.06

1990 9.48 11.22 10.15 8.97 10.04

1991 9.48 11.22 10.14 8.97 10.03

1992 9.49 11.23 10.16 8.98 10.06

Rel. %

Change +0.42 +0.45 +0.40 +0.45 +0.60

48
Table 10b. Comparison of Percent Transmittance Values Obtained From 1974 to 1992

for SRM 930, Set No. 100, Filter with a Nominal 20% Transmittance

Wavelength

Date ttu mil nun I1III DHUJ.A lilll jy\f son nrn 11111 lilll

1i 074. y /

1

17.48 19.78 18.37 16.78 18.21

1 075 17.49 19.80 18.39 16.79 18.23

1076 17.48 19.79 18.39 16.80 18.23

1077 17 / / 17.48 19.77 18.37 16.78 18.22

1078 17/0 17.49 19.79 18.37 16.78 18.22

1070 i y 1 y 17.50 19.79 18.38 16.79 18.23

1080 17.49 19.79 18.39 16.79 18.23

1081 1 70

1

17.50 19.80 18.41 16.81 18.23

JL108? 70Zr 17.50 19.82 18.42 16.82 18.25

1083 1 70J 17.49 19.79 18.39 16.80 18.23

1984 17 40 10 80 1 8 40 16 81 1 8 94

1985 17.49 19.79 18.40 16.82 18.25

1986 17.51 19.81 18.43 16.84 18.28

1 hot 19o/ 17.51 19.82 18.42 16.84 18.27

1988 17.52 19.82 18.43 16.84 18.28

1989 17.52 19.82 18.42 16.84 18.28

1990 17.52 19.81 18.40 16.82 18.26

1991 17.53 19.82 18.40 16.82 18.27

1992 17.51 19.81 18.40 16.82 18.26

Rel. %

Change +0.17 +0.15 +0.16 +0.24 +0.27

All the measurements discussed thus far indicate clearly that the glass filters can be used as a secondary transfer optical transmittance standard with an uncertainty of <0.5 percent relative.

We would like to briefly mention, however, a case where the glass material exhibited a greater

instability [34] . This property was recorded on several sets of SRM 930b where an instability

with time of about twice the certified uncertainty was observed. This instability was traced to

the initial grinding and polishing of the glass material by the manufacturer. The problem was

eliminated by fine grinding and polishing at the NIST optical shops, and by aging the material

in our laboratory for a least six months before its issuance as an SRM. Further information

concerning the stability of the optical transmittance of glass filters is found in a paper by W.R.

Blevin [35].

49
Table 10c. Comparison of Percent Transmittance Values Obtained From 1974 to 1992

for SRM 930, Set No. 100, Filter with a Nominal 30% Transmittance

Wavelength

Date TtU I111I *+U*7 /t£%£Z urn 1 1111 JIUil 11111 UJJ 11111

1974 30.85 34.22 32.73 30.60 31.55

1975 30.86 34.25 32.78 30.66 31.60

1976 30.82 34.20 32.75 30.64 31.61

1977 A. ^ 1 1 30.81 34.20 32.72 30.62 31.59

1978 30.85 34.19 32.72 30.62 31.58

1979 A. J 1 s 30.84 34.23 32.76 30.65 31.61

1980 A. KJ\J 30.85 34.24 32.78 30.66 31.61

1981 X S VJ A. 30.83 34.23 32.78 30.67 31.62

1982 30.82 34.22 32.80 30.68 31.64

1983 30.81 34.21 32.76 30.67 31.61

1984 30 81 34 1Q 3? 77 30 fifi 31 61

1985 30.80 34.20 32.77 30.67 31.61

1986 30.84 34.23 32.82 30.71 31.66

30.83 34.22 32.81 30.71 31.67

1988 30.84 34.25 32.82 30.73 31.69

1989 30.83 34.22 32.79 30.70 31.67

1990 30.81 34.19 32.74 30.65 31.63

1991 30.81 34.20 32.74 30.66 31.64

1992 30.80 34.20 32.73 30.65 31.63

Rel. %

Change -0.16 -0.06 0.00 +0.16 +0.25

5.5 Interlaboratory Transmittance Measurements

A set of SRM 930 was measured at NIST and at the National Physical Laboratory (NPL)

in England by F.J. J. Clarke and his associates using their high-accuracy instrument. Both NIST

and NPL measurements were made with noncollimated, convergent-beam geometry. A

rectangular surface area of the filter, about 3x8 mm, was used at NPL. At NIST, a surface

area of about 8x0.5 mm was used for the transmittance measurements. The results given in

table 11 indicate that an average difference of 0. 19 percent of the measured values was obtained

between the measurements carried out at NPL and at NIST.

50
Table 11. Comparison of Transmittance Measured on Three Schott NG

Glass Filters at NPL and NIST

%T

Rplativp Pprrpnt

Wavplpnoth NIST NPL Tiif

f

prpnpp

1 flVIST-NPLl

440.0 12.92 12.93 -0.08

465 0 14.97 15.01 -0 27

590 0 11.67 11.67 0\J . 00\J\J

635 0 12.70 12.72 -0 16

440.0 19.60 19.62 -0.10

465.0 22.37 22.43 -0.27

590.0 19.01 19.01 0.00

635.0 20.41 20.47 -0.29

440.0 32.88 32.98 -0.30

465.0 35.53 35.66 -0.36

590.0 31.13 31.21 -0.26

635.0 32.54 32.62 -0.25

Average relative percent difference -0.19%

Another comparative study, which included three clinical laboratories, was performed to

determine the reproducibility of transmittance measurements on the Schott NG glass filters.

Three filters having nominal percent transmittances of 10, 20, and 30%T were first measured

at four wavelengths on a conventional spectrophotometer at NIST. This instrument included a double monochromator with quartz prisms used in a double-beam mode, and a transmittance

scale with 1000 divisions between 0 and 100 percent transmittance. The transmittance accuracy

of this scale was established using SRM 930. The same filters, including instructions describing

the technique to be used in measuring their transmittances, were then sent to three laboratories,

arbitarily designated A, B, and C.

The results obtained are summarized in table 12 and include the average percent

transmittance (%T) values, the standard deviation (S.D.), the percent relative standard deviation

(%R.S.D.), and the percent difference (% Diff.). The percent difference values were calculated

using the NIST data as the reference values. This study demonstrates that transmittance data can

be reproduced under actual laboratory conditions with a general uncertainty of less than 1 percent. From the total of 36 transmittance measurements, only 3 measurements exceed 1 percent (all by laboratory B), and this was due probably to inadequate spectral bandpass of the

particular spectrophotometer used.

51
Table 12. Comparison of Transmittance Measurements Performed by

Four Laboratories on Three Schott NG Glass Filters

Laboratory A %T

tliter No. 440 nm 465 nm 590 nm 635 nm

1 Ave. 12.91 14.99 11.68 12.71

S.D. 0.0063 0.004 0.008 0.005

07 T> C 1~\ /o K.o.D. 0.049 0.027 0.070 0.041

To Dirt. -0 4 -0V/. 07v/ / -0 W.J

2 Ave. 19.61 22.37 19.06 20.50

S.D. 0.004 0.010 0.004

% R.S.D. 0.021 0.046 0.022

% Diff. -0.5 -0.3 -0.3 -0.1

3 Ave. 32.81 35.44 31.12 32.58

S.D. 0.0080 0.0080 0.0050 0.0050

% R.S.D. 0.023 0.023 0.017 0.016

% Diff. +0.3 +0.3 +0.1 +0.2

Laboratory B %T

Filter No. 440 nm 465 nm 590 nm 635 nm

1 Ave. 12.91 14.87 11.56 12.59

S.D. 0.041 0.041

% R.S.D. 0.276 0.36

% Diff. -0.4 -0.9 -1.6 -1.4

2 Ave. 19.59 22.39 18.90 20.51'

S.D. 0.053

% R.S.D. 0.281

% Diff. -0.6 -0.2 -1.1 -0.05

3 Ave. 32.58 35.23 30.90 32.36

S.D. 0.093

% R.S.D. 0.264

% Diff. -0.4 -0.3 -0.6 -0.4

52
Table 12. Comparison of Transmittance Measurements Performed by

Four Laboratories on Three Schott NG Glass Filters (Continued)

%T

Filter No. 440 nm 465 nm 590 nm 635 nm

1 Ave. 12.88 14.96 11.66 12.65

\j.Lr. - 0.014 0.011 0.018

% R.S.D. - 0.09 0.09 0.15

% Diff. -0.6 -0.3 -0.8 -0.9

2 Ave. 19.54 22.34 19.12 20.37

S.D. 0.024

% R.S.D. 0.13

% Diff. -0.9 -0.4 +0.05 -0.7

3 Ave. 32.88 35.40 30.97 32.43

S.D.

% R.S.D.

% Diff. +0.5 +0.1 -0.4 -0.2

NIST Laboratory %T

Filter No. 440 nm 465 nm 590 nm 635 nm

1 Ave. 12.96 15.00 11.75 12.77

S.D. 0.0053 0.0037 0.0048 0.0057

% R.S.D. 0.041 0.025 0.041 0.045

2 Ave. 19.71 22.44 19.11 20.52

S.D. 0.0049 0.0052 0.0038 0.0049

% R.S.D. 0.025 0.023 0.020 0.024

3 Ave. 32.72 35.35 31.09 32.50

S.D. 0.0045 0.0056 0.0040 0.0043

% R.S.D. 0.014 0.016 0.013 0.013

53
6. SCATTER, HOMOGENEITY, POLARIZATION, AND POSITIONING OF SCHOTT

NG GLASS FILTERS

The conditions required for a material to be useful as a reference standard were

enumerated at the beginning of this work. Among those mentioned earlier, and which will be

discussed further here, include: freedom of scatter and polarization, and homogeneity.

6.1 Influence of Scatter, Homogeneity, and Polarization on Transmittance

Scatter and homogeneity measurements in polarized radiation were performed on three

Schott NG glass filters by K.L. Eckerle in the Radiometric Physics Division at NIST using the

instrument described in reference 36. All measurements were performed at 546.1 nm, and the

results are given in table 13.

Table 13. Measurement of Scatter, Homogeneity, and Polarization on

Three Schott NG Glass Filters

(x,y) ft T(0°) ATR

(0°) T(90°) ATR(90°) T(Ave) ATR

(Ave)

Set #537 Filter 1

(+1,0)

(-1,0)

(0,0)

(0,0)

(0,0)

0.0045

0.0045

0.0045

0.0012

0.014

0.10038

0.10047

0.10042

0.10046

0.10039

0.000014

0.000015

0.000012

0.000013

0.000013

0.10042

0.10042

0.10035

0.000012

0.000026

0.000020

0.10042

0.10044

0.10037

0.000008

0.000015

0.000012

Set #537 Filter 2

(+1,0)

(-1,0)

(0,0)

(0,0)

(0,0)

0.0045

0.0045

0.0045

0.0012

0.014

0.19240

0.19216

0.19254

0.19252

0.19237

0.000023

0.000031

0.000022

0.000026

0.000024

0.19250

0.19249

0.19239

0.000028

0.000021

0.000027

0.19252

0.19251

0.19238

0.000018

0.000017

0.000018

Set #537 Filter 3

(+1,0)

(-1,0)

(0,0)

(0,0)

(0,0)

0.0045

0.0045

0.0045

0.0012

0.014

0.30875

0.30872

0.30877

0.30875

0.30848

0.000024

0.000019

0.000020

0.000042

0.000028

0.30874

0.30874

0.30847

0.000028

0.000049

0.000035

0.30875

0.30874

0.30847

0.000017

0.000032

0.000023

54
The quantities (x,y) are the coordinates of the center of the illuminating spot with respect to

the center of the filter in millimeters when looking in the direction of the source. The

parameter, Q, is the approximate solid angle in steradians. The quantities T(0°), T(90°), and

T(Ave) are the transmittances for the electric vector of the illuminating radiation horizontal,

vertical, and average of horizontal and vertical, respectively, measured at 546. 1 nm. Their

respective standard errors are ATR(0°), ATR

(90°), and ATR

(Ave).

The radiation scatter measurements were made by determining the transmittance of the

filters successively at distances from the averaging sphere of 210 mm, 370 mm, and 715 mm

corresponding to solid angles of about 0.014, 0.0045, and 0.0012 steradians, respectively. The

diameter of the circular entrance aperture at the averaging sphere was 28 mm. To assess the

uniformity of the filters, the transmittance measurements were made at a constant solid angle

of 0.0045 steradians. From the data in table 13 it can be concluded that, within the

experimental measuring conditions, there is no significant change of transmittance produced

by scatter and polarization, and that the Schott NG glass has good homogeneity.

The absence of polarization influence on transmittance was verified, again on a set of

SRM 930, as shown in table 14. This table shows a comparison of transmittance

measurements performed on the same SRM 930 set at 465.0 and 546.1 nm using the two

high-accuracy spectrophotometers at NIST in the Inorganic Analytical Research Division

(IARD) and the Radiometric Physics Division (RPD).

Table 14. Transmittance of a Set of SRM 930 at Two Wavelengths Obtained at NIST

Using the IARD and the RPD High-Accuracy Spectrophotometers

HighAccuracy

Instrument

Nominal 30%T

Filter

Nominal 20%T

Filter

Nominal 10%T

Filter

465.0 nm 546.1

nm

465.0

nm

564.1

nm

465.0 nm 546.1 nm

IARD

RPD

0.3427

0.3430

0.3280

0.3284

0.1980

0.1983

0.1840

0.1844

0.1120

0.1121

0.1014

0.1015

A(RPD)* -0.00002 -0.00007 -0.00008 -0.00005 -0.00001 -0.00002

* The difference, A(RPD), is the difference in transmittances measured by

illuminating the filters with polarized radiation having the electric vector in

either the horizontal or vertical direction.

55
6.2 Influence of Positioning of the Filters on Transmittance

The transmittance error resulting from positioning the glass filter at various angles to

the incident radiation beam was determined. In these measurements, the filter was placed

in the high-accuracy spectrophotometer on a vertical holder attached to a 10-cm diameter rotary

horizontal table. This table had a scale divided into 360° with a vernier reading to 5 min, and

was held on the single-sample carriage of the spectrophotometer with a vertical mounting rod.

This remote-controlled carriage has a smooth transversal movement, and the sample positioning

in and out of the beam can be reproduced to within 0.025 mm.

Table 15 presents the comparative results for transmittance measurements made at

590.0 nm on a Schott NG neutral glass filter, 2-mm thick, having a nominal percent

transmittance of 30%T, for normal incidence and for angles of 3°, 6°, and 9°. From these

results it can be concluded that a deviation of up to 3° from a normal incidence position can

be tolerated. Since this parameter is instrument dependent, however, the results from table

1 5 are valid only for the high-accuracy spectrophotometer used to perform these measurements.

Table 15. Percent Transmittance Measured at 590.0 nm on a Schott NG Glass Filter,

2-mm Thick, Having a Nominal 30% Transmittance for Normal

Incidence and for Angles of 3°, 6°, and 9°

Angle %T

0° 28.13

3° 28.10

6° 28.03

9° 27.98

7. EXAMPLE OF USING SRM 930 FOR TRANSMITTANCE SCALE

VERIFICATION AND CALIBRATION

In another series of tests, a set of SRM 930 was used to verify and correct the

transmittance scale of a conventional spectrophotometer. The difference in scales was

determined by measuring the transmittances of the certified glass filters on the tested

instrument, and by comparing the results with the certified values. These results are presented

in table 16.

56
Table 16. Comparison of Percent Transmittance Measurements Performed on a Schott

NG Filter Using the NIST Inorganic Analytical Research Division Instrument and

a Conventional Spectrophotometer

NIST Conventional Relative

Wavelength (nm) Instrumen Instrument Difference %

t Difference

440.0 32.82 32.75 -0.07 -0.21

465.0 35.51 35.40 -0.11 -0.31

590.0 30.98 30.93 -0.05 -0.16

635.0 32.38 32.32 -0.06 -0.19

Average Relative % Difference -0.22

From the data in table 16, it can be concluded that the transmittance scale of this

conventional spectrophotometer provides transmittances that are lower than the accurate values

by an average difference of 0.22 percent. This average difference can be added to the

measurements made with the conventional instrument to obtain the accurate transmittance

values. Another method to perform the correction is to use an interpolation procedure using

a graph established by plotting on the abscissa the accurate transmittance (or absorbance)

values of SRM 930, and on the ordinate the values found on the instrument for this material.

In this discussion it was implicitly assumed that all other parameters that can affect the

transmittance accuracy have been considered and minimized. The stability of the instrument,

the wavelength accuracy, the spectral bandpass, stray light, and temperature are the parameters

that require special attention. Correction of the transmittance or absorbance scale of a

spectrophotometer should be applied only when these conditions are fulfilled.

8. TRANSMITTANCE TRANSFER STANDARDS FROM THE NATIONAL

PHYSICAL LABORATORY IN GREAT BRITAIN AND THE PHYSICS AND

ENGINEERING LABORATORY, DSIR, IN NEW ZEALAND

The National Physical Laboratory in Teddington, Great Britain, is issuing on a regular

basis semi-transparent evaporated metal filters on fused-silica substrates as transfer standards

in the ultraviolet, visible, and near-infrared spectral ranges. The metal used is a specially

selected nichrome alloy. The exposed metal surface of these filters is not protected. These

filters are certified at any selected densities over the spectral range as required by the user. Neutral optical glasses of the Schott NG type are also available for work in the visible spectral

range, and those available from stock have nominal absorbances of 0.04, 0.15, 0.25, 0.5, 1.0,

1.5, 2.0, 2.5, and 3.0 [37-39].

57
Neutral glass filters, similar to the NIST SRM 930 discussed in this publication, are described by Bittar and Havelin. They are certified using a high-accuracy spectrophotometer

designed and built at the Physics and Engineering Laboratory in New Zealand [40].

9. SUMMARIZING REMARKS

The acquisition of meaningful measurements by spectrophotometry requires that every

phase of the chemical processes involved in the analytical procedure, and every parameter of

the instrumental spectrophotometric technique be well-known and understood. The analyst

must be able to assess the importance of these two factors in a quantitative manner, and to

evaluate their individual contributions to the uncertainty of the final measurement. Some of

the parameters involved in the instrumental spectrophotometric technique have been examined

in this publication, and particular attention was given to those instrumental parameters that play

a more significant role: the short- and long-term instrumental stability, the wavelength

accuracy, the spectral bandpass problem, stray radiation, and the transmittance accuracy.

Means and methods have been discussed, and Standard Reference Materials have been

described and produced, that permit to determine to a large extent the magnitude of these

important parameters and to assess and correct their individual contributions toward the

deterioration of spectrophotometric precision and accuracy.

Instrumental stability, wavelength accuracy, adequate spectral bandpass, freedom of

stray radiation, and transmittance accuracy are basic parameters that play a determinant role

in accurate transmittance measurements of a transparent material in the spectral range of

interest. Without the fulfillment of these conditions no accurate transmittance measurements

could be performed. SRMs 930 and 1930 seem to be a useful means to assume that the

measurements are accurate.

The usefulness of glass filters has also been established by the fact that they are used

by several manufacturers of spectrophotometers to establish and monitor the accuracy of the

transmittance scale of their instruments. While the use of SRM 930 and SRM 1930 do not

solve all problems in spectrophotometry, their availability have contributed appreciably to the

production of more accurate and, therefore, more meaningful measurements in all fields where

spectrophotometry is applied.

The production of accurate chemical analytical data, that is the capability to determine

accurately the amount of a particular chemical species in a given matrix, requires, on the other

hand, spectrophotometric precision, as determined by the instrumental stability and sensitivity.

The accuracy of the analytical measurements is determined in this case by the accuracy

achieved in the preparation of the analytical samples and in the establishment of the reference

curve. These in turn depend on the stability of the chemical reactions involved in the

spectrophotometric process, and on the ability to prepare adequate reference solutions having

accurately known concentrations of the chemical species of interest in a matrix identical to that

of the analytical sample. Under these circumstances, analytical accuracy can be achieved using

58
spectrophotometers which exhibit good stability and sensitivity. The wavelength accuracy, the

spectral bandpass, and the accuracy of the transmittance or absorbance scale play a lesser role

in this analytical application. The lack of accuracy of these parameters could affect the

sensitivity and linearity, and in some cases, the specificity of spectrophotochemical analyses.

Standard Reference Materials 930 and 1930, Glass Filters for Spectrophotometry, are

transfer standards that are calibrated for transmittance, in the spectral range from 440.0 nm to

635.0 nm, using a high-accuracy spectrophotometer which is a primary transmittance standard.

The transmittance values were certified with a relative uncertainty of 0.7 to 0.3 percent,

depending on the transmittance level of the filter. The validity of these transmittance values

with time will depend on the stability of the glass and the conditions under which it is used.

The measurements from table 10 prove that the intrinsic stability of the certified transmittance

values are reproducible, within the uncertainty indicated, over a length of time of at least 19

years. This assumes that the filters have been used with proper care, following the instructions

given in the Certificate. Deviations from these instructions could result in degradation or loss

of accuracy. In particular, the glass surface should never be touched with the fingers, the

filters should not be exposed to dust or corrosive reagents, and when not in use, they should

always be kept in the container provided for this purpose and with the shutters inserted.

Should the glass surface of the filters become accidentally contaminated, no attempts

should be made to wash it, since the user does not have means to determine if this cleaning

treatment has not altered the properties of the glass surface and degraded the accuracy of the

certified transmittance values. Since SRMs 930 and 1930 are transfer standards, the only

means available to determine their integrity is by remeasuring their transmittance with a primary standard instrument, and this can be done only with a high-accuracy spectrophotometer

like the one described in this work. Should there be a need to verify the accuracy of the

certified transmittance data, the glass filters may be returned to NIST for verification of the

transmittance data assigned to that particular set, and for cleaning and recalibration if

necessary. Before returning the glass filters, it is recommended that proper arrangements be

made by contacting the Atomic and Molecular Spectometry Group in the Inorganic Analytical

Research Division at NIST to establish the best conditions in which such a verification can be

made.

SRMs 930 and 1930 are delicate and costly units that have required an appreciable

amount of careful work for preparation and certification. These materials should be handled

with great attention to ensure the production of accurate measurements and long life. This is

particularly true when the working conditions in certain laboratories require frequent use of

these SRMs. To protect these reference materials in such cases, it might prove desirable to

produce "in house" secondary glass standards derived from SRMs 930 and 1930, and use these

secondary standards for daily control of the spectrophotometers used in the laboratory. When

these standards indicate a significant change in the measurements, only then should SRMs 930

and 1930 be used as final verification of the data.

Secondary glass standards could be produced by purchasing the optically neutral glass

used for the production of SRMs 930 and 1930, cut to size as described in this publication, and

placed in holders similar to those illustrated in detail in figures 17 through 21. The

transmittance of these filters should be established at the desired wavelengths with the best

59
spectrophotometer available in the laboratory after its transmittance scale was verified with

SRMs 930 and 1930.

10. UNCERTAINTY ESTIMATION FOR SRM 930D

The uncertainty estimation for SRM 930d is detailed in the table below. The source

of each estimated uncertainty component in SP 260-116 is indicated in the table, and each is

discussed by number in the text. Uncertainty components for items 1-6 are derived from an

estimate of the range, ±ai5 with the assumption that the uncertainty is uniformly distributed.

The (normally distributed) standard uncertainty component is then approximated as ajA/3.[41]

Items 2-4 have ranges not symmetrically distributed about zero, but are estimated in the same

fashion, and the negative going components (items 3 and 4) combine in quadrature to yield

the same uncertainty as the positive going component (item 2), so the three are combined as a single uncertainty component, normally designated as "stability/geometry".

Table 17. Estimation of Overall Uncertainty for SRM 930d

i Description of Standard Uncertainty

Component, Uj

note

(Rel.%)

1 Uniformity of Filter over Designated Area a 0.173

2 Two-year Stability of Filter b

3 Filter Orientation with respect to Optical Axis c 0.140

4 Exposure to Light and Laboratory Atmosphere d

5 Temperature Effects over Permitted Range e 0.115

6 Instrumental Linearity f 0.058

7 Short-Term Reproducibility g 0.010

Overall Estimated Uncertainty, U h 0.515

a. Acceptance value for optical homogeneity test described in Section 4.2.2.

b. Estimated from a graphical study of Tables 7, 8, and 10 in Section 5.4.

c. Estimated from the data of Table 15, Section 6.2.

d. Estimated from Table 6, Section 5.3, p. 44, and Table 9, Section 5.4.

e. Estimated from a graphical study of the data in Table 5, Section 5.2.

f. Appendix 2, Figure 16 and related discussion.

g . Table 4 , Section 5.1.

h. See "Overall estimated uncertainty" below for the error propagation formula

used.

1 . The reference spectrophotometer irradiates an area of approximately 8 mm by 1 mm,

which is approximately centered in the filter. Users instruments may irradiate an area of

either different size, different location, or both. To cover this eventuality, the filters are

60
tested for homogeneity in a densitometer as described in section 4.2.2. The irradiation area

in the densitometer is approximately 1 mm by 4 mm, and the 9 locations examined span an

area of 5 mm by 24 mm. A filter is rejected if the transmittance at any of the eight

peripheral sampling locations differs from that at the center location by 0.3% relative or

more. Many commercial instruments irradiate an area which is below the centerline of the

cuvette, to better accomodate partially filled cuvettes.

2. The time dependence of filter transmittance in tables 7, 8, and 10 is confounded with

other error components, such as u3 and u5

. However, the data of these tables reveal a clear

increase in transmittance with time, in accord with the proposed explanation discussed in

Section 5.4. A fit of the data of table 7 averaged over all wavelengths and all three

transmittance values is consistent with a possible change in transmittance of 0.25% relative

over the period of time between 0.5 years and 2.5 years following the grinding and polishing

of a filter. Since the filter is aged for 0.5 years at NIST before the final certification, and is

certified for a period of 2 years, this figure is taken as an appropriate estimate of the positive

(increasing transmittance) uncertainty component. If the filter is returned to NIST for

cleaning and recalibration on 2-year intervals, as is recommended, the time drift effect

should be considerably lower over 2-year intervals subsequent to the first one.

3. The transmittance of the filter is a function of angle of incidence because the reflectance

from the entrance and exit surfaces increases with increasing deviation from normal

incidence. From the data of table 15, it may be determined that a loss in transmittance

(negative bias) of 0.2% accomodates a tolerance of ±4 degrees of arc in the alignment of the

filter with respect to normal. Furthermore, the f/10 converging optics in the NIST reference

spectrophotometer imply that the angle of incidence in the irradiating light varies from

normal to as much as the half-angle of the illuminating cone — or arctan(0.1/2)«3 degrees

of arc — off of normal. This negative bias in the IARD spectrophotometer used to certify

SRM 930 may account for the consistent negative bias indicated by table 14 for

intercomparisons with the reference instrument located in the Radiometric Physics Division

(RPD), which employs collimated beam optics.

4. From tables 6 and 9, it is evident that the effect of exposure to light and laboratory

atmosphere is to reduce the transmittance fractionally. The error estimate here is somewhat

subjective, and is designed with the intent that the true value of transmittance fall within the

stated uncertainty limits over the two-year period of certification, assuming that the user has

handled the filters as recommended, and kept them stored in the containers provided for the

majority of the time between infrequent calibrations. Each of the exposures of table 6

represents three years of around-the-clock exposure to typical ambient light levels, so that

this effect is essentially negligible for the recommended procedure. On the other hand, the

estimated error component is less than the effect of laboratory atmosphere indicated by the

data of table 9, especially considering the offsetting effect of error component u2. The

anecdotal data set of table 9 is thus considered to be less compelling than the experience with

NIST reference filters kept in-house over a period of 19 years.

5. A graphical study of the data of table 5 reveals that the relative percent difference

between a transmittance measured at 25.5 °C and at 20.5 °C is clearly a function of both the

wavelength and the nominal transmittance of the filter. The value given is scaled linearly

61
from the result of the experiment to the total tolerance of 4 °C (±2 °C) considered

acceptable for use of the filter. The temperature effect is most notable for low transmittance

and for the extremes of wavelength certified. Users operating outside of the recommended

temperature range of 21 ±2 °C may expect to encounter difficulties with the nominal 10 %T

filter at the wavelength extremes.

6. The inherent accuracy of a single measurement of the apparent transmittance of a filter or

sample depends on the accuracy of the zero (intercept) and the linearity of the detector

response, as well as the stability of the source during the measurement of the incident and

transmitted intensities. The NIST spectrophotometer is nulled frequently to < 0.01% of the

full scale reading for the incident light, which approaches 0.1% relative for the worst case

and the 10 %T filter. The linearity of the detector was evaluated initially using the double

aperture method as described in Section 1.1 and Appendix 2, and a polynomial correction

function was fitted and incorporated into the measurement system. The linearity is

periodically confirmed by means of the double-aperture method, and has been found to be

stable. Every transmittance is computed from a measured transmitted intensity and two

bracketing measurements of the incident intensity (blanks), with the measurements made at

time intervals of approximately six seconds. Thus, the incident intensity at the time of

measurement of the transmitted intensity is a linear interpolation of two values measured at

twelve second intervals. The absolute accuracy of this estimate of incident intensity depends

on any drift in lamp intensity being linear over this time period.

7. This value represents a pooled estimated standard deviation of the measured transmittance

for 6 replications using 7 filters (35 degrees of freedom), reported in table 4, Section 5.1,

and is probably dominated by photon shot noise, or the random Poisson distribution of

photon statistics.

10.1 Overall Estimated Uncertainty

The expanded uncertainties of the certified transmittance values, U, are given in relative

percent, as determined from the component standard uncertainties (i.e., estimated standard

deviations) and a coverage factor k=2 based on the t-distribution for infinite degrees of

freedom. The expanded uncertainty defines an interval within which the unknown value of

the transmittance can be asserted to lie with a level of confidence of approximately 95

percent. This uncertainty includes "Type A" uncertainties, which are evaluated by statistical

methods (item 7 in the table), and "Type B" uncertainties, which are evaluated by other

means (items 1-6 in the table). The uncertainties are combined by the root-sum-of-squares

method. The methods used to treat uncertainties are described in NIST Technical Note

1297[41].

62
Acknowledgments

The authors gratefully acknowledge the assistance of the following organizations and

persons:

The National Institute of General Medical Sciences of the National Institutes of

Health for supporting, in part, the work reported in this publication;

George N. Bowers, Jr., M.D., of Hartford Hospital, Hartford, Connecticut; Royden

N. Rand, Ph.D., of the Eastman Kodak Co. Research Laboratories, Rochester, New York;

and Donald S. Young, M.D., Ph.D., of the Mayo Clinic, Rochester, Minnesota, for their

support and cooperation;

H.L. Hoover, Corning Glass Works, for providing the transmittance information of

the didymium and holmium oxide glass filters;

J.R. DeVoe, R.L. Watters, Jr., and J.C. Travis, NIST Inorganic Analytical Research

Division; and K.L. Eckerle, NIST Radiometric Physics Division, for scientific review of the

manuscript;

J.J. Reed, NIST Chemical Science and Technology Laboratory, for typing the

manuscript.

Note

This work is a revised and augmented edition of an earlier NBS-SP 260-51

publication issued in 1975 on the selection, preparation, certification and use of SRM 930

"Glass Filters for Spectrophotometry" [14]. To the material included in this early work, an

appreciable amount of new data, mainly from references 15 and 16, were incorporated in

this updated edition which includes a description of the new Standard Reference Material

1930 issued in 1987. SRM 1930 extends the transmittance range of SRM 930, issued in

1971, by providing three neutral glass filters having nominal percent transmittances of 1, 3,

and 50%T.

63
References

K.D. Mielenz, Anal. Chem., 48, 44-45 (1976); see Appendix 1.

R.N. Rand, J. Res. Nat. Bur. Stand. (U.S.), 76A (5), 499-508 (1972); reprinted in

NBS Spec. Publ. 378, 125-134 (1973).

R.N. Rand, Clin. Chem., 15 (9), 839-863 (1969).

C. Burgess and A. Knowles, (Eds.), "Standards in Absorption Spectrometry,"

Chapman and Hale, 1981, 142 pp.

W.P. Belk and F.W. Sunderman, Amer. J. Clin. Pathol., 17, 853 (1947).

I.D.P. Wooton and E.J. King, Lancet I, 470 (1953).

P. I. A. Hendry, Report Adelaide Meeting, College of Pathologists of Australia, 1961,

p. 8.

D.B. Tonks, Clin. Chem., 9, 217 (1963).

F.B. Desmond, New Zealand Med. J., 63, 716 (1964).

C. Burgess and K.D. Mielenz, (Eds.), "Advances in Standards and Methodology in

Spectrophotometry," Elsevier Science Publ., Amsterdam, 1987, 403 pp.

"Spectrophotometry," Sections in NBS Tech. Note 544; 1969 June - 1970 June, pp.

3-41, and ibid. 584; pp. 1-52; 1970 July - 1971 June containing the Summaries of

Activities of the Analytical Coordination Chemistry Section reported by various

authors and edited by O. Menis and J.I. Shultz.

K.S. Gibson, "Spectrophotometry," NBS Circular 484 (1949) 48 pp.

R. Mavrodineanu, J. Res. Nat. Bur. Stand. (U.S.) 76A(5), 405-425 (1972); also NBS

Spec. Publ. 378, 31-51 (1973). See Appendix 2, and reference 11 above.

R. Mavrodineanu and J.R. Baldwin, NBS Spec. Publ. 260-51 (1975) 107 pp.

R.W. Burke and R. Mavrodineanu, NBS Spec. Publ. 260-81 (1983) 126 pp.

R. Mavrodineanu and R.W. Burke, in "Advances in Standards and Methodology in

Spectrophotometry," C. Burgess and K.D. Mielenz, (Eds.), Elsevier Science Publ.,

Amsterdam, 1987, pp. 125-174.

64
F.J.J. Clarke, J. Res. Nat. Bur. Stand. (U.S.) 76A(5), 375-403 (1972); also NBS

Spec. Publ. 378, 1-29 (1973).

K.D. Mielenz and K.L. Eckerle, Appl. Optics, U (10), 2294-2302 (1972).

K.D. Mielenz, K.L. Eckerle, R.P. Madden, and J. Reader, Appl. Optics, 12 (7),

1630-1641 (1973); also NBS Tech. Note 729 (1972) 55 pp. and (1976) 43 pp..

K.D. Mielenz, in NBS Spec. Publ. 378 (1973) pp. 81-93.

NBS Spec. Publ. 378, "Accuracy in Spectrophotometry and Luminescence

Measurements," (1973), 255 pp., R. Mavrodineanu, J.I.Shultz, and O. Menis, (Eds.).

This volume contains the 18 papers presented at the Conference on Accuracy in

Spectrophotometry and Luminescence Measurements held at NBS, Gaithersburg,

MD, March 22-24, 1972. The same papers were published previously in J. Res.

Nat. Bur. Stand. (U.S.), 76A, Nos. 5 and 6, 375-510, 547-654 (1972).

NBS Spec. Publ. 466, "Standardization in Spectrophotometry and Luminescence

Measurements," (1977), 150 pp., K.D. Mielenz, R.A. Velapoldi, and R.

Mavrodineanu, (Eds.). This volume contains the 15 papers presented at the

Workshop Seminar on Standarization in Spectrophotometry and Luminescence

Measurements held at NBS, Gaithersburg, MD, November 19-20, i975. The same

papers were published previously in J. Res. Nat. Bur. Stand. (U.S.), 80A, Nos. 3

and 4 (1976).

J.G. Vinter, in reference 4, pp. 111-120.

V.R. Weidner, R. Mavrodineanu, K.D. Mielenz, R.A. Velapoldi, K.L.Eckerle, and B.

Adams, J. Res. Nat. Bur. Stand. (U.S.), 90 (2), 115-125 (1985). See Appendix 4,

and NBS SP 260-102 (1986) 56 pp.

K.A. Gschneider, Jr., and L.E. Yring, "Handbook of the Physics and Chemistry of

Rare Earths," Vol. 3, North-Holland, 1979.

American Society for Testing and Materials "Manual on Recommended Practices in

Spectrophotometry," 1969, 136 pp.

K.D. Mielenz, V.R. Weidner, and R.W. Burke, Appl. Optics, 21 (18), 3354-3356

(1982).

W. Budde, Proc. Fourth IMEKO Symposium Prague, 1969, p. 167. See also K.L.

Eckerle, NBS Tech. Note 913 (1976) 39 pp.

R. Mavrodineanu, R.W. Burke, M.V. Smith, F.C. Ruegg, and W.A. Bowman III,

NBS Technical Activities, Center for Analytical Chemistry, NBSIR 85-3272 (1985)

pp. 37-38.

65
[30] B.P. Adams, Standardization News, 5 (1), 26-57 (1977), Amer. Soc. for Testing and

Materials.

[31] L. Holland, "The Properties of Glass Surfaces," 1964, Chapman and Hall, 556 pp.

See also T. Putner, British J. Appl. Phys., 10, 332-336 (1959); and E.B. Butler and

W.H. Johnston, Science, 120, 543-544 (1954).

[32] T. Heslin and R. Hunkeler, Material Technology Report MTR No. 755-011, 11 pp.,

April 1975, Goddard Space Flight Center.

[33] A.T. Young, Mon. Not. R. Astr. Soc, 135, 175-180 (1967).

[34] R. Mavrodineanu and U.W. Drews, Anal. Chem., 48 (1976); in Editor's Column.

[35] W.R. Blevin, Intern. J. Optica Acta, 6, 99-101 (1959).

[36] K.L. Eckerle, NBS Tech. Note 913 (1976) 43 pp.

[37] F.J.J. Clarke, M.J. Downs, and W. McGivern, UV Spectrometry Group Bull. 5,

104-108 (1977).

[38] G.H.C. Freeman, in "Advances in Standards and Methodology in

Spectrophotometry," C. Burgess and K.D. Mielenz, (Eds.), Elsevier Science Publ.,

Amsterdam, 1987, pp. 69-86.

[39] J.F. Verrill, in reference 10, pp. 111-124.

[40] A. Bittar and J.D. Hamlin, Report No. 769 (1981) 8 pp. Physics and Engineering

Laboratory, DSIR, Private Bag, Lower Hurt, New Zealand. Also A. Bittar, PEL

Report No. 713 (1981).

[41] B.N. Taylor and C.E. Kuyatt, "Guidelines for Evaluating and Expressing the

Uncertainty of NIST Measurement Results", NIST Tech. Note 1297 (1993) 15 pp.

66
Appendix 1

Reprinted from ANALYTICAL CHEMISTRY, Vol. 48, Page 1093, June 1976

Copyright 1976 by the American Chemical Society and reprinted by permission of the copyright owner

Comments on Spectrometry Nomenclature

Sir: As an optical physicist who collaborates with analytical chemists in the development and use of absorption and

luminescence spectrometers, I have become increasingly aware of the fact that our vocabularies are not always the same. Although some of these differences have been pointed out before (1) it seems worthwhile to familiarize the

readers of this journal with the currently prevailing terminology of physical optics. Perhaps the following comments

may be helpful to editors, authors, and nomenclature committees in establishing a common and consistent terminology that can be used in all branches of spectrometry and

would save readers a good deal of puzzlement and reading

between lines. The main purpose of these comments is to emphasize the need for interdisciplinary efforts in defining

spectrometry nomenclature without perpetuating the in- consistencies that exist at present. They do not represent an official position of the National Bureau of Standards.

The basic concept of modern optical terminology (2, 3) is to combine nouns and adjectives in order to describe quantities and properties as explicitly as necessary, rather than

using glossaries of short names for them. This "Chinese

Restaurant" method of nomenclature (4) offers flexibility in introducing new terms, and has recently been extended

to include the photon quantities used in photochemistry

and similar disciplines dealing with the interaction of light

and matter (5). The basic list of nouns describing the transport of energy

according to the laws of geometrical optics is: Energy, Q.

Energy density, u = dQ/'dV. Energy per unit volume.

Flux, <£ = dQ/dt. Time rate of energy flow. Flux density, W = d<t>/cL4. Flux per unit area. Intensity, I = d$/dQ. Flux per unit solid angle.

Sterance, L = d2<i>/d(Acos9)dQ. Flux per unit projected area and unit solid angle.

(A number of alternative and additional terms have been

proposed. For example: pointance to replace the continually misused term intensity; incidance and exitance, or inci- dent areance and {transmitted, emitted, etc.) areance for the flux densities arriving at and leaving a surface; steri- sent for the sterance generated per unit path length by

emission or scattering into the direction of propagation;

and fluence for the surface energy density incident upon a volume element irradiated from within a large solid angle

of rays (F. E. Nicodemus, private communication). As these

have not yet been accepted generally, they were not included here. However, the new term sterance was included as it avoids the misnomer photon radiance that has appeared in papers on luminescence spectrometry.)

These nouns are modified by the adjective radiant, and

the above symbols are written with a subscript 'e' (for energy), when radiometric units are used. The modifiers lumi- nous and photon are used in conjunction with subscripts

V (for visual) and 'p' (for photon) to indicate the use of

photometric and photon units, respectively. For example:

Radiant energy density, ue [J m

-3

]. Luminous intensity, Iv[lm sr-1

].

Photon sterance, LP [E m-2

sr-1

s-1

]. (The basic unit for photon energy used here is the einstein

[E], defined as one mole of photons. It is not an SI unit, but

is used so extensively in photochemistry and photobiology

that its acceptance as a supplementary SI unit may be desirable.)

The additional modifier spectral and subscripts X and a

are used to denote derivatives of radiometric and photon

quantities with respect to wavelength and wavenumber, re- spectively. Thus:

Spectral radiant energy density, it e,x = dujd\ [J m-4

]. Radiant energy density per unit wavelength interval.

Spectral photon intensity, IPt<, = dip/'da [E sr-1

s -1 m].

Photon intensity per unit wavenumber interval. Any of these can be modified further; such as: fluorescence

photon flux, transmitted spectral radiant flux density, or absorbed luminous energy. Usually, it is possible to drop

most of the modifying adjectives as well as the subscripts e, v, or p, once the context has been clearly established or whenever a distinction is not necessary. The same method of nomenclature also provides a simple and logical way of specifying the precise meaning of the

quantities and material properties commonly measured in analytical spectrometry. Thus radiant absorptance a* and

photon absorptance ap should be used for the ratios of the

radiant or photon fluxes absorbed by a sample to those in- cident upon it, when measured with a large bandwidth so that these two ratios are not the same. On the other hand,

spectral absorptance a(X) is sufficiently accurate in the

case of measurements made with a small bandwidth. Similarly, luminescence yields (the ratios of the radiant or photon fluxes emitted by a sample to those absorbed by it) should be designated as radiant yield i)e or photon yield ijp (not energy yield or quantum yield). In this case, spectral

radiant yield, rje>tr = drjjda, and spectral photon yield, ^Pi<r = drip/da, are different quantities even in the limit of infi- nitely narrow bandwidths, and thus should be referred to by these names.

[The dependence on wavelength or wavenumber is indicated by a subscript (r?p, 0) when the spectral distribution is defined as a derivative, but in functional form [a(X)] in the

case of spectral distributions that are not derivatives. This

is an important distinction that must also be borne in mind

in the presentation of spectra. The positions of the peaks

and valleys in derivative spectra depend on the units used,

so that these should not be published in the form of a single

graph with dual scales (such as wavelength and wavenumber, or spectral radiant and photon yield).] In addition to this general description of current optical

terminology, the following comments are made in direct

reference to the nomenclature list in the December 1975

issue of Analytical Chemistry (6).

1) This list cautions not to use optical density instead of

absorbance, but defines the latter as "the logarithm to the

base ten of the reciprocal of transmittance"—which is the

definition of optical density. Internal transmittance

should have been substituted for transmittance in this defANALYTICAL CHEMISTRY, VOL. 48, NO. 7, JUNE 1976 • 1093
inition of absorbance. The International Lighting Vocabulary suggests transmission density and internal transmission density instead of optical density and absorbance.

This is more determinative, and also fits into a general

scheme where reflection density is defined as the negative

logarithm to the base ten of reflectance. On the other hand,

transmission and reflection density are operational quantities that merely express measured data on a logarithmic

scale, whereas absorbance is directly related to molecular

constants through the Lambert-Beer and Strickler-Berg

equations. Thus, in my opinion, it should be retained as a separate term. I believe that all confusion would be avoided

by defining:

Transmittance, t. Ratio of the flux transmitted by a sample to the flux incident upon it. Internal transmittance, j\. Transmittance exclusive of

losses at boundary surfaces and effects of interreflections between them.

Transmittance density, D = —logins. Negative logarithm to base ten of transmittance.

Absorbance (Internal transmission density), A = ~logio^i. Negative logarithm to base ten of internal

transmittance.

2) The definitions of absorptivity in chemistry and physics are not the same. In chemistry, it means absorbance per

unit path length and unit concentration (A/bc), whereas

the International Lighting Vocabulary (2) defines it as in- ternal absorptance per unit path length (dati/db). Similarly,

it specifies transmissivity as internal transmittance per

unit path length and reflectivity as the reflectance of a

thick layer (so that a further increase in thickness will no

longer change its value). Generally, terms ending in -ance

represent sample properties, whereas terms ending in -ivity

denote material properties that are independent of sample

geometry. The quantity A/bc does not fall in this latter cat- egory as it is also independent of sample concentration.

The International Union for Pure and Applied Chemistry,

apparently aware of this discrepancy, has suggested calling

it absorption coefficient. However, as this name has been

given different and mutually inconsistent meanings in the

past, a less ambiguous word—perhaps specific absorbance—

would have been better. Therefore it is proposed to define:

Absorptivity, a = daj/do [m -1

]. Internal absorptance per

unit path length.

Specific absorbance, e = A/bc [kg-1 m2

]. Absorbance per

unit path length and unit concentration.

Specific molar absorbance, em = A/bcm [mol-1 m2

]. Absorbance per unit path length and unit molar concentration.

3) The December 1975 nomenclature list also implies

that the Beer-Lambert laws are the same. The correct definitions are: Beer's law: Absorbance is proportional to concentration.

Lambert's law: Absorbance is proportional to path

length. Also called Bouguer's law.

4) The definition of the angstrom unit in terms of the

red line of cadmium has been abrogated several years ago.

It is now defined as 10-10

m, exactly. However, the angstrom is not an SI unit, and has been sanctioned by the In- ternational Committee on Weights and Measures only as a supplementary unit that will eventually be abandoned (7). Therefore, authors should be encouraged to use micrometers or nanometers.

5) The December 1975 list defines spectrometry as the

"measurement of spectra", but restricts the meaning of

spectrometer to "instrument with an entrance slit, a dis- persing device, and with one or more exit slits . . .". As this

excludes non-dispersive and slit-less instruments, such as Fourier and Girard spectrometers, it would be more consis- tent to call any instrument used for spectrometry a spectrometer. Therefore:

Spectrometer: Instrument for the measurement of spectra. The general term for instruments that measure spectral

distributions of radiometric quantities is spectroradiometer. However, as this implies measurements in absolute

units, spectrometer is a better term for the simpler, usually

ratio-forming instruments used in analytical spectrometry.

Thus, a spectrophotometer could also be called absorption

spectrometer, and fluoroescence spectrometer would end

the controversy of spectrofluor imeter vs. spectrofluorometer.(Strictly speaking, the word spectrophotometer is a mis- nomer. A photometer is an instrument that measures lumi- nous flux in lumens. Since the adjective luminous implies

the integral effect of visual radiation as perceived by the

human eye, the spectral analysis of luminous flux has no

physical meaning. However, in view of the firmly established meaning of spectrophotometer, it is not suggested to change it, although a scanning of the recent literature

shows an increasing usage of absorption spectrometer.)

LITERATURE CITED

(1) G. A. W. Rutgers, in "Accuracy in Spectrophotometry and Luminescence

Measurements," Nat. Bur. Stand. (U.S.), Spec. Publ. 378, 53 (1973).

(2) "International Lighting Vocabulary", 3rd ed.. Publication CIE No. 17,

Paris, 1970.

(3) Editor's Page. J. Opt. Soc. Am., 57, 854 (1967).

(4) J. Geist and E. Zalewski, Appl. Opt, 12, 435 (1973).

(5) J. J. Murray, F. E. Nicodemus, and I. Wunderman, Appl. Opt., 10, 1465

(1971).

(6) "Spectrometry Nomenclature", Anal. Chem., 47, 2526 (1975).

(7) "The International System of Units (SI)", Nat. Bur. Stand. (U.S.), Spec.

Publ. 330(1971).

Klaus D. Mielenz

Analytical Chemistry Division

Institute for Materials Research

National Bureau of Standards

Washington, D.C. 20234

RECEIVED for review September 26, 1975. Accepted December 23, 1975.

1094 • ANALYTICAL CHEMISTRY, VOL. 48, NO. 7, JUNE 1976
Appendix 2

JOURNAL OF RESEARCH of the Notional Bureau of Standards — A. Physics and Chemistry

Vol. 76A, No. 5, September-October 1972 An Accurate Spectrophotometer for Measuring the

Transmittance of Solid and Liquid Materials

R. Mavrodineanu

Analytical Chemistry Division, National Bureau of Standards, Washington, D.C. 20234

(May 31, 1972)

The optical transmittance of solids and liquids as well as the molar absorptivity of various chemical

species are parameters of fundamental significance in characterizing these materials. Meaningful trans- mittance data can be obtained only when the measurements are performed with well-known accuracy and precision. To perform such measurements, a high accuracy spectrophotometer was designed and

assembled at NBS, Analytical Chemistry Division, and will be described in this paper. This singlebeam instrument is composed of a constant radiation source, a monochromator, a sample carriage, an

integrating sphere-photomultiplier assembly followed by appropriate electronics, and a read out system

consisting of a digital voltmeter and a computer data acquisition and handling provision. The accuracy

of transmittance measurements is determined by the light-addition principle used in conjunction with a

two-aperture arrangement. The spectrophotometer can be used in manual or automatic modes of operation. A detailed discussion of the data obtained with this instrument, used in both modes, will be pre- sented together with its application to the certification of solid and liquid Standard Reference Materials

for checking the photometric scales of conventional spectrophotometers.

Key words: Absorbance; automation of accurate spectrophotometer; instrumentation, spectrophotometric; spectrophotometry, high accuracy; standard reference material in spectrophotometry;

transmittance.

Transmittance is the ratio of two radiation flux in- tensities. It is therefore necessary that the photometric

scale of the spectrophotometer used to perform the

measurements be accurate. The transmittance of a particular material is also a function of wavelength; hence

the wavelength scale of the monochromator should also

be accurate, and appropriate spectral bandpasses

should be used. The measurements should be made

using collimated radiations. Such radiations define un- ambiguously the actual path length through the trans- mitting medium, the reflection losses, and eliminate

the effects of polarized radiations that are produced at the surface of the sample. Other important factors

which must be considered are: homogeneity and stability of the sample, radiation scatter inside the sample,

interference phenomena, stray radiation, polarization,

fluorescence, temperature, particulate matter, and sur- face conditions. Since transmittance measurements

depend on a diversity of factors, meaningful values can

be obtained only by defining the experimental conditions for obtaining transmittance data [1, 2]. 2 Spectrophotometers are used to perform two types of measurements:

(1) Quantitative determination of chemical species

using the relation between optical transmission of the

material, and the concentration as a measuring parameter. Under these circumstances, the photometric scale

; Figures in brackets indicate the literature references at the end of this paper.

I. Introduction

Optical transmittance is due to an intrinsic property

of matter and characterizes a particular transparent

material. Since this parameter is not known a priori, it must be determined by experimental procedures.

True transmittance values can be obtained only by

using accurate measuring techniques and by taking

into consideration all factors which can affect and dis- tort the data. 1

1 The optical transmittance of a solid material includes the reflection losses which occur

at the air-solid interface. The internal transmittance is defined as the transmittance of the material corrected for reflection losses (2). This internal transmittance can be calculated in principle from the transmittance by using the well known Fresnel equations (1, pp. 98 to 100). For collimated radiation the reflectance A, for a material with an index of refraction, n, and an absorptivity, a, at wavelength. A, is given through:

(i»»-l)' + n» a{

' (/u + D' + nJaj

For a nonabsorbing material and collimated radiation:

For glass, n is approximately l.S in the visible region of the spectrum, and R will be about 4 percent at every air-glass interface. When noncollimated radiation is used:

sin8 (q-/3)> R

»~sinM<» + 0h

for perpendicular polarized radiation, and

tan* (a-/3U

£11= — ' tan2 (o + /8)»

for parallel polarized radiation, where a and 0 are the angles of incidence and refraction,

respectively. _

In coUimated radiation and in air, a = (3 = 0 and Jl1 = /?" = «.

405
of the spectrophotometer is calibrated in meaningful

units, using a series of ref*»r*»n<-o solutions having

known concentrations of the species to be determined,

rather than values of optical transmittance.

The accuracy of the measurements is related solely

to the accuracy with which the concentration of the

reference solutions is known and to the precision (stability, sensitivity, reproducibility) of the spectrophotometry method and instrument used. The accuracy of

the photometric scale per se, is not a critical factor in

such measurements.

The precision, stability, and reproducibility of the

instrument can be checked before each series of measurements by careful use of solid or liquid reference

filters having well established transmittance values.

(2) Determination of the optical transmission characteristics of solid or liquid materials, and the determination of molar absorptivities of chemical compounds.

In both cases the accuracy of the photometric scale of

the measuring instrument, among other things, is essential to provide true values. Ways to establish

and check this important parameter are critically

needed.

Since conventional spectrophotometers do not provide means to check photometric accuracy or to evaluate the possible sources of systematic errors, it was decided in 1969 to design and construct a re- search spectrophotometer on which transmittance

measurements could -be performed with well defined

accuracy. Such an instrument would be used to determine optical transmittance of selected solids and

liquids at various wavelengths. These materials can be

used as standard reference materials (SRM's) to check

the accuracy of the photometric scale of conventional

spectrophotometers. The same certified SRM's could

likewise be used to monitor the precision, stability,

and reproducibility of those instruments [3, 4]. After a comprehensive examination of the literature

in this field [5 to 34] arranged in chronological order, an instrument was developed which is similar in

principle to the instrument at the National Physical

Laboratory (NPL), Teddington, England, where a long

tradition of high accuracy spectrophotometry exists.

The instrument described in this work performs 1 measurements of radiant energy in the visible and

ultraviolet region of the spectrum, with well established

and high photometric accuracy. Transmittance meas- urements on solids and liquids can be made with this

instrument using collimated as well as noncollimated

beam geometry. The wavelength accuracy and spectral

bandpass achievable are adequate to avoid degradation of photometric accuracy, and the other interferences mentioned have been given careful consideration, and, in most cases, have been assessed quantitatively.

The transmittance measurements on the optically

neutral glass filters discussed in this work have been

made with a noncollimated beam geometry corresponding to an aperture of about f:10. The image of the exit

slit of the monochromator (8 mm x 0.5 mm) was

produced at the center of the entrance face of the

filter. All measurements have been made against air

for the nonattenuated radiation flux, and no correction for reflection losses was made. Transmittance

measurements made with noncollimated radiation by

projecting the image of the exit slit of the monochromator on the entrance face of the sample using an opening of f:10 (total angle of about 7° or 8°), may

differ by several parts in 104 of the value when compared with similar measurements made with colli- mated radiations, as indicated in this Journal by

K. Mielenz.

Noncollimated beam geometry was applied in this

work to approach the measuring conditions used in

most of the conventional spectrophotometers which

are available today. A brief description of this instrument was given earlier in reference [3].

II. Description of the Instrument3

The high accuracy spectrophotometer, completed

and tested in 1970, is a single beam instrument which

contains the following components: (a) a constant

radiation source, (b) a monochromator, (c) a sample

holder, (d) a system to check the accuracy of the

photometric measurements, (e) an integrating sphere

attached to a photomultiplier-digital voltmeter unit,

and (f) the data presentation system. Figure 1 illus- trates schematically the arrangement of these various

components. A circular neutral wedge is placed after

the light source to select various levels of radiation

intensities required for measurements. A description

of the components is presented in the following

sections.

a. The Radiation Source. Since the instrument is a single-beam type, it is essential that the radiation source be constant and homogeneous. Additional

desirable conditions are: capability of monitoring the

current supplied to the source and radiation similar

to that from a Planckian radiator. The source is similar in design to that developed and used at NBS

by H. J. Kostkowski and R. D. Lee of the Institute for

Basic Standards. This source was duplicated in our

instrument with the kind assistance of its developers.

The source is used in the spectral range 360 nm to 800 nm and consists of a tungsten incandescent fila- ment lamp with a tungsten ribbon 8 mm long by 2 mm

wide. The connections to the lamp terminals are soldered to minimize contact problems (see fig. 6). The direct current required to operate this lamp at approximately 3000 K is 18 A across a 6 V drop; our source is operated at 5 V and 15 A. The d.c.

power supply is capable of delivering 15 V and 50 A,

and can be operated in constant current or constant

voltage modes. To achieve the constant current mode

an external sensing resistor of 0.1 fl and 50 A and

a current control circuit are placed in series with the

power supply. A feedback voltage across this resistor

is connected to the sensing system. The character

1 The commercial instruments and parts used in the construction of the spectrophotometer are identified in the addendum.

406
Precision Optical Bench

III 1111. 1 IIII IMI-MCtliCT

sriCTiiriiTiiiru

Lent

Lens

1m Grating Monochromator (Czerny -Turner)

Carriage

Sample

1

Precisian Optical Bench in Lightproof Enclosure

I Light Constant Power

Source Supply

I

Monitor

Integrating Sphere

Blank

Figure 1. Principle of the single beam high accuracy spectrophotometer. The aperture unit is placed on the optical bench only when linearity

measurements are performed.

istic of this function is the ability to automatically

change its output voltage to maintain a constant

current to the load resistor, which, in our case, is the lamp source. The nominal current regulation

obtained is better than 0.01 percent, and the stability over an 8 hour period, at constant load temperature,

is better than 0.02 percent. The stability of the current

delivered to the lamp is monitored with a high accuracy

potentiometer used in conjunction with a null meter.

This meter is sensitive to variations in the current

supplied to the lamp from 1 part in 1000 to 1 part in

1,000,000 per division (fig. 1 and fig. 14). The potentiometer is connected to the current source across a resistor (0.01O and 100 A) placed in series with the

lamp.

The demagnified (2 to 1) image of the ribbon fila- ment is projected on the entrance slit of the predisperser by a fused quartz (nonfluorescent Si02>

lens whose focal distance is 254 mm and diameter is 44 mm. This and the other lenses used in the optical

system, were calculated by K. Mielenz of the Institute

for Basic Standards at NBS. The lenses are mounted

in carriers which permit orientation in any position.

A circular neutral wedge is placed between the light source and the predisperser. This wedge, evaporated

inconel on a fused quartz disc (150 mm diam), is linear

in density and provides a light attenuation of 100 to 1. The wedge is motor driven (1 rev. per s) to select

proper radiation intensity levels as required by the

measurements (figs. 2,3, and 4). The radiation source used for measurements in the ultraviolet region to 275 nm is a single coil tungsten-bromine incandescent

lamp (fig. 5) supplied by an adequate power source; below 275 nm, a deuterium discharge lamp is con- templated.

b. The Monochromator. The monochromator is a 1-m Czerny-Turner type grating instrument with a dispersion of 0.8 nm/mm. The flat grating has 1200

grooves per mm covering a surface of 100 x 100 mm.

The monochromator is provided with a predispersing

attachment to reduce the stray light (fig. 3). This preFigure 2. General views of the spectrophotometer. Rear: optical

bench carrying the tungsten-halogen radiation source used for checking the alignment of optical components, followed by a quartz

lens, the circular quartz neutral wedge, and a flat mirror. Left: the 1-m Czerny-Turner grating monochromator (the predisperser

is not illustrated here). Front: optical bench carrying a quartz

lens, the single-sample and blank carriage, a second quartz lens,

and the integrating sphere with the photomultiplier housing.

407

470-063 OL - 72 - 3
Figure 3. Close view of the tungsten ribbon filament lamp on its adjustable holder, followed by the quartz lens — circular neutral wedge assembly, and by the flat mirror in its adjustable holder. The 30° quartz prism Littrow-type predisperser is located at the entrance slit of the 1-m grating monochromator. Extreme left: neon gas laser used to check the optical alignment, and mercury

discharge lamp for wavelength calibration. When in use, the tungsten ribbon lamp is surrounded by an enclosure with black

walls (50 cm x 50 cm x 70 cm high). Rear: enclosure containing the

optical units illustrated in figure 2.

disperser is a small quartz prism monochromator

connected to the scanning system of the 1-m instrument. A wavelength counter permits readings to

0.1 nm and the scanning speed can be varied from 0.05

nm to 200 nm/min by a 12 speed synchronous electric

motor.

The optical components are placed on precision lathe

bed type optical benches which are 160 and 120 cm

long, and are equipped with appropriate carriers

provided with x-y-z adjustments.

c. Sample Carrying Systems. The spectrophotometer is provided with two sample carrying systems. One system measures one sample and its blank,

while the other system permits sequential measure- ments for seven samples and eight reference reading

positions against air, and can be operated manually

or automatically through a computer interfaced

with the instrument.

The single sample carrying unit consists of a platform provided with two vertical holders which can

accept f-in (14 mm) rods and a variety of sample

supports (fig. 2). These holders can be moved laterally

through a rack and pinion arrangement. The platform

is mounted on 4 ball bushings which ride on two

horizontal rods and can be moved pneumatically

across the optical axis. The pneumatic operation was

recommended by G. E. Moore and J. T. Sterling of

the Institute for Materials Research at NBS and

by L. Owen, a guest worker at NBS. The travel distance is 8 in (20 cm) and the linear movement is smooth; the position of the platform and the sample

in and out of the optical beam, can be reproduced

within 0.025 mm. This unit is illustrated in figure 2

Figure 4. Close view of the circular, neutral wedge. The front plate

which carries the fused silica lens was removed to show the fused

silica disc with the evaporated metal layer.

and is located between the two quartz lenses. The

sample holder is designed to accept conventional

solid or liquid filter holders which fit most spectrophotometers. These holders are provided with a thermostating jacket, and can be rotated in the

horizontal plane through a 10 cm diameter rotating

table.

A filter holder which permits the rotation and

scanning of the sample in the x-y direction is also

available (fig. 7). It is provided with micrometer

screws having a total linear motion of 25 mm with

0.01 mm per division. The seven-sample carrying

unit is illustrated in figures 8 and 9 and consists

of a semicircular aluminum-alloy plate placed horizontally on an appropriate carrier on the optical bench

along the optical axis. This plate, which is 32 cm in

diameter and 2.5 cm thick, can be rotated clockwise

through a pneumatically operated precision ratchet

system in increments of 12°. The stepwise rotation

utilizes a solenoid valve which is operated electrically

by a switch located outside the enclosure. This

switch can be operated manually or automatically by

computer (fig. 14).

408
FIGURE 5. Single coil tungsten halogen lamp in the adjustable

holder.

The semicircular plate carries seven sample holders

similar to those used for the single sample system

described earlier. The holders are placed at 24°

intervals and are separated by blank spacings. About

1 atm of air pressure is used to operate the plate and

the rotation is set at 2 s per 12° step when the automatic computer operating mode is used.

d. System to Check the Accuracy of the

Photometric Reading. Since the high accuracy

spectrophotometer is single beam, accurate photometric data are obtained when there is a linear relation

between the measured radiation flux and the corre- sponding response of the photodetector.

Linearity of photodetectors can be measured by

several means: the inverse square law [7, 15]; the use of

optical elements having a known transmittance which

can be determined by other means [17] and the light

addition principle of Elster and Geitel using a plurality

of light sources [5, 6, 8, 9, 10, 13, 18, 19, 20, 28. 31. 33,

34] or multiple apertures [11, 12, 14, 16, 21, 23. 25. 26,

27, 30]. A novel approach to the problem of accurate

Figure 6. The tungsten ribbon filament lamp in the newly designed

adjustable holder. 1 he platform which carries the lamp is similar

to that described in figure 5 and can be oriented in the horizontal plane through the six screws spaced around the edges of the platform at 6CP intervals. Three screws push the platform while

the other three pull. The current-supplying wires are soldered directly to the lamp

terminals to eliminate contact problems.

photometric measurements was described by O. C.

Jones and F. J. J. Clarke [24, 29] and by F. Desvignes

and J. Ohnet [32]. A critical discussion of some

aspects of accurate spectrophotometry will be found

in an NBS manuscript by Gibson and associates [22].

The radiation addition principle, using two apertures

with one source of radiation, was chosen for our work.

The aperture method for checking the linearity of

photometric data was in use at the National Physical

Laboratory from about 1930 onwards, and one form of it was described by Preston and Cuckow [11] in conjunction with a single beam spectrophotometer, using a

five aperture screen. One year later, Buchmuller and

Konig [12] described and used a two aperture unit. At

NBS, Barbrow [14] used a 10 aperture arrangement,

while Harding [16] and Cordle and HabeU [25] at NPL

described a two aperture system. Multiapertures were

used by Hoppmann [21], Bischoff [23], Sanders [26]

and Nonaka and Kashima [27]. Finally, Clarke f30]

409
Figure 7. Same as figure 2 except tor the sample holder which

in this case is capable of rotating the sample 360° and to displace

it in the x-y direction through the micrometer screws.

Figure 9. Same as figure 8. The pneumatic cylinder which rotates the circular platform through a ratchet mechanism is visible at the rear of the platform. The integrating sphere with its pneumatic

shutter is seen at right.

Figure 8. Circular platform carrying seven filter holders. The lb

position switches (7 sample positions and 8 blank positions) are

visible along with the two quartz lenses. The exit slit of the monochromator is at left.

discussed in detail the use of a two aperture system to check the accuracy of photometric data obtained on the

spectrophotometer at NPL. It is this two aperture

system which is used at NBS.

The two aperture unit consists of a metal plate

(130 mm by 100 mm) containing two rectangular

Figure 10. Same as figure 2. In this case the dual-aperture unit for linearity control is located on the optical bench after the exit

slit of the monochromator.

windows, A and B, (20 mm by 8 mm) located one above

the other (figs. 10, 11. 12). Each aperture can be closed

by a light-tight shutter which is operated pneumatically

by remote control (fig. 14). The aperture plate is placed

in the optical path after the exit slit of the monochromator and within the optical solid angle of the

instrument. The image of the apertures is then projected on the target of the integrating sphere. A fused

quartz lens with a focal distance of 190 mm and a diameter of 60 mm is used for this purpose. The arrangement is illustrated in figure 10. No optical element

410
FIGURE 11. Detail of the dual-aperture unit showing its construction and the pneumatic system which operates the two shutters. One aperture is open, the other is closed.

should be placed between the aperture plate and the

monochromator. The linearity check consists of

measuring the photocurrent produced when aperture

A is open then closed, and then aperture B is open and

then closed. The value of (A) + (B) is compared with the

values obtained with both apertures (A + B) open. If the

system is linear these two values should be identical:

(A)+(B)=(A+B).

If this is not the case, the system shows nonlinearity

which is proportional to the amount by which the

sum of (A) + (B) differs from (A + B). This difference is then used to correct the transmittance values measured

on the solid or liquid filters.

e. Integrating Sphere and Photomultiplier

Arrangement. The radiations emitted from the exit

slit of the monochromator and passing through the

aperture or the filter are received on the target of the

integrating sphere. This sphere is illustrated in

figures 2, 7, 9, and 10. A block of aluminum made

from identical halves was cut to produce a half sphere

in each block. The halves were joined together to form

a hollow sphere. Its diameter is 125 mm and a target,

made from a circular plate, 35 mm in diameter, is located at the center of the. sphere. The front surface

of the sphere has a 20 mm diameter opening. This

Figure 12. Front view of the dual-aperture unit, with both

apertures open.

opening can be closed by a shutter which is operated

remotely by a pneumatic system. A 50 mm diameter

opening is at the opposite end to which the housing of

the photomultiplier is attached by an "O" ring to provide a light-tight joint. The inside of the sphere is coated using a suspension of BaSCX; the outside is painted black.

Under these circumstances the sensitive surface of

the photodetector receives the radiations originating

from the exit slit of the monochromator only after

these radiations have undergone at least two diffuse

reflections.

The photomultiplier is a 50 mm flat-faced, silica

end window tube with a 44 mm cathode and 11 Venetian blind dynodes having CsSb secondary emitting

surfaces. The cathode is an S-20 or tri-alkali type.

The spectral range of this tube is from below 200.0 nm

to 850.0 nm. The operating voltage used is 850 V. The

photomultiplier output is supplied to a current-tovoltage converter consisting of an operational amplifier with high precision feedback resistors with

values of 106

, 3 x 106

, 107

, 3 x 107, and 10 8H. Dark

current compensation is also available. This electronic

system, described in figure 13 was designed and

assembled by K. W. Yee of the NBS Electronic In- strumentation Section. The output from the currentto-voltage unit is connected to a digital voltmeter,

illustrated in figure 14, with one microvolt resolution on the 1 V full scale range.

411
CUBjg SHIELDED 101

OUTPUT

iv m iinutED

FULL SOLE

ALL KOHfLECIROLITIC CAPACITORS

ARE ktalKD HILAR UNLESS OTHERWISE HOTED

SELECTED FOR J 333 I RATIO 10 IITHli 0 1*

ALL RESISTORS BETTER THAU 5% TOtEUIICE HAVE It

OR LESS

AWE DEVICES

, SIC 310* 'ret

+ I5Vo

C °

-I5VO

OUT o

8»Lo

Figure 13. Schematic of the current-to-vohage circuitry. Courtesy of K. W. Yee.

The optical components located after the exit slit of the monochrornator, including the photomultiplier

tube, are enclosed in a light-tight box 200 cm long, 70

cm wide and 76 cm deep (fig. 3). The removable

front panel is provided with a sliding door to permit

rapid access to the filter-holder system. The box

contains outlets for the compressed air which operates

the apertures, sample carriage and integrating sphere

shutter, and for the electrical connection from the

photomultiplier. The inside walls are lined with thermal

insulation painted black. When in use. all nonblack

metal parts are covered with a black cloth to reduce

stray light. The entire equipment is placed on a vi- bration isolation table 3.66 m by 1.52 m. The optical

benches and the monochrornator are secured by stops

which are attached to the table surface. The alignment of the optical parts is made and checked periodically with a low-power laser shown in figure 3 (CW gas

laser, output power 2 mW, A. 6328 A) and with a high

intensity tungsten-halogen lamp shown in figure 5.

f. Data Collection and Presentation Systems.

The data output from the digital voltmeter (DVM),

corresponding to the current generated at the photomultiplier tube by the radiations passing through the

aperture system (A. B. A + B) or the samples (/) and

blanks (7o), can be obtained by visual means or computer operation. Both methods have been used in

this work with good results. In the visual mode, the

operator examines the digital voltmeter display and

takes a mental average of the data. The display rate

is adjusted to about one reading per second.

When measurements are taken by computer, the

display of the digital voltmeter is adjusted to a faster

rate; for instance, 10 to 20 data per second, depending

on the capabilities of the instrument and measurement

requirements. In our work, we use 10 data per second

and collect 50 individual data for each measurement.

This information is fed to the computer which calculates and prints the results as averages with the cor- responding standard deviation, relative standard

412
FIGURE 14. Console containing the power supply for the photomultiplier tube, the dc null detector, the current-to-voltage

converter, the digital voltmeter, the command panel for computer

operation, and the command panel for the pneumatic operation

of the shutter, aperture system, and single sample carriage. The

electric switches for operating the circular neutral wedge are also located on this panel.

Middle right: potentiometer for monitoring the dc current supplied to the tungsten ribbon filament lamp. At bottom left: teletype for data presentation.

Right: light panel which indicates the position of the automatic seven sample holder.

deviation, and sample position number to identify

the measurement. When transmittance measurements

are made on individual samples or when linearity

checks are performed, the readings are initiated

manually for every position. When the seven sample

holder is used for sequential measurements, the

operation is performed automatically by the computer. It is programmed to take a predetermined

number of individual DVM readings (50), print the

arithmetic average, followed by the standard deviation, relative standard deviation, percent transmittance and sample position (fig. 15). At the conclusion

of each measurement, the computer initiates a signal

which rotates the holder to the next position. This is followed by the data taking and sample changing

sequence until the measurements are stopped manually or automatically by a provision made in the

computer program.

CALIBKATIC1N OF SRM930 AT 440 NM 8-2-72

SAMPLE NUMBER AND POSITION:

1-70 IN 2; 1-79 IN MS 2-79 IN 6; REFERENCE IN fi; 3-79 IN 10;

1-91 IN 12; 3-91 IN 14

POS AV s S/AV PERCENT T

1 .2001 700E 01 •4388E-03 .21 92F.-03

2 •6413441E 00 . 3256E-03 •5077F-03

3 .2001230E 01 .4320E-03 .21 59E-03 32 .044

4 .6528346E 00 .4804E-03 . 7359E-03

5 .2001040E 01 .4254E-03 •2126E-03 32.623

6 .4221 1 10E 00 • 2293E-03 . 54 32E-03

7 .200099SF 01 .3041E-03 . 1520E-03 2 1 .095

8 •6560142E 00 .3189E-03 •4861E-03

9 .2001402F 01 .4476E-03 .2236E-03 32.781

10 .23S6762E 00 . 1425E-03 .6039E-03

1 1 •2001539E 01 .4 1 14E-03 •2055E-03 1 1.785

12 •6739426E 00 •2848E-03 .4226E-03

13 .200 1205E 01 .4678E-03 •2338E-03 33.674

11 .2386761E 00 . 1064E-03 .4456E-03

15 .2001 101E 01 •4150E-03 .2074E-03 1 1 .927

1 .2001622E 01 •3187E-03 • 1 592E-03

2 •6412983E 00 •1321E-03 .2060E-03

3 •2002043E 01 •6139E-03 •3067E-03 32.036

4 .6526482E 00 •2474E-03 . 3790E-03

5 .2001551E 01 •4756E-03 .2376E-03 32.603

6 •4222282E 00 • 2174E-03 •5149E-03

7 .2001507E 01 .4355E-03 •2176F-03 21.095

8 •6562004E 00 • 3344E-03 .5095E-03

9 •2001529E 01 .5337E-03 •2667F-03 32.785

10 •2359933E 00 . 1886E-03 .7992E-03

1 1 .2001634E 01 •4532E-03 .2264E-03 1 1 .790

12 .674 1099E 00 .3418E-03 . 5070F-03

13 .2001589F 01 .6368E-03 .3182F-03 33.678

14 •2386828F 00 . 1604E-03 .6720E-03

15 .200I829F 01 .3388E-03 . 1693E-03 1 1 .924

1 •200P2 14E 01 •2826E-03 . 14 12E-03

2 •6413472E 00 .3518E-03 .S4R6E-03

3 .2001882E 01 •388RF-03 . 1939E-03 32.035

4 .65261 77E 00 .2S13E-03 .3851E-03

5 •2001741E 01 •3841F-03 . 1919F-03 32.601

• *J/too e, c i/i ilor. h r •2957F-03 . 7000E-03

7 .2002075E 01 •4892F-03 .2443F-03 21 .097

8 •6562083E 00 . 1365E-03 •2081E-03

9 •2001871F 01 • 3708E-03 . 185HF-03 32.778

10 •2358786F 00 . 1 596E-03 .6767E-03

1 1 •2002130E 01 . 5036E-03 •2515E-03 I 1 .762

12 •6740563E 00 .3022E-03 •4484F-03

13 •2001997E 01 .4545F-03 .2270F-03 33-668

14 .2388354F 00 .1704E-03 .7133E-03

1 5 .20024 78

F

01 •2975E-03 . I48 5F-03 1 1 .928

Figure 15. Computer data presentation.

The programming of the entire computer operation

was developed by J. Aronson, R. Freemire, and J. Wing. The computer-instrument interfacing was

performed by F. Ruegg and R. Shideler of the NBS

Analytical Chemistry Division, Technical Service

Group, under the supervision of J. DeVoe.

III. Stability of the Electronic System

As a rule, before taking measurements with the

spectrophotometer, a warmup period of one hour is required. The room temperature is kept at 24 ±1 °C,

and the relative humidity is 35 percent. The particulate matter is controlled through special filters which

rates the room in the 100,000 class.

The dark current of the photomultiplier tube was

measured by taking 15 replications each consisting of

the average of 50 individual digital voltmeter readings.

These measurements were made using 850 V at the

anode. The average dark current under these cir- cumstances produced 0.000682 V with a relative

standard deviation of 0.71 percent.

In all of our work, a dark current buck-out arrangement was used. A series of measurements were performed to determine the stability of this dark current

compensation. To this effect, 15 consecutive measurements, each representing the average of 50 indi413
vidual digital voltmeter readings, were made and the of the 15 values was calculated along with the cor- average dark current value was 0.000024 V with a responding percent standard deviation. These measrelative standard deviation of 23.1 percent. urements were then repeated three times. The results

Four tests were made to determine the stability of are summarized in table 1. This table also presents

the electronic system and the radiation source using the values for the first group and the average values

the computer data acquisition mode. and corresponding percent standard deviation for the

a. Stability of the Current-to-Voltage Con- two consecutive groups. It can be seen from the

verier. A constant voltage was supplied to the con- stability of the current-to-voltage unit that measureverter using the dark current compensation provided ments can be performed with a reproducibility of on the unit. Fifty individual measurements were taken about 0.0012 (at the 67% confidence level) expressed

every 5 seconds and the average value was printed as percent standard deviation for a single determialong with its percent standard deviation. The meas- nation. The time interval between the first and last

urements were then repeated 15 times and an average group of measurements was 15 min.

Table 1. Stability of the current-to-voltage converter alone measured in three groups of 15 replications each

Average of 50 individual Percent standard

lication measurements; volts deviation

1 1.003494 0.0013s

2 1.0034% 0.00129 3 1.003482 0.00146 4 1.003507 0.0011,

5 1.003515 0.00134 6 1.003508 0.0013«

7 1.003497 0.001

l

g 8 1.003498 0.00133 9 1.003505 O.OOllo

10 1.003510 0.0014,.

11 1.003518 0.00133

12 1.003521 0.0012o

13 1.003522 0.0013,

14 1.003507 0.00128

15 1.003527 0.00137

Average of replications Percent standard

deviation

Average of replications Percent standard

deviation

1.003507

0.0012

1.003535

0.0015

1.003545

0.0010

First group

Second group

Third group

Average of replications Percent standard

deviation

b. Stability of the Current-to-Voltage Converter Plus the Photomultiplier Tube Supplied

with 850 V and in Total Darkness. The measure- ments were made as previously described and the

results are presented in table 2.

c. Stability of the Current-to-Voltage Converter and the Photomultiplier Tube Supplied

with 850 V and Exposed to the Radiation of a Tritium Activated Fluorescence Source. A constant radiation source consisting of a tritium activated

phosphor was placed before the integrating sphere

and a series of measurements were taken following

the technique described above. Table 3 shows the

results.

d. Stability of the Current-to-Voltage Converter, the Photomultiplier Tube Supplied

with 850 V, and the Tungsten Ribbon Filament

Lamp. The same measuring procedure as mentioned

in a, b, and c was used here. In this case, however,

the incandescent tungsten lamp was used as the source of radiation. Table 4 summarizes the results

of four groups of measurements over a period of 20

min. This last series of measurements indicate that

the single-beam spectrophotometer is capable of

producing measurements of radiation fluxes with a percent standard deviation of about 0.022s for single

measurements with 2.00 V at the photomultiplier tube

anode.

414
Table 2. Stability of the current-to-voltage converter and the photomultiplier tube at 850 V in total darkness

Average of 50 individual Percent standard

Replication measurements; volts deviation

1 1.012329 0.027, 2 1.012342 0.0488 3 1.012322 0.0272 4 1.012320 0.033s 5 1.012394 0.043s 6 1.012421 0.0153 7 1.012404 0.0184 8 1.012406 0.0296 9 1.012365 0.019e 10 1.012402 0.019,

11 1.012465 0.0252 12 1.012412 O.O6I5

13 1.012451 0.0235 14 1.012417 0.029„

15 1.012481 0.0248

Average of replications Percent standard

deviation

Average of replications Percent standard

deviation

Average of replications Percent standard

deviation 0.0035

In these measurements the stability of the direct

current (nominal 5 V; 14 A) supplied to the tungsten

ribbon lamp was monitored with the potentiometer,

and the variation of this current was less than one

part in 105 during a series of 15 consecutive meas- urements (5 min).

Following the four stability tests discussed earlier, a consecutive series of six measurements were made

to determine the reproducibility of transmittance

measurements. To this effect seven Schott NG-4

neutral glass filters were placed in the automatic

sample carrying system and the data acquisition and

sample changing operations were performed automatically through the computer unit. As mentioned

previously, the sample carrying system can accept seven samples in positions 2; 4; 6; 8; 10; 12; 14, and

eight intermediate positions 1; 3; 5; 7; 9; 11; 13; 15.

The odd numbers correspond to measurements of

the nonattenuated radiation beam passing through

air and are marked /o, while the even numbers cor- respond to measurements of the attenuated radiations

after passing through the absorbing material and are marked /. The uncorrected transmittance, T, is then

The radiation flux from the tungsten ribbon filament

lamp was attenuated with the circular neutral wedge

First group

Second group

Third group

until a photocurrent corresponding to about 2.0020 V

was obtained for the nonattenuated beam /o. The photomultiplier tube was supplied with 850 V and the 30

Mil resistor was used at the current-to-voltage converter. For every position, 50 digital voltmeter readings were taken by the computer at a rate of 10 to 15 per

second. The average value was printed along with

the sample position, the standard deviation, the relative standard deviation, and the transmittance values

for the glass niters 2; 4; 6; 8; 10; 12; and 14:

T = ^2 j- — I* f __/<•_. 1 2 jo _i_ jo'-1 14 jo a. jo

'-1 1

6

/o_i_fo' elc- • • •

'l _

l 3 3 5 5 7 2 2 2

until the seven glass filters were measured. This

sequence was repeated six times and the results are given in table 5. As can be seen from these data, the reproducibility

of sequential transmittance measurements can be

performed with an average standard deviation of

0.010 percent for a single determination.

IV. Wavelength Calibration

The wavelength scale of the monochromator is provided with a counter which indicates wavelength

directly in angstroms. This counter is checked for

1.012395

0.0050

1.012467

D.0033

1.012510

415
Table 3. Stability of the current-to-voltage converter and the photomultiplier tube at 850 V and exposed to the

radiation of a tritium activated fluorescent source

Average of 50 individual Percent standard

plication measurements; volts deviation

1 1.536345 0.0275 2 1.536326 0.0302 3 1.536196 0.022k 4 1.536289 0.0235 1.536106 O.O3O9

U 11 . \J\J\J 536117 111 O.O3I4

7 1.535916 0.025,

8 1.536065 0.023„ 9 1.536179 0.0293

10 1.536003 0.0233

11 1.536083 0.021s

12 1.535961 O.O263

13 1.536052 0.0313 14 1.536095 0.0266

15 1.536092 0.0262

Average of replications Percent standard

deviation

Average of replications Percent standard

deviation

1.536122

0.0082

1.535768

0.0095

First group

Second group

Average of replications Percent standard

deviation

1.535522

0.0054

Third group

accuracy with a low pressure mercury discharge

lamp placed before the entrance slit of the monochromator. The following wavelengths were used

for calibration: 3650.2 A; 4046.6 A: 4077.8 A; 4339 2

A; 4347.5 A; 4358.4 A; 4916.0 A; 5460.7 A: 5769.6 A;

and 5790.7 A. If additional reference wavelengths

are needed, a Cd-Hg or a He-discharge lamp could be

used for calibration. The wavelength counter was

then checked using the procedure recommended by

Gibson [2], and a slit of 0.1 mm which is equivalent to an effective spectral bandpass of 0.08 nm. The deviation of the wavelength counter from the true value

was found to be less than ±0.1 nm; hence no wavelength correction was applied to the measurements

discussed here.

V. Stray Radiation

Tests were made to determine the stray radiant

energy (SRE) in the monochromator proper, as well as in the photometric arrangement. The measure- ment of stray radiation in the monochromator, that is. the radiation energy at wavelengths different from

those of the nominal spectral bandpass transmitted

through the instrument, is not easy or infallible. A

detailed discussion of this instrumental parameter was

given in an ASTM Tentative Method [35] and the procedure recommended in this work was used to determine SRE in the blue and yellow spectral range. In this

procedure, a solution of methylene blue, which has a

strong absorption in the range from \ 600 to 660 nm is used. The SRE using a slit of 1 mm (0.8 nm) was equal

to or less than five parts in 10s

. The SRE generated inside the photometric system is defined as the radiant energy which falls on the

photosensitive detector without passing through the

absorbing sample. This SRE is usually produced by

reflections and scattering of radiations on the optical

and mechanical parts located between the exit slit of

the monochromator and the integrating sphere. The

measurements were performed using a slit of 1 mm by

placing a front surface mirror at the sample position,

which reflects to the instrument all radiations received

from the exit slit imaged at the mirror surface. The size

of this image was about 8 mm high and 1 mm wide. In

this way, a maximum SRE was generated in the

spectrophotometer. The measurements were then

performed at \ 577.3 nm, using a radiation flux intensity

five times greater than that used in routine transmiitance measurements, by determining the dark cur- rent of the photomultiplier with the shutter in the

closed position at the integration sphere. An average

dark current of 0.040 mV was observed. The mirror

was then placed at the sample position, the shutter

416
Table 4. Stability of the current-to-voltage converter, the photomultiplier tube at 850 V, and the tungsten ribbon

filament lamp

Replication

1

234

56

789

10

1112

13

1415

Average of 50 individual measurements; volts

2.002395

2.001356

2.002145

2.000975

2.001944

2.000925

2.001832

2.000825

2.001551

2.000960

2.001739

2.000851

2.001729

2.000825

2.001557

Percent standard

deviation

0.038o

0.022b

0.024,

0.0262 0.0207 0.028,

0.026, 0.0235 0.0263 0.021 2 0.023, 0.0244 0.0282 0.0230 0.0244

Average of replications Percent standard

deviation

2.001441

0.026

First group

Average of replications Percent standard

deviation

2.001517

0.012 Second group

Average of replications Percent standard

deviation

2.000826

0.025

Third group

Average of replications Percent standard

deviation

2.001268

0.027

Fourth group

Table 5. Reproducibility of transmittance measurements on seven Schott NG-4 glass filters No. 2; 4; 6; 8; 10;

12; and 14

Percent transmittance

Replication No.

Average

Percent cr

Average percent cr

was opened and measurements were made again

2 4 6 8 10 12 14

33.327 21.711 12.236 50.990 33.377 20.906 13.473 33.325 21.710 12.237 50.983 33.377 20.903 13.471 33.321 21.711 12.241 50.992 33.383 20.900 13.474 33.320 21.708 12.240 50.988 33.375 20.901 13.470 33.323 21.710 12.239 50.983 33.379 20.901 13.474 33.325 21.710 12.238 50.986 33.377 20.904 13.470

33.32 21.710 12.238 50.987 33.378 20.902

^ 13.472

0.0080 0.0051 0.0150 0.0072 0.0083 0.0108 0.0141

0.010

r - VI. Linearity Control

"

—o vi* MiiwMiiy vviiiivi

The average value found was 0.037 mV. This indicated

that no SRE could be detected under the experimental The single-beam static optical system described in

circumstances. this work permits the unequivocal use of the radiation

417
addition principle by means of the double-aperture

method for determining departure from linearity of the

entire optical, photometric, and electronic system, and

thus of the photometric accuracy of transmittance

measurements.

The double-aperture and its positioning on the

optical bench was described earlier. Its use will now

be illustrated, and follows the procedure developed

and used at the National Physical Laboratory.

Since the linearity of photometric data for a given

photomultiplier tube depends on the anode voltage,

the values at the current -to- volt age converter, and

the ambient temperature, all measurements were made

using identical experimental conditions. These same

conditions were maintained when transmittance meas- urements were performed. Since the linearity is,

within 1 part in 104

, not usually a function of wavelength [36], all measurements were performed at \

565.0 nm. A recent study of this parameter at NBS by

Mielenz and Eckerle indicates that there may be a

relation between wavelength and linearity at the level

of 1 part in 105 [38].

The intensity of the radiation flux produced by the

tungsten ribbon lamp was attenuated with the circular

neutral wedge until a photocurrent equivalent to 2.0020 V was obtained when both apertures, A and

B, were open. A setting of 850 V was used at the photomultiplier tube with a 30 MH resistor at the currentto-voltage converter. Fifty individual DVM readings were taken and the average value for (A + B) was

printed. Aperture B was then closed, and 50 DVM

readings were taken. The average value for aperture

A was printed. The average value for aperture B was

then obtained in a similar manner by closing aperture

A and opening aperture B. This sequence was repeated

three times, ending with an (A + B) value.

Identical measurements were made over a range of

attenuation corresponding to 4 cascaded steps of 2

to 1 as illustrated in the actual example which follows:

Step 1 (A + B)

2.00147 2.00156 2.00206 2.0021!

1.0159«

1.0159s

I.OI6O9

B

0.9864,

0.9864o

0.98625 1.01598 + 0.98635 = 2.00233

Av.

Diff.

% Corr.

Step 2

2.00181

2.00233 - 2.00181 = -0.026

+ 0.00052

Av.

Diff.

% Corr.

Step 3

(A + B)

1.00049 1.00078 1.00066 1.0009q

1.00071

1.00112 —

= -0.041

(A + B)

0.50062

B

0.49108 0.4909,

0.4911s

0.51020 0.50996 0.51004

0.51007 + 0.49105=1.00112

1.00071 = + 0.00041

A

0.25657 B0.24430

Av.

Diff.

% Corr.

Step 4

0.50056 0.25673 0.24436 0.50070 0.2566s 0.24432 0.50062 0.25665 + 0.24433 = 0.50098

0.50063

0. 50098 - 0. 50063 = + 0. 00035 = -0.069

(A + B)

0.25020 0.25028 0.25019

0.25023

A

0.12872 0.12856 0.12854

B

0.1217s

0.12168 0.12166 0.12861 + 0.12170 = 0.25031

Av.

Diff.

% Corr.

0.25023

0.25031-0.25023: = -0.031

+ 0.00008

The correction curve is established from these data

by plotting voltages on the abscissa and the corre- sponding additive correction value on the ordinate.

These are tabulated below and illustrated in figure 16.

625 12.5 25

FIGURE 16. Linearity correction curve.

Voltage % T % Correction of % T

2.00 100 0.0

1.00 50 0.026

0.50 25 0.067

0.25 12.5 0.14 0.125 6.25 0.167

When transmittance measurements are performed,

the /o reading is initially set with the circular neutral

wedge to a value near 2.0020 V. The / value is then

measured. If the initial Io = 2.00214 V and final

/„ = 2.0022s V and /= 0.54220 V, then percent T is:

0.54220

2.00214 + 2.00228

X 100 = 27.081

which is the noncorrected value. To correct this value,

one takes from the ordinate of figure 16 the value

corresponding to 0.54220 on the abscissa which, in

this case, is 0.072. The corrected percent T value

then:

27.081 - f

7"81^0- 072

)

=27.061.

IS

418
Mielenz and Eckerle have studied recently the

louble aperture method for testing photometric

inearity, and have used a curve-fitting procedure

lor finding the nonlinearity correction rather than

the method described earlier (38).

VII. Sample Position

A series of measurements were performed to determine the magnitude of error which could occur when

the sample is oriented with its entrance face at an

angle to the incoming radiation beam. The single

sample holder provided with the rotating table, as described in section II, paragraph c, was used. Transmittance measurements were performed by producing

the image of the exit slit of the monochromator at the

entrance face of the sample (aperture f:10). The data

are shown in table 6. The consequence of this condition on transmittance measurements is discussed by

Mielenz in this Journal.

Table 6. Percent transmittance (%T) , measured on

three neutral glass filters 1.0; 1.5; and 2.0 mm

thick at A. = 440 nm, at three angles of

incidence

Angle of Filter Filter Filter incidence 1.0 mm 1.5 mm 2.0 mm

Normal incidence 32.9U 19.838 11.606 r 32.899 19.833 11 604 2° 32.897 19.81 9 11.599 3° 32.88, 19.81 2 11.589

Similar measurements were made to determine the

identity of positions on the seven-sample automatic

changer described in section II, paragraph c. For

this experiment, seven neutral glass filters A; B; C;

D; E; F; and G were used and were positioned in

holders 2; 4; 6; 8; 10; 12; and 14 in three different

arrangements as described by Garfinkel, Mann and

Youden [39].

Table 7. Evaluation of the identity of the seven stations of the automatic sample changer

Percent Transmittance (%T) at station number (St. No.) 2: 4; 6; 8; 10; 12; and

14 for filters A, B, C, D, E, F, and G at K= 465.0 nm

Run No. B D

%T

St. No.

27.08 16.44 46.39 26.12 •15.22

10

37.34

12

23.23

14

%T 27.07

II St. No. 14

%T 27.08

III St. No. 10

16.40 46.39 26.11

2 4 6

16.43 46.39 26.11

12 14 2

15.21 37.34 23.23

8 10 12

15.21 37.33 23.23

4 6 8

Transmittance measurements were then performed

on all filters for the three different arrangements and

the results are given in table 7. From these data it can be concluded that the seven stations are interchangeable and will produce measurements which will not

differ by more than one part in one thousand.

VIII. Influence of Polarized Radiations on

Transmittance Measurements

This effect was determined by measuring the transmittance of a Schott NG-4 neutral glass filter at four

wavelengths using radiations emerging from the predisperser-monochromator unit, and by projecting the

image of the exit slit (8 by 0.5 mm) at the entrance

face of the filter with a convergent beam geometry

corresponding to an f : 10 opening. The glass filter was

checked prior to measurements with a polariscope for

freedom of internal tensions. Column one of table 8

shows the results obtained when transmittance meas- urements were made using the radiations produced by

the spectrophotometer. Column two shows the results

obtained when a polarizing sheet, with the vibration

plane horizontal, was placed in front of the glass filter. The measurements obtained with the vibration plane

in vertical position, are given in column three.

These measurements show that polarized radiations :an affect transmittance measurements of solid

glass filters when noncollimated beam geometry is used. This effect is predicted by the Fresnel equations

mentioned in the introduction and should disappear

when collimated radiations are used (1, pg. 100).

XI. Comparison of Transmittance

Measurements

Two sets of solid filters were used in a comparative

test to determine the reproducibility of transmittance

measurements between two laboratories. One set was

made from three neutral glass Schott NG-4 filters having nominal percent transmittances of 10; 20; and 30.

The second set was made as described elsewhere [4]. Three evaporated metal (Inconel) on fused quartz

(nonfluorescent) plates having nominal percent

419
Table 8. Effect of polarization on percent transmittance (%T) measured at four wavelengths on a Schott NG-4

glass filter

Wavelength nm

Produced by

spectrophotometer

%T

Polarizer,

plane of vibration horizontal

Polarizer,

plane of

vibration

vertical

% Difference

%T, - %T3

440.0

465.0

590.0 635.0

19.8U

22.597 19.178 20.61,

19.800 22.600 19.170 20.602

transmittances of 25; 50; and 75 were used. The trans- mittance measurements were performed on two sets of

filters at the National Physical Laboratory (NPL) in

England using their high accuracy spectrophotometer, and at NBS on the instrument described in this

paper. The measurements at NBS were carried out

before and after the measurements at NPL. All

measurements were made with noncollimated conver- gent beam geometry. A rectangular surface of the

filter about 3 mm by 8 mm was used at NPL and the

beam was only slightly convergent. At NBS an area about 8 mm by 0.5 mm was used for the transmittance

measurements.

The results given in table 9 indicate that an average

difference of —0.19 percent of the values was obtained

between the measurements carried out at NPL and at NBS. An average difference of —0.30 percent of the

value was found when similar measurements were

19.78„ 22.569 19.099 20.547

- 0.15 -0.12

-0.41 - 0.31

performed on the inconel-on-quartz filters, as shown in

table 10.

X. Standard Reference Materials for

Spectrophotometry

The need for providing means and materials tc check the proper functioning of a spectrophotometer

was discussed in some detail in previous publications [3, 4]. At that time it was established that the

accuracy of the photometric scale is a critical and most

demanding parameter in spectrophotometry. Hence,

particular attention was given to a number of ways for

checking this parameter. Investigations showed that

solid colored glass filters, exhibiting optical neutrality over the spectral range from 400.0 nm to 700.0 nm,

would constitute an acceptable Standard Reference

Material (SRM). From the various colored glass

Table 9. Comparison between the percent transmittances (%T) measured on three Schott NG-4 glass filters at NPL and NBS

Wavelength NBS, %T NBS, %T NBS, %T NPL, %T % Diff. nm March 12. May 18. average February NBS to NPL

1971 1971 1971

440.0 12.92 12.91 465.0 14.96s 14.98 590.0 11.70 11.64 635.0 12.72 12.68

12.91 5 12.93 -0.11

14.973 15.01 - 0.25 11.67 11.67 0.0 12.70 12.72 - 0.16

440.0

465.0

590.0

635.0

440.0

465.0

590.0

635.0

19.62s 22.38s 19.06

20.45s

32.89

35.52

31.16s

32.56s

19.58 22.35

18.95 20.37

32.86

35.54 31.10

32.52

19.60s 22.36,

19.00s 20.41s

32.87s 35.53

31.13s 32.543

19.62 22.43

19.01 20.47

32.98

35.66

31.21 32.62

-0.09

-0.28

-0.03

-0.23

-0.32

-0.36

-0.25

-0.24

Average difference between NBS and NPL percent T values = — 0.19 percent.

filters available, Schott NG-4 "neutral glass" was by NBS as a means to check the photometric scale of

selected, prepared and characterized. It is now offered spectrophotometers.

420
Table 10. Comparison between the percent transmittances (%T) measured on three inconel-on-silica filters at NPL and NBS

Wavelength NBS, %T NBS, %T % Diff. nm 1 2 average NPL. %T NBS to NPL

450.0 24.87 94. RR 94 R7, 9d4. Lr. 7j — n 1 r 550.0 23.78 91 R9 93 RO 4.0.00 — u.zo 650.0 23.38 23.39 23.385 23.46 -0.32

450.0 49.35 49.33 49.34 49.56 - 0.44* 550.0 47.60 47.60 47.60 47.81 - 0.44 650.0 46.85 46.85 46.85 47.14 -0.64

450.0 72.17 72.20 72.185 72.30 - 0.16 550.0 72.05 72.11 72.08 72.20 - 0.17 650.0 72.20 72.34 72.27 72.33 -0.08

Average difference between NBS and NPL percent T values = —0.30 percent.

This filter had a flaw in the form of a crack which was sometimes visible and other times invisible. The larger differences found in the measurements of this filter may be due to this flaw.

SRM 930, developed in the Analytical Chemistry

Division and available since March 1971 consists of

three glass filters. Each filter bears an identification

number, and the upper left corner has been removed

to indicate correct orientation in the metal holder

(fig. 17).

The transmittance measurements were made with

the high accuracy spectrophotometer described in

this paper, and are certified with an uncertainty of

±0.5 percent of the value. This uncertainty is the

sum of the random errors of ±0.1 percent (2SD

limit) and of estimated biases which are ±0.4 percent. These biases are due to possible systematic

errors originating principally from the inherent

inhomogeneity and instability of the glass as well as from positioning of the filter. Measurements were made at 24 °C, and variations within several degrees Celsius of this temperature will not significantly affect the calibration of the filters. The neutral

NG-4 glass for the filters was provided by Schott

of Mainz, Germany and is designated as "Jena Colored

and Filter Glass." Nominal transmittance for a

filter 1.5 mm thick is 20 percent at 400.0 nm wavelength and 32 percent at 700.0 nm wavelength. Between these limits the transmittance varies in a monotonic manner. The filter is held in a frame and the size and shape

of the filters and frames were selected, for practical

considerations, to conform to the dimensions of the

standardized cuvettes for which holders are supplied

in most conventional spectrophotometers. The filters are approximately 1.0, 1.5, and 2.0 mm thick. Corresponding to these thicknesses are nominal trans- mittances of 30, 20, and 10 percent, respectively.

These thicknesses were selected to provide a means for

calibrating the photometric scale at three different

levels.

The effective spectral bandpasses used to determine

the certified values were equal to or smaller than 2.2 nm

at 440.0 nm; 2.7 nm at 465.0 nm; 5.4 nm at 590.0 nm;

and 6.0 nm at 635.0 nm. The transmittance measure- ments are made by producing the image of the slit (about 8 mm by 0.5 mm) using a convergent beam

geometry with an opening of f:10 corresponding

to an angle of 7° to 8° in the middle of the entrance

face of the filter. This beam geometry was used to reproduce the average experimental conditions

found in most of the conventional spectrophotometers available today. Prior to the certification, each

filter is examined for surface defects and thoroughly

cleaned. If, through handling, the surface of the

filter becomes contaminated, it may be cleaned

with a small soft brush attached to a rubber tube

connected to a vacuum source [40]. If contamination

results from fingerprints, they must be removed

before making measurements. This may be accomplished by removing the filter from its holder, breathing lightly on it, and rubbing the surface gently

with optical lens tissue. The clean filter is then properly

positioned in its holder. To remove and replace the

filter in the holder, the spring-loaded plate should

be lifted with care to prevent damage to the filter. As little handling as possible is recommended. SRM

930 should be used according to the directions on

the certificate; consult the manufacturer of the

instrument if differences are obtained that exceed

those specified by the manufacturer.

Under no circumstances should other cleaning

procedures which make use of detergent solutions,

organic solvents, etc. be applied.

When a filter has become contaminated beyond

cleaning by the procedure described in the certifi- cate, it should be forwarded to NBS. After proper

cleaning, the filters will be checked and, if needed,

421
Figure 17. NBS Standard Reference Material 930 and calibration certificate.

recalibrated using the high accuracy spectrophotometer described in this work.

It was already stated that the accuracy of photometric scales defines only one of the parameters re- quired for obtaining accurate transmittance values

and molar absorptivities. Other factors must also be

established. These are wavelength accuracy, adequate

spectral bandpass, stray light, cell parameters (when

solutions are measured), fluorescence, polarization,

reflection, and temperature coefficient. Some of these

variables were discussed in NBS Technical Notes 544

and 584 and are also examined in this paper.

The transmittance data given in the certificate

which accompanies each SRM 930 depend not only on the intrinsic properties of the glass and the ex- perimental measurement conditions, but on the surface

state of the glass. This parameter varies with time

and exposure conditions. When glass is exposed to normal room atmosphere and temperature, its surface

is corroded to an extent depending on the composition, time of exposure, concentration, temperature

and nature of the glass surface acting agents. This

action produces a change in the reflecting and trans- mitting properties of the material [41]. For instance, a well-known phenomenon called "blooming" of the

glass is due to the formation of an SiO layer at the

surface of the glass. This layer, which increases the

transmittance, acts as an antireflection coating. The

speed with which such a layer is formed varies with

the composition of the glass, the atmosphere and time.

Generally speaking, several years are needed for a

fresh surface to reach equilibrium. This, and similar

422
phenomena are presently being studied, along with

means to stabilize the surface state of glass filters. Until more information is acquired in this field, we

recommend that the colored glass filters issued as SRM 930 be rechecked annually to determine whether

any physicochemical changes, which might affect the

transmittance values, have occurred.

Another important factor is the need for defining

and producing a clean glass surface. Until now the

final cleaning of the NG-4 filters was made with re- distilled ethyl alcohol and pure water (thermally

distilled and deionized). Other cleaning procedures

are under consideration. The use of isopropyl alcohol

in vapor or liquid form associated with mild ultrasonic action is being investigated [41].

The transmittance characteristics of the SRM 930

limit the use of this material to the visible region of

the spectrum from about 400 nm to the near infrared.

Since the ultraviolet region, from about 200 nm is also

important to the analyst who uses spectrophotometric

methods, exploratory work is underway to select and

certify solid material for checking the photometric

scale in this spectral region. Optical filters exhibiting

small transmittance-wavelength dependence in the

spectral range 200 nm to near infrared can be obtained

by evaporating thin semitransparent layer of a metal on a suitable transparent substrate [42, 4], and such

filters have been considered in this work. The metals

selected were inconel and chromium which exhibit

adequate transmission characteristics and good adhesion to the substrate. The substrate was nonfluorescent fused quartz. A series of filters were prepared

by the optical shop at NBS according to the following

specifications: a number of nonfluorescent optical

quality fused quartz plates, 10 mm x 25 mm and 1 mm

thick, were cut and polished, Inconel or chromium

metal was evaporated on the surface to produce

nominal transmittances of 25, 50, and 75 percent.

The surface bearing the evaporated metal was coated

with a layer of optical cement which was transparent

to the visible and ultraviolet radiations down to 230 nm. A clear plate of the same material was used to cover and protect the evaporated metal layer.

The filter assembly was then marked at one corner

to insure its proper positioning and the finished filter was placed in a metal holder of conventional size

(approximate o.d. 13 x 13 x 57 mm) fitting the cuvette

holder found in most spectrophotometers. The metal

holder was also marked at one side to permit positioning of the filter in a reproducible manner.

In addition to the evaporated metal filters, a number

of units were prepared using only the clear uncoated

fused quartz plates and assembled with the same

optical cement. When desired, these clear filter assemblies could be used as reference samples in

the blank compartment.

Before submitting the evaporated metal filters to transmittance measurements, a study was made of the

effect of radiations on their transmittances. A filter was exposed to an accelerated test in which radiations

had the same spectral distribution as the fluorescent

lighting of the laboratory, except that they were 1000

times more intense. The filter was exposed for an

equivalent of 36.000 hours of continuous irradiation.

This test was made on a radiation accelerator made

available by the Building Research Division of NBS.

The percent transmittance was measured before and

after the exposure and gave the following results:

Transmittance, percent

Wavelength, nm 250 380 500 650

Before exposure 44.48 51.35 48.90 47.41

After exposure 44.11 51.34 48.92 47.47

The relative standard deviation for a single determination of these measurements was 0.01 percent.

As can be seen, the only significant relative change in

transmittance of about 0.84 percent of the value

occurred at 250 nm.

Several sets of these filters were calibrated at five

selected wavelengths, 250 nm; 350 nm; 450 nm; 550

nm; and 650 nm, using the cleaning and measuring

procedures outlined for Schott NG-4 colored glass

neutral filters. The results indicated that the reproducibility of transmittance measurements is good

(percent standard deviation 0.009 to 0.024) and is comparable to those obtained for the colored glass

filters at all wavelengths except 250 nm. From the

experimental data, it is evident that the transmittance

of the evaporated metal filter at 250 nm is critical

and, at present, no satisfactory explanation for this

phenomenon can be given. A limitation of the evaporated metal filters is that they attenuate the intensity

of radiation by reflecting a part of it, rather than absorbing. This can produce, in certain circumstances,

undesirable stray fight in the instrument and make the

transmission measurements dependent on the geometry of the optical beam. However, since these filters are closer to optical neutrality than the colored glass

filters, and since they can be used in the ultraviolet

region as well, they were included in this work.

This limitation was apparent from the data obtained in a cooperative study conducted at C. Zeiss

by A. Reule using conventional spectrophotometers.

On the other hand, a similar comparative test, made

on the same filters by F. J. J. Clarke at NPL has

produced the results presented in table 10. One can

observe that, in spite of the limitations mentioned

above, an agreement within —0.30 percent of the

value was obtained between NBS and NPL measure- ments at the indicated wavelength.

Further studies will be needed to assess unambiguously the transmittance characteristics of evaporated

metal-on-quartz filters, with or without a protective

quartz plate, and to assess their suitability as Standard

Reference Materials to check the photometric scale

of spectrophotometers in the ultraviolet and visible

part of the spectrum.

423

470-063 OL - 72 - 4
XI. Addendum

The identification of commercial instruments and

products, is given in the Addendum only to permit

reproduction of the work described in this paper.

In no instances does such identification imply recom- mendation or endorsement by the National Bureau of

Standards, nor does it imply that the particular equipment or product is necessarily the best available for the

purpose.

Radiation source for visible — Microscope lamps,

type 18A/T10/1P-6V: General Electric Co.. Lamp

Division, Nela Park, Cleveland, Ohio 44112. For ultraviolet: Atlas single coil halogen (Bromine) lamp,

type Pl/8, 30V, 250W: GTE Sylvania, Inc., 6610

Electronic Drive, Springfield, Virginia 22151.

Power supply for microscope lamp, Kepco, Model

JQE 15-50-M-VP: Kepco, Inc., 131-38 Sanford

Avenue, Flushing, New York 11352. For tungstenhalogen single filament lamp: same manufacturer,

Model JQE-36-30 Mt-VP.

Potentiometer: Leeds and Northrup Model K3 with

null meter and power supply. Resistors: Leeds and

Northrup 0.1, 50 A and 0.01, 100 A: Leeds and

Northrup, Sumneytown Pike, North Wales, Pa.

19454.

Nonfluorescent fused silica: Dynasil Corporation of

America, Berlin, New Jersey 08009.

Neutral Density Attenuator and BaS04 white paint:

Eastman Kodak Co., Special Products Sales, Kodak

Apparatus Division, Elmgrove Plant, Rochester, New

York 14650.

Monochromator with predisperser: McPherson In- strument Corp., 530 Main Street, Acton, Massachusetts 01720.

Optical benches with carriers and x — y sample

holder with micrometer control: Gaertner Scientific

Corp., 1201 Wrightwood Ave., Chicago, Illinois 60614.

Lens holders: Ardel Instrument Co., Inc., P. O.

Box 992, Jamaica, New York 11431.

Ball bushing and rails: Thompson Industries, Inc.,

Manhasset, New York 11030.

Pneumatic cylinders and accessories: Clippard

Instrument Laboratory, Inc., Cincinnati. Ohio 45239.

Rotating table: Ealing Optics Division. 2225 Massachusetts Avenue, Cambridge, Massachusetts 02140.

Thermostating holders for glass cells and glass

filters: Gary Instruments, 2724 South Peck Road,

Monrovia, California 91016.

Pneumatic ratchet system: Allenair Corp., P. O.

Box 350, 255 East 2nd Street, Mineola, New York

11501.

Black paint -Nextel 101-c 10 Black: Reflective

Products Division 3M, 2501 Hudson Road, St. Paul,

Minnesota 55101.

Photomultiplier tube EMI-9558QA: Gencom Division, 80 Express Street, Plainview, New York 11803.

Power supply for photomultiplier tube: Model 415B

and digital voltmeter 8400A: John Fluke Manufacturing

Co., P.O. 7428, Seattle, Washington 98133.

Thermal insulation: Photoshroud, Shumway

Optical Instruments Corp., 2118 Beechgrove Place,

Utica, New York 13501.

Vibration isolation table: Lansing Research Co.,

705 Willow Avenue, Ithaca, New York 14850.

Low power laser; Model 195 cw gas laser, output

power 2 mw: Optics Technology, Inc., 901 California

Avenue, Palo Alto, California 94304.

Tritium activated fluorescent source, Beta light

Marker HM-110: Canrad Precision Industries, Inc.. 630 Fifth Avenue, New York, New York 10020.

Colored glass neutral filters, Schott NG-4: FishSchurmann Corp., 70 Portman Road, New Rochelle,

New York 10802.

Spectral lamp: Oriel Optics Corp., 1 Market Street,

Stamford, Connecticut 06902.

Polarization filters: Polaroid Corp., 119 Windsor

Street, Cambridge, Massachusetts 02139.

Computer: 24K memory and 16 bit words. EMR

computer, Division of Weston Instruments, Inc.,

Schlumberger Co., 8001 Bloomington Freeway,

Minneapolis, Minnesota 55420.

The author gratefully acknowledges the assistance

of the following persons, in addition to those already

mentioned in the text: F. J. J. Clarke, for the considerable assistance given

through personal discussions on the philosophy and

principles of high accuracy spectrophotometry at NPL.

Anne Compton, for advice and help during the ex- perimental work carried-out at the high accuracy

spectrophotometer at NPL.

G. E. V. Lambert, for information concerning glass

filters (NPL).

K. D. Mielenz (NBS), for constructive discussions on

the principles of spectrophotometry and for critically

reviewing the manuscript.

W. H. Venable (NBS), for critically reviewing the

manuscript.

K. L. Eckerle (NBS), for advice in measuring stray

radiations and polarization produced in the monochromator.

C. R. Yokley (NBS), for advice on the radiation

stability of incandescent tungsten filament lamps.

W. K. Haller (NBS), for valuable information on the

properties of glass surfaces and optical glass filters. H. H. Ku (NBS), for advice on interpretation of the

data discussed in this paper.

NBS Instrument Shop, for constructing the mechanical parts described in this work.

NBS Optical Shop, for grinding the quartz lens used

in this work.

D. S. Bright (NBS guest worker), for the programs

written at various times during the initial phase of this

work.

V. E. Gray (NBS), for assistance in performing the

accelerated radiation tests. H. J. Hbfert and A. Reule (C. Zeiss, Oberkochen,

Germany), for valuable information on linearity of

424
photodetectors and for demonstration of the radiation

addition method.

R. J. Carpenter (NBS), for constructive discussions on electronic circuitry and photodetectors.

L. Marzetta (NBS), for advice and assistance in

assessing various functioning parameters of photomultiplier tubes.

O. Menis, Section Chief, Analytical Coordination

Chemistry (NBS), for approving and encouraging the

work reported in this paper.

J. I. Shultz, Assistant Section Chief, Analytical

Coordination Chemistry (NBS), for critically reviewing

the manuscript.

XII. References

[1] Kortiim, G., Kolorimetrie-Photometrie und Spektrometrie

464 p., 4th ed. (Springer-Verlag, 1962).

[2] Gibson, K. S., Spectrophotometry, Nat. Bur. Stand. (U.S.). Circ. 484, 48 pages (1949).

[3] Mavrodineanu. R., Spectrophotometry, instrumental development, O. Menis and J. I. Shultz, Eds., Nat. Bur. Stand.

(U.S.), Tech. Note 584, pp. 9-21 (1971).

[4] Mavrodineanu, R., Solid materials to check the photometric

scale of spectrophotometers, O. Menis and J. I. Shultz, Eds., Nat. Bur. Stand. (U.S.), Tech. Note 544, pp. 6-17 (1970).

[5] Elster, J. and Geitel, H., Un tne comparison of light intensities by photoelectric methods, Ann. Physik, Chemie 48, 625-635

(1893).

[6] Elster, J. and Geitel, H., The photoelectric effect on potassium

at low light levels, Phys. Ztsch. 13 , 468-976 (1912).

[7] Koller, L. R., and Breeding, H. A., Characteristics of photoelectric tubes. General Electric Rev. 31, 476-479 (1928).

[8] Campbell, N. R., The experimental proof of a fundamental

photoelectric law, Trans. Opt. Soc. (London) 32, 61-65

(1930-1931).

[9] Fleury, P., An addition method for the precise study of the

variation of the current output of a photoelectric cell as a function of the incident luminous flux, Compt. Rend. Acad.

Sci. (France) 199, 195-197 (1934).

[10] Preston, J. S., and McDermott, L. H., The illumination re- sponse characteristics of vacuum photoelectric cells of the

Elster-Geitel Type. Proc. Phys. Soc. (London) 46. 256-272

(1934).

[11] Preston, J. S., and Cuckow, F. W., A photoelectric spectrophotometer of high accuracy, Proc. Phys. Soc. (London)

48,869-880 (1936).

[12] Buchmiiller, F., and Konig, H., Precision measurements on

incandescent lamps using selenium photocells. Assoc.

Suisse Electriciens, Bulletin 28, 89-99 ( 1937)

.

[13] Atkinson, J. R., Campbell, N. R., Palmer, E. H., and Winch.

G. T., The accuracy of rectifier-photoelectric cells, Proc. Phys. Soc. (London) 50, 934-946 (1938).

[14] Barbrow, L., A photometric procedure using barrier-layer

photocells, J. Res. Nat. Bur. Stand. (U.S.), 25, 703-710

(1940) RP1348.

[15] Kaiser, H., Photographic-photometric calibration of step

filters, Spectrochimica Acta 3, 518-537 (1947-1949).

[16] Harding, H. G. W., Precautions necessary for accurate measure- ments of optical density standards, Photoelectric Spectrometry Group Bulletin 4, 79-86 ( 1951 )

.

[18

[19

[20

[21

[22

[23

[24

[25

[26

[27

(28

[29

[30

[31

[32

[33

[34

[35

[36

[37

[38

[39;

[40

[41

[42

Kortiim, G., and Maier, H., Dependence of photocurrent and

illuminating intensity in photocells and photomultipliers, Z. Naturforschung 8A, 235-245 ( 1 953 )

.

Hansen, G., Check of photometric function of spectrophotometers, Mikrochimica Acta 1955, 410-415.

Hermann, W., Noise and linearity check on photomultipliers, Z. Naturforschung 12 A, 1006-1013 (1957). Reule, A., Checking the photometer scale of absorption instru- ments, Zeiss-Mitt. 1, 283-299 (1959). Hoppmann, H., An instrument for checking the proportionality

of physical radiation receivers, Technisch-Wissenschaftliche

Abhandlungen Der Osram-Gesellschaft 7, 306-312 (1958). Gibson, G. L., Hammond, H. K.. Ill, Holford, W. L., and

Nimeroff, I., Calibration of photometers, NBS Manuscript, Nov. 1, 1960. Bischoff, K., Measurement of proportionality of radiation re- ceivers over a large radiation intensity range, Z. Instr. 69,

143-147 (1961). Jones, O. C, and Clarke, F. J. J., A new photometric technique

using a variable shutter device. Nature 191, 1290 (1961). Cordle, L. C, and Habell, K. J., Photometry of telescopes and

binoculars, National Physical Laboratory, Notes on Applied

Science No. 14, 18 pp. (1962). Sanders, C. L., A photocell linearity tester, Appl. Optics 1,

207-271 (1962). Nonaka, M., and Kashima, T., Linearity characteristics of multiplier phototubes, Japanese Journal Applied Physics 2, 785-

791 (1963). Hofert, H. J., and Loof, H., Calibration of photometric scales of

a reflection photometer, Z. Instrumentenkunde 72, 139-143

(1964). Clarke, F. J. J., Time ratio photometry. Lecture to the Optical

Group of the Institute of Physics and the Physical Society

(June 9, 1966). Clarke, F. J. J., High accuracy spectrophotometry of three filters, NPL; Report No. 3042, 14 pp. (Nov. 4, 1968).

Reule, A., Testing spectrophotometer linearity, Appl. Optics 7,

1023-1028(1968).

Desvignes, F., and Ohnet, J., Characteristics and measurement

of properties of radiation receivers, Techniques Philips N.6,

1-18 (1968). Kunz, H., Representation of the temperature scale above 1337.58 K with photoelectric direct current pyrometers, Metrologia 5,

88-102 (1969). Kostkowski, H. J., Personal communication (NBS 1970).

ASTM Tentative Method of Estimating Stray Radiant Energy,

ASTM:E 387-69T.

Clarke, F. J. J., Personal communication (NPL, August 1970). Mielenz, K. D.. and Eckerle, K.. Accuracy of Dolarization attenuators (to be published), Appl. Optics (1972). Mielenz, K. D., and Eckerle, K., Spectrophotometer linearity

testing using the double-aperture method, Appl. Opt. 11,

2294 (1972). Garfinkel, S. B., Mann, W. B., and Youden, W. J., Design and

statistical procedures for the evaluation of an automatic

gamma-ray point-source calibrator, J. Res. Nat. Bur. Stand. (U.S.),70C (Eng. and Instr.). No 2.53-63 (1966).

Edisbury, J. R., Practical Hints on Absorption Spectrophotometry (Plenum Press, New York, 1967).

Holland, L., The Properties of Glass Surfaces (Chapman and

Hall, LondonJ964). _ Hass, G., and Thun, R. E., Editors, Physics of Thin Films

(Academic Press, 1967).

(Paper 76A5-729)

425

Appendix 3

January

1955

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

WASHINGTON, D. C. 20234

Letter

Circular

LC-1017

Re-issued

January 1967

STANDARDS FOR CHECKING THE

CALIBRATION OF SPECTROPHOTOMETERS

(200 to 1000 nm)

Contents

1. Introduction.

2. Checking the wavelength scale.

2.1 Non-recording spectrophotometers.

2.2 Recording spectrophotometers.

2.2.1 General Electric recording spectrophotometers.

2.2.2 Cary Model 14 recording spectrophotometers.

References.

I. Introduction.

In continuation of a type of activity carried on for many years at

the National Bureau of Standards, there is described in this letter circular the various types of standards that are available for issuance by

the Bureau for the purpose of checking or maintaining the over-all reliability of spectrophotometers in the ultraviolet, visible and near-infrared regions of the spectrum. Certain other information of similar purpose

is also included.

Experience has shown that spectrophotometers can easily get out of

adjustment. Although repeated trials may give the same values over and

over again, indicating high sensitivity and precision, and the instrument

may otherwise appear to be functioning perfectly, gross errors in wavelength may nevertheless be rendering the values obtained highly unreliable

Other causes of error may likewise be present and unsuspected, particularly with the photoelectric spectrophotometers now comprising so large a per

centage of the total in use. The use of the various standards described

herein has been found of considerable assistance in detecting and eliminating errors that would otherwise be present, or in confirming that the

instrument is in fact giving reliable results.

Reference is made in this letter circular to the NBS test fee

schedules currently in effect. Those relating to spectrophotometry are

designated as 202.105, Spectrophotometry Standards, and 202.106,

Spectrophotometric Measurements. These schedules are obtainable from the

National Bureau of Standards.
- 2 -

2. Checking the Wavelength Scale.

Most of the present-day spectrophotometers have a direct-reading

wavelength scale; that is, the scale, instead of being divided in uniform

linear or circular measure, is divided and engraved directly in nanometers (formerly called millimicrons). This greatly facilitates setting

the instrument at any desired wavelength. The accuracy of many of these

direct-reading wavelength scales is remarkably good, when put in the best

average adjustment, considering the difficulties of quantity production

of such scales. When so adjusted, it is not uncommon to find them in

error by not more than 1 nm throughout the ultraviolet and visible spectrum.

However, if one wishes the uncertainties in his wavelength settings to be

of the order of 0.1 nm, a careful check of these direct-reading scales is

necessary.

Certain sources and wavelengths that have proved especially suitable

for the calibration of various types of spectrophotometers are listed in

tables la and lb which are similar in scope and purpose to table 1 of

Circular 484 (1)*.

^Numbers in parentheses refer to the References.

All values of tables la and lb are consistent with those published in the

M.I.T. wavelength tables (2). Which of these sources to use, and which

wavelengths of the several sources are the most suitable, will depend on

the type of instrument. Furthermore, the procedure will vary importantly

depending on whether the instrument is a non-recording or a recording

spectrophotometer.
- 3 -

Table la. Wavelengths of Sources Suitable for Calibration of

Spectrophotometers

.

Wavelength

(nm)

Note Wavelength

(nm)

Note Wavelength Note Wavelength

(nm) (sun)

Note

Mercury Arc in Quartz (Same wavelengths in glass above 300 nm)

*

1*

2*

3*

4*

5*

296.73

302. 15^

302.351

302.561

302. 75j

312.57

313.15

313,

334,

349.28

365. 0i)

365.481

366.29

366. 33j

390.64

1.181

^15J

6*

7*

8*

398.40

400.63

404.66

407.78

435.83

491.60

546 . 07

576,

579,

623.44

671.62

690.72

1014.0

1128.7

.u/

.96*1

• .07J

9*

Helium Discharge Tube in Glass

318.77

361.36

363.42

370.50

381.96

388.86

396.47

402.62

412.08

414.38

438.79

443.75

447 . 15

471.31

492.19

501.57

504.77

587.56

667.81

706.52

728.13

1083.0 it

These lines have been found most useful on the Beckman DU

spectrophotometer.

1. A value of 239.95 is recommended for the unresolved pair.

2. A value of 248.3 is recommended when the 3 lines are unresolved.

3. The intensity of 253.48 is negligible compared to that of 253.65.

The latter value should be used when the lines are unresolved.

4. The 265.20 line is somewhat stronger than the others and a value

of 265.3 is recommended when the three lines are unresolved.

5. ' These two lines are of approximately the same intensity and a

value of 280.40 is recommended for the unresolved pair.

6. The two shorter lines are considerably stronger than the other

two. It is probable that a value of 302.25 should be used for the unresolved

lines.

7. A value of 313.16 is recommended for the unresolved pair.

80 With the arc used on the Beckman DU spectrophotometer the ratio of

intensities for 365.01 : 365.48 : 366.33 is 100 : 48 : 36, approximately.

The intensity of the 366.29 line appears negligible relative to that of

366.33.

9. These two lines are of approximately the same intensity and a

value of 578.0 is recommended for the unresolved pair.
- 4 -

Table lb. Wavelengths of Sources Suitable for Calibration of Spectrophotometers

Neon Discharge Tube Aluminum Hydrogen Arc

Spark in Air

Wave- Relative

length Intensity Wavelength Wavelength

(nm) (nm) (run)

585.25 5 216.88 434,05

588.19 4 217.40 486 . 13

594.48 8 220.46 656.28

597.55 2 221.00

603.00 2 226.35

607.43 8 226.91 Sodium Arc

609.62 13 236.71 Wavelength

614.31 25 237.21 (nm)

616.36 6 237.31 589.00

621.73 4 237.34 589.59

626.65 11 237.84

630.48 4 256.80

633.44 20 257.51 Cesium Arc

638.30 23 257.54 Wavelength

640.22 100 263.16 (nm)

650.65 39 265.25 852.11

653.29 8 266.04 894.35

659.90 12 281.62

667.83 23 308.22

671.70 14 309.27

692.95 23 358.69

702.41 2 394.40

703.24 45 396.15

705.91

717.39 5

724.52 17

743.89 4

748.89

753.89 mm mm 754.40
- 5 -

2.1 Non-Recording Spectrophotometers ,

The best procedure for checking the wavelength scale of a nonrecording spectrophotometer is by direct use of a source of radiant

energy having spectral lines of suitable intensity and adequately

spaced throughout the spectral range of interest. Various sources

are available and can be recommended for such purpose. How many sources,

or how many wavelengths, to use in such a calibration depends, of course,

on the desires of the individual investigator.

In this connection it should be noted that the number of significant figures of importance in spectrophotometry (including "absorption

spectroscopy") is of a different order of magnitude than that used in

emission spectroscopy or in standard wavelength tables. In the visible

spectrum with the usual type of spectrophotometer it seems impossible

to maintain the wavelength calibration with uncertainties less than about

0.1 run. While the uncertainty may be less in the ultraviolet with a

prism instrument, there seems no purpose served in giving standard wavelengths to better than 0.01 nm for spectrophotometric calibration.

Two suitable sources for wavelength calibration are the mercury

lamp and the helium lamp. A mercury lamp in a quartz envelope is by far

the best single source for wavelength calibration from 205 to 1014 nm.

A mercury lamp in a glass envelope provides the same spectral lines except

that below about 300 nm they are not transmitted by the glass envelope.

The helium lines are especially well placed for wavelength calibration

in the visible spectrum, and the strong lines at 388 and 1083 nm are also

often very useful. Many other sources, flame or arc, are available for

visual wavelength calibration (2, 3) but most of these are too unstable for

accurate calibration with a photoelectric detector.

These same sources and many others are also useful for the wavelength

calibration of spectrographs used in photographic spectrophotometry. Between 200 and 400 nm the series of doublets obtained from the aluminum

spark in air is very useful because they are so readily recognized.

Not all of the lines for any of the sources are given in tables la

and lb but only those that are considered especially suitable for the purpose. Furthermore, not even all of those listed for any one source may

be suitable for any one particular instrument. The mercury arc in quartz

is an example. All of the lines listed (and still others) can be used for

wavelength calibration of a photographic spectrophotometer over the range

of sensitivity of the plate used. The lines from 404.7 to 690.7 nm can

be used for visual calibration of a spectrophotometer. But not all of the

lines are suitable for calibration of a photoelectric instrument, and

those that prove adequate will depend on the sensitivity and slit widths

characteristic of any particular instrument. One must be very careful

that other lines are not included, in addition to the one on which the

settings are supposedly being made, of sufficient intensity to affect

the wavelength setting.
- 6 -

Special attention should perhaps be called to the use of a cesium

arc at 852.1 and 894.3 nm (4). From tables la and lb it is apparent that

there are few suitable lines between 706.5 and 1014.0 nm, particularly

from steady sources necessary or desirable in the calibration of photoelectric spectrophotometers. The neon discharge tube gives many lines

between 750 and 1000 nm (2) but these have not been found satisfactory

in the calibration of photoelectric spectrophotometers. In the orange

and red the neon lines are useful for visual calibration and many of

these can be used to calibrate photoelectric spectrophotometers (5) if

the sensitivity is such that very narrow slits can be used. The relative

intensities (6) given in table lb will help in case of overlapping.

The best technique to use in wavelength calibration of non-recording

spectrophotometers, given a suitable source, will vary from instrument to

instrument and method to method. A few general principles can be given

here, however.

In photographic spectrophotometry it usually is sufficient to

photograph a known spectrum at the top and bottom of the plate, unless

the source used for the absorption spectra itself carries such known

reference lines. A few of these reference lines will then serve to

correlate that particular plate with whatever complete calibration curve

has previously been established by more extensive measurements with the

various sources.

On visual and photoelectric non-recording spectrophotometers, it

usually is necessary, for highest precision, to have a basic reference

line to which all of the other wavelengths are compared by direct check.

At the Bureau the Hg yellow lines have proved most suitable for the

Konig-Martens visual spectrophotometer (7). At the slit widths used the

overlapping of the two lines gives a central brighter "line" taken as

578.0 nm with a luminous background against which the slit jaws are

readily seen. A luminous background, or slight illumination of the

ocular slit, always facilitates calibration when an eyepiece is used.

Visual calibration without an eyepiece is usually less precise unless

very narrow slits are used.

Two techniques have been used at the Bureau in the calibration of

non-recording photoelectric spectrophotometers. On the Gibson spectrophotometer (8) the slits are always 0.1 mm wide or greater and the most

reliable calibration is obtained by plotting galvanometer deflections

at closely adjacent wavelengths. The most probably value for the wavelength reading is given by the intersection of the two straight lines

resulting from a plot of the data for any given line, the correction

being given by the difference between this value and the true wavelength.

This is illustrated in reference (1).
- 7

-

On the Beckman DU spectrophotometer the same method has been used

(9), but at the Bureau it has seemed preferable and is much more rapid,

to calibrate with a narrow slit and record the wavelength dial reading

for the maximum left deflection of the galvanometer needle as the wavelength dial is slowly turned. The most suitable reference line on two

of the Bureau's instruments has proved to be the Hg green line at

546.07 nm (5).

2.2 Recording Spectrophotometers .

The initial wavelength calibration of a recording spectrophotometer,

such as the manufacturer must carry out in connection with cutting his

cams or preparing his reading scale, is not here considered, but only

the check of such a calibration by the user of the instrument.

Such a user can, of course, follow the procedure prescribed above

for checking the wavelength calibration of non-recording spectrophotometers. However, there are two important reasons for following a

different procedure for recording spectrophotometers. For such instruments it is desirable to have a calibration that is made with the instrument operating. It is further desirable in most kinds of work to have

this calibration appear on the graph sheet so that difficulties connected

r?ith positioning of the sheet, expansion or contraction of the paper with

humidity or temperature, or instrumental variations can be eliminated.

Wavelength calibrations of this kind can be made if a material is

available having a number of strong and narrow absorption or transmission

bands suitably spaced over the spectral range of interest. Two materials

have been used or suggested for this purpose: (a) Glasses containing

rare-earth oxides, such as didymium glasses and holmium oxide glasses,

have been used for many years at the National Bureau of Standards (10, 11),

(b) quartz -Polaroid combinations have been proposed (12) and may prove

useful for such work.

The use of a didymium glass or a holmium oxide glass in this manner

would not in general be accurate unless it is calibrated at nearly the

same slit widths as are to be used. Most of the absorption bands that are

usable for the purpose are multiple bands and the wavelengths of maximum

absorption often depend on the slit widths. This has been illustrated in

previous publications (1, 10).

While the use of a didymium glass or a holmium oxide glass for checking the wavelength calibrations of a recording spectrophotometer is highly

recommended, there are two other uses of these glasses which are not

recommended. First, these glasses are not well suited for checking the

photometric scale of any spectrophotometer, recording or non-recording.

Transmittances at the peaks of the absorption bands are too dependent on

slit widths, and transmittances on the steep parts of the curve are too

dependent on slight wavelength errors, both as illustrated in Fig. 1 of

reference (10) and in Fig. 8 of reference (1). Second, the use of these

glasses to check the wavelength calibration of a non-recording spectro-
- 8 -

photometer is considered much inferior from the standpoints of time,

convenience and reliability to the direct use of line sources as

described in Sec. 2.1. The National Bureau of Standards has consistently refused to accept didymium glasses or holmium oxide glasses for

calibration for either of these two purposes.

2.2.1. General Electric Recording Spectrophotometer .

The NBS didymium glass standards were carefully calibrated by

point-by-point measurements on the Konig-Martens visual and Gibson

photoelectric spectrophotometers with slit widths approximating the

10- and 20- nanometer slits used on the NBS General Electric spectrophotometer. Some of these values have been published (13).

The most suitable didymium glass for the purpose, considering

type of curve and availability, is a Corning 5120 glass of 3.0 mm

thickness. While it is not known how much the wavelengths of maximum

absorption of this 5120 glass might vary from melt to melt, glasses

from at least three melts have been measured, and there has never been

any certain variation among the samples tested. For much work it is

probably safe to use the values given in table 2.

Table 2. Wavelengths of maximum absorption for Coming 5120 glasses

of 3.0 mm thickness as obtained at the National Bureau of

Standards for the slit widths indicated.

For those who wish greater certainty, however, the Bureau has

obtained a supply of Corning 5120 glass in 2-inch polished squares

and of 3.0 mm thickness. These are measured and the values reported

in accordance with NBS test fee schedule 202.105, items d to f. The

measurements consist of recording a curve of the test glass on the

same sheet as the curve of the NBS standard glass and deriving values

of the wavelengths of minimum transmittance of the former relative to

Wavelength

of

Maximum Absorption

(nm)

Approximate Spectrum

Interval

Transmitted by Slits

(nm)

10

10

10

10

10

10

20

20

20

20
- 9

-

those of the latter. The over-all uncertainties of the values so

reported are considered to be not greater than +1 nm from 441.0 run to 743.5 nm, and not greater than + 2 ran from 745 to 1067 nm.

Methods of use of a calibrated didymium glass on a G. E. recording

spectrophotometer are described in references (1), (10), and (13).

2.2.2. Cary Model 14 recording spectrophotometer .

The NBS holmium oxide glass standards were carefully calibrated

by using a Cary Model 14 recording spectrophotometer, the wavelength

indicating dial of which had previously been calibrated by means of

a number of sources having wavelengths throughout the ultraviolet and

visible spectral regions. Measurements of the wavelengths of minimum

transmittance were made as functions of slit width over the range 0.06

to 5.0 nanometers of spectral width. Eleven sharp absorption bands

were found to be sufficiently symmetrical that the wavelengths of

minimum transmittance indicated by the recorder remained constant for

slit widths up to about 2 nanometers.

The most suitable holmium oxide glass for the purpose is a Corning

3130 glass of approximately 2.5 mm thickness. It is not known how much

the wavelengths of minimum transmittance of this 3130 glass might very

from melt to melt. It is known that, for some 3130 glasses, the absorption of the base glass prevents the use of the glass for wavelength

calibration in the ultraviolet near 241 nanometers. For much work it is

probably safe to use the values given in the following table.

For those who require that the base glass of the standard transmit

sufficiently for the standard to be useful at 241 nanometers, the

Bureau has obtained a supply of Corning 3130 glass in 2-inch polished

squares and of 2.5 mm thickness. These are measured and the values

reported in accordance with NBS test fee schedule 202.105, items g to i.

Wavelengths of minimum transmittance for Corning 3130

glasses of 2.5 mm thickness as obtained at the National

Bureau of Standards for slit widths less than 2 nm.

Useful wavelengths

between 240 and 370 nm

Useful wavelengths

between 360 and 650 nm

241.5

279.3

287.6

333.8

360.8

360.8

385.8

418.5

453.4

459.9

536.4

637o5
- 10 -

The measurements consist of recording a curve of the test glass on

the same sheet as the curve of the NBS standard glass and deriving

values of the wavelengths of minimum transmittance of the former

relative to those of the latter. The over-all uncertainties of the

values so reported are considered to be not greater than + 0.5 nm.

The present supply of holmium oxide glass contains striae, and in

some cases strains, which have not appreciably altered the wavelengths of minimum transmittance.

The methods of use of a calibrated holmium oxide glass on a

Cary Model 14 recording spectrophotometer are similar to those

described in references (1), (10), and (13) relating to the method

of use of a didymium glass on a G. E. recording spectrophotometer.
- 11

References .

1. Kasson S. Gibson, Spectrophotometry (200 to 1000 millimicrons) NBS Circular 484 (1949).

2. Massachusetts Institute of Technology Wavelength Tables,

measured and compiled under the direction of George R.

Harrison (John Wiley & Sons, Inc., New York, 1939).

3. Spectrophotometry, report of O.S.A. Progress Committee

for 1922-23, K. S. Gibson, Chairman, J. Opt. Soc. Am. &

Rev. Sci. Inst. 10, 169 (1925).

4. No C. Beese, Cesium vapor lamps, J. Opt. Soc. Am. 36, 555

(1946).

5. Kasson S. Gibson and Margaret M. Balcom, Transmission

measurements with the Beckman quartz spectrophotometer,

J, Research NBS 38, 601 (1947) RP1798; also J. Opt. Soc.

Am. 37, 593 (1947).

6. M. Garbuny, Relative Intensitaten und Ubergangswehrscheimlichkeiten der rotgelben Neonlinien, Zeit. Physik 107 , 362

(1937)

.

7. H. J. McNicholas, Equipment for routine spectral transmission and reflection measurements, BS J. Research

Y, 793 (1928) RP30.

8. K. So Gibson, Direct -reading photoelectric measurement of

spectral transmission, J. Opt. Soc. Am. and Rev. Sci. Inst.

7, 693 (1923); also described in J. Opt. Soc. Am. 21, 564

(1931)

.

9. H." H. Gary and Arnold 0. Beckman, A quartz photoelectric

spectrophotometer, J. Opt. Soc. Am. 31, 682 (1941)

10. Kasson S. Gibson and Harry J. Keegan, Calibration and operation

of the General Electric recording spectrophotometer of the

National Bureau of Standards, J. Opt. Soc. Am. 28, 372 (1938).

11. H. J. Keegan, J. C. Schleter, and V. R. Weidner, Ultraviolet

wavelength standard for spectrophotometry, J. Opt. Soc. Am.

51, 1470 (1961).

12. George L. Buc and E. I. Stearns, Uses of retardation plates in

spectrophotometry, II, Calibration of wavelength, J. Opt. Soc.

Am. 35, 465 (1945).

13. Harry J. Keegan and Kasson S. Gibson, On the use of working

standards of didymium and Vitrolite glasses for spectrophotometry measurements, J. Opt. Soc. Am. 34, 770 (1944).

Appendix 4

Journal of Research of the National Bureau of Standards

Volume 90, Number 2. March-April 1985

Spectral Transmittance Characteristics

of Holmium Oxide

in Perchloric Acid Solution

Victor R. Weidner, Radu Mavrodineanu, Klaus D. Mielenz, Ranee A. Velapoldi,

Kenneth L. Eckerle, and Bradley Adams

National Bureau of Standards, Gaithersburg, MD 20899

Accepted: November 28, 1984

The work describes the methods and procedures used to determine the wavelengths of minimum trans- mittance of holmium oxide in perchloric acid solution. Measurements of spectral transmittance of the solutions were made by means of a high precision spectrophotometer over the wavelength range 200 nm to 680 nm. The

wavelength scale accuracy of this instrument was verified by extensive measurements of mercury and deuterium

emission lines. The measurements of spectral transmittance of the holmium oxide solutions were made as a function of temperature, purity, concentration, and spectral bandwidth. Analysis of the uncertainties associated with these parameters and the uncertainties associated with the calibration of the instrument wavelength scale and the data analysis have resulted in an estimated uncertainty of ±0.1 nm for the determination cf the wavelengths of minimum transmittance of the holmium oxide solution.

Key words: holmium oxide; spectral bandwidth; spectrophotometer calibration; spectral transmittance; wavelength calibration; wavelength standard.

1. Introduction

This work describes the methods and procedures used

to determine the wavelengths of minimum spectral

transmittance of holmium oxide (Ho2 Oj) in perchloric

acid (HC104) solution in the spectral region 200 to 680 nm. The object of this activity was to develop a standard

for verifying the wavelength scale of uv/visible spectrophotometers, and to provide assistance toward improving the accuracy of measurements in the fields of

About the Authors: Victor R. Weidner is with

NBS' Radiometric Physics Division, headed by

Klaus D. Mielenz and served also by Kenneth L. Eckerle; Radu Mavrodineanu is associated with the

Bureau's Inorganic Analytical Research Division as a reemployed annuitant; Ranee A. Velapoldi is deputy

director of the NBS Center for Analytical Chemistry;

and Bradley Adams served the Bureau as a student

worker.

molecular absorption spectrometry or spectrophotometry [i]

1

. Holmium oxide in a glass matrix has been issued by

the National Bureau of Standards (NBS) as a wavelength standard for the ultraviolet and visible spectrum

since 1961 [2]. Didymium glass wavelength standards

have been issued since 1945. The didymium glass wavelength standards are available from NBS as Standard

Reference Material 2009; 2010; 2013; and 2014 [3].

2. Experimental

2.1 Instrumentation

Measurements of spectral transmittance of the holmium oxide solutions were made by means of a Varian

Cary Model 2390 recording spectrophotometer2.

'Numbers in brackets indicate literature references.

: Certain commercial equipment or products are mentioned in this paper in order to adequately document the work. In no case does this imply that the equipment or product is being endorsed by NBS or that

it is necessarily the best equipment or product for the application.

115
The optical system of this double beam instrument

consists of a filter predisperser and a double-pass

double-sided grating monochromator. A deuterium

lamp is used over the wavelength range 185 to 340 nm

and a tungsten-halogen lamp is used over the wavelength range 340 to 3150 nm as a source. The spectral

bandwidths are selectable from 0.07 nm to 3.6 nm. Data

can be recorded by a digital readout system with thermal printer and an analog chart display.

2.2 Materials

2.2.1 Holmium Oxide

The holmium oxide solutions were prepared by dissolving the powder in a 10% perchloric acid in distilled

water. The purity of the holmium oxide specimens used

in the preparation of these solutions was indicated by the

manufacturer [4] to be 99.99% (Lot No. Ho-0-4-007)

and 99.999% (Lot No. Ho-O-5-007). Solutions were prepared with 2%, 4%, and 6% holmium oxide. These

solutions were placed in 10 mm pathlength nonfluorescent fused silica cuvettes.

The actual form in which Ho is formed when Ho2 03

is dissolved in HC104 is that of an aquo ion of the general

formula Ho(H2 0)n + . The term "holmium oxide" and

the spectral transmittances reported in the manuscript

refer to this chemical species. Aqueous solutions of

perchloric acid are used in this study to dissolve Ho2 03 since the resulting aquo ion is least likely to form complexes when subjected to changes in temperature and

concentration [5]. The holmium oxide powder is stated by the manufacturer to have an average particle size of about 2 micrometers. The perchloric acid was a nominal 70-72%

reagent grade (considered here as 100%). The distilled

water was produced by thermal distillation. The aque- ous solutions of holmium oxide in 10% perchloric acid

were prepared by weighing 2, 4, or 6 g of the oxide and

adding 10 mL of distilled water and 10 mL of perchloric

acid. The holmium oxide was dissolved by heating at about 80 °C for one hour. The clear solution was trans- ferred quantitatively to a 100 mL volumetric flask and

was brought to volume with distilled water at room

temperature.

2.2.2 Cells

The cells used for the transmission measurements of

the holmium oxide solutions were conventional nonfluorescent, fused silica, cuvettes with a nominal pathlength of 10 mm. These cuvettes were provided with

graded quartz-to-pyrex tubes with rubber caps.

2.3 Measurement Techniques

2.3.1 Calibration of the Spectrophotometer

Wavelength Scale

The spectophotometer wavelength scale error was

evaluated by measuring the emission spectrum of the

instrument's deuterium lamp and the emission spectrum

of a mercury pen lamp [6,7,8].

The wavelength calibration was performed at the beginning of the holmium oxide solution measurements

and again at the completion of the measurements. A

number of emission lines of mercury and two emission

lines of deuterium were used over the wavelength range

230 nm to 690 nm (see fig. 1). The wavelength scale errors were determined for spectral bandwidths of 0.

1

nm, 1 nm, 2 nm, and 3 nm. Each emission line was scanned at a rate of 0.01 nm

per second and recorded on a scale of 0.2 nm per centi-

.2 ~

£ .i <ro

£ o

UJ

200

WAVELENGTH SCALE CALIBRATION

O MERCURY EMISSION LINES

X DEUTERIUM EMISSION LINES

JULY 18

AUGUST 9

Figure 1-Wavelength scale cali- bration of the spec- trophotometer.

300 400 500

WAVELENGTH (nm)

600 700

116
meter. The recorded emission peaks were bisected, us- ing a proportional divider, to determine the wavelengths at the center of the recorded triangular lines. Using this technique, the wavelength scale was read to the nearest 0.01 nm with a precision of ±0.005 nm. The

differences between the measured wavelengths of mer- cury emission lines and the true wavelengths are plotted

in figure 1. A linear least squares fit of these wavelength

differences for the two dates July 18 and August 9 (also

shown in fig. 1) was used to correct the measured values

of the holmium oxide transmission minima for errors in the wavelength scale of the instrument.

The wavelength error of the instrument was essentially the same for spectral bandwidths less than 1 nm. However, at spectral bandwidths of 2 nm and 3 nm, the

wavelength error was slightly different and a different

correction was required.

The deuterium lamp emission lines were measured at two wavelengths each day during the holmium oxide

solution characterization to verify the accuracy of the

wavelength scale. These measurements showed that the

calibration of the wavelength scale is constant to ±0.02

nm after a nominal warm-up of one hour. These results,

as well as the repeatability of the calibration with the

mercury line source, indicate that the overall stability of

the instrument wavelength scale was better than ±0.05

nm during the period of time required to complete the

wavelength characterization of the holmium oxide solution.

The mercury pen lamp is mounted in the lamp positioning device that ordinarily holds the deuterium lamp

of the spectrophotometer. The arc tube of the mercury

pen was aligned parallel to the entrance slit of the monochromator. The lamp holder is equipped with screw

adjustments for vertical and horizontal alignment of the source. The entrance slit cannot be directly observed.

Therefore, the source was adjusted with instrument

operating in the single-beam mode until a maximum signal is observed. To test the effect of the positioning of

the mercury line source on the observed emission line

maxima, the pen lamp was moved horizontally across the field of view of the entrance slit to the monochromator in approximately 0.22 mm steps for a total of

1 2 steps or a 2.64 mm distance. The results of this experiment showed that the recorded emission maxima varied

by less than ±0.01 nm for lamp positions within ± 1 mm

of the center position. The center position corresponded

to the position of maximum signal.

2.3.2 Confirmation of Spectral Bandwidths

The mercury line source was used to confirm the

spectral bandwidth settings of the spectrophotometer.

The mercury emission line at 435.8 nm was scanned for

spectral bandwidths of 0.1 nm, 0.25 nm, 0.5 nm, 1 nm, 2

nm and 3 nm. The emission peak was normalized to 100% on the chart recorder by adjusting the instrument

gain. The bandwidth at half peak height is approximately equal to the spectral bandwidth. The natural

bandwidth of the emission line is much less than the

instrument bandwidth. The recorded curve has a triangular symmetry for all settings of the monochromator

slitwidths. For the above-mentioned nominal spectral

bandwidth settings of the spectrophotometer, the measured spectral bandwidths were 0.092 nm, 0.228 nm,

0.468 nm, 1.04 nm, 2.18 nm, and 3.20 nm, respectively.

This technique for determining spectral bandwidths has

some uncertainties due to assumptions made. However,

it serves to confirm that the desired spectral bandwidths

are closely approximated when the instrument is programmed to provide those settings; hence the effect on

the transmittance minima is negligible.

2.3.3 Determination of the Wavelengths

of Minimum Transmittance

The spectral transmittance of holmium oxide in an aqueous solution of perchloric acid exhibits many absorption bands in the ultraviolet and visible spectrum.

The spectrum is shown in figure 2 for a 0. 1 nm spectral

bandwidth scan. The total number of observed absorption bands varies as a function of the spectral bandwidth

used during the recording of the spectrum. There are approximately 14 major absorption features or bands

between 200 and 650 nm that can be observed clearly for a wide range of spectral bandwidth settings. Most of the

major absorption bands have lesser bands between them

or in close association. These smaller bands are better

resolved at bandwidths less than 1 nm. Only the major

absorption bands that could be of use as possible wavelength standards were selected for detailed study. The

various parameters affecting the measured wavelengths

of minimum transmittance of the holmium oxide solution are discussed in section 3. These include such parameters as temperature, purity, concentration, and

spectral bandwidth.

The spectral transmittance of the holmium oxide solution was digitally recorded on a thermal printer at 0.1

nm intervals with the monochromator scanning at a rate of 0.05 nm/s. The transmittance was simultaneously

recorded on a chart with a wavelength display of 0.5

nm/cm.

The holmium oxide solution was contained in a 10

mm pathlength fused silica cuvette. The transmittance

of the solution in this cuvette was measured relative to an air-only path in the reference beam.

The determination of the wavelengths of the trans- mittance minimum of the holmium oxide solution was

derived from an analysis of the recorded digital output.

The 0. 1 nm interval transmittance data were plotted on

1 17
I TRANSMITTANCE (%)

3r

8

I

m(-m

2

o

o 3 — jo" ° 5 ^ 3

a V

Z R B.3

n —

C5 ~

C BS a g

5- i.

B) 3

5 2

5'3^

3' C

5'3

3"

0_I

C

3o

Xa

to

I TRANSMITTANCE (%)

I

m

— CaI = o

3 O

TRANSMITTANCE (%)

0)o

00o

TJ CO CO =- o —

5. Z =

5' o

x

o

3

c3

118
graph paper on a scale of 0.1 nm per cm with subdivisions of 0.01 nm per mm so that the wavelength

interval of this plotted digital data could be read to the

nearest 0.01 nm between the measured data points. The

location of the wavelength of minimum transmittance

for a given holmium oxide band was determined graphically (fig. 3) by drawing a curve through the data

points and bisecting the horizontal grid lines between

the two slopes of the curve representing the absorption

feature. Several of these bisection points locate the line

between the two slopes that intersects the minimum

transmittance point, (usually at the lowest point of the

curve). The wavelength at this point of intersection was

taken as the measured wavelength of minimum trans- mittance for the absorption feature. The true wavelength of the minimum was determined by applying a wavelength correction to the instrumental wavelength

scale, as determined in section 2.3.1.

2.3.4 Accuracy of the Wavelengths of

Minimum Transmittance

The overall uncertainty in the location of the wavelengths of minimum transmittance is believed to be no

greater than ±0.1 nm at the 95% confidence limit. This

conclusion is based on the reproducibility of the following calibration procedures:

z

<

1 3

Bl SECHON OF C JRVE

s

-3S

i

( > 1! 1

M T~

PLQ

/

|r of

+ ',1

0

(O.I

GITAL nm IN

DAT/

TERV/ A.)

:

\ V

WAVE

MINIf LENG

IUM m oi

TRAf ISMIT TANCE

WAVELENGTH (nm)

Figure 3-Graphical technique used to determine the transmittance

minima from digital spectral transmittance data.

• The uncertainty of the calibration of the instrument wavelength scale using the mercury and deuterium lamps, and the long-term and day-to-day instabilities of the instrument wavelength scale ( less than

±0.05 nm).

• The imprecision of the graphical technique for

deriving the measured wavelengths of minimum transmittance. (±0.02 nm).

• The dependence of the measured wavelengths

of minimum transmittance on variations in temperature or concentration of the solution.

(±0.02 nm).

These uncertainties have been discussed in section 2.3.1. The imprecision of the graphical technique is illustrated

in table 1, where data are shown for three absorption

features. Ten sets of digital results were produced by the

instrument for each of these features. The digital data

were plotted and the wavelengths of minimum trans- mittance for each absorption feature were determined

for the 10 sets of data by the graphical technique. The

standard deviation and standard error for the 10 determinations is also given in table 1.

3. Measurements

3.1 Influence of the Blank Cuvette, Solvent, and Water

The spectral transmittances of an empty fused silica

cuvette, a cuvette filled with distilled water, and a

Table 1. Ten cycle repetitive measurements of three holmium

oxide transmittance minima. Listed minima are obtained

by graphical techniques, using the digitally recorded

0. 1 nm interval transmittance measurement.

(spectral bandwidth = 0.1 nm)

Minima Minima Minima

Cycle No. 1 No. 12 No. 14

1 240.88 nm 485.08 nm 640.28 nm

2 240.87 485.10 640.30

3 240.86 485.11 640.30

4 240.84 485.11 640.30

5 240.85 485.10 640.29

6 240.84 485.11 640.30

7 240.85 485.11 640.29

8 240.85 485.11 640.30

9 240.84 485.10 640.30

10 240.85 485.11 640.29

Average: 240.853 485.104 640.295

Standard

Deviation: 0.0134 0.0097 0.0071

Standard

Error: .0042 .0031 .0022

Note: The data shown in this table have not been corrected for the wave'

length scale error of the spectrophotometer.

1 19
cuvette filled with the diluted perchloric acid (without

holmium oxide) are illustrated in figure 4. These measurements are relative to an air path in the reference

beam.

Measurements of the wavelength of minimum trans- mittance for the band at 241 nm were made with the

holmium oxide in perchloric acid solution versus a cuvette containing only the perchloric acid in the refer- ence beam. The wavelength of minimum transmittance

of this band was found to be the same when the holmium

oxide solution was measured relative to air in the refer- ence beam and when it was measured relative to a cuvette containing the perchloric acid solution. Since

the 241 nm band is within the spectral range showing a slope in the transmittance of the cuvette-perchloric acid

spectra, (see fig. 4) it was considered to be the band most

likely to be influenced by this slope. However, no measurable influence was detected in the location of the

wavelength of minimum transmittance of this band due

to these spectral features associated with the solvent or

cuvette.

3.2 Influence of Temperature

The wavelengths of minimum transmittance of the

holmium oxide solution were determined at 20 °C,

25 °C, and 30 °C for spectral bandwidths of 0.1 nm and

1 nm. If there is a temperature-related influence on the

location of the wavelengths of minimum transmittance,

it was not detected within these temperature ranges.

The measured differences were attributed to random

uncertainties.

100

y 60

z

<

<

I 40-

20-

^Water filled quartz cuvette

Empty quartz cuvette

/ 10% perchloric acid

/ in quartz cuvette

_L _L J_

200 300 400 500

WAVELENGTH (nm)

600

Figure 4-Spectral transmittances of an empty fused silica cuvette, a cuvette filled with water, and a cuvette filled with a solution of 10%

perchloric acid in water.

3.3 Influence of Purity

Complete spectral scans of solutions made with the

99.99% and 99.999% purity holmium oxides showed no

spectral differences except in the extreme ultraviolet

cut-off at wavelengths less than 230 nm. The differences

are illustrated in figure 5 for the wavelength range 200

to 300 nm. The wavelengths of the minimum trans- mittances for the 14 selected absorption bands were

found to be the same for solutions prepared from these

two lots of holmium oxide.

100

80

Holmium Oxide Purity

99.999 a f * r V

> A i i/

_L

200 250

WAVELENGTH (nm)

300

Figure 5-Spectral differences of solutions prepared from holmium

oxide specimens of 99.99% purity and 99.999% purity.

3.4 Influence of Concentration

The location of the wavelengths of minimum trans- mittance of the holmium oxide solution as a function of

concentration was tested with concentrations of 2%,

4%, and 6% in the perchloric acid solution. The general

spectral scan indicated that the changes in concentration

affected the measured transmittance, as would be ex- pected but did not influence the location of the trans- mittance minima. To verify this further, three of the

bands were evaluated by the graphical technique described in section 2.3.1. The results of these concentration measurements for these bands are shown in table 2.

120
Table 2. Influence of holmium oxide concentration on the wavelengths of minimum transmittanee.

(spectral bandwidth = 0.1 nm)

Holmium oxide Minima Minima Minima

concentration No. 1 No. 12 No. 14

2 % 240.84 nm 485.11 nm 640.30 nm

4 % 240.86 485.1 1 640.32

6 % 240.84 485.12 640.31

Note: The data shown in this table have not been corrected tor the wavelength scale error of the spectrophotometer.

3.5 Influence of Spectral Bandwidth

The wavelengths of minimum transmittanee of the

holmium oxide solution were determined for spectral

bandwidths of 0. 1 nm, 0.25 nm, 0.5 nm, 1 nm, 2 nm, and

3 nm. Measurement of the instrumental spectral bandwidths is discussed in section 2.3.2. The influence of

spectral bandwidth was by far the most important parameter affecting the location of the measured wavelengths of minimum transmittanee. The results of this

study are shown in figures 6 through 19 for the 14 minima identified in figures 2-1, 2-2, and 2-3. The data for

bandwidth effects are also listed in tables 3 and 4. The

measurements indicate that for most of the holmium

oxide bands, the location of the wavelengths of minimum transmittanee does not change significantly for

spectral bandwidths of less than 1 nm. From these data

it can be seen that a 0.1 nm spectral bandwidth is adequate to define the wavelengths of minimum trans- mittanee within the stated uncertainties. For some bands

the location of the minimum transmittanee shifts only

slightly for spectral bandwidths greater than 1 nm. However, many do show large shifts for larger bandwidth settings. These results indicate that for instruments with spectral bandwidth settings of less than

1 nm, the holmium oxide solution can serve as an excellent wavelength standard. For instruments having bandwidth settings between 1 nm and 3 nm the standard can

still be of use if the instrument bandwidth is known.

manuscript as a key to associate the data in the tables

with the spectral features illustrated in the figures.

4.2 Selection of Useful Wavelengths

of Minimum Transmittanee

The selection of holmium oxide bands that are considered useful for calibration purposes was based on the

influence of spectral bandwidth on the location of these

minima. The transmittanee minima of small side bands

associated with major absorption features usually shift

in wavelength with bandwidth setting or are not re- solved over the normal instrumental bandwidth range.

The 14 major absorption bands are listed in tables 3 and

4. The absorption bands not listed in the tables are not

considered useful for wavelength calibration purposes.

4.3 Numerical Data

The wavelengths of minimum transmittanee of the

4% holmium oxide solution are listed in tables 3 and 4

for six spectral bandwidths. The results of the measurements as a function of temperature are also listed for

0.1 nm and 1 nm spectral bandwidths at temperature

settings of 20 °C, 25 °C, and 30 °C.

The shift in wavelengths of minimum transmittanee as a function of spectral bandwidth is shown in figures 6

through 19 for the 14 selected minima. The recommended values of minimum transmittanee are listed in table

3 and 4 for the measurements made at 25 °C.

4.4 Uncertainty of Measured Wavelengths

The uncertainties associated with the data listed in tables 3 and 4 have been discussed in section 2.3.4. The

uncertainty in the determination of the wavelengths of

minimum transmittanee for the holmium oxide solution

is believed to be no greater than ±0.1 nm at the 95%

confidence limit using the instrumentation and techniques described in this paper.

4. Results

4.1 Transmittanee of Holmium Oxide Solution

The general spectral signature of the holmium oxide

solution is illustrated in figures 2-1, 2-2, and 2-3 for a spectral bandwidth of 0. 1 nm. Some of the finer spectral

features shown in these figures will be absent when the

spectrum is recorded at bandwidths greater than 1 nm. The major transmittanee minima selected for this study

are indicated by numbers 1 through 14 as shown in these

figures. These band numbers are used throughout the

4.5 Comparison of Results with Measurements

Made on the NBS Reference Spectrofluorimeter

The wavelengths of minimum transmittanee of

several absorption bands of holmium oxide solution

were determined by analysis of spectral data obtained

with the NBS Reference Spectrofluorimeter [9]. This

reference instrument is primarily designed to be used as a research tool for high accuracy spectral analysis of

fluorescent materials and in the development of stan- dards for use in this area of research. The versatile design of the instrument allows for its use as a high accu121
Figures 6-19—Fourteen holmium oxide transmittance minima as a function of spectral bandwidth.

333.9

3338

333.7 -

X

333.6

333i5

333.4

333?

333,2

333.1

(nm)

HOLMIUM OXIDE MINIMA NO. 5

>o-o oJ_J L

.1 .25 .5 IP 2.0 SPECTRAL BANDWIDTH (nm)

HOLMIUM OXIDE MINIMA NO. I

(nm)

.1 25 .5 IP 2P

SPECTRAL BANDWIDTH (nm)

250.2

250.1 -

250.0 ^ 249.9 -

(nm) 2498 K

249.7 -

249.6 2495

HOLMIUM OXIDE MINIMA NO. 2

U L

.1 -25 .5 1.0 2p

SPECTRAL BANDWIDTH (nm)

.1 .25 .5 lp 20

SPECTRAL BANDWIDTH (nm)

.1 .25 .5 IP 2.0 SPECTRAL BANDWIDTH (nm)

3.0

30

3.0

345.9 -

345.8 -

345.7 -

345.6 - (nm)

345.5 -

345.4 -

345.3 -

36 1.6 i361.5 -

X

361.4 -

361,3 " (nm) 3612 -

361.1

361.0-

386.1 r

.1 .25 .5 1.0 2P SPECTRAL BANOWIDTH (nm)

J .25 .5 1.0 2P SPECTRAL BANDWIDTH (nm)

A

(nm)

.1 .25 .5 lp 2P SPECTRAL BANDWIDTH (nm)

(nm)

.1 .25 .5 1.0 2P

SPECTRAL BANDWIDTH (nm)

122
451.6

451.5

X 451.4

451.3

451.2

451,1

4510

(nn>)

HOLMIUM OXIDE MINIMA NO. 10

QPl IT^ INTO

- TWO BANDS O

-ONE BAND —-O

1 1 1 1

1

1

.1 .25 .5 1.0 20

SPECTRAL BANDWIDTH (nm)

3.0

468.2 468 l _ HOLMIUM OXIDE MINIMA NO. 1

1

.1 .25 10 20

SPECTRAL BANDWIDTH (nm)

.1 .25 .5 1.0 20

SPECTRAL BANDWIDTH (nm)

(nm) 4852 " HOLMIUM OXIDE MINIMA NO. 12

485.1 _i i i i 1

.1 .25 .5 IP 2P

' 1 6403L_L_I 1 1

3.0 .1 .25 .5 10 2P

I

3.0 SPECTRAL BANDWIDTH (nm)

SPECTRAL BANDWIDTH (nm)

Table 3. Results of holmium oxide solution analysis for transmittance minima 1 through 7 for spectral

bandwidths of 0.1, 0.25. 0.5, 1. 2 and 3 nm. 4?r holmium oxide in a 10% perchloric acid solution.

Minimum Temp. Spectral Bandwidths

No. fC) 0.1 nm 0.25 nm 0.5 nm 1 nm 2 nm 3 nm

1 20 240.99 nm 241.13 nm

1 25 240.99 240.97 nm 241.01 nm 241.13 241.08 nm 240.90 nm

1 30 240.99 241.15

2 20 249.79 249.88

2 25 249.83 249.78 249.79 249.87 249.98 249.92

2 30 249.79 249.86

3 20 278.14 278.09

3 25 278.15 278.14 278.13 278.10 278.03 278.03

3 30 278.14 278.09

4 20 286.99 287.17

4 25 287.01 287.00 287.01 287.18 287.47 287.47 4 30 287.01 287.18

5 20 333.48 333.45

5 25 333.47 333.44 333.43 333.44 333.40 333.32

5 30 333.48 333.45

6 20 345.57 345.47 6 25 345.55 345.55 345.52 345.47 345.49 345.49

6 30 345.62 345.49

7 20 361.38 361.33

7 25 361.36 361.35 361.33 361.31 361.16 361.04

7 30 361.38 361.31

Note: The uncertainty in the wavelengths of minimum transmittance is ±0.1 nm. An extra decimal place is given for rounding purposes.

123
Table 4. Results of holmium oxide solution analysis for transmittance minima 8 through 14 for spectral

bandwidths of 0.1, 0.25, 0.5, 1, 2, and 3 nm. 4% holmium oxide in a 10% perchloric acid solution.

NiviI inmii min 1u

1 mii i Temp. Spectral Bandwidths

No. <j <_. 0.1 nm0.25 nm0.5 nm 1 nm2 nm3 nm

ao 20 385.45 nm 385.66 nm

8 25 385.45 385.42 nm 385.50 nm 385.66 385.86 nm 386.01 nm

8 30 385.44 385.68

qy 20 416.10 416.27

9 25 416.07 416.07 416.09 416.28 416.62 416.84

9 30 416.13 416.30

in 20 * 451.30

10 25 * * _* 451.30 451.30 451.24

10 30 * 451.34

1

1

20 467.83 467.84

11 25 467.82 467.82 467.80 467.83 467.94 468.07

11 30 467.84 467.84

12 20 485.30 485.29

12 25 485.28 485.28 485.27 485.29 485.33 485.21

12 30 485.31 485.30

13 20 536.53 536.65

13 25 536.54 536.53 536.54 536.64 536.97 537.19

13 30 536.45 536.63

14 20 640.48 640.52

14 25 640.51 640.49 640.49 640.52 640.84 641.05

14 30 640.52 640.49

* Splits into 2 minima for spectral bandwidths less than ! nm.

Note: The uncertainty in the wavelengths of minimum transmittance is ±0.1 nm. An extra decimal place is given for rounding purposes.

racy spectrophotometer in some applications. The

wavelength scale of the spectrofluorimeter has been

carefully calibrated by extensive measurements of emission line sources and is known to have an uncertainty of

±0.1 nm for spectral bandwidth of 0.1 nm.

This reference instrument was used to confirm the

results obtained with the calibrated high-precision commercial spectrophotometer used for the holmium oxide

measurements. A comparison was made to confirm the

results for three of the transmittance minima at one spectral bandwidth setting. The results of this comparison

are shown in table 5. The two instruments provided data

for these three transmittance minima that agree to within 0.1 nm. The wavelength scale uncertainty for

both instruments is ±0.1 nm.

4.6 Other Measurements Outside NBS

A list of the wavelengths of minimum transmittance

of the holmium oxide solution reported by other workers is given in table 6. (Ref. 10 offers details of these

measurements.)

Table 5. Comparison of values for holmium oxide

transmittance minima with values obtained with

the NBS Reference Spectrofluorimeter.

(spectral bandwidth = 0.1 nm)

Minima Minima Minima

Instrument No. 1 No. 7 No. 14

Varian (Cary) 240.995 nm 361.361 nm 640.507 nm

Model 2390

NBS Reference 240.970 nm 361.313 nm 640.469 nm

Spectrofluorimeter

The data shown in table 6 indicate that workers in other laboratories are in generally good agreement with one another and that the NBS data also agree well with

these workers' previously published data. These workers also found that the wavelengths of minimum trans- mittance of holmium oxide in similar solution in

perchloric acid are not sensitive to variations in temperature and concentration. They also concluded that

the wavelengths of minimum transmittance were least

affected by changes in spectral bandwidth for band124
Table 6. Comparison of values for holmium oxide

transmittance minima with values obtained

by other workers (see Reference 1J).

NBS

Min. Transmittance No. Minima (1) (2) (3) (4)

1 240.99 nm* 241.15 nm 241.0 nm 241.1 nm 241.1

2 249.83 * 249.75 250.0 249.7 249.7

3 278.15 * 278.2 277.8 278.7 278.2 4 287.01 * 287.15 287.5 287.1 287.2

5 333.47 * 333.5 333.3 333.4 333.3

6 345.55 * 345.6 345.5 345.5 345.0

7 361.36 * 361.5 361.0 361.5 361.2

8 385.45 . * 385.6 385.6 385.5 385.6

9 416.07 * 416.2 416.0 416.3 416.6

10 451.30 # 450.7 450.4 450.8 451.0

11 467.82 467.75 468.0

12 485.28 485.25 485.2 485.8 485.2

13 536.54 536.3 536.8

14 640.51 640.5

* Spectral bandwidth = 0.1 nm

# Spectral bandwidth = 1 nm

widths less than 1 nm, but that large shifts can be en- countered at bandwidths in excess of 1 nm.

5. Conclusions

The reported wavelengths of minimum transmittance

of the holmium oxide solution appearing in tables 3 and

4 are estimated to be uncertain by no more than ±0.1

nm at the 95% confidence limit. These wavelengths of

minimum transmittance were found to be essentially unaffected by changes in temperature at 25 °C ±5 °C.

They were also unaffected by variations in the concentration for solutions containing 2%, 4%, and 6% holmium oxide. The critical parameter affecting the measured values of minimum transmittance was found to be

the spectral bandwidth setting of the spectrophotometer. For spectral bandwidths less than 1 nm

the wavelength shift is generally less than 0.2 nm. Users can most effectively determine the wavelength error associated with their instrument by using the NBS data

listed in tables 3 and 4 that are representive of the spec

tral bandwidth setting ordinarily used with the instru

ment. The wavelengths of minimum transmittance of

the holmium oxide solution for spectral bandwidths

greater than 3 nm have not been evaluated.

The authors gratefully acknowledge the assistance of

Chenq-Tsong Chang, guest worker from Taiwan, for

his contribution in measuring the transmittance minima

of the holmium oxide solution on the NBS Reference

Spectrofluorimeter; Jack J. Hsia, for constructive discussions; Robert W. Burke, for assistance in the preparation of various Ho2 03 solutions; and the NBS Optical

Shop for preparation of the fused silica cuvettes.

6. References

[1] The terminology used in this work was defined earlier by K. D. Mielenz in Anal. Chem. 48, 1093 (1976).

[2] Keegan, H. J.; J. C. Schleter and V. R. Weidner, Ultraviolet Wavelength Standard for Spectrophotometry, J. Opt. Soc. Am. 51. 1470 (1961), (abstract).

[3] Venable, W. H., Jr., and K. L. Eckerle, Standard Reference

Material: Didymium Glass Filters for Calibrating the Wavelength Scale of Spectrophotometers (SRM 2009, 2010. 2013.2014).

[4] The holmium oxide was obtained from Research Chemicals, a Division of Nucor Corp., Phoenix, Arizona.

[5] Gschneider, K. A., Jr., and L. E. Yring, Handbook of the Physics and Chemistry of Rare Earths, Vol. 3 North-Holland

(1979).

[6] Harrison, G. R., MIT Wavelength Tables, MIT Press, Cambridge, MA (1960).

[7] ASTM Manual on Recommended Practices in Spectrophotometry, Published by American Society for Testing and

Materials, 1916 Race St.. Philadelphia. PA 19103 (1969).

[8] CRC Handbook of Chemistry and Physics, 63d Edition, CRC

Press, Inc., (1982-1983).

[9] Mielenz, K. D., in "Measurement of Photoluminescence" (K. D.

Mielenz, Editor), pp 17-19, Academic Press (1982).

[10] Burgess, C, and A. Knowles, Eds. Standards in Absorption

Spectrometry, Chapter 7, Wavelength Calibration,

Chapman and Hall, London (1981).

125

Appendix 5

Reprinted from Applied Optics, Vol. 21, page 3354, September 15, 1982

Copyright © 1982 by the Optical Society of America and reprinted by permission of the copyright owner.

Heterochromatic stray light in UV absorption spectrometry:

a new test method

K. D. Mieienz, V. R. Weidner, and R. W. Burke

A new method of estimating the amount of heterochromatic stray light in UV spectrophotometers is described. The method uses the same solution filters with sharp UV absorption edges as ASTM Test Method

E387, but one measures the apparent absorbance of a 10-mm path-length cell in the sample beam relative to a 5-mm cell in the reference beam. Scanning toward shorter wavelengths, one records an apparent ab- sorbance maximum which is a direct measure of the stray light. This method was found to be in satisfactory

agreement with the ASTM method in comparative tests of several spectrophotometers at different wavelengths between 200 and 390 nm, using KC1, KI, Nal, acetone, and NaN02 solution filters. The new method

proved to be simpler, the main advantage being that the apparent absorbance maximum occurs at considerably lower scale values than the corresponding absorbance plateau measured by the ASTM method. This

reduces the need for successive attenuations of the reference beam every time the spectrophotometer runs

off scale. In many instances the new method required no attenuation at all.

I. Introduction

Stray light (stray radiant energy or SRE) is a common

problem in spectrophotometry. Edisbury1 called it "villainous company that hath been the spoil of me"

(Shakespeare, Henry IV) as it has caused a good deal of

grief to spectrophotometrists, from biased absorbance

readings to nonlinear Beer's law plots and false absorption peaks. These errors are often unsuspected and

may not be apparent to the user unless stray-light tests have been performed to ensure that the spectrophotometer meets specifications and is used properly.

The practical importance of such tests is manifest

from a 1975 survey conducted by Beeler and Lancaster2

to assess the extent of stray-light problems in routine

clinical spectrophotometry. When the absorbance of

solutions of sodium iodide at 240 nm was measured by

the 159 laboratories which participated in this survey,

15% of these laboratories reported results that suggested a problem caused by stray-light levels in excess of 1%.

The laboratories also measured the absorbances of acid

potassium dichromate, alkaline potassium chromate,

and nickel sulfate solutions between 240 and 563 nm.

The authors are with U. S. National Bureau of Standards, Washington, D.C. 20234; R. W. Burke is with the Inorganic Analytical

Research Division, Center for Analytical Chemistry, and the other

authors are with the Radiometric Physics Division, Center for Radiation Research.

Received 13 May 1982.

Those that had stray-light problems reported absorbances that were skewed in distribution and showed low

mean values as well as wide instrument-to-instrument

variation. On the other hand, the laboratories without

stray-light problems reported results which followed a symmetrical distribution, showed less instrument-toinstrument variation, and provided an accurate estimate

of the true absorbance of the solutions. Beeler and

Lancaster emphasized that this survey showed how

stray light in a spectrophotometer "can quickly bo

identified by a single reading on a single solution a' single wavelength."

While stray light at the 1% level is easily detected, i. determination of smaller percentages is more difficult.

The stray light in a well-designed modern spectrophotometer is typically only a small fraction of 1%, and for

double-monochromator instruments it is often <0.001%.

As the measurement of such a small percentage falls outside the normal operating range of spectrophotometers, special test methods are required. In general,

these methods involve tedious procedures poorly suited

for routine testing. The new method described in this

paper is simpler and, therefore, more likely to be carried

out by practicing spectrophotometrists.

II. Definitions

In general terms, stray light is spurious radiant energy

that has departed from its regular path in a spectrophotometer and then reenters the path so that it is sensed by the detector and causes false readings of

transmittance and absorbance. The kind of stray light

most often encountered in routine spectrophotometry

3354 APPLIED OPTICS / Vol. 21, No. 18 / 15 September 1982
1 9 • ni Q|0 Acid <HP \ \HoNO?

kf«ch\ \

M«CI 2 1

NaBr 1

orHal

Acaton* \ \

\\

KCI \ \

200 250 \

500 U350 V4 00

Fig. 1. Absorption spectra of solution cutoff filters suitable for

stray-light testing in the UV.

is heterochromatic SRE, i.e., stray radiant energy

comprised of wavelengths outside the nominal bandpass

of the monochromator. It usually arises from scatter

and spurious reflections inside the monochromator.

Therefore, it consists predominantly of wavelengths X', where the source flux <fv, the monochromator transmittance T(A'), and the detector responsivity R(X') are high, and is most detrimental for nominal wavelength

settings X where these parameters fall off to produce a decrease in the main signal without a commensurate

decrease of the stray-light signal. In general, these

conditions occur near the wavelength limits of a spectrophotometer so that the stray light that matters most

in practical UV absorption spectrometry is heterochromatic SRE from near-UV and visible wavelengths

X', which is present at short-UV wavelength settings X

< X'. This type of heterochromatic SRE can be detected readily with a sharp cutoff filter that absorbs the

nominal wavelength X while transmitting the longer

stray wavelengths X'. If such a filter is placed in the

sample compartment of a spectrophotometer and the

monochromator is scanned to a wavelength X below the

cutoff, the instrument will register an apparent transmittance of

x(\) =\s *yT(X')R(X')d\' [<t> x T(X)«(X)AX), (1)

where AX is the nominal bandwidth. This quantity x

is called the instrumental stray-light ratio of the spectrophotometer at the wavelength X. Figure 1 shows the absorption spectra of several

cutoff filters suitable for this purpose. One of these,

potassium iodide, is available as NBS Standard Refer- ence Material SRM No. 2032.3

III. ASTM Test Method E387

The usually recommended method of estimating

stray light in UV spectrophotometry is ASTM Test

Method E387,4 which is quite simple in concept: a cutoff filter of the type mentioned above is used as the

sample, the spectrophotometer is scanned toward short

wavelengths until an apparent absorbance plateau A'

is recorded, and hence the stray-light ratio defined by

Eq. (1) is given by x = 10~ A '.

In practice, this test method poses the problem that

most spectrophotometers run off-scale long before the

final absorbance plateau is reached. When this happens, the scan must be halted to place an attenuating

screen into the reference beam. The attenuation thus

introduced must be noted, and then the scan can be

resumed until the constant absorbance reading is obtained or (more likely) the instrument runs off-scale

again. In the latter case another attenuator must be

added, and often this procedure must be repeated several times.

Figure 2 shows an example in which two attenuators were needed to test a double-monochromator instrument with a maximum absorbance readout of 3.0, using

a 10 g/liter aqueous solution of KI as the cutoff filter near 255 nm. The two attenuation steps were 1.92 and

1.13 absorbance units, respectively, and the final absorbance reading was 2.37. Thus, the actual absorbance

plateau was A' = 1.92 + 1.13 + 2.37 = 5.42, and hence

the stray-light ratio was calculated as x — 10~ A ' = 3.8

X 10-6

. Practical details on the ASTM method will be

found in Sec. V.

IV. The New Method

The new test method to be described originated from an idea to use a gradual attenuation of the reference

beam instead of the step-wise attenuation recom- mended by ASTM. It was thought that a shorter

path-length of the cutoff solution could provide this

gradual attenuation. Thus, the same solution cutoff

filters are used but one measures a 10-mm path-length

cell in the sample beam against a 5-mm cell placed in the

reference beam. Again scanning toward high absorbances, one should obtain a zero reading (100% transmittance) at the longer wavelengths, where both cells are transparent, and once more at the shorter wavelengths, where they are opaque to the main beam but

transmit equal stray-light fluxes. Hence, it is intuitively clear that an apparent absorbance maximum A"

should be recorded at some intermediate wavelength

X, and that this maximum should represent a direct measure of the heterochromatic stray-light ratio of the

spectrophotometer. Furthermore, the spectrophotometer should be less likely to run off scale, since only

o 2.5-

2 1.5-

f

STRAY LIGHT: \O

«. ^10""". 9il0"*

(a)

260 250 WAVELENGTH (nm|

(b)

Fig. 2. Stray-light tests of a single-beam, double-monochromator

spectrophotometer with 0-3 absorbance scale, using KI cutoff filters near 255 nm: (a) ASTM Test Method E387, (b) new method. The

ASTM method required two screens in the reference beam for successive attenuations of 1.92 and 1.13 absorbance units, respectively.

15 September 1982 / Vol. 21, No. 18 / APPLIED OPTICS 3355
70 -

6.0 -

so -

4.0 -

3.0 -

2.0-

3.0 4.0 9.0 6.0 -log« (ASTM Mtthod E387)

Fig. 3. Comparative results obtained in fifteen stray-light tests by

the new method and by ASTM Test Method E387, showing the equivalence of both methods within ±10%.

one-half of the absorbance of a 10-mm cell is seen by the

spectrophotometer and since it is well known that

stray-light effects are amplified in differential spectrophotometry.5 These ideas were tested in a series of

test runs. In the example shown in Fig. 2(b), a maximum of only 2.36 absorbance units was recorded for the

same test conditions as in Fig. 2(a).

Mathematically, this apparent absorbance maximum

is given as follows. Let x be again the stray-light ratio

defined by Eq. (1), r the true transmittance of the

10-mm cutoff solution, and y/r that of the 5-mm solution. Then, if these solutions are transparent to the

stray light, the sample and reference signals will be

proportional to (r + x) and {\fr + x), respectively, so that the apparent transmittance recorded by the spectrophotometer is

r' = (r + *)/(v

/7 + *>. (2)

Setting the derivative dr'/dr equal to zero, we find

V7 + x = V2 (r + x)V7, V2 t = *(% + v7).

Thus, if \fr « V2. the minimum apparent transmittance, r" = r'mi n, occurs where the transmittance of the

10-mm cell is equal to the stray-light ratio x, then t =

x. Equation (2) then shows that r" = 2y/x/{l + \fx)

or, since \fx « 1, the ratio becomes t" = 2\fx. Hence,

the stray-light ratio can be calculated as follows in terms

of t" or in terms of the corresponding absorbance

maximum A" = —log(T // ):

x = V4 (r") 2 = V4 10- (3)

In the case of Fig. 2(b), this expression gave a stray-light

ratio, x = 4.6 X 10~6

, which agrees well with the corre- sponding result obtained by the ASTM method.

V. Comparison

In addition to the comparison illustrated by Figs. 2(a)

and 2(b), further tests were performed to compare both

methods. All in all, a series of fifteen measurements

was completed on the following four types of spectrophotometer:

Single-beam, single-prism monochromator, meter

readout from 0 to 3 absorbance units.

Single-beam, single-grating monochromator with

prism predisperser, unlimited digital readout.

Single-beam, double- (prism-grating) monochromator, meter readout from 0 to 3. Double-beam, double- (prism/prism) monochromator, chart recording from 0 to 2. The tests were performed at several wavelengths

between ~200 and 400 nm, using the following solution

cutoff filters: 10 g/liter KC1, 10 g/liter KI, 10 g/liter

Nal, acetone, and 50 g/liter NaNC>2. Each test was done

according to the ASTM recommendations for Test

Method E387, and then repeated by the new method.

The relative (10 vs 5 mm) absorbances were obtained

directly on the double-beam spectrophotometer and by

taking the difference of separate absorbance readings

for each cell with the single-beam instruments.

Readings were taken in 1- or 2-nm intervals in the direction of decreasing wavelength.

The results of these comparative tests are shown in

Fig. 3 in the form of a logarithmic plot of the stray-light

values obtained by the new method vs those given by the

ASTM method. The average ratio of the corresponding

values of —log(x ) is 1.01 ± 0.10. Hence, these data can

be represented by the 45° line shown in the figure, and

it can be concluded that both methods give equivalent

results within random experimental errors. The

standard deviation of ±10% of this mutual agreement

was judged satisfactory in view of the low signal levels

being measured.

As expected, the use of either method was straightforward as long as the spectrophotometer stayed on

scale during the test, but the new method proved to be

considerably simpler when small amounts of stray light were measured.

References

1. J. R. Edisbury, Practical Hints on Absorption Spectrometry

(Plenum Press, New York, 1967), pp. 151-164.

2. M. F. Beeler and R. G. Lancaster, Am. J. Clin. Pathol. 63, 953

(1975).

3. K. D. Mielenz and R. Mavrodineanu, "Crystalline Potassium Io- dide, Heterochromatic SRE Standard for UV Absorption Spectrophotometry," (NBS Standard Reference Material 2032: 1979); obtainable from Office of Standard Reference Materials, National

Bureau of Standards, Washington, D.C. 20234.

4. ASTM Committee E-13, Manual on Recommended Practices in Spectrophotometry (American Society for Testing and Materials,

Philadelphia, 1969), pp. 94-105.

5. W. Luck, Z. Elektrochem. 64, 676 (1960).

3356 APPLIED OPTICS / Vol. 21, No. 18 / 15 September 1982

Appendix 6

JOURNAL OF RESEARCH of the National Bureau of Standards — A. Physics and Chemistry

Vol. 77A, No. 6, November-December 1973

Reflection Correction for High-Accuracy Transmittance

Measurements on Filter Glasses

K. D. Mielenz and R. Mavrodineanu

Institute for Materials Research, National Bureau of Standards, Washington, D.C. 20234

(July 27, 1973)

Multiple reflections in the sample compartment of a spectrophotometer constitute a source of systematic bias in transmittance measurements on filter glasses. This bias may be removed by applying

a numerical correction obtained from measurements on tilted samples in polarized light. For a high- accuracy spectrophotometer, this correction was found to be of the order of several 10 ~4 transmittance

units, independent of polarization, but slightly wavelength dependent.

Key words: Correction for reflections; reflections, multiple; spectrophotometry, high accuracy;

systematic bias in spectrophotometry; transmittance, correction.

1. Introduction

The sample compartment of most spectrophotometers is designed such that the monochromator exit

slit is focused into the sample by a lens, and is re- focused into the detector by another lens (fig. 1). This

beam geometry is preferred by instrument designers

for intensity reasons and other convenience gains, but

may constitute a source of significant systematic

errors of the measured data. One of these errors is caused by multiple reflections between the two lenses

and the sample surfaces [1,2, 3]. 1 For high-accuracy applications this error can be

eliminated by means of a numerical correction, A7\

which converts the measured transmittance, T.u, into the correct value

t=7\, + A7\ (1)

' Figures in brackets indicate the literature references at the end of this paper.

A simple but accurate method to determine this cor- rection is described in this paper.

The specific spectrophotometric application discussed is the measurement of the transmittance of

glass filters to an accuracy of ± 10~4 transmittance

units. For the particular high-accuracy spectrophotometer used [4], the correction AF was found to be

of the order of several 10~4 transmittance units,

independent of polarization throughout the visible

spectrum, and applicable for filters with widely dif- ferent indices of refraction. The correction was found

to be slightly wavelength dependent.

2. Theoretical Background

A detailed discussion of the reflection errors

affecting the measurement of filter transmittances

may be found in reference [3]. A summary is given

here in order to provide the theoretical foundation

for the experimental work described in section 3.

MONOCHROMATOR

EXIT SLIT SAMPLE

FIGURE 1. Sample-compartment optics of a focused-beam spectrophotometer.

699
The purpose of the measurement is assumed to be

the determination of the normal-incidence transmittance.

r,(l-r) 2

1 r*T,2 (2a)

of a glass filter surrounded by air. The filter is characterized by its thickness t and complex refractive

index n(l + i/<), so that for each wavelength X the

normal-incidence internal transmittance Tj and

Fresnel reflectance r appearing in eq (2a) are given by

• . = 0

—4irnK</\

r —

n-l

n+l

(2b)

(2c)

This quantity r can be measured accurately with

specially designed instrumentation in which the sample

is placed normal to a colli mated beam of light, the

collimation being effected by off-axis mirror optics in a manner which reduces reflection errors to a negligible

level [3, 5]. A conventional spectrophotometer does not

permit such a direct measurement of t because of the

above-mentioned reflection error, even if its lenses

were arranged to produce a collimated rather than the

usual focused beam.

The cause of this error is illustrated in figures 2a and

2b. According to the first of these, the radiant flux

received by the detector during the clear-space

measurement is

(/>o=r1 r2 (i+i? 1 i? 2 + .), (3a)

where the initial flux is taken as unity, and where T\ , R\, T2 , and R2 are the fractional fluxes transmitted or

reflected into the detector by all components (lenses,

slit jaws, detector housing, etc.) in front of and behind

the sample area, respectively. When a sample with

transmittance T and reflectance R is inserted, the

flux into the detector is

(f> = Tl TT2 (l+R l R2 T2 + RR2 + R 1 R + . . .), (3b)

as indicated in figure 2b. Within the approximation

that all contributions due to four or more reflections are considered negligibly small, the measured transmittance is

Tii— 4>l4>o = T[\ + (Ri+R 2 )R-R l R 2 (l-r-)]. (3c)

The correction which must be applied to Tn to eliminate the reflection bias is T-T„ = AT+±T\

(a) i\ L

\1 /

(b)

T,T,

Ti R g R | T 2

T,TT 2

T, TR 2 TR, TT 2

T, TR 2 RT 2

T, RR, TT 2

Figure 2. Reflected beams in a spectrophotometer with lenses.

&T=-(Ri+R2 )RT,

A7"=/? 1 /? 2 7

, (l-7'2 ).

(4b)

(4c)

The first of these two error terms, A7\ is due to the

fact that the insertion of the sample has created the

two additional beams shown at the bottom of figure 2b.

This error is greatest when the sample is normal to the

optic axis, and may be eliminated by sufficiently tilting

the sample so that these two beams no longer reach the

detector. The other error component, AT" , arises from

the fact that the second beam from the top in figure 2b

passes the sample three times, whereas the direct

beam passes it only once. Since this residual error cannot be avoided by tilting the sample, it is clear that

tilting is an effective means to eliminate reflection errors only if AT" is substantially smaller than A7\

Equations (4b, c) suggest that this should be the case

for any well-designed spectrophotometer for which

these reflection errors are not unduly large. To show

this, we assume that Ri and R2 are both of the order

of a few percent, and that glass filters with an average

transmittance T ~ 0.5 and reflectance R ~ 0.08 are used as samples. This leads to A77A7" ~ 10, showing

AT" to be a significantly smaller error in such circumstances. Whether or not this estimate holds must be

ascertained individually for any given spectrophotometer.

Tilting the sample introduces obliquity errors, and

therefore does not yield an accurate measurement of

the normal-incidence transmittance t unless further

corrections are applied. These errors arise from a de- crease of internal transmittance due to increased

pathlengths in the sample for oblique light incidence

and, secondly, from a variation of reflection losses at the sample boundaries with angle of incidence. The

latter also introduces a dependence of measured

transmittance on polarization.

For glass filters measured with a conventional spectrophotometer in which the monochromator exit slit is imaged onto the sample, these errors are given by

where

(4a) T=t[1+ (l/2/i2

) (lnri) (0

2r +| 6%)

± (4r/ra) d\ + (5)

700
where T is the measured transmittance (corrected for

reflection errors); t, n, and r are the normal-incidence

values of transmittance, internal transmittance, and

Fresnel reflectance as given by eqs (2a, b, c); 9T and

do are the tilt angle and the cone angle of the incident

light; and where the upper and lower signs of the last

term pertain to the two cases in which the light is S or P polarized in the tilt plane. Equation (5) was

derived in reference [3] under the assumption that the

monochromator exit slit is sufficiently short to approximate a point source, and that the focusing lens is underfilled so that the grating or prism constitutes the limiting aperture. This aperture is assumed to be square,

subtending the same angle 20o in the horizontal and

vertical planes. The sample is assumed to be tilted only

in one of these planes. The small-angle approximation

given in eq (5) is estimated to be accurate to 10~4 transmittance units for angles 0r and do up to about 10°.

For average transmittances and tilt angles of a few

degrees, these obliquity effects are of the order of

10-3 transmittance units. Since this is the same magnitude as estimated above for the reflection error, it is clear that for a spectrophotometer with lenses an

accurate measurement of the normal-incidence transmittance t cannot be achieved without numerical cor- rection for either source of error. If the sample is tilted in order to reduce the reflection error, an obliquity correction must be applied. Vice versa, a reflection correction is necessary if the sample is normal to the optic axis so that obliquity effects are minimized.

Although these two alternatives are equivalent, the

latter was chosen in this work since it constitutes the

simpler approach for routine measurements of transmittance.

3. Experimental Procedure

The preceding theoretical discussion does not permit

a precise calculation of the required reflection cor- rection. Representing fractional fluxes which actually

reach the detector, the quantities R\ and R2 appearing

in equations (4b, c) are complicated and generally

unknown functions of the relative positions of sample

compartment elements, lens curvatures, location of

aperture stops, and other h*»am-geometry factors.

This makes it necessary to measure the correction.

Such a measurement may be performed as follows.

Each of a series of filters with different transmittances is measured for different known tilt angles

6t, and for S and P polarization of the incident light.

This yields a set of raw data T.u, such as plotted in

figure 3a. These are transformed into corrected data

T by adding the tilt-correction given by eq (5),

(AD mt = -riW [(l/2n2) In (y^)±(4r/n)]«J, (6)

where the approximate values Tm and 7V/(1 — 2r)

were substituted for t and T( on the right-hand side, and

where estimates of n and r may be used. As indicated

by figure 3b, these corrected transmittances T are constant for sufficiently large positive or negative tilt

t r

(b) -9y

I I I I 1 1 -6 -4 0 4 8

Figure 3. (a) Measured transmittance TM versus tilt angle 6T for 55 percent filter in S and P polarized light at X = 574 nm.

(b) The same data after application of tilt correction.

angles, but near normal incidence exhibit the 're- flection hump' whose height represents the desired

reflection correction A T to be applied for this particular value of transmittance. From the measurements

performed on all of the filters one may then determine

the dependence of AT on T, as illustrated by figure 4. According to eq (4b), this dependence should be linear.

In the work described here, these measurements were made as follows.

A series of four Schott NG-4 glass filters with nominal transmittances 0.1, 0.2, 0.3, and 0.5, an evaporated

metal-on-glass filter with a nominal transmittance

0.4, and a clear glass plate were used. All of these samples were 50 mm X 50 mm X 2 mm in size, and flat and plane-parallel to within a few fringes of mercurygreen light. The spectrophotometer used to perform

the measurements was fully described in a previous

issue of this journal [4], and will not be discussed here.

The filters were placed in the sample compartment of

this spectrophotometer on a vertical rectangular

holder adapted to a rotary horizontal table with 10 cm

diameter. This table had a scale divided in 360° with

a vernier reading to 5 min, and was held on the singlesample carriage of the spectrophotometer (see ref.

[4], fig. 2) with a vertical mounting rod. This sample

carriage unit consists of a platform provided with

vertical holders which can be moved laterally by a

701
0.5 —

FIGURE 4. Reflection correction AT versus T for K = 574 nm.

rack and pinion arrangement. The platform, mounted

on four ball bushings which ride on two horizontal

rods, can be moved pneumatically across the optical

axis. This remote-controlled movement is smooth, and

the sample position in and out of the beam is reproducible within 0.025 mm.

A sheet polarizer of 9.5 cm diam was used to produce the S and P polarized radiation. This polarizer

was held in a ball bearing rotating mount with a circular

scale divided from 0 to 360°. It was placed on the

optical bench in front of the entrance slit of the predisperser-monochromator aggregate of the spectrophotometer between the circular neutral wedge and

the flat front-surface mirror (see ref. [4], fig. 3). This

placement of the polarizer on the source side of the

spectrophotometer avoids the additional reflection errors which would occur if the polarizer were put in

the sample compartment of the spectrophotometer,

but requires that the monochromator does not significantly depolarize the incident radiation or rotate its plane of polarization. This was ascertained by testing

the monochromator between crossed polarizers at the

wavelengths at which the measurements were performed.

Each transmittance measurement was performed

in S and P polarized radiation and for tilt angles varying

by ± 10° about the normal position. Each measurement

resulted from the ratio I/Io of the attenuated to the

unattenuated radiation value, each / and Io being

the average of 50 individual measurements. The

measurements were repeated four times.

4. Results

Using the procedures outlined .in section 3, the

numerical values of the reflection correction AT

were determined for the four neutral-density glass

filters and the clear glass plate, at the wavelength

A. = 574 nm. This yielded the result represented by the

straight line in figure 4; namely,

AT/T=-S.9X 10- for A. = 574 nm. (7a)

Each of the individual values of AT could be measured

to a precision better than 10-4 transmittance units.

There was no evidence suggesting a dependence of

AT on polarization.

Secondly, the measurements on the clear glass

plate were repeated for A. = 400 nm and 650 nm.

Together with the previously obtained result for 574

nm, this showed the wavelength dependence indicated in figure 5. This dependence is also linear,

and may be expressed as

A77r=-1.26x 10-6 (A-574 nm), for T=0.92.

(7b)

These results were then combined into the final

empirical expression.

A77r = -8.9X 10-4 [1 + 1.4X 10-3 (A-574 nm)],

(8)

for the correction AT to be applied for different trans- mittances T and wavelengths A. This formula agrees

with all individual measurements of AT to within

± 5 x 10-5 transmittance units.

The correction AT so obtained represents only the

first of the two error terms appearing in eq (4a), above.

The exact value of the second term A7" cannot be

determined within the context of this paper, but a

quantitative estimate may now be obtained as follows.

From eqs (4b) and (7a) it may be seen that, for this

spectrophotometer and for the appropriate value R ~ 0.08 for the filters used,

12 (Ri + Rz) ~ 6x 10- (9a)

Provided that this arithmetic average of Ri and R2 is not substantially different from their geometric average

(R1R2) 112

, it follows from eq (4b) that

AT ~ 3.6 x 10-5 T(l-P), (9b)

400 500 600 700

Figure 5. Reflection correction AT versus K for T = 0.92

702
or A7" -1.4X10-* for T=0.577, for which this error is a maximum. Although this argument is not

mathematically exact, it suggests that the residual

reflection error A7" is entirely negligible for the

spectrophotometer used here.

According to eq (4b), the correction A71 depends on

the reflectance R and, therefore, on the refractive index n of the sample. Using eqs (4b) and (2c) it is easily seen

that

d(AT) =- Sr(R l + R2 )T

(n+1) 2 dn. (10)

For n= 1.5, r=0.04, r=0.5, and the above value

(9a) for (Rt + R2 ), this is reduced to d{&T) - 3 X lO"4 dn, showing that AT

7 will not be affected by more than

10 -4 transmittance units as long as dn < 0.3, or 1.2

< n < 1.8. Hence it may be concluded that the above

values of AT are applicable to virtually all filter glasses.

Other types of samples, such as coated glasses, will

of course require a different correction. For example,

measurements on the evaporated metal-on-glass filter with transmittance 0.4 at 474 nm yielded a value,

AT = — 1.4 X 10 -3

, which is about four times larger

than indicated by figure 3 for uncoated samples.

5. Conclusion

Based upon this work, all transmittance measure- ments on glass filters with this high-accuracy spectrophotometer are performed with the sample accurately

aligned normal to the optic axis. In addition to the re- flection correction given by eq (8) and the detector

nonlinearity correction discussed in reference [4], an /-number correction is applied to remove the residual bias due to the nonparallel radiation in the sample

compartment. This latter correction is given by eq

(5) as

(Ar) / =-(W)(in T^2;)^;

its magnitude is 1.3 X 10-4 transmittance units, or less,

for the fl 10 cone of light (0O = O.O5 rad) used. With

these corrections applied, all measurements made with

this high-accuracy spectrophotometer yield the normalincidence transmittance t defined by eq (2a) within a 10"4 limit of instrumental accuracy.

This spectrophotometer was designed and con- structed to permit calibration of solid and liquid filters for transmittance with a well-defined accuracy (4). These filters are used as Standard Reference Materials

(SRM's) to check the photometric scale of conventional

spectrophotometers. SRM 930a consists of a set of

three glass filters (Schott NG—4) with nominal trans- mittances 0.1, 0.2, and 0.3. Their transmittance values

are certified to ±0.5 percent of the value, this uncertainty being the sum of random and systematic errors. The latter are principally due to the inherent in- homogeneity and instability of the glass, surface

effects, and positioning the filters. They exceed by a large margin the instrumental corrections discussed

in this paper. Hence, the certified transmittances of

previously issued standard filters remain unaffected

by these corrections.

6. References

[1] Gibson, K. S., and Balcom, M. M., J. Res. Nat. Bur. Stand.

(U.S.), 38, 601 (1947).

[2] Goldring, L. S., Hawes, R. C, Hare, G. H., Beckman, A. O., and Stickney, M. E., Anal Chem. 25 , 869 (1953).

[3] Mielenz, K. D., J. Res. Nat. Bur. Stand. (U.S.), 76A (Phys. and

Chem.), No. 5, 455-467 (Sept.-Oct. 1972).

[4] Mavrodineanu, R., J. Res. Nat. Bur. Stand. (U.S.), 76A (Phys.

and Chem.), No. 5, 405-425 (Sept.-Oct. 1972).

[5] Mielenz, K. D., Eckerle, K. L., Madden, R. P., and Reader, J., Appl. Optics 12, 1630 (1973).

(Paper 77A6-792)

703

Appendix 7

JOURNAL OF RESEARCH of the National Bureau of Standards - A. Physics and Chemistry

Vol. 78A, No. 5, September-October 1974 Adaptation of a High-Accuracy

Spectrophotometer for Ultraviolet Work

K. D. Mielenz, R. Mavrodineanu, and E. D. Cehelnik*

Institute for Materials Research, National Bureau of Standards, Washington, D.C. 20234

(May 31, 1974)

A high-accuracy spectrophotometer, originally designed for work at visible wavelengths, was

modified to permit measurements in the ultraviolet without degradation of its original performance.

This was accomplished by equipping the spectrophotometer with a stable deuterium arc source, a highly efficient averaging sphere with fluorescent wavelength converter, a new grating, and achromatic

sample-compartment optics. The modified spectrophotometer will be used for the development of new

Standard Reference Materials, as well as for materials research, in the region between 200 and 300 ran.

Key words: Averaging sphere; deuterium arc lamp; fluorescent wavelength converter; grating; spectrophotometry; standard reference materials; ultraviolet; UV achromats; visible.

1 . Introduction

A primary goal of the current NBS program in

spectrophotometry has been the design of highaccuracy instrumentation for the certification of calibration standards and the development of improved

measurement techniques. For transmittance meas- urements at visible wavelengths, this goal was accomplished by specially constructing two highly accurate

single-beam spectrophotometers [1, 2],

1 which are presently used in the Institutes for Materials Research

(IMR) and Basic Standards (IBS) for standards work

at visible wavelengths (400 to 800 nm, approximately).

The instrumental accuracy achieved with these

spectrophotometers is 10-4 transmittance units (0.01

percent). Although similar in some design aspects,

these two spectrophotometers differ significantly in

others, and therefore can be employed for in-house

comparisons to ensure the accuracy and consistency

of all spectrophotometric measurements at NBS. The

purpose of this work was to extend these capabilities

into the ultraviolet spectral region by developing the

means for modifying either spectrophotometer for

applications down to about 200 nm. The particular instrument that was modified was

the IMR spectrophotometer described in reference

[1]. As originally designed, this spectrophotometer

employed a current-stabilized, 100-W tungsten ribbon

lamp as the radiation source; a 1-meter, f/8.7 Czerny-

*This paper describes a cooperative project on the part of the three authors. The basic design concepts and the accomplishment of a functioning instrument are due to K. D. Mielenz and R. Mavrodineanu. The optimization of the fluorescent wavelength converter for the averaging sphere was achieved by E.D. Cehelnik.

1 Figures in brackets indicate the literature references at the end of this paper.

( NOTE. See also USA Patent 3,968,363,

Turner grating monochromator (1200 lines/mm, 500-nm

blaze) with a fused-silica prism predisperser as the

dispersing element; and an 11-stage, S-20 photomultiplier tube attached to a 125-mm, BaSCVcoated

averaging sphere as the signal detector. The tungsten

lamp is imaged on the predisperser entrance slit by

a fused-silica lens. Two additional fused-silica lenses

are used in the sample compartment to focus the

monochromator exit slit at the sample, and to refocus

it into the averaging sphere [3]. The photomultiplier

signal is measured by means of a current-to-voltage

converter and a digital voltmeter, interfaced with a computer.

It may be estimated that, for a spectrophotometer of

this type, a clear-space signal of at least 10~7 A is required if measurements are to be made with a standard deviation of at least 10-4 transmittance units [4]. The actual clear-space signal current obtained with

this spectrophotometer at different wavelengths and

for typical values of tungsten-lamp power, monochromator slit width, photomultiplier anode voltage,

etc. is shown as curve (a) in figure 1, and thus indicates an inadequate performance of the instrument in

the spectral region below approximately 370 nm. In

order to effect the desired improvement of ultraviolet

performance, the spectrophotometer was equipped

with a deuterium arc source, a re-designed averaging

sphere, and a differently blazed grating. These components were chosen to achieve a wide spectral range

(200 to 800 nm) of the modified spectrophotometer,

requiring only a change back to the tungsten lamp

for work above 400 nm. In order to facilitate the usage

of the instrument throughout this extended range, it was necessary to replace the two sample-compartment lenses by fused-silica, lithium-fluoride achromats.

July 6, 1976
2. Modification of Spectrophotometer

2.1. Deuterium Arc Lamp

To a significant extent, the sharp drop of ultraviolet

signal level shown by curve (a) in figure 1 is attribi<r6

300 400 WAVELENGTH, nm

FIGURE 1. Successive improvement of clear-space signal level of

original spectrophotometer (curve a) obtained by:

(b) deuterium arc lamp, (c) improved averaging sphere,

and (d) new grating.

utable to the tungsten lamp of the original spectrophotometer. A rapid decrease of radiant intensity

with decreasing wavelength is typical for incandescent

filament lamps, and therefore makes any source of

this kind a poor choice for work at short wavelengths

[5]. In contrast, a deuterium arc lamp exhibits an in- crease of radiant intensity with decreasing wavelength, and thus provides much better power levels

in the ultraviolet.

A 60-W deuterium lamp with high-purity fusedsilica window was chosen for the modified spectrophotometer. This lamp has a line-free continuous

emission in the ultraviolet extending to below 180 nm,

and is rated to have the same output at 200 nm as a 150-W xenon arc lamp. In comparison to a 250-W

tungsten-bromide filament lamp, its output is about

1000 times greater at 250 nm, and 4 times greater at 300 nm. For work above 400 nm, tungsten lamps are preferable because of the decreasing intensity of the

deuterium-arc spectrum and its line structure in the

visible. The clear-space signal obtained after the deuterium lamp was installed in the spectrophotometer

is shown as curve (b) in figure 1. The stability of the radiant-intensity output of the

deuterium lamp was tested by performing regularly

repeated measurements of the spectrophotometer

signal over extended periods of time. The long-term

stability of lamp power so observed may be seen from

figure 2, showing a gradually decreasing signal drift at the rates of 1.5, 0.6, and 0.08 percent per hour after

the lamp had been operated for 1, 3, and 5 h, respectively. The average short-term instability (random

noise plus drift) of the deuterium lamp was found to be

0.02 percent for a total of 15 sets of 20 individual readings, taken at 5-s intervals and at arbitrary times during an 8-h period of lamp operation. Since all data obtained with this spectrophotometer are derived by

averaging a large number of individual measurements

of transmittance, performed in a time-symmetrical

sequence which eliminates the effects of drifting, this

stability of the deuterium lamp is adequate for routine

2.00 -

z

Uic

K

3

O

-l<

z

o

35

1.95 -

OPERATING HOURS

FIGURE 2. Long-term signal drift observed

with deuterium lamp.

632
measurements with a precision better than 10 ~4 transmittance units. The above results for the deuterium lamp compare favorably with the stability data

found for a current-stabilized tungsten lamp [2].

2.2. Averaging Sphere

The original averaging sphere of the spectrophotometer was designed as shown in figure 3a. It has an

internal diameter of 125 mm, and circular entrance and

exit ports 20 and 50 mm in diameter, respectively.

The circular target, 35 mm in diameter, is located at the center of the sphere. The inside of the sphere,

both sides of the target, and the thin target support

rod are coated with several layers of specially prepared barium sulfate and polyvinyl alcohol coatings

as developed by Grum and Luckey [6], The photomultiplier tube is attached to the sphere, so that its 50-mm cathode is located 16 mm behind the exit port.

The efficiency of this sphere was determined by suc- cessively measuring photomultiplier signals obtained

at different wavelengths with and without sphere,

using the deuterium arc lamp. These measurements

showed a steep decrease of sphere efficiency from about

15 percent for visible wavelengths near 500 nm to

SH

Figure 3. Geometry of (a) original averaging sphere and {b) improved

sphere.

A: entrance port. A': exit port, D: detector, F: fluorescent dye cell. S: sphere wall, SH:

shutter, T: target.

less than 0.1 percent at 200 nm, and thus indicated the

necessity to equip the spectrophotometer with an improved, more efficient sphere [7J. According to theory [8], the efficiency of an averaging sphere of the type considered is, approximately,

4>dI4> = [rz

l ( 1 - r ) ] ( 1 - eld)dW ( 1

)

where

<f> = radiant flux into sphere,

(f)o — flux reaching detector,

r= wall reflectance (assumed close to, but not equal

to unity),

s = sphere radius,

d = radius of sensitive area of detector (assumed

equal to radius of exit-port) e = distance between exit port and detector.

It may be seen from this equation that, to a large

extent, the poor efficiency of the original spectrophotometer sphere is due to inadequate coupling of exit

port and detector. The collection efficiency of the

detector (i.e.; the fraction of the diffuse flux from the

exit port which actually reaches the photocathode)

appears in Eq (1) as the factor (l-e/d), which in this case has the numerical value 0.64 and thus shows

that more than one third of the available flux is lost

through the gap between exit port and detector.

Therefore, a large increase in efficiency could be

gained by modifying the sphere as shown in figure 3b.

The sphere wall was tapered in a pear-like fashion

and was extended into the detector housing, so that

the exit port is butted directly against the photocathode. This simple modification of the sphere re- sulted in a large increase of efficiency (from 15 to 70 percent at 500 nm). However, the efficiency in the

ultraviolet (now 2.5 percent at 200 nm) was still inadequate.

The averaging effectiveness of this sphere was

tested by Mr. K. L. Eckerle of the Optical Radiation

Section of NBS. The sphere was found to be insensitive to beam displacements from the target center of

±1.5 mm within a ± 10~4 limit of signal variation.

This is to be compared with a ±5 percent variation

for a ± 1 mm beam displacement on the same photomultiplier used without the sphere.

The high efficiency of the modified sphere in the

visible suggested that a near-optimal sphere geometry

had been achieved, and that the limiting factor in the

ultraviolet was now the reduced reflectance of the barium-sulfate sphere coating. In order to overcome this re- maining difficulty, a fluorescent dye was used to shift

the incident short-wave radiation into the longer

wavelength region for which the reflectance of barium

sulfate is high. This dye, which is contained in a cell

placed in front of the sphere target as shown in figure

3b, had to be chosen such that virtually all of the incident flux at short wavelengths is absorbed and is converted into fluorescence with a quantum efficiency as nearly equal to unity as possible. In order to avoid

impairing the high sphere efficiency at longer wavelengths, it was also required that visible light be not

absorbed, so that it passes through the dye unaffected

and is reflected from the target as before. The fluores633
cent material selected was a 0.923 g/1 solution of 2,5-

diphenyloxazole (PPO) in rc-hexane in a 10 X 10 X 2

mm high-purity fused-silica cell. It is effectively transparent for wavelengths above 360 nm, and below 340

nm absorbs more than 99 percent of the incident radiation. Its estimated quantum efficiency is 0.9, with the

peak emission occurring near 370 nm. The use of this

dye proved highly successful, and resulted in a sphere

efficiency of 20 percent at 200 nm. The marked enhancement of signal level obtained

with this improved sphere is shown by comparing

curve (c) in figure 1 with curve (b). The measured

efficiency of the sphere is roughly constant and equal

to 20 percent between 200 and 300 nm, followed by a gradual increase to a constant level between 60 and

65 percent for wavelengths from 400 to beyond 650 nm. A further increase of sphere efficiency was obtained in a final design by reducing the sphere diameter and

further optimization of the dye [8]. It was also ascertained that the fluorescence wavelength converter did not affect the linearity of the

detection system. Theoretically, the radiant intensity

of fluorescence from the dye can be expressed as

Ie = QIa, (2a)

where Q is the quantum efficiency of the dye and/a is the intensity absorbed. The latter is given by Beer's

law as

/a = /o (l-10-«-&

) (2b)

where I0 is the initial intensity, and where e, c, and b

denote molar absorptivity, concentration, and path

length, respectively. Thus, Ie is proportional to I0 under the applicable assumptions that Beer's law is valid and that the quantum efficiency of the dye is independent of intensity. This conclusion was verified

by measuring the linearity of the detection system at one wavelength (250 nm) for which the dye is effective,

and at another (400 nm) for which it is not. These two

measurements, which were made using the doubleaperture method [9], yielded indistinguishable results

within the limits of experimental uncertainty, and also

agreed with previously performed linearity measure- ments of the same photomultiplier tube at 575 nm [1].

2.3. Grating

The previously obtained clear-space signal level, curve (c) in figure 1, was still judged inadequate for

accurate work at short wavelengths. Since the original

grating of the spectrophotometer (blazed for 500 nm in

the first order) had been chosen for work in the visible,

it was obvious that a further improvement could be

effected by substitution of another grating. This new

grating has 600 lines/mm, is blazed for 200 nm in the

first order, and resulted in the final clear-space signal

current plotted as curve (d) in figure 1. Since the signal

level near 200 nm can easily be raised by adjusting the

photomultiplier gain, the original goal of achieving a

clear-space signal of at least 10 -7 A throughout the

ultraviolet had thus been accomplished.

The loss of signal level in the visible due to the

change of gratings [curves (d) and (c) in figure 1J is relatively small, and is more than offset by the higher

efficiency of the new sphere [curves (c) and (b)]. Thus

the modified spectrophotometer can be used above 400

nm by merely interchanging deuterium and tungsten

lamps, without need to substitute gratings.

2.4. Achromatic Sample-Compartment Optics

In view of the wide spectral range of the modified

spectrophotometer, the chromatic aberration of the

original sample-compartment lenses proved to be a

limiting factor of instrumental performance. Significant

losses of signal level were caused by the fact that

these lenses, when initially focused by eye, were out

of focus in the ultraviolet and thus caused less-thanoptimal illumination of sample and detector.

If the monochromator exit slit is focused at the

sample with unit magnification at a wavelength \o,

the diameter of the blur circle in the sample plane for a wavelength k < \o can easily be shown to be equal to

5 = D[2/(\o)//(\)-l], (3)

where / is the focal length and D is the effective diameter of the focusing lens. For the particular lenses of the

original spectrophotometer [/(200 mm) =168 mm,

/(500 nm) = 200 nm, D = 38 mm], and assuming that

the focusing was done at 500 nm, the computed blurcircle diameter at 200 mm is 14 mm. This numerical

example illustrates the large defocusing caused by

chromatic lenses in the sample compartment, although

it should be pointed out that due to the opposing effect

of the second lens a smaller blur circle is incurred at the detector. Nevertheless, it was found empirically

that the ultraviolet signal level of the spectrophotometer could be improved as much as 30 percent by re- positioning the sample-compartment lenses.

The most effective way to overcome this deficiency

of the spectrophotometer would have been to use

mirror optics, but since this would have required a redesign of the sample compartment and its enclosure

it was more expedient to employ achromatic lenses.

The lenses selected are f/3 lithium-fluoride, fusedsilica (non fluorescent) achromats with 200 mm focal

length, best correction for longitudinal chromatic aberration between 200 and 400 nm, and with anti -reflection

coating for 400 nm. On account of the limited spectral regions for which

the deuterium and tungsten lamps of the modified

spectrophotometer are employed, it was not found

necessary to use an achromat in the source compartment as well.

3. Conclusion

. ii a result of the work reported here, the modified

hi ^curacy spectrophotometer is now used, throughout ; spectral region between 200 and 800 nm, for

634
the certification of solid and liquid Standard Reference

Materials for routine applications in spectrophotometry, as well as for materials research aimed at establishing inorganic and organic molar absorptivity

standards.

The modified spectrophotometer may also serve as a model for improved commercial instrumentation. In

particular, it is anticipated that the development of a

highly efficient averaging sphere for visible and ultraviolet wavelengths will lead to a more widespread

use of spheres in routine spectrophotometry.

4. References and Notes

[1] Mavrodineanu. R., J. Res. Nat. Bur. Stand. (U.S.), 76A (Phys.

and Chem.), No. 5, 405-425 (Sept.-Oct. 1972).

[2] Mielenz. K. D., Eckerle. K. L.. Madden. R. P., and Reader. J., Appl. Optics 12, 1630-1641 (1973); Mielenz, K. D.. and

Eckerle. K. L., Nat. Bur. Stand. (U.S.), Tech. Note 729. 60

pages (June 1972).

[3] The systematic error due to interreflection between the samplecompartment lenses is accounted for in all measurements

made with this spectrophotometer. See Mielenz. K. D., and

Mavrodineanu, R., J. Res. Nat. Bur. Stand. (U.S.), 77A (Phys.

and Chem.), No. 6. 699-703 (Nov.-Dec 1973).

[4] K. D. Mielenz, J. Res. Nat. Bur. Stand. (U.S.), 76A (Phys. and

Chem.). No. 5, 455-467 (Sept.-Oct. 1972).

[5| Although tungsten-halogen lamps are frequently used in ultra- violet spectrophotometry, such a source was unsuitable for

this application. A 250-W, single-coil tungsten-bromide lamp

was tried, but merely resulted in an over-all increase of signal

level which could also have been achieved by choosing a more

intense tungsten lamp or increasing the photomultiplier gain.

The tungsten-bromide lamp exhibited essentially the same

steep drop of signal toward shorter wavelengths as the tungsten

lamp, and was found to be inadequate for accurate work

below 320 nm. A further difficulty is that tungsten-halogen

lamps are generally too unstable for high-accuracy spectrophotometry in a single-beam geometry. See Clarke, F. J. J., J. Res. Nat. Bur. Stand. (U.S.), 76A (Phys. and Chem.), No.

5, 375-403 (Sept.-Oct. 1972).

[6) Crum, F., and Luckey, G. W., Appl. Optics 7, 2289-2294 (1968).

[7| A significant improvement of the visible and ultraviolet signal

levels was found with a flux averager consisting of two double- ground fused-silica plates, 1 mm in thickness and spaced by a

diffusely reflecting, cylindrical \\ 2 03 tube with 50 mm length

and internal diameter. Although this type of averager has been

used successfully for other applications (See Budde, W., Proc. Fourth Imeko Symposium, Prague 1969, p. 167), the averaging

effectiveness of the particular unit used in this work was in- adequate for this application since a beam displacement of ±1 mm at the entrance window of the averager still caused a

0.4 percent change in the photomultiplier signal. These meas- urements were performed by Mr. K. L. Eckerle of the Optical

Radiation Section of NBS.

[8| Mielenz, K. D., Mavrodineanu. R., and Cehelnik, E. D., Appl.

Optics (to be published).

[9] Mielenz. K. D. and Eckerle, K. L.. Appl. Optics 11, 2294-2303

(1972).

(Paper 78A5-S38)

552-907 OL - 74 - 6 635

Sintered mixtures of phosphors in polytetrafluoroethylene resin for fluorescence

standards

Victor R. Weidner, Radu Mavrodineanu, and

Kenneth L. Eckerle

U.S. National Bureau of Standards, Gaithersburg, Maryland 20899

Received 9 January 1986.

The National Bureau of Standards is investigating the

fluorescence properties of sintered mixtures of inorganic

phosphors and polytetrafluoroethylene (PTFE) resin as possible reference materials for fluorescence standards. Four

phosphors were selected from a collection of NBS standard

phosphors 1 that produce emission spectra in the blue, green,

yellow, and orange wavelength regions of the visible spectrum. The spectral properties of PTFE make it ideal for this

application because of its low absorption in the UV and

visible spectrum.

Analysis of the fluorescence properties of these specimens was done on the. NBS Reference Spectrofluorimeter.2 The

initial testing involved an examination of the specimen stability under exposure to UV radiation. Figure 1 shows the

emission spectra of the four phosphor-PTFE specimens before and after an 8-h exposure to a 450-W xenon source at a distance of 15 cm. The observed differences between the

emission spectra before and after the exposure to the xenon

source were too small to be shown in Fig. 1. The sintered

specimens appear to be unaffected by this intensity of exposure. Further studies are being conducted to determine the

effects of phosphor concentration on the measured emission

spectra and the effects of varying the excitation wavelengths.

The emission spectra illustrated in Fig. 1 were obtained

from specimens containing 10% phosphor by weight in

PTFE. The sintered specimens were produced in 2-mm

thick disks of ~50-mm diameter in crucibles of the design

shown in Kig. 2. The powder mixture is pressed to a density

of 1 g/cm3 between two ceramic disks in the crucible. The

specimen volume shrinks by ~10% during the sintering process, w hich takes place in an electric furnace at 370°C. The

specimen temperature is monitored by means of a thermocouple probe inserted into the base of the crucible. The

process requires ~3 h for the temperature of the crucible to

rise to 370°C, after which it remains at this temperature for

~1 h before being allowed to cool. The powder specimen is placed between ceramic disks to reduce the chances of con- tamination from the metal parts of the crucible that develop

an oxidation coating during the high-temperature cycle. After removal from the crucible, the specimen is buffed on an

abrasive cloth to remove surface irregularities and produce a

flat diffuse surface on the finished disk. Rectangular specimens are sliced from the sintered disk and mounted in 10-

mm square cuvette holders for analysis. These cuvette holders have apertures on all four sides so that the specimen can

be illuminated by the excitation radiation through one aperture and the emission radiation viewed through an aperture

at 90° from the optical axis of the excitation beam. The

specimen is mounted so that the incident excitation beam is 60° from the normal to the plane of the specimen. This

geometry allows the specular component of the reflected

excitation radiation to be excluded from measurement of the

fluorescence spectra.

References

1. C. F. Shelton, ••Emission Properties of NBS Standard Phosphor

Samples Under Photo-Excitation," Natl. Bur. Stand. U.S. Tech.

Note 417 (Mar. 1968).

2. K. D. Mielenz in Measurement of Photoluminescence, K. D.

Mielenz. Ed. (Academic New York. 1982), pp. 17-19.

Appendix 8

Applied Optics

25, No. 6, 828 (1986)

1.0

< - c

o

.6 a

UJ

ISI -J .4 <

2

<ro

z 2

BLUE 4 \ WCSH-j

N^O«*H0^

'V /

l L~~~ 4000 5000 6000

WAVELENGTH (A)

7000 7500

Fig. 1. Emission spectra of four sintered specimens containing 10%

phosphor in PTFE resin.

PISTON

CYLINOER —

CERAMIC

OISKS

3=£ ^0

PISTON

~ STOP

POWDER

SPECIMEN

BASE

Fig. 2.

SINTERING CRUCIBLE

Design of the crucible used to sinter the mixtures of phosphors in the PTFE resin.

Appendix 9

Netitnnal Knsiiiute of §>ian£nrfis Sc

CUrrttftratF

Standard Reference Material 930d

Glass Filters for Spectrophotometry

This Standard Reference Material (SRM) is intended for use in the verification of the transmittance and absorbance

scales of spectrophotometers in the visible spectral domain. SRM 930d consists of three individual glass neutral

density filters in separate metal holders and one empty filter holder. The exposed surface of the glass is approximately 29 x 8 mm, measuring from a point 1.5 mm above the base of the filter holder (see figure 1). The

filter holders are provided with shutters that protect the glass filters when not in use. Each filter-containing holder

bears a set identification number and a filter number (10, 20, or 30), which corresponds to the nominal %

transmittance (100 x transmittance) of the filter.

Certified Transmittance Values: Certified transmittance values independently determined for each filter at five

wavelengths in the visible portion of the electromagnetic spectrum are given in Table 1 . The relative uncertainty

of the certified transmittance values is ± 0.5% at 21 ± 2 °C for a period of two years from the date of certification

specified below in Table 2. This uncertainty includes the effects of the random and systematic errors of the

calibration procedure, as well as estimated systematic errors associated with alignment of the filters and material

properties (e.g. aging of the glass which may cause some filters to change transmittance by about + 0.25% over

the two-year period). Uncertainty estimation is described in the NIST Special Publication 260-116. [1]

Certified Transmittance Densities: The transmittance densities given in Table 2 are calculated from the certified

transmittance (T) as -log 10 T. These values should be indicated by the absorbance (A) scale of the

spectrophotometer if the filters are measured against air. The overall uncertainty in transmittance density is ± 0.002 absorbance units (AU) for a period of two years from the date of certification below Table 2.

The research, development, and initial production of this SRM were conducted in the NIST Inorganic Analytical

Research Division by R. Mavrodineanu and J.R. Baldwin.

The transmittance measurements were performed in the NIST Inorganic Analytical Research Division by

J.C. Travis, M.V. Smith, and N.K. Winchester.

The overall direction and coordination of technical measurements leading to certification were performed in the

NIST Inorganic Analytical Research Division by J.C. Travis and R.L. Waiters, Jr.

The technical and support aspects involved in the preparation, certification, and issuance of this SRM were

coordinated through the Standard Reference Materials Program by J.C. Colbert.

Gaithersburg, MD 20899 Thomas E. Gills, Acting Chief

October 18, 1993 Standard Reference Materials Program

(Revision of certificate dated 9-27-93)

(over)
NOTICE AND WARNINGS TO USERS

Storage and Handling: SRM 930d is stored in a black anodized aluminum container fitted with a threaded cap

made of the same metal. Contamination of the glass filter surface with paniculate matter due to static charge is minimized by the metallic nature of the container. Each filter is placed in a cylindrical cavity to prevent any contact

between the filter face and the walls of the storage container. Each filter holder is provided with a flat leaf spring

which is inserted into the cylindrical cavity to minimize damage during transportation. These springs can be

removed during normal use in the laboratory. Improper storage or handling of the filters may cause changes in the

transmittance. [1] It is recommended that the filters in the holders be handled only by the edges with soft plastic

(polyethylene) gloves and optical lens tissue. When not in use they should be stored in their holders and in the

container provided for this purpose. Extended exposure to laboratory atmosphere and dusty surroundings should

be avoided. Should the surface of the glass filter become contaminated, no attempt should be made to clean it unless the user has the facilities to demonstrate that the cleaning treatment has not altered the glass surface or compromised the certified values. As this SRM is a transfer standard, the only means available to verify its integrity is by remeasuring its transmittance with a primary standard instrument similar to that used in this

certification. [2-4]

Expiration of Certification: This certification is valid for 2 years from the date of certification specified below

in Table 2.

Recaiibration: The filters should be returned to NIST for cleaning and recalibration at two-year intervals to revalidate the filters. For recalibration, please contact M.V. Smith at (301) 975-4115 or N.K. Winchester at (301)

975-3152. Filters for recalibration should be shipped to M.V. Smith, NIST, Bldg. 222, Rm. B222, Gaithersburg,

MD 20899.

Instrument Dependence Warning: Some samples of SRM 930d may cause a small (<0.02 AU) increase in the

apparent absorbance, resulting from a minor deviation of the optical axis, in instruments for which wavelength

dispersion occurs after the light has passed through the filter. If such effects are detected or suspected, the user should contact J.C. Travis, NIST Inorganic Analytical Research Division at (301) 975-4117, for assistance and

instructions.

Source and Preparation of Material: The neutral glasses for the filters were provided by Schott of Mainz,

Germany, and are designated as "Jena Color and Filter Glass." Glass material of types NG-4 and NG-5 were

selected for best homogeneity and a minimum of inclusions and striae. The filters were cut from plates which were ground and polished in the NIST optical shop to appropriate thicknesses to achieve the nominal transmittances of

0.1, 0.2, and 0.3. [1,5] Prior to certification measurements, the glass filters were aged at NIST for at least six

months and each filter was examined for surface defects and thoroughly cleaned. [1]

Determination of Transmittances: The transmittance measurements were made against air (an empty filter holder)

at an ambient temperature of 21.0 + 1.0 °C using the high-accuracy spectrophotometer designed and built in the

NIST Inorganic Analytical Research Division. [2] This instrument represents the primary transmittance standard;

its transmittance accuracy was established using the double-aperture method of linearity testing. [1,2,6,7] The

effective spectral bandpass used to determine the certified values was 0.8 nm. The transmittance measurements were made by producing the vertical image of the slit (about 8 mm by 1 mm), using a convergent beam geometry

with an aperture ratio f: 10, in the middle of the entrance face of the glass filter. The filter is mounted in a multiplefilter carousel in the spectrophotometer. Each transmittance value reported in Table 1 is the average of three

transmittances determined over an eight-min period required for three carousel rotations. The transmittance is measured in this way several times during an aging period of at least six months, and only the final measurement

is reported. Each transmittance measurement is calculated from a measurement of the intensity transmitted through

2
the filter and bracketing measurements of the intensity transmitted through an empty filter holder, with a settling

time of approximately 5 s and a signal integrating time of approximately 1 s for each measurement. The filters were

measured in the spectrophotometer in a position perpendicular to the incident light beam as shown in figure 1

.

Uniformity: The transmittance uniformity of each filter comprising SRM 930d was established over an area 5 mm

wide by 24 mm long and located symmetrically about the center face of each filter. The transmittance was required

to vary by less than the estimated systematic error component for uniformity of 0.3%, relative, over the specified

area.

Instructions for Use: The transmittance of the filters depends upon the intrinsic properties of the material,

wavelength, spectral bandpass, geometry of the optical beam, and can be affected by other factors such as stray

light, temperature, and positioning of the filter. A change of ambient temperature of ± 2 °C from 21.0 °C will

not significantly affect the calibration. [1] Changes in the transmittance may be caused by changes in surface

conditions, aging of the glass, exposure to a harmful atmosphere or careless handling as indicated under "Storage

and Handling". [1,3,5,6] The spectral bandpass values indicated in parentheses in this certificate are maximum

values that should not be exceeded when accurate measurements are contemplated. The empty filter-holder is provided to be used in the reference beam of the spectrophotometer so that approximately equivalent conditions of

stray radiation are achieved for both beams. The shutters provided with each filter must be removed at the time

of measurement and be replaced after the measurements have been completed. Measurements performed outside

of these specified conditions, and the optical geometry described under "Determination of Transmittances", will

produce transmittance values that might differ from the certified data.

The cooperation of G.N. Bowers, Jr., M.D., of Hartford Hospital, Hartford, CT; R.N. Rand, Ph.D., of the

Eastman Kodak Co. Research Laboratories, Rochester, NY; D.S. Young, M.D. and Ph.D., of the Mayo Clinic,

Rochester, MN; and B. Mueller, Ph.D, of Hewlett-Packard GmbH, Waldbronn, Germany are gratefully

acknowledged.

REFERENCES

[1] Mavrodineanu, R., et al. Glass Filters as a Standard Reference Material for Spectrophotometry - Selection,

Preparation, Certification, and Use, SRM 930 and SRM 1930, NIST Spec. Publ. 260-116, (1993).

[2] Mavrodineanu, R., An Accurate Spectrophotometer for Measuring the Transmittance of Solid and Liquid

Materials, NBS Journal of Research 76A, No. 5, pp. 405-425, (1972).

[3] Collected Papers from NBS Conference on Accuracy in Spectrophotometry and Luminescence

Measurements, NBS Journal of Research 76A, No. 5, pp. 375-510, (1972).

[4] Burke, R.W., and Mavrodineanu, R., Accuracy in Analytical Spectrophotometry, NBS Special Pub.

260-81,(1983).

[5] Mavrodineanu, R., Solid Materials to Check the Photometric Scale of Spectrophotometers, NBS Tech.

Note 544, Menis, O., and Shultz, J. I., eds., pp. 6-17, (Sept. 1970); ibid, NBS Tech. Note 584, pp. 2-21,

(December 1971).

[6] Gibson, K.S., Spectrophotometry, NBS Circ. 484, (Sept. 1949).

[7] Mielenz, K.D., and Eckerle, K.L., Spectrophotometer Linearity Testing Using the Double-Aperture

Method, Appl. Optics 11, pp. 2294-2303, (1972).

3
Figure 1. Metal Holder for the Colored Glass Filters

Shutter Removed

Colored Glass Filter

Exposed Surface

29 x 8 mm

Incident Light Beam

Shutter

( Partially Removed )

Metal Filter Holder

57.0 x 12.75 x 12.75 mm

1.5 mm

-4-
Appendix 10

Naitnrtal Dnsittuip of §>tan&tr5s Sc QlFrtjnniaijtj

CUFrttftraiF

Standard Reference Material 1930

Glass Filters for Spectrophotometry

This Standard Reference Material (SRM) is intended as a reference source for the verification of the transmittance

and absorbance scales of spectrophotometers in the visible spectral domain. It is complementary to SRM 930d,

providing levels of transmittance/absorbance that are outside the range covered by that SRM. SRM 1930 consists

of three individual glass neutral density filters in separate metal holders and one empty filter holder. The exposed

surface of the glass is approximately 29 x 8 mm, measuring from a point 1 .5 mm above the base of the filter holder

(see figure 1). The filter holders are provided with shutters that protect the glass filters when not in use. Each

filter-containing holder bears a set identification number and a filter number (1, 3, or 50), which corresponds to the nominal % transmittance (100 x transmittance) of the filter.

Certified Transmittance Values: Certified transmittance values independently determined for each filter at five

wavelengths in the visible portion of the electromagnetic spectrum are given in Table 1 . The relative uncertainty

of the certified transmittance values is ± 1.2% for filters 1 and 3 and ± 0.4% for filter 50, for a period of two

years from the date of certification specified below Table 2. This uncertainty includes the effects of the random

and systematic errors of the calibration procedure, as well as estimated systematic errors associated with alignment

of the filters and material properties (e.g. aging of the glass which may cause some filters to change transmittance

by as much as 0.4% over the two-year period). Uncertainty estimation is described in the NIST Special Publication

260-116. [1]

Certified Transmittance Densities: The transmittance densities given in Table 2 are calculated from the certified

transmittances (T) as -log10 T. These values should be indicated by the absorbance (A) scale of the

spectrophotometer if the filters are measured against air. The overall uncertainty in transmittance density is ± 0.003 absorbance units (AU) for filters 1 and 3 and ± 0.0015 AU for filter 50, for a period of two years from

the date of certification below Table 2.

( The research, development, and initial production of this SRM were conducted in the NIST Inorganic Analytical

Research Division by R. Mavrodineanu and J.R. Baldwin.

The transmittance measurements were performed in the NIST Inorganic Analytical Research Division by

J.C. Travis, M.V. Smith, and N.K. Winchester.

The overall direction and coordination of technical measurements leading to certification were performed in the

NIST Inorganic Analytical Research Division by J.C. Travis and R.L. Watters, Jr.

The technical and support aspects involved in the preparation, certification, and issuance of this SRM were

coordinated through the Standard Reference Materials Program by J.C. Colbert.

Gaithersburg, MD 20899 Thomas E. Gills, Acting Chief

September 27, 1993 Standard Reference Materials Program

(Revision of certificate dated 3-30-93)

(over)
NOTICE AND WARNINGS TO USERS

Storage and Handling: SRM 1930 is stored in a blue anodized aluminum container fitted with a threaded cap made

of the same metal. Contamination of the glass filter surface with paniculate matter due to static charge is minimized

by the metallic nature of the container. Each filter is placed in a cylindrical cavity to prevent any contact between

the filter face and the walls of the storage container. Each filter holder is provided with a flat leaf spring which

is inserted into the cylindrical cavity to minimize damage during transportation. These springs can be removed

during normal use in the laboratory. Improper storage or handling of the filters may cause changes in the

transmittance. [I] It is recommended that the filters in the holders be handled only by the edges with soft plastic

(polyethylene) gloves and optical lens tissue. When not in use they should be stored in their holders and in the

container provided for this purpose. Extended exposure to laboratory atmosphere and dusty surroundings should

be avoided. Should the surface of the glass filter become contaminated, no attempt should be made to clean it unless the user has the facilities to demonstrate that the cleaning treatment has not altered the glass surface or compromised the certified values. As this SRM is a transfer standard, the only means available to verify its integrity is by remeasuring its transmittance with a primary standard instrument similar to that used in this certification. [2-4]

Expiration of Certification: This certification is valid for 2 years from the date of certification specified below

Table 2.

Recaiibration: The filters should be returned to NIST for cleaning and recalibration at two-year intervals to revalidate the filters. For recalibration, please contact M.V. Smith at (301) 975-4115 or N.K. Winchester at (301)

975-3152. Filters for recalibration should be shipped to M.V. Smith, NIST, Bldg. 222, Rm. B222, Gaithersburg,

MD 20899.

Instrument Dependence Warning: Some samples of SRM 1930 may cause a small (<0.02 AU) increase in the

apparent absorbance, resulting from a minor deviation of the optical axis, in instruments for which wavelength

dispersion occurs after the light has passed through the filter. If such effects are detected or suspected, the user should contact J.C. Travis, NIST Inorganic Analytical Research Division at (301) 975-4117, for assistance and

instructions.

Source and Preparation of Material: The neutral glasses for the filters were provided by Schott of Mainz,

Germany, and are designated as "Jena Color and Filter Glass." Glass material of types NG-3 and NG-11 were

selected for best homogeneity and a minimum of inclusions and striae. The filters were cut from plates which were ground and polished in the NIST optical shop to appropriate thicknesses to achieve the nominal percent

transmittances of 1, 3, and 50%. [1,5] Prior to certification measurements, the glass filters were aged at NIST

for at least six months and each filter was examined for surface defects and thoroughly cleaned. [1]

Determination of Transmittances: The transmittance measurements were made against air (an empty filter holder)

at an ambient temperature of 21.0 ± 1.0 °C using the high-accuracy spectrophotometer designed and built in the

NIST Inorganic Analytical Research Division. [2] This instrument represents the primary transmittance standard;

its transmittance accuracy was established using the double-aperture method of linearity testing. [1, 2, 6, 7] The

effective spectral bandpass used to determine the certified values was 0.8 nm. The transmittance measurements were made by producing the vertical image of the slit (about 8 mm by 1 mm), using a convergent beam geometry

with an aperture ratio f: 10, in the middle of the entrance face of the glass filter. The filter is mounted in a multiplefilter carousel in the spectrophotometer. Each transmittance value reported in Table 1 is the average of three

transmittances determined over an eight-min period required for three carousel rotations. The transmittance is measured in this way several times during an aging period of at least six months, and only the final measurements

are reported. Each transmittance measurement is calculated from a measurement of the intensity transmitted through

the filter and bracketing measurements of the intensity transmitted through an empty filter holder, with a settling

time of approximately 5 s and a signal integrating time of approximately 1 s for each measurement. The filters were measured in the spectrophotometer in a position perpendicular to the incident light beam as shown in figure 1.

Uniformity: The transmittance uniformity of each filter comprising SRM 1930 was established over an area 5 mm

wide by 24 mm long and located symmetrically about the center face of each filter. The transmittance was required

to vary by less than the estimated systematic error component for uniformity of 0.9%, relative, for filter 1, 0.8%

for filter 3 and 0.2% for filter 50, over the specified area.
Instructions for Use: The transmittance of the filters depends upon the intrinsic properties of the material:

wavelength, spectral bandpass, geometry of the optical beam, and can be affected by other factors such as stray

light, temperature, and positioning of the filter. A change in ambient temperature of ± 2 °C from 21.0 °C will

not significantly affect the calibration. [1] Changes in the transmittance may be caused by changes in surface

conditions, aging of the glass, exposure to a harmful atmosphere or careless handling as indicated under "Storage

and Handling". [1,3,5,6] The spectral bandpass values indicated in parentheses in this certificate are maximum

values that should not be exceeded when accurate measurements are contemplated. The empty filter-holder is provided to be used in the reference beam of the spectrophotometer so that approximately equivalent conditions of

stray radiation are achieved for both beams. The shutters provided with each filter must be removed at the time

of measurement and be replaced after the measurements have been completed. Measurements performed outside

of these specified conditions, and the optical geometry described under "Determination of Transmittances", will

produce transmittance values that might differ from the certified data.

The cooperation of G.N. Bowers, Jr., M.D., of Hartford Hospital, Hartford, CT; R.N. Rand, Ph.D., of the

Eastman Kodak Co. Research Laboratories, Rochester, NY; D.S. Young, M.D. and Ph.D., of the Mayo Clinic,

Rochester, MN; and B. Mueller, Ph.D, of Hewlett-Packard GmbH, Waldbronn, Germany are gratefully

acknowledged.

REFERENCES

[1] Mavrodineanu, R., et al., Glass Filters as a Standard Reference Material for Spectrophotometry - Selection,

Preparation, Certification, and Use, SRM 930 and SRM 1930, NIST Spec. Publ. 260-116, (1993).

[2] Mavrodineanu, R., An Accurate Spectrophotometer for Measuring the Transmittance of Solid and Liquid

Materials, NBS Journal of Research 76A, No. 5, pp. 405-425, (1972).

[3] Collected Papers from NBS Conference on Accuracy in Spectrophotometry and Luminescence

Measurements, NBS Journal of Research 76A, No. 5, pp. 375-510, (1972).

[4] Burke, R.W., and Mavrodineanu, R., Accuracy in Analytical Spectrophotometry, NBS Spec. Publ. 260-81,

(1983).

[5] Mavrodineanu, R., Solid Materials to Check the Photometric Scale of Spectrophotometers, NBS Tech.

Note 544, Menis, O., and Shultz, J.I., eds., pp. 6-17, (Sept. 1970); ibid, NBS Tech. Note 584, pp. 2-21,

(December 1971).

[6] Gibson, K.S., Spectrophotometry, NBS Circ. 484, (Sept. 1949).

[7] Mielenz, K.D., and Eckerle, K.L., Spectrophotometer Linearity Testing Using the Double-Aperture

Method, Appl. Optics 11, pp. 2294-2303, (1972).
Figure 1. Metal Holder for the Colored Glass Filters

Shatter Removed

Colored Glaaa Filter

Exposed Surface

29 x a

Iacideac Light Bea

Baao

Shatter

( Partially Removed )

Metal Filter Holder

57.0 x 12.75 x 12.75

1.3

4
National JJnsittxjte of Spaniards $c tEerlfnoingij

Appendix 11

Standard Reference Material 2031

Metal-on-Quartz Filters for Spectrophotometry

This Standard Reference Material (SRM) is intended for use in the verification of the transmittance and absorbance

scales of conventional spectrophotometers at selected wavelengths in the ultraviolet and visible spectral domains.

SRM 203 1 consists of three individual neutral density filters in their metal holders and one empty filter holder. The

exposed surface of each filter is approximately 29 x 8 mm, measuring from a point 1.5 mm above the base of the

filter holder (see Figure 1). The metal holders for these filters are provided with shutters to protect the filters when

not in use. Each filter containing holder bears a set identification number and a filter number (10, 30, or 90), which

corresponds to the nominal percent transmittance (100 x transmittance) of the filter.

Certified Transmittance Values: Certified transmittance values independently determined for each filter at ten wavelengths in the visible and ultraviolet spectral domains are given in Table 1 . The relative uncertainties of the

certified transmittance values are ± 1.0% for filters 10 and 30, and ± 0.5% for filter 90 at an operational

temperature range of 21 ± 2 °C for a period of two years from the date of certification specified under Table 2. These uncertainties include 0.5% for random and systematic errors of the calibration procedure for all three filters

in the set, as well as 0.5% for error sources unique to the metal film filters (numbers 10 and 30). The estimates

are based on a study of 19 filter sets [1], and include the inherent effects of temperature and humidity variation,

positional reproducibility, and apparent changes in the transmittance over a two-year period. In addition, the

uncertainty includes components to account for potential differences between the NIST and user measurement

conditions. These differences might include a wider temperature range, non-normality of the filter with respect to

the optical axis (± 3 degrees of arc), the size of the spectrophotometer beam, and its position on the filter (see

"Uniformity", page 3).

Certified Transmittance Densities: The transmittance densities given in Table 2 are calculated from the certified

transmittance (T) as -log10 T. These values should be indicated by the absorbance (A) scale of the

spectrophotometer when the filters are measured against air. The overall uncertainty in transmittance density is ± 0.004 absorbance units (AU) for filters 10 and 30, and ± 0.002 AU for filter 90 for a period of two years from

the date of certification under Table 2.

The research, development, and initial production of this SRM were performed in the NIST Inorganic Analytical

Research Division by R. Mavrodineanu and J.R. Baldwin.

The transmittance measurements were performed in the NIST Inorganic Analytical Research Division by M.V.

Smith, N.K. Winchester, and J.C. Travis.

The overall direction and coordination of technical measurements leading to certification were performed in the

NIST Inorganic Analytical Research Division by J.C. Travis and R.L. Watters, Jr.

The technical and support aspects involved in the preparation, certification, and issuance of this SRM were

coordinated through the Standard Reference Materials Program by J.C. Colbert.

Gaithersburg, MD 20899 Thomas E. Gills, Acting Chief

October 28, 1993 Standard Reference Materials Program

(Revision of certificate dated 1-25-93)

(over)
NOTICE AND WARNINGS TO USERS

Storage and Handling: SRM 203 1 is stored in a black-anodized aluminum container provided with a threaded cap

made of the same metal. Each filter is placed in a cylindrical cavity to prevent any contact between the filter face

and the walls of the storage container. Contamination of the filter surface with paniculate matter due to static charges is minimized by the metallic nature of the container. A flat leaf spring is inserted into the cylindrical cavity

with each filter holder to minimize damage during transportation. These springs can be removed during normal

use in the laboratory. Changes in the transmittance of the filters may be caused by surface conditions, aging of the

material, exposure to a harmful atmosphere, or careless handling. [3, 5-7] It is recommended that the filters in the

holders be handled only by the edges with soft plastic (polyethylene) gloves and optical lens tissue. When not in use, the filters should be stored in their holders with the shutters in place and in the metal container provided for

this purpose. Extended exposure to laboratory atmosphere and dirty surroundings should be avoided. Should the

surface of the filter become contaminated, no attempt should be made to clean it unless the user has the facilities

to demonstrate that the cleaning treatment will not alter the surface or degrade the accuracy of the certified values.

As SRM 2031 is a transfer standard, the only means available to verify its integrity is to remeasure its transmittance

with a primary standard instrument similar to that used in this certification. [1,3]

Expiration of Certification: Normal surface degradation and aging of the metallic film may cause the transmittance

of the filters to change by an appreciable fraction of the certified uncertainty over a period of two years. This

certification is therefore valid for 2 years from the date of certification under Table 2.

Recalibration: The filters should be returned to NIST for cleaning and recalibration at two-year intervals to validate the filters' integrity. For recalibration, please contact M.V. Smith at (301) 975-4115 or N.K. Winchester

at (301) 975-3152. Filters for recalibration should be shipped to M.V. Smith, NIST, Bldg. 222/Rm 222,

Gaithersburg, MD 20899.

Instrument Dependence Warning: In some commercial instruments, the metal-on-quartz filters can generate

reflection effects in the sample compartment that can degrade the accuracy of the measured transmittances. During

the development of SRM 2031, the presence and magnitude of reflection effects were studied and were found

negligible, within the uncertainty specified, in all spectrophotometers tested (see Ref. 3, pp. 16-30 for additional

details of this study). However, for certain instruments these effects could become significant. If such effects are detected or suspected, the user should contact J.C. Travis, NIST Inorganic Analytical Research Division at (301)

975-4117, for assistance and instructions.

Source and Preparation of Material: The evaporative coating of quartz substrates with thin chromium films was

performed by Evaporated Metal Films, Ithaca, NY. The quartz plates were provided by Dynasil Corp. of America,

Berlin, NJ. Except for the chromium coatings, the filters that constitute SRM 2031 were fabricated by the NIST

Optical and Instrument Shops. Two filters, having nominal transmittances of 0.1 and 0.3, were produced by

evaporating different thicknesses of chromium metal onto 1.5 mm thick fused silica plates that had been precision

ground and polished. These metal films are protected by 1 .5 mm thick clear fused silica cover plates optically

contacted to the base plates. The third filter is a single fused silica plate 3 mm thick, having a nominal

transmittance of 0.9. Prior to certification measurements, the filters were aged at NIST for at least six months and

each filter was examined for surface defects and the condition of the optical contact. [3]
Determination of Transmittances: The transmittance measurements were made against air (an empty filter holder)

at an ambient temperature of 21.0 ± 1.0 °C using the high-accuracy spectrophotometer designed and built in the

NIST Inorganic Analytical Research Division. [5] This instrument is a primary transmittance standard; its transmittance accuracy was established using the double-aperture method of linearity testing. [5-7] The effective

spectral bandpass used to determine the certified values was 1.6 nm in the ultraviolet and 0.8 nm in the visible

region. Transmittance measurements for SRM 203 1 were made by producing the vertical image of the slit (about

8 mm x 1.5 mm), using a convergent beam geometry with an aperture ratio f: 10, in the middle of the entrance face

of the filter. The filter is mounted in a multiple-filter carousel in the spectrophotometer, and is perpendicular to the incident light beam when in the measurement position. Each transmittance value reported in Table 1 is the

average of three transmittances determined over an eight-min period required for three carousel rotations. The

transmittance is measured in this way several times during an aging period of at least six months, and only the final

measurement is reported. Each transmittance measurement is calculated from a measurement of the intensity

transmitted through the filter and bracketing measurements of the intensity transmitted through an empty filter holder, with a settling time of approximately 5 s and a signal integrating time of approximately 1 s for each

measurement.

Uniformity: The transmittance uniformity of each filter comprising SRM 203 1 was established over an area 5 mm

wide by 24 mm long and located symmetrically about the center face of each filter. The variation in transmittance over the specified area was less than 0.8% for filters 10 and 30, and 0.2% for filter 90. Further information

concerning the selection, preparation, and properties of SRM 2031 will be found in reference. [1]

Instructions for Use: The measured transmittance of the filters depends upon the intrinsic properties of the

material, wavelength, spectral bandpass, geometry of the optical beam, and can be affected by other factors such

as stray light, temperature, and positioning of the filter. While changes of ± 2 °C in ambient temperature from

21.0 °C have not significantly affected the calibration, the effect of temperature variations exceeding ± 2 °C have

not been investigated. The empty filter holder provided is to be used in the reference beam of the

spectrophotometer so that approximately equivalent conditions of stray radiation are achieved for both beams. The

shutters provided with the filters must be removed at the time of measurement and be replaced after the

measurements have been completed. Because the transmittance of these filters exhibits an appreciable optical

neutrality, the dependence of transmittance on bandpass is not critical, and wider bandpasses than that employed

for certification (see "Determination of Transmittances", page 3) may be used in routine measurements. For a

quantitative discussion of this subject, the user should consult reference [1], p. 32. Measurements performed outside

of the specified conditions, and the optical geometry described under "Determination of Transmittances", will

produce transmittance values that might differ from the certified data.

We wish to acknowledge the cooperation of G.N. Bowers, Jr., M.D., of Hartford Hospital, Hartford, CT; R.N.

Rand, Ph.D., of the Eastman Kodak Co. Research Laboratories, Rochester, NY; and D.S. Yound, M.D., Ph.D.,

of the Mayo Clinic, Rochester, MN. H.E. Bennett and J.M. Bennett of the Michelson Laboratory, Physical Optics

Branch, Naval Weapons Center, China Lake, CA, who helped in the initial production of SRM 2031, are also

gratefully acknowledged.
REFERENCES

[1] Mavrodineanu, R. and Baldwin, J.R., Metal-on-Quanz Filters as a Standard Reference Material for

Spectrophotometry, SRM 2031; NBS Special Publication 260-68; (1979).

[2] Mavrodineanu, R., Solid Materials to Check the Photometric Scale of Spectrophotometers, NBS Tech.

Note 544, Menis, O. and Shultz, J.L, eds., pp. 6-17, (Sept. 1970); ibid, NBS Tech. Note 584, pp. 2-21,

(Dec. 1971).

[3] Mavrodineanu, R., Considerations for the Use of Semi-Transparent Metallic Thin Films as Potential

Transmittance Standards in Spectrophotometry. NBS Journal of Research 80A. No. 4, pp. 637-641,

(1976).

[4] Burke, R.W. and Mavrodineanu, R., Accuracy in Analytical Spectrophotometry; NBS Spec. Pub. 260-81;

(1983).

[5] Mavrodineanu, R., An Accurate Spectrophotometer for Measuring the Transmittance of Solid and Liquid

Materials; NBS Journal of Research 76A, No. 5, pp. 405-425, (1972).

[6] Gibson, K.S., Spectrophotometry, NBS Circ. 484; (Sept. 1949).

[7] Mielenz, K.D. and Eckerle, K.L., Spectrophotometer Linearity Testing Using the Double-Aperture

Method, Appl. Optics 11, pp. 2294-2303, (1972).

Figure 1 . Metal Holder for the Metal-on-Quanz Filters

Shatter

( Partially Removed )

Metal Filter Holder

57.0 x 12.75 x 12.75 mm

Incident Light Beam

1.5 mm

-4-
Appendix 12

Nattnnal ilnstttntf of g>tanbar6s Sc (Eerltnolngy

GlFrttftratr

Standard Reference Material 93 le

Liquid Absorbance Standard for

Ultraviolet and Visible Spectrophotometry

This Standard Reference Material (SRM) is intended primarily for routine critical evaluation of daily working

standards used in spectrophotometry, and for use in the calibration and checking of accuracy of the photometric

scale of spectrophotometers that provide a narrow effective spectral bandpass, not to exceed 1.5 nm at 302 tun, 2.0

nm at 395 nm, 3.3 nm at 512 nm, and 8.5 nm at 678 nm. [1]

SRM 93 le is certified as solutions of known net absorbances at four (4) specific spectral wavelengths for a 10.00-

mm measurement pathlength (see Preparation of Filter Solutions for details). Each unit of SRM 93 le consists of

3 sets of liquid filters (12 ampoules total), each set consisting of a blank solution and 3 absorbance levels (I, II, and

III) of the filter (nominal absorbances of 0.3, 0.6, and 0.9, respectively, for a 10- mm pathlength). Approximately

10 mL of each liquid filter is individually flame-sealed in a glass ampoule which has been prescored for easy

opening. Each set of liquid filters in the SRM unit is individually packaged in a tray.

The certified net absorbances are given below for the three solution levels at four wavelengths, for a 10.00-mm

pathlength cell and a temperature of 22 ± 1 °C. The uncertainties of the certified values include all known sources of random and possible systematic errors (see Certification of Net Absorbance for 10.00-mm pathlength).

Wavelength, nm

Filter 302 395 512 678

Level I 0.2978 ± 0.0015 0.3060 ± 0.0015 0.3013 ± 0.0015 0.1153 ± 0.0015

Level II 0.5946 ± 0.0025 0.5954 ± 0.0025 0.6001 ± 0.0025 0.2245 ± 0.0015

Level III 0.9160 ± 0.0035 0.9009 ± 0.0035 0.9055 ± 0.0035 0.3402 ± 0.0015

Note: All certified values have been corrected for absorbances due to the blank solution.

The preparation of the filter solutions and the performance of the transmittance measurements for the certification

process were performed in the NIST Inorganic Analytical Research Division by J.D. Messman.

The overall coordination of technical measurements leading to certification was performed in the NIST Inorganic

Analytical Research Division by R.L. Watters, Jr. and J.C. Travis.

The statistical analysis of the data was performed in the NIST Statistical Engineering Division by S.B. Schiller .

The technical and support aspects involved in the preparation, certification, and issuance of this Standard Reference

Material were coordinated through the Standard Reference Materials Program by J.C. Colbert.

Gaithersburg, MD 20899 Thomas E. Gills, Acting Chief

May 26, 1993 Standard Reference Materials Program

(Revision of certificate dated 3-3-93)

(over)
NOTICE AND WARNINGS TO USER

Stability and Expiration of Certification: While no long-term stability studies have been made on this lot (93 le),

studies on previous lots (931, 931a, 931b, and 931c) over three-year periods showed no degradation of the material

when stored in the original sealed ampoules. Therefore, this material is certified only for use within three years

following the date of shipment from NIST.

Preparation of Filter Solutions: The filter solutions were prepared by dissolving high-punty cobalt and nickel in a mixture of nitric and perchloric acids. The absorbance of nitrate ion was adjusted to a comparable level by

evaporation and, if necessary, subsequent addition of small amounts of nitric acid. The nominal weights of the

cobalt and nickel metals and the typical volumes of nitric and perchloric acids used in the preparation of the three

absorbance levels of solutions are given below for information. The pH of these solutions is about I.

Liquid Filter Cobalt (g) Nickel (g) HNQ3 (mL) HCIO^ (mL)

Level I 7.4 6.9 30 50

Level II 14.7 13.9 45 90

Level III 22.1 20.7 65 125

To facilitate dissolution of the metals and removal of excess nitric acid, the requisite weights of metal needed to produce the 20-L volume of solution for each absorbance level were divided into ten equal portions.

The dissolutions were performed in 1-L glass beakers. Following dissolution and evaporation of excess nitric acid

for each portion on a hot plate, the solutions were filtered through Whatman No. 2 filters directly into 2-L

volumetric flasks to remove any residual insoluble particles. The solutions were then diluted to calibrated volume

with distilled water and mixed thoroughly. The ten 2-L portions for each absorbance level were combined in a 20-L

glass carboy and the contents were swirled thoroughly to produce the final solutions. The blank solution (0. IN

HC104in distilled water) was produced by preparing ten 2-L portions of distilled water containing 1 8 mL of HC104 and combining the portions in a 20-L glass carboy.

Following preparation of the 20-L volumes of the blank solution (0. IN HCIO4) and the three absorbance level

solutions, the solutions were ampouled, each containing approximately 10 mL of solution.

The maxima in the absorbance spectrum (see figure 1) at 302 and 512 nm are due to absorbance by NO3" and

Co(H20)6 + +

, respectively. The maximum at 395 nm and the plateau at 650-700 nm are due to Ni(H20)6

.

Certification of Net Absorbance for 10. 00-mm Pathlength: The transmittance measurements leading to the

certification of this SRM were performed at an ambient temperature of 22 ± 1 °C using the NIST high accuracy

spectrophotometer. The design and construction of this instrument have been described previously. [2] The

instrument is a primary transmittance standard; its accuracy has been verified using the double aperture radiationaddition principle. The effective spectral bandpass used to determine the certified values was 0.8 nm. The

transmittance measurements were made by producing the vertical image of the slit (about 8 mm by 0.5 mm), using

a convergent beam geometry with an aperture ratio f: 10, in the middle of the entrance face of the sample cuvette. The cuvette was oriented in a position perpendicular to the incident light beam.

The liquid absorbance filter solutions were calibrated at the wavelengths and conditions indicated by measuring the

transmittance, T, of the "Blank" and solutions "I", "II", and "III" against air as a reference. Each transmittance

measurement is calculated from a measurement of the intensity transmitted through a 10-mm cuvette containing the

SRM solution and bracketing measurements of the intensity transmitted through an empty cuvette holder, with a

settling time of approximately 5 s and a signal integrating time of approximately 1 s for each measurement.

For the certification measurements, 18 ampoules were randomly selected from each of the four solutions (blank and

3 absorbance levels). Randomized measurements were made in four runs on each of three days. In each run, six

unknown samples (SRM 93 le) and one control sample (SRM 93 Id) were measured in triplicate using cuvettes with

the pathlength known to ± 0.0005 mm. On each day, six samples from each of the four levels (blank and levels

I, II, and III) were measured. Two samples from three of the four levels were measured in each run. A blank was

run in each cuvette on each day.
The values of T were used to calculate the corresponding values of the apparent absorbance, A, using the

relationship A = -log 10T. The net absorbances were obtained by subtracting the apparent absorbance of the blank

solution from the apparent absorbances calculated for solutions I, II, and III. The certified net absorbances are the

grand mean values for the three levels and four wavelengths. Statistical analysis revealed no apparent "day" or

"cuvette" effects in the data. A small apparent "run" effect was consistent with the temperature coefficients given

below, and with the normal temperature variation in the laboratory. The random error (95% confidence interval) was about a third of the total estimated error given in the table, which includes systematic error estimates to accommodate a ± 1 °C temperature variation and a small angular uncertainty in cuvette orientation.

Temperature Dependence: Absorbances at various temperatures (17° to 35 °C) may be calculated using the

equation:

AT = A22[l+CA

(T-22)]

where: AT = Absorbance at temperature T (°C)

A22 = Absorbance certified at 22.0 °C

CA = Fractional change in absorbance per °C

The values of CA, at the four wavelengths, are given below.

Note: At wavelength 302 nm, absorbance decreases with increasing temperature; at the other wavelengths,

absorbance increases with increasing temperature.

Wavelength, nm CA

302 - 0.0014

395 +0.0014

512 +0.0018

678 +0.0014

Wavelength Accuracy and Bandpass Requirements: Due to the spectral features in the absorbance spectrum (see

figure 1), the absorbances of these liquid absorbance standards will depend not only on the accuracy of the

photometric scale, but also on the wavelength accuracy and the spectral bandpass of the spectrophotometer. A

mercury lamp is recommended for checking the wavelength scale. In addition, for those spectrophotometers having

a hydrogen (H) or deuterium (D) source, the two emission lines at 486. 1 and 656.3 nm (H) or 486.0 and 656. 1 nm

(D) may provide a convenient check at these wavelengths. To ensure that the measured absorbances are not

significantly different from the certified values, the wavelength scale of the spectrophotometer should be calibrated

to within ± 0.5 nm, and the effective spectral bandpass should not exceed 1.5, 2.0, 3.3, and 8.5 nm at 302, 395,

512, and 678 nm, respectively.

3
Instructions for Use: This Standard Reference Material should be kept in the original sealed ampoules. Once

opened, the material should be used immediately. No attempt should be made to reseal the ampoule.

The instructions below are for use with the standard 10-mm rectangular cuvette, and apply to either single-beam

or double-beam spectrophotometers. For calibration of the several spectrophotometric systems used in various

automated instruments, the user is referred to the instruction manual for the particular instrument.

1. Select a clean 10.00-mm cuvette, free of scratches and fitted with a ground glass or Teflon stopper, for all SRM measurements.

2. Mark the cuvette to assure reproducible orientation in the spectrophotometer in case the cuvette should need

to be removed during the calibration procedure (not recommended).

3. Place the cuvette in the sample holder and fill with distilled water. (Borosilicate Pasteur-type pipettes fitted with rubber bulbs are recommended for transferring all solutions to and from the cuvettes. Soft glass pipettes,

which are available commercially, contain residual amounts of ultraviolet absorbing material, but may be used

after proper cleaning. Several nnses, first with isopropyl alcohol and then with distilled water, are generally

adequate).

4. Obtain the apparent absorbance of the cuvette containing distilled water against air (nothing in the reference

holder) at 302, 395, 512, and 678 nm, using an effective spectral bandpass not to exceed 1.5 nm, 2.0 am,

3.3 nm, and 8.5 nm at the respective wavelengths.

5. Empty the cuvette by suction without removing it from the sample holder (if at all possible), refill with

distilled water and measure the apparent absorbance against air again at each of the above wavelengths.

6. Repeat the emptying and refilling operation until constant apparent absorbance readings are obtained. Empty

the cuvette by suction without removing it from the sample holder.

7. Using the liquid filters provided, measure, in turn, the apparent absorbance of the "Blank" and of solutions

"I", "II", and "III" against air in the following manner. Shake each ampoule before opening to remix any

condensate which may have been collected in the neck. (The ampoules have been prescored directly below

the gold band to facilitate opening.) Rinse the sample cuvette in the sample holder with at least two complete

fillings of the SRM solution from each 10-mL ampoule before taking final apparent absorbance readings for

that ampoule at the wavelengths and bandwidths of step 4.

8. Subtract the apparent absorbance of the "Blank" from the apparent absorbances obtained for solutions "I" "II", and "III" to yield the net absorbances for the three levels. These net absorbances should agree with the

certified values within the uncertainties specified. Consult the manufacturer of the instrument if they do not.

REFERENCES

[1] Burke, R.W., Deardorff, E.R., and Menis, O., J. Research, Nat. Bur. Stand. 76A, 469-482 (1972).

[2] Mavrodineanu, R., J. Research, Nat. Bur. Stand. 76A. 405-425 (1972).

Note: References [1,2] are also published in NBS Special Publication 378, Accuracy in Spectrophotometry and

Luminescence Measurements, R. Mavrodineanu, J.I. Shultz, and O. Menis, Editors, U.S. (1973).

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I.S. Department of Commerce

C. William Verily Secretary Appendix 13

Naitnral Bumu ul Standard*

I- mm AmNrr. Dim**

Rational ^Suram of Standards

Certificate of ^nal^gis

Standard Reference Material 935a

Crystalline Potassium Dichromate

for Use as an Ultraviolet Absorbance Standard

This Standard Reference Material (SRM) consists of crystalline potassium dichromate of established purity. Solutions of

known concentrations of this SRM in 0.001 N perchloric acid are certified for their apparent* specific absorbances**, £

a

, at

This SRM is intended to be used as a reference standard for the verification of the accuracy and linearity of the absorbance

scale at 235, 257, 313, 345, and 350 nm of absorption spectrometers that can provide an effective spectral bandpass of 1.6 nm

or less. Such verification is accomplished by comparing the measured apparent absorbances, Aa

, to the Aa

calculated from

the certified ea values as described under "Instructions for Use."

Table 1 (shown on the next page) gives the certified values of ea

in kg-g^cnr1 for ten concentrations of the SRM 935a

potassium dichromate in 0.001 N perchloric acid at 23.5 °C and the indicated wavelengths and spectral bandpasses for a 1-cm

internal pathlength.

The sample preparations and technical measurements leading to the certification of this SRM were performed by R.W. Burke

of the Inorganic Analytical Research Division (now retired).

The technical and support aspects involved in the preparation, certification, and issuance of this Standard Reference Material were coordinated through the Office of Standard Reference Materials by RJL. McKenzie.

•The term "apparent" is used because no corrections have been applied to the data for the effects of internal multiple reflections

within the cuvette or for buoyancy, i.e., the weights used to express concentrations have not been corrected to vacuum. These

combined corrections do not exceed 0.2 percent. The specific absorbances are given in reference 1.

**The nomenclature used in this certificate is that recommended by K.D. Mielenz, Anal. Chem. 48, 1093-1094 (1976), which is reproduced in the Appendix of MBS Special Publication 260-54.

23.5 °C.

Gaithersburg, MD 20899

April 25, 1988

Stanley D. Rasberry, Chief

Office of Standard Reference Materials

(over)

n
Table 1. ea, Apparent Specific Absorbance, kg-g" 1 -cmNominal Wavelength and (Bandpass) nm

Concentration

g-kg" 1 235.0(1.2) 257.0(0.8) 313.0(0.8) 345.0(0.8)" 350.0(0.8) UncertaintyD

0.020 12.260 14.262 4.805 10.604 10.672 ± 0.034

0.040 12.304 14.318 4.811 10.603 10.682 ± 0.020°

0.060 12.347 14.374 4.816 10.602 10.692 ±0.020°

0.080 12.390 14.430 4.821 10.601 10.701 ± 0.020°

0.100 12.434 14.486 4.827 10.600 10.711 ± 0.020°

235.0(1.6)

d 257.0(1.6)

d 313.0(1.6)d 345.0(1.6)d 350.0(1.6)d

0.1 2QF 12.480 14.541 4.835 10.600 10.722 ±0.04f

0.140° 12.524 14.605 4.840 10.599 10.731 ±0.04f

0.160° 12.567 14.658 4.846 10.599 10.742 ±0.04f

0.180° 12.609 14.711 4.851 10.598 10.751 ±0.04f

0.200° 12.649 14.763 4.856 10.597 10.759 ±0.04f

a Wavelength 34S.0 nm is near one of the two isosbestic points in HCrQ^j/C^Q^ spectra. Because it is on the slope of the

composite spectrum, reproduction of the ea values is dependent on wavelength accuracy. Measurements at this wavelength

should be made only for verification of the linearity of the absorbance scale. b

£ a values are not corrected for the effects of internal multiple reflections within the cuvette, nor have the weights been

corrected to vacuum. With these two exceptions, each uncertainty given is the 95 percent confidence interval of the mean and

includes all known systematic errors.

c At wavelength 313.0 nm, the uncertainty is reduced to ± 0.010.

dThe increase in spectral bandpass was necessary to obtain an adequate signal. Expected changes in specific absorbance due to

this increase do not exceed 1 part in 1,000.

e e

a values are given to the third decimal place to preserve the smooth variation of the data with concentration, although the

uncertainties are in the second decimal place.

*At wavelength 313.0 nm, the uncertainty is reduced to ± 0.02.

PREPARATION AND CERTIFICATION

The preparation and certification of SRM 935a followed the procedures used for the preparation and certification of SRM 935,

which are described in detail in NBS Special Publication 260-54, Certification and Use of Acidic Potassium Dichromate

Solutions as an Ultraviolet Absorbance Standard[2]. This publication should be referred to every time SRM 935a is to be

used. Briefly, the transmittances, T, of the solutions prepared from the undried, as received, material were measured with the

NBS Institute for Materials Research high accuracy transmission spectrometer[3].

The ea values were calculated for each wavelength using the relation:

a bxc bxc w

where: ea = apparent specific absorbance

Aa = apparent absorbance

Ds = transmittance density of the sample solution, -logioTs Db = transmittance density of the blank solution, -logioT0

b = internal cuvette pathlength, cm

c = concentration, by weight, of K^Cr^Oy solution, g-kg-1

2
The crystalline potassium dichromate used for SRM 935a is a special lot of analytical reagent grade material obtained from

the J.T. Baker Chemical Co., Phillipsburg, NJ.

Assay: A coulometric assay of the purity of the undried material was performed by G. Marinenko of the NBS Center for

Analytical Chemistry. The purity was found to be better than 99.97 percent. In addition, the material was examined by

optical emission spectrometry for trace elemental impurities by J.A. Norris of the NBS Center for Analytical Chemistry.

The only significant impurities detected were sodium and rubidium. Their concentrations were estimated to be in the range of

0.02 and 0.03 weight percent, respectively. Drying at 105 °C for 12 hours showed that the surface moisture of this material

was less than 0.01 percent.

Stability: Solutions prepared from SRM 935a in the concentration range indicated in table 1 and made according to the

instructions given in NBS SP 260-54 have been found to be stable within the uncertainties given in table 1 for at least six

months when stored at room temperature and protected from evaporation and exposure to light.

INSTRUCTIONS FOR USE

The use of SRM 935a as an absorbance standard requires the careful preparation of a series of solutions of known

concentrations, c, of the potassium dichromate in 0.001 N perchloric acid. These solutions are transferred sequentially to a

quartz cuvette of known pathlength, b, and their apparent absorbances measured at wavelengths 235, 257, 313, and 350 nm,

using the spectral bandpass requirements given in table 1. The preparation and measurement of these solutions are described

in detail in Section 5 of NBS SP 260-54.

The accuracy of the absorbance scale of the spectrometer being tested is ascertained by comparing the measured apparent

absorbances, Aa

, of a series of 0.001 N perchloric acid solutions containing 0.020 to 0.200 gram K2Cr207/kg to the Aa values calculated from the certified ea values. Although the ea data in table 1 are given for nominal concentrations of 0.020,

0.040, 0.060, 0.080, 0.100, 0.12, 0.14, 0.16, 0.18 and 0.20 g K2Cr207/kg, the ea values for concentrations between these

nominal concentrations can be determined by linear interpolation. Using the appropriate ea

values, the calculated Aa values at 23.5 °C are obtained from the expression:

Aa = ea x b x c (2)

Calculations:

An example of the calculation of Aa for one concentration of K2Cr207 under a specified set of conditions is shown below.

Calculations of Aa for other concentrations and wavelengths are performed in a similar manner.

Conditions: Wavelength = 350 nm, spectral bandpass 0.8 nm or less

b = 0.9982 cm

c = 0.04375 g-kg- 1

t = 23.5 °C

From column 6, table 1, the ea for concentrations of 0.040 and 0.060 g-kg- 1 are 10.682 and 10.692, respectively. The

corresponding ea for c = 0.04375 g-kg- 1 is:

£a = 10682 + SW(1°-692 • ia682)

e

a = 10.682 + 0.0019

e

a = 10.684

3
The calculated apparent absorbance, Aa

, from equation 2, is:

Aa = 10.684 x 0.9982 x 0.04375

Aa = 0.4666

The uncertainty, AAa

, in the calculated Aa is determined from the combined uncertainties in e a

, b and c in equation 2,

provided no other systematic errors are present. Thus:

AAa = bclAea

l + e aclAbl + £ablAcl (3)

To evaluate AAa, A£ais taken from column 7 of table 1 and the Ab and Ac values must be determined experimentally.

In the experiments performed to obtain the ea values in table 1, the uncertainties for b and c did not exceed 1 part in 104 and 2

parts in 104

, respectively.

The solution of equation 3 gives:

AAa = 1(0.044) (0.020) + 10.7(0.044) (0.0001) + 10.7(1) (0.0000088) = 0.0010

Thus, the uncertainty of Aa

, for the above set of conditions, is ±0.0010.

The correction of the absorbance scale of the absorption spectrometer under test is determined by plotting the differences

between Aa measured and Aa calculated as a function of absorbance. A typical plot of such a graph is shown in figure 1. The

apparent absorbances measured on this instrument at 350 nm are accurate when the indicated correction is subtracted from the

corresponding absorbance scale reading, provided that the conditions of wavelength accuracy, spectral bandpass, and absence of

stray light are fulfilled as specified in NBS SP 260-54. Correction curves for wavelengths 235, 257, and 313 nm are obtained

in a similar manner.

10 r

13

U

<

ea

C9 <

0.2 0.4 0.6 0.8 1.0

Absorbance

Figure 1 . Correction curve for the absorbance scale of a precision spectrometer. The error bars are the sum of the errors

arising from the uncertainties in the certified apparent specific absorbances, ea, cuvette pathlength, b, and concentration, c.

4
Temperature Correction:

Although ea values in table 1 are certified at 23.5 °C, SRM 935a can be used as an absorbance standard at other temperatures

in the range 20 to 30 °C provided corrections are made to the ea values. Over this range the apparent specific absorbances

decrease linearly with increasing temperature for all the wavelengths given in table 1. The corresponding temperature

coefficients, k, for these wavelengths are given in table 2.

Table 2. Variation of ea with Temperature Over the Range 20 to 30 °C.

X, nm Temperature Coefficient, k

Percent per degree Celsius

235 -0.05

257 -0.05

313 -0.02

345 -0.08

350 -0.05

The value of eaat any temperature in the range 20 to 30 °C can be calculated from the certified value and the appropriate

temperature coefficient using the relation:

e

a = E

a

3 ' 5

( 1+

750

(t " 23- 5»

where: e

a = apparent specific absorbance at temperature t (°Q

23 5 e

a = apparent specific absorbance certified at 23.5 °C.

k = temperature coefficient, percent per °C.

ACKNOWLEDGEMENTS:

We wish to acknowledge the following contributions to the development of SRM 935, which was the basis of SRM 935a:

K.D. Mielenz, NBS Analytical Chemistry Division, for his many valuable discussions and suggestions, and H.H. Ku, NBS

Applied Mathematics Division, for his statistical treatment of the data; the cooperation and early support of the National

Institute of General Medical Sciences in the research leading to this SRM; and the encouragement of George N. Bowers, Jr., M.D., Hartford Hospital, Hartford, Conn., and Royden N. Rand, Ph.D., Eastman Kodak Co., Research Laboratories,

Rochester, N.Y.

REFERENCES:

1. Burke, R.W., and Mavrodineanu, R., Acidic Potassium Dichromate Solutions as Ultraviolet Absorbance Standards, J. Res. Nat. Bur.Stand. (U.S.), 80A (Phys. and Chem.), No. 4, 631-636 (July-Aug. 1976).

2. Burke, R.W., and Mavrodineanu, R., Certification and Use of Acidic Potassium Dichromate Solutions as an Ultraviolet

Absorbance Standard, NBS Spec. Publ. 260-54 (1977). Copies may be obtained from the Office of Standard Reference

Materials, National Bureau of Standards, Gaithersburg, MD 20899.

3. Mavrodineanu, R., An Accurate Spectrophotometer for Measuring the Transmittance of Solid and Liquid Materials, J. Res.

Nat. Bur. Stand. (U.S.), 76A (Phys. and Chem.), No. 5, 405-425 (1972).

5

S. Department ot Commen t luanita M. Kreps

Secretary

Appendix 14

Naiiunrtl Bureau ol Sunrinrds

Krnest Ambler. Director

^aitmtal ^

itram of jitatutartte

Standard Reference Material 2032

Crystalline Potassium Iodide

Heterochromatic Stray Radiant Energy Standard for

Ultraviolet Absorption Spectrophotometry

K. D. Mielenz and R. Mavrodineanu

This Standard Reference Material consists of reagent-grade, crystalline potassium iodide (KI) to be used to

assess heterochromatic stray radiant energy (stray light) in ultraviolet absorption spectrophotometers in the

spectral region below 260 nm. Stray light is assessed by measuring the spectral absorbance of aqueous solutions

of S RM 2032 of known pathlengths and concentrations, and comparing the result with the certified values ofthe

specific absorbance, t.

Specific Absorbance*

t (L g" 1 cm"

1 ), vs Wavelength, A (nm), at 23.5 °C

K 240 245 250 255 260 265 270 275

e 25.6 10.67 3.66 1.05 0.260 0.0560 0.0121 0.0031

""Specific absorbance" is defined here as absorbance per unit pathlength and unit concentration. The term "absorptivity" has been avoided

since it is ambiguously defined. See K. D. Mielenz, Anal. Chem. 4JJ, 1093-1094 (1976).

The estimated uncertainty of these values is ± 5%, which includes the random and systematic errors of the

calibration procedure, as well as variations due to possible instability of the KI. (See, Certification Procedure.)

The material used to produce SRM 2032 was obtained from the J. T. Baker Chemical Co., Phillipsburg, N.J., in

two bottles, as reagent-grade (99.8% purity) potassium iodide.

SRM 2032 was issued with the technical assistance of J. R. Baldwin, R. W. Burke, A. L. Cummings, B. I. Diamondstone, and G. A. Sleater, and under the overall direction of I. L. Barnes.

The technical and support aspects concerning preparation, certification, and issuance of this SRM were

coordinated through the Office of Standard Reference Materials by R. W. Seward.

Washington, D C. 20234 George A. Uriano, Chief

October 31, 1979 Office of Standard Reference Materials

(over)
Supplementary Information

Material Testing:

Material homogeneity was tested by measuring the absorbance of 1% aqueous solutions ofthe KI sampled from

the top, middle, and bottom of each bottle. These measurements, at 265, 267, and 270 nm, showed no evidence of

inhomogeneity. Tests for moisture content (Karl Fischer method) gave an average of 0.007%. This value was

sufficiently small that all certified data are based on weighed samples of the undried, as received, material.

Stability of the material to UV and visible light was tested by exposing the KI salt in a low-actinic glass bottle to

radiation from a 3 kW xenon-arc lamp at a distance of 25 cm for four days at room temperature. The average

absorbances at five wavelengths between 240 and 270 nm of aqueous solutions of the exposed material were

1.1% lower than those of solutions of the unexposed material. In view ofthe severity ofthis test, this change was

considered minor. Nonetheless, the 1.1% was included in the overall uncertainty of the certified values.

Certification Procedure:

The certified specific absorbances were measured in the NBS Center for Analytical Chemistry high-accuracy

spectrophotometer [1, 2], equipped with a deuterium lamp and UV averaging sphere [3, 4]. Thirteen aqueous

solutions of KI with concentrations ranging from 0.03 to 30 g L"

1 were prepared and measured against distilled

water, using standard fused-silica cuvettes with pathlengths between 10.001 and 10.009 mm . The absorbance

measurements were performed at the temperature, t = 23.5 ± 0.5 °C. A 0.2 nm bandpass was used, and

wavelength settings were made with an accuracy of 0.05 nm. The measurements were made on at least three

concentrations at each wavelength. These concentrations were chosen so that most absorbance readings fell between 0. 1 and 1.0 to minimize stray light and bandwidth errors. The data were found to obey Beer's law.

Thus, the equation,

e = A/cg (1)

was used to calculate the specific absorbances, e, from the measured absorbances, A, and the known values of

concentration, c, and pathlength, £ .

Limits to random error (two standard deviations) of these measurements (including the photometric

imprecision of the spectrophotometer, errors in the repositioning of cuvettes, concentration errors, pathlength

uncertainties, temperature errors, Beer's law uncertainties, and bandwidth errors) were determined to be ±

2. 4%. The error due to the 0.05 nm uncertainty of the wavelength calibration of the spectrophotometer is ± 1 . 1 %

The uncertainty of 5% stated on the face of this certificate represents the sum of these errors and the above

mentioned 1.1% uncertainty due to instability of the KI . No corrections were applied for the effects of internal

reflections inside the sample and reference cuvettes, nor were the weights corrected to vacuum. These and all other sources of error were considered negligible.

The temperature coefficient at 260 nm was determined to be

-f-l = 0.031 -C-.

(2)

For precision measurements, it is recommended that SRM 2032 be used in thermostated cuvettes at 23.5 °C, or

that a temperature correction according to Eq. (2) be applied.

b Issued by NBS as SRM 932. Quartz Cuvettes for Spectrophotometry.
Instructions for Use

Storage and Preparation:

SRM 2032 should be stored in the original, low-actinic glass bottle and the cardboard container in which it was

issued to protect it from unnecessary exposure to light and humidity. When so stored, the expected stability of

this material is at least three years. Thus, until additional stability data are obtained, this material should be used

within three years of the date of purchase.

All solutions prepared from SRM 2032 should be made in borosilicate glass containers using distilled water and

transfer pipettes (Pasteur type) of the same glass and fitted with rubber bulbs

c

. Use clean spectrophotometer

cuvettes, free of scratches, made of non-fluorescent fused silica, and fitted with ground-glass or Teflon stoppers

to minimize evaporation; or preferably use NBS SRM 932. Mark all cuvettes to assure the same orientation in

the spectrophotometer, and place them into their respective holders. Using transfer pipettes of the type

mentioned, rinse each cuvette several times with distilled water. Prepare a solution of KI in distilled water (e.g.

1%), fill the sample cuvette with the KI solution and the reference cuvette with distilled water. Measure the

absorbance, leaving the cuvettes in their holders, empty them (using the pipettes), and repeat the rinsing and

filling operations until constant absorbance readings are obtained. Fresh solutions should be made before every

test.

Measurements:

A 1% KI solution (c = 10 g L ') with a 1 cm pathiength exhibits a sharp cutoff in transmittance near 260 nm; i.e.,

it transmits more than 90% above 273 nm, but less than 0.01% below 258 nm. Therefore, with the

monochromator set for a wavelength below 260 nm, any appreciable amount of light detected is heterochromatic stray light, which consists of wavelengths above the cutoff. The amount of stray light in the

spectrophotometer at wavelength A may be determined from the equations:

T(A)+»<A>

(3)

1 + x(A)

xm _ im - T(X)

X(X) (4) ~ 1 - T'(A)

W

where:

x(A) is the stray light ratio,

T'(A) is the apparent transmittance*

T(A) is the true transmittance.

The stray light ratio, x(A), is the proportion of heterochromatic stray radiant energy in the spectrophotometer

for the wavelength setting A. Equation (4) forms the theoretical basis for the determination ofthe stray light ratio

by comparing apparent transmittance to true transmittance. For SRM 2032, the true transmittance, T(A), of KI

solutions may be calculated as:

T(A)=10-

€(X)Cg

where e(A) is the certified specific absorbance given on the face of this certificate.

c Soft glass containers and pipettes contain residual amounts of UV absorbing material, but can be used after proper cleaning. Several rinses,

first with isopropyl alcohol and then with distilled water, are generally adequate.
The apparent transmittances of KI solutions depend not only on the stray light ratio of the spectrophotometer,

but also on the wavelength accuracy and the spectral bandwidth, which can significantly affect the results

obtained because of the steep slope of the absorbance cutoff of KI. A low-pressure mercury discharge lamp is suggested for verifying the wavelength scale, and a bandwidth as narrow as compatible with adequate signal-tonoise ratios should be used.

Many instruments do not permit the direct measurement of transmittances below certain limits, e.g., 1% or

0. 1.. For such instruments, the use of SRM 2032 will only measure stray light ratios above these limits. To

measure stray light ratios below these limits, the reference beam of the spectrophotometer must be attenuated to

extend the transmittance scale into the low-transmittance region.

References

1. R. Mavrodineanu, J. Res. NBS 76A, 405^25 (1972).

2. J.F. Barkley, F. C. Ruegg, and R. Mavrodineanu, 29th Pittsburgh Conf., Abstract 452, Cleveland, Ohio,

(1978).

3. K. D. Mielenz, R. Mavrodineanu, and E. D. Cehelnik, J. Res. NBS 78A, 631-635 (1974).

4. K. D. Mielenz, R. Mavrodineanu, and E. D. Cehelnik, Appl. Optics 14, 1940-1947 (1975).
. S. Department of Commerce

Philip M. Klutznick

Secre,ary Appendix 15

National Bureau of Standards

Ernest Ambler, Director

^National ^mtnn of ^tamtards

(Eerttfttate

Standard Reference Material 2033

Crystalline Potassium Iodide with Attenuator

Heterochromatic andlsochromatic Stray Radiant Energy

Standard for Ultraviolet Absorption Spectrophotometry

K. D. Mielenz and R. Mavrodineanu

This Standard Reference Material consists of reagent-grade crystalline potassium iodide (KI) and a radiation attenuator

to be used to assess the heterochromatic stray radiant energy (stray light) in ultraviolet absorption spectrophotometers in

the spectral region below 260 nm.. The radiation attenuator consists of two semitransparent evaporated metal-on-fused

silica (non-fluorescent) filters, each having a nominal transmittance of 10%. One filter is mounted in a cuvette-style

holder that can be inserted into the sample compartment of the spectrophotometer. The other is mounted in one of the

two shutters of the holder. The use of this attenuator permits the expansion of the transmittance scale into the lowtransmittance region by providing a two-step attenuation of the reference beam of the spectrophotometer to about 1%.

The attenuator can also be used to assess isochromatic stray light by following the instructions given in this certificate.

The heterochromatic stray light is assessed by measuring the spectral absorbance of aqueous solutions of SRM 2033 of

known pathlengths and concentrations, and comparing the results with the certified values of the specific absorbance, e.

Specific Absorbance

e(Lg" cm"'), vs Wavelength, a (nm), at 23.5 °C

A 240 245 250 255 260 265 270 275

e 25.6 10.67 3.66 1.05 0.260 0.0560 0.0121 0.0031

"Specific Absorbance" is defined here as absorbance per unit pathlength and unit concentration. The term "absorptivity" has been avoided since it is ambiguously defined. See K. D. Mielenz. Anal. Chem. 48. 1093- 1094 ( 1976).

The estimated uncertainty of these values is ± 5%, which includes the random and systematic errors of the calibration

procedure, as well as variations due to possible instability of the KI. (See Certification Procedure.)

The material used to produce SRM 2033 was obtained from the J.T. Baker Chemical Co.. Phillipsburg, N.J., in two

bottles, as reagent-grade (99.8% purity) potassium iodide.

The transmittance of the filter mounted in the holder at A.255 nm is ; the transmittance of both filters at A.255 is . SRM 2033 was issued with the technical assistance of J. R. Baldwin, R. W. Burke, A. L. Cummings, B. I. Diamondstone,

and G. A. Sleater, and under the overall direction of E. L. Garner, NBS Inorganic Analytical Research Division.

The technical and support aspects concerning preparation, certification, and issuance of this SRM were coordinated

through the Office of Standard Reference Materials by R. W. Seward.

Washington, D.C. 20234 George A. Uriano, Chief

May 9, 1980 Office of Standard Reference Materials

(over)
Potassium Iodide Material Testing:

Material homogeneity was tested by measuring the absorbance of 1% aqueous solutions of the KI sampled from the

top, middle, and bottom of each bottle. These measurements, at 265, 267, and 270 nm, showed no evidence of

inhomogeneity. Tests for moisture content (Karl Fischer method) gave an average of 0.007%. This value was sufficiently

small that all certified data are based on weighed samples of the undried, as received, material.

Stability of the material to UV and visible light was tested by exposing the KI salt in a low-actinic glass bottle to radiation

from a 3 kW xenon-arc lamp at a distance of 25 cm for four days at room temperature. The average absorbances at five

wavelengths between 240 and 270 nm of aqueous solutions of the exposed material were 1.1% lower than those of

solutions of the unexposed material. In view of the severity of this test, this change was considered minor. Nonetheless,

the 1.1% was included in the overall uncertainty of the certified values.

Potassium Iodide Certification Procedure:

The certified specific absorbances were measured in the NBS Center for Analytical Chemistry high-accuracy spectrophotometer [1,2], equipped with a deuterium lamp and UV averaging sphere [3,4]. Thirteen aqueous solutions of KI

with concentrations ranging from 0.03 to 30 g L

1 were prepared and measured against distilled water, using standard

fused-silica cuvettes with pathlengths between 10.001 and 10.009 mm . The absorbance measurements were performed

at the temperature, t = 23.5±0.5°C. A 0.2 nm bandpass was used, and wavelength settings were made with an accuracy

of 0.05 nm. The measurements were made on at least three concentrations at each wavelength. These concentrations were

chosen so that most absorbance readings fell between 0. 1 and 1 .0 to minimize stray light and bandwidth errors. The data were found to obey Beer's law.

Thus, the equation,

e = A/cfi, (1)

was used to calculate the specific absorbances, e, from the measured absorbances, A, and the known values of concentration, c, and pathlength, ft .

Limits to random error (two standard deviations) of these measurements (including the photometric imprecision of the

spectrophotometer, errors in the repositioning of cuvettes, concentration errors, pathlength uncertainties, temperature

errors, Beer's law uncertainties, and bandwidth errors) were determined to be ± 2.4%. The error due to the 0.05 nm

uncertainty of the wavelength calibration of the spectrophotometer is ± 1.1%.

The uncertainty of 5% stated on the face of this certificate represents the sum of these errors and the above mentioned

1.1% uncertainty due to instability of the KI. No corrections were applied for the effects of internal reflections inside the

sample and reference cuvettes, nor were the weights corrected to vacuum. These and all other sources of error were

considered negligible.

The temperature coefficient at 260 nm was determined to be

— • ^ = 0.031 °C"'. (2)

e dt

For precision measurements, it is recommended that SRM 2033 be used in thermostated cuvettes at 23.5 °C, or that a temperature correction according to Eq. (2) be applied.

b Issued by NBS as SRM 932, Quartz Cuvette for Spectrophotometry.

SRM 2033

Page 2
Instructions for Use of Potassium Iodide

Storage and Preparation:

SRM 2033 should be stored in the original, low-actinic glass bottle and the cardboard container in which it was issued

to protect it from unnecessary exposure to light and humidity. When so stored, the expected stability of this material is at

least three years. Thus, until additional stability data are obtained, this material should be used within three years of the

date of purchase.

All solutions prepared from SRM 2033 should be made in borosilicate glass containers using distilled water and transfer

pipettes (Pasteur type) of the same glass and fitted with rubber bulbs

c

. Use clean spectrophotometer cuvettes, free of

scratches, made of non-fluorescent fused silica, and fitted with ground-glass or Teflon stoppers to minimize evaporation;

or preferably use NBS SRM 932. Mark all cuvettes to assure the same orientation in the spectrophotometer, and place

them into their respective holders. Using transfer pipettes of the type mentiond, rinse each cuvette several times with

distilled water. Prepare a solution of KI in distilled water (e.g., 1%), fill the sample cuvette with the KI solution and the

reference cuvette with distilled water. Measure the absorbance, leaving the cuvettes in their holders, empty them (using

the pipettes), and repeat the rinsing and filling operations until constant absorbance readings are obtained. Fresh

solutions should be made before every test.

Measurements:

A 1% KI solution (c = 10 g L ') with a 1 cm pathlength exhibits a sharp cutoff in transmittance near 260 nm; i.e., it transmits more than 90% above 273 nm, but less than 0.01% below 258 nm. Therefore, with the monochromator set for a wavelength below 260 nm, any appreciable amount of light detected is heterochromatic stray light, which consists of

wavelengths above the cutoff. The amount of stray light in the spectrophotometer at wavelength X may be determined

from the equations:

T(X) + x(X) (3)

T'(X) = —— — 1 + x(X)

I - T"(X)

where:

x(X) is the stray light ratio,

T'(X) is the apparent transmittance,

T(X) is the true transmittance.

The stray light ratio, x(X), is the proportion of heterochromatic stray radiant energy in the spectrophotometer for the

wavelength setting X. Equation (4) forms the theoretical basis for the determination of the stray light ratio by comparing

apparent transmittance to true transmittance. For SRM 2033, the true transmittance, T(X), of KI solutions may be

calculated as:

T(X) =

10-

t,Mcil

where e(X) is the certified specific absorbance given on the face of this certificate.

'Soft glass containers and pipettes contain residual amounts of UV absorbing material, but can be used after proper cleaning. Several rinses, first with

isopropyl alcohol and then with distilled water, are generally adequate.

SRM 2033

Page 3
The apparent transmittances of K.I solutions depend not only on the stray light ratio of the spectrophotometer, but also

on the wavelength accuracy and the spectral bandwidth, which can significantly affect the results obtained because of

the steep slope of the absorbance cutoff of K.I. A low-pressure mercury discharge lamp is suggested for verifying the

wavelength scale, and a bandwidth as narrow as compatible with adequate signal-to-noise ratios should be used.

Optical Attenuator:

Many instruments do not permit the direct measurement of transmittances below certain limits, e.g., 1% or 0. 1%. To

measure stray light ratios below these limits, the reference beam of the spectrophotometer must be attenuated to extend

the transmittance scale into the low-transmittance region.

As mentioned on the face of this Certificate, the optical attenuator consists of two filters, each of which has a nominal

transmittance of 10 percent. One filter is mounted in a metal holder that can be inserted in the sample compartment of

the spectrophotometer (5). This holder is provided with a front and rear shutter, one of which has a window in which the

second filter is mounted. The holder and shutters are flat black. This unit was produced in the NBS Instrument Shops.

Instructions for Use of the Attenuator

This unit can be used to attenuate the incident radiation in the reference beam of the spectrophotometer in two steps

by a total factor of about 100, corresponding to about 1 percent transmittance (6). To attenuate by a factor of 10, the

attenuator is inserted in the sample compartment of the spectrophotometer facing the incident beam, with both shutters

removed. A further attenuation by a factor of about 10 is obtained when the shutter carrying the second filter is inserted

in the holder, in front of the first filter. Under these conditions, and with an instrument that cannot measure transmittances lower than 1 percent, the use of the attenuator will permit heterochromatic stray light to be measured down to about 0.01 percent. This attenuation procedure can be used with spectrophotometers capable of scale expansion.

Isochromatic Stray Light:

The radiation attenuator can be used to assess the isochromatic stray light that results from reflection of the incident

radiation at the surface of the sample and various optical components, and reaches the photodetector without passing

through the sample. It is implicitly assumed that the sample compartment of the spectrophotometer is light-tight.

The measurement is performed by placing the attenuator in the sample beam of the spectrophotometer, with the

opaque shutter placed at the rear of the filter holder. Under these conditions, if a signal is detected, it is caused by

reflection at the surface of the filter exposed to the incident radiation. This radiation is scattered from the walls and other

components of the sample compartment and reaches the photodetector without passing through the sample (5). This

signal is the isochromatic stray light.

Isochromatic stray light, which passes through the sample, is generally caused by interreflections between lenses and/or

other sample compartment elements. Tests for this stray light component, which is not detected by the methods described

above, are discussed in references 7 and 8.

References:

1. R. Mavrodineanu, J. Res. NBS 76A, 405-425 (1972).

2. J. F. Barkley, F. C. Ruegg, and R. Mavrodineanu, 29th Pittsburgh Conf., Abstract 452, Cleveland, Ohio, (1978).

3. K. D. Mielenz, R. Mavrodineanu, and E. D. Cehelnik, J. Res. NBS 78A, 631-635 (1974).

4. K. D. Mielenz, R. Mavrodineanu, and E. D. Cehelnik, Appl. Optics 14, 1940-1947 (1975).

5. R. Mavrodineanu, J. Res. NBS 80A, 637-641 (1976).

6. Estimating Stray Radiant Energy, ASTM Manual on Recommended Practices in Spectrophotometry, pp. 94-105,

3rd. Edition, 1969, 1916 Race St., Philadelphia, Pa. 19103.

7. K. D. Mielenz, J. Res. NBS 76A, 455-467 (1972).

8. K. D. Mielenz and R. Mavrodineanu, J. Res. NBS 77A, 699-703 (1973).

SRM 2033

Page 4
S. Pepartrfijsn vbf Commerce MalcolartBaldrige

National puraMtol Standards

Ernest

Appendix 16

^National £3uram of j&tatulartte

Certificate

Standard Reference Material 2009a

Didymium Glass Filter for Checking the

Wavelength Scale of Spectrophotometers

SERIAL NO.

Kenneth L. Eckerle

This Standard Reference Material (SRM) is a 10 x 30 x 3 mm didymium glass filter. SRM 2009a is intended for use in calibrating the wavelength scale in the visible wavelength region of scanning spectrophotometers having nominal bandwidths in the range 1.5 to 10.5 nm. Depending upon the bandwidth of the spectrophotometer, 12 to 22 wavelength

corrections can be determined from 389 to 760 nm. Detailed instructions on the use of this SRM and examples of its use

are given in the accompanying NBS Special Publication 260-66. Each didymium glass filter is identified by serial number.

All measurements on these filters were made at 25 °C with a high-accuracy reference spectrophotometer that has a wavelength accuracy of 0.04 nm.

Table 1 gives the wavelengths of the transmittance minima as obtained from measurements on two filters representative

of this melt of didymium glass. These values are given for seven equally spaced values of the half-height width of

triangular passbands. The minima numbers are shown on Figure 1, which illustrates the spectral transmittance as a function of wavelength.

Table 2 gives the wavelength values of nine points of inflection on the spectral transmittance curve as obtained on two

filters. These inflection points are representative of this glass melt and are also shown on Figure 1. These inflection

points should only be used with the transmittance minima as described in Sections 2.2 and 2.3 in SP 260-66. Table 2 also

indicates the range of the measured wavelengths of the inflection points.

Table 3 gives the estimated random and systematic errors of the transmittance minima given in Table 1 (as obtained from

4 sets of measurements on a single filter).

Table 4 gives the spectral transmittance as a function of wavelength for a filter representative of this melt. These values

are not certified but are provided for use as specified in SP 260-66. They should not be used to check the photometric

scale of a spectrophotometer.

Trial calibrations made on several instruments, using both minima and inflection points, indicate that wavelength

corrections made with these SRM's can be accurate to 0.2 nm. The uncertainty of a calibration, however, will depend

upon the stability and other characteristics of a particular instrument.

This filter should be handled only by its edges; when not in use it should be stored in the box provided. If cleaning is necessary, wet the filter with water and rub gently with optical lens tissue soaked with a mild soap solution, rinse with

distilled water, rinse with isopropyl alcohol, and rinse again with distilled water. Dry after each rinsing by wiping lightly

with optical lens tissue.

July 25, 1984

Gaithersburg, MD 20899

Stanley D. Rasberry, Chief

Office of Standard Reference Material
Figure I

. Spectral transmittance of didymium glass as a function of wavelength. The numbers indentify minima, and the

letters identify selected inflection points.

LU

<

<

n

i

- 10.5 nm BANDWIDTH

I—I .5

in

o

t—i—i—i—I—i—i—i—i—I—i—i—i—i—j i—i—

1.5 nm BANDWIDTH

J I L 1

9-

j_j i 10J I I L 1 J_J L

400 500 600 700

WAVELENGTH [nm]

Page 2

SRM 2009a
TABLE 1

Certified Wavelengths (nm) of the Transmittance

Minima for the Indicated Bandwidths

Dane wiain

Minimum No.

1.5 nm 3.0 nm 4.5 nm 6.0 nm 7.5 nm 9.0 nm 10.5 nm

i2 431.58

JA

AAt\ 7ft 44U. to AAfi Oft A A 1 /4 1 44 1 .4

1

A A \ CtA AA1 1 Q AA*) 1 7 A A 1 07 1.0/

*fcJ ah fn All A1 All 1

1

4 /Z.J I

6 481.20 480.68 480.37 479.67 478.36 477.26 476.18

7 513.66 513.76 513.94 514.25 514.87

8 529.37 529.47 529.26 529.11 529.08 528.98 528.74

9 573.12 573.88 574.75 575.72

10 585.50 585.68 585.86 586.03 585.91 585.29 584.39

11 623.85 624.42

12 629.51 629.26 628.17 627.17 627.17

I 13 684.71 684.74 684.78 684.80 684.77 684.73 684.65

14 739.61 740.20 740.58 741.33 742.55 743.50 744.16

15 748.64 748.48

TABLE 2

Wavelengths and Transmittances at Nine Selected

Points of Inflection

Point

Identification

Wavelength

(nm)

Range*

(nm)

+

Transmittance

A 388.68 +.02

-.02

0.3027

B 429.67 +.06

-.10

.6268

C 450.21 +.03

-.03

.5569

D 485.75 +.08

-.12

.6210

E

t

536.39 +.04

-.06

.5597

F 568.40 +.06

-.03

.3969

G 599.57 +.02

-.02

.3375

H 733.55 +.05

-.03

.4493

I 757.22 +.01

-.02

.3959

The range of wavelengths within which the wavelength for the given trans- mittance will fall for symmetric triangular passbands with half-height bandwidths from 1.5 to 10.5 nm.

These values of transmittance are not certified.

Page 3

SRM 2009a
TABLE 3

Estimated Random and Systematic Errors of the Transmittance Minima

Nominal Standard Deviation for Indicated Bandwidth

Wavelength

Band of Minimum 1.5 nm 3.0 nm 4.5 nm 6.0 nm 7.5 nm 9.0 nm 10.5 nm

Number Transmittance

1 402 nm

2 431 0.013 nm

(0.06)* 0.010 nm

3 440

0.021

(0.05) 0.013 0.050 nm 0.028 nm 0.009 nm 0.005 nm

0.007 nm

(0.25)

4 446 --•

5 473

U.UI nnii1

(0.08) 0.012 0.032

6 481 0.015

(0.15) 0.013 0.009 0.009 0.009 0.014

0.017

(0.25)

7 513

0.022

(0.08) 0.016 0.014 0.013 0.010

8 530

A A 1 0 U.UI I

(0.21) 0.010 0.010 0.01

0.010 0.011 A A t A

(0.25)

9 573 A flflA

(0.05) 0.010 0.010 0.012 0.014

10 585

0.007

(0.06) 0.004 0.007 0.008 0.008 0.007

0.007

(0.25)

II 624

0.058

(0.06) 0.061

12 630

0.210

(0.02) 0.120 0.171 0.133 0.091

13 685

0.019

(0.05) 0.029 0.024 0.017 0.014 0.014

0.012

(0-25)

14 740

0.009

(0.25) 0.013 0.011 0.010 0.009 0.009

0.010

(0.25)

15 748

0.020

(0.04) 0.016

Values in parentheses are estimates of the systematic error. The estimates for the 1 .5 nm bandwidth were obtained from the data taken at

1.5 nm intervals as compared to data taken at 0. 1 5 nm intervals. The method of estimating the systematic errors for the 10.5 nm bandwidth

is described in Section 3.4 in SP 260-66.

Page 4

SRM 2009a
TABLE 4

Transmittance (T) as a function of wavelength from 380 to 780.5 nm for a bandwidth of 1.5 nm. The estimates of

random and systematic errors are also provided. These data are not certified.

Wavelength At At Wavelength At At

(nm) T Standard Error Systematic (nm) T Standard Error Systematic

3 80.0 0 •14580 •000027 .00010 381.50 .17003 .000030 •00010

383.00 .19554 •000013 .00010 384.50 •22249 •000015 •00010

386.00 •25140 •000030 •00010 387.50 • 28 048 •000036 •00010

389.00 .30929 •000054 .00010 390.50 .33718 •000059 •00010

392.00 •36465 .000036 .00010 393.50 .39159 .000027 •00010

395.00 .41903 •000065 .00010 396.50 •44370 •000034 •00010

39b.Q0 •46633 •000062 .00010 399.50 .48 705 •000055 •00010

4 01 .00 . 50766 •000057 .00010 402.50 •52624 •000068 •00010

4 04.00 •54799 •000057 •00010 405.50 •56861 •000032 •00010

407.00 .58745 •000094 .00010 408.50 • 60604 •000060 •00010

4 10.00 .62234 .0000 38 .00010 411,50 •63656 •000068 •00010

4 13.00 .64951 •C00059 .00010 414.50 •66131 •000065 •00010

416.00 .671 52 •000046 •00010 417.50 •67987 .00005 7 •00010

419.00 .68701 •000067 .00010 420.50 •69555 •000103 •00010

4 22.00 .70151 •000030 .00010 423.50 •70430 •000041 •00010

425.00 .70965 .000033 .00010 426.50 .71287 •000023 •00010

4 2b. 00 .702 85 •000049 .00010 429.50 .63752 •000022 •00010

431.00 .54947 .000024 •00010 432.50 .55 494 • 000013 •00010

4 34.00 .55895 •000046 •00010 435.50 •54368 .000026 •00010

4 37.00 .52045 •000033 .00010 438.50 •49497 • 000054 •00010

440.00 .47901 •000032 .00010 441.50 •47848 •900021 •00010

443.CC •48766 .000036 •0001G 444.50 .49576 •000022 •00010

4 46.0 0 • 499 20 •000024 .00010 447.50 •50825 •000046 •00010

449.00 •53230 .000030 .00010 450,50 •56323 •000034 • 00010

4 52 .00 .58958 •000041 .00010 453.50 .60902 •000039 •00010

4 55.00 •62269 •000032 •00010 456.50 .62779 •000015 •00010

4 56.00 •622 18 .000031 .00010 459.50 • 60829 .000019 •00010

461.00 .59143 •000041 .00010 462.50 •57854 •000026 •00010

4 64.00 .57327 .000014 •00010 465.50 .56913 •000036 • 00010

4 67.00 •55998 •0C0018 .00010 468.50 .54595 •000016 .00010

4 7C.00 .53141 •000028 •00010 471,50 •51638 •000030 •00010

473.00 .50873 •0G0021 .00010 474.50 •53083 •000021 •00010

476.00 .55022 •C00031 .00010 477.50 •52368 •000028 •00010

4 79.00 .50331 .000028 •00010 480.50 .49395 •C00037 •00010

482.00 .49597 •000022 •00010 483.50 .53422 •000031 • 00010

4 85.30 •59664 •000041 •00010 486.50 •65144 • 00 0 06 0 •00010

488.00 •693t8 •000055 .00010 489.50 .72897 .000037 •00010

491.00 .754 82 •000024 .00010 492.50 .76958 •000047 •00010

4 94 .00 .77582 •000044 .00010 495.50 .77650 •000051 •00010

497.00 .77208 .000054 .00010 498.50 .76286 .000025 •00010

Page 5

SRM 2009a
TABLE 4 (Continued)

Transmittance (T) as a function of wavelength from 380 to 780.S am for a bandwidth of 1.5 am. The estimates of

random and systematic errors are also provided. These data are not certified.

Wavelength AT At Wavelength At At

(nm) T Standard Error Systematic (nm) T Standard Error Systematic

5 00.0 0 .75007 • 000038 .00010 501.50 .73534 .000045 •00010

503.00 .71949 •000049 .00010 504.50 .70383 .000045 •00010

5C6.00 .68654 .000029 •00010 507.50 .66262 •000024 •00010

5 09.0 0 •62231 .000040 .00010 510.50 .56707 •000031 •00010

512.00 .51720 •000029 •00010 513.50 • 49346 •000056 • 00010

515.00 .50708 .000031 .00010 516.50 . 53946 .000027 •00010

518.00 •56851 •000043 .00010 519.50 •58209 •000030 •00010

521. CO •57281 •000Q36 •00010 522.50 .51543 •000052 •00010

524.00 •43289 •000017 .00010 525.50 .39072 •000015 •00010

527.00 .37679 •000020 .00010 528.50 .35483 •000018 •00010

5 30.00 •35279 .000016 .00010 531.50 •37015 •000025 • 00010

533.00 •39916 •000017 •00010 534.50 .45853 •00004 2 •00010

536.00 •54124 •000035 .00010 537.50 .61975 .000031 •00010

5 39.0 0 •68105 •000047 .00010 540.50 .72896 .000044 •0001Q

5 42.0 0 .76653 .000033 .00010 543.50 .79325 •000059 .00010

545.00 .81171 . 000029 .00010 546.50 •82444 •000040 •00010

548.00 .83233 •000043 •00010 549.50 .83615 •000081 • 00010

5 51 .00 .83719 .000045 •00010 552.50 •83660 • 000048 •00010

554.00 .83571 •000022 .00010 555*50 .83426 •000029 •00010

557.00 .83133 •000029 .00010 558*50 •82586 •000025 •00010

5 60.0 0 .81679 •000044 .00010 561.50 .80277 •000038 •00010

5 63.00 .780 36 •000034 •00010 564.50 .74242 •000038 .00010

5 66.0 0 •67208 •000029 .00010 567.50 •53386 •000026 •00010

569.00 .31171 •000019 .00010 570.50 •11909 •000013 •00010

572.00 •044 66 •000020 •00010 573.50 .03514 •000016 •00010

5 75.0 0 •04138 •000008 •00010 576.50 .04491 •000012 •00010

5 76.0 0 •05465 • 000013 •00010 579.50 .06821 •000009 • 00010

5 PI .00 •06696 •000015 .00010 582.50 •04148 •000013 •00010

5 84.0 0 •01915 •000006 •00010 585.50 .01262 •000006 •00010

587.00 .01731 .000007 •00010 588.50 •03031 .000012 •00010

590.00 •04805 •000019 .00010 591.50 •06187 •000007 •00010

5 93.0 0 •08229 •000008 .00010 594.50 ,12409 •000025 •00010

596.00 •18771 •000025 •00010 597.50 .25829 •000021 •00010

599.00 ,31676 • 000014. •00010 600 . 50 .37306 .000031 •00010

6C2.00 •43501 •000046 •00010 603.50 .49640 •000049 •00010

605.00 •S5363 .000037 •00010 606.50 .5992 5 •000042 •00010

603.00 .631 07 •000066 .00010 609.50 •65692 •000042 .00010

611 .00 •68896 •C00030 •00010 612.50 .73245 •000043 •00010

614.00 •78043 •000024 •00010 615.50 •82013 •00002 7 •00010

617.00 •64419 •000022 •00010 618.50 .85409 .000056 .00010

Page 6

SRM 2009a
TABLE 4 (Continued)

Trantmitunce (T) as a function of wavelength from 380 to 780.S nm for a bandwidth of l.S nm. The estimates of

random and systematic errors are also provided. These data are not certified.

Wavelength AT At Wavelength At At

(nm) T Standard Error Systematic (nm) T Standard Error Systematic

6 20.00 • 85400 •0000 56 •00010 621.50 .84 757 •000045 •00010

6 23.0 0 •84081 •000045 •00010 624.50 • 84027 .000029 •00010

6 26.00 •84212 •000048 •00010 627.50 •84185 • 000 048 • 00010

629.00 .83965 • 000035 .00010 630.50 •84039 •00004 7 •00010

632.00 •84500 •0C0046 •00010 633*50 •85109 . 000062 •00010

6 35.0 0 .85677 •000038 .00010 636.50 .86029 •000032 •00010

6 38.0 0 •86270 •000045 •00010 639.50 •86501 •000022 •00010

641 .00 •86684 •000035 •00010 642.50 •86847 •000026 •00010

6 4 4.00 •86991 •000050 • 000 10 645.50 •87114 •000069 •00010

647.00 •87189 •000053 • 000 10 648.50 •87232 •000097 •00010

6 5C.00 •87268 •000030 •00010 651.50 .87294 •000108 •00010

653.00 •8 73 22 •000052 .00010 654.50 .87322 .000064 •00010

6 56.0 0 .87320 •000078 •00010 657.50 .87276 •C00059 •00010

6 59 .00 •87216 •000017 •00010 660.50 •87106 .000029 •00010

6 6* .0 0 •86924 •000035 .00010 663.50 •86636 •000018 .00010

665.00 .86192 •000120 •00010 666.50 •85616 •OO00S4 •00010

668.00 •84947 •000030 .00010 669.5.0 .84273 •000043 •00010

671.00 •83688 .000020 •00010 672.50 .83157 •000038 •00010

674.00 •82668 •000032 .00010 675.50 •82088 •000031 • 00010

677.00 •81184 •000039 •00010 678.50 .79967 •00005 8 • 00010

680.00 •78648 .000047 •00010 681.50 .77033 .000071 •00010

683.00 .75090 .000047 •00010 684.50 .73735 •000031 •00010

686.00 .74509 .0000 39 •00010 687.50 .76335 •000040 •00010

689.00 .77874 •000044 •00010 690.50 •794 70 •000040 •00010

6 92.00 .81070 •000036 •00010 693.50 .82509 •000060 •00010

6 95.0 0 .83668 •000063 •00010 696.50 •84584 .000038 .00010

698.00 • 852 96 •000065 •00010 699.50 •85807 •000074 •00010

7C1.00 .86156 •000057 •00010 702.50 •86388 .000070 •00010

704.00 •86559 •ooooso •00010 7C5.50 .86695 .000087 •00010

707.00 .86777 •000188 •00010 708.50 •86848 • 000069 • 00010

710.00 •86848 •000063 •00Q10 711.50 •86809 .000113 •00010

713.00 •86701 •000069 •00010 714.50 .86522 •000073 •00010

7 16.00 •86278 •000142 .00010 717.50 •85960 •000047 •00010

7 19.00 .85552 •000123 .00010 720.50 .85002 •000116 •00010

7 22.0 0 •84260 .000084 .00010 723.50 .83304 •000031 •00010

725.00 .81990 .000114 .00010 726.50 .80031 .000070 •00010

726.00 .77176 .000077 .00010 729.50 .72617 •00005 7 •00010

7 31.00 .65583 •000101 .00010 732.50 • 54 906 .000059 .00010

7 34.00 .41047 .000065 .00010 735.50 •27146 .000070 •00010

737.00 .17783 . 000032 .00010 738.50 .14350 .000025 .00010

Page 7

SRM 2009a
TABLE 4 (Continued)

Transmittance (T) as a function of wavelength from 380 to 780.3 nm for a bandwidth of 1.5 nra. The estimates of

random and systematic errors are also provided. These data are not certified.

Wavelength At At Wavelength At At

(nm) T Standard Error Systematic (nm) T Standard Error Systematic

740.00 .14029 .000039 •00010 741.50 •14205 •000027 •00010

743.00 •15422 •000013 •00010 744.50 •17506 •000027 •00010

746.00 .17950 •000044 •00010 747.50 •16393 •000038 •00010

749.00 .15817 •000026 •00010 750.50 .17714 •000030 •00010

752.00 •21278 •000036 •00010 753.50 •75589 •000065 •00010

755.00 • 30634 •000047 .00010 756*50 • 36590 • 00005 0 .00010

7 58 .00 •42896 •000027 •00010 759.50 •48660 .000056 •00010

761.00 •53598 •000089 •00010 762.50 .57747 •000099 •00010

764.00 •61508 •00006S •00010 765.50 •65041 .000071 •00010

7c7.00 •68350 •000108 •00010 768.5 0 .71193 •000061 .00010

7 70.00 •73432 •000079 •00010 771.50 .75 005 •000086 •00010

7 73 .00- •76019 •000081 •00010* 774.50 • 76S05 •000141 • 00010

776.00 •764 38 .000145 •00010 777.50 •75658 • 000066 •00010

779. CO .74102 •000081 •00010 780.50 .71767 •000142 •00010

Page 8

SRM 2009a
Appendix 17

Naitnnal ilnsitiute of §>tan£>ari>s $c QJerfynnlngij

GJertiftratF

Standard Reference Material 2034

Holmium Oxide Solution Wavelength Standard from 240 to 650 nm

Series No. 94

This Standard Reference Material (SRM) is a certified transfer standard intended for the verification and calibration

of the wavelength scale of ultraviolet and visible absorption spectrometers having nominal spectral bandwidths not

exceeding 3 nm. SRM 2034 is batch-certified for wavelength location of minimum transmittance of 14 bands in the

spectral range from 240 to 650 nm for six spectral bandwidths from 0.1 to 3 nm.

SRM 2034 is an aqueous solution prepared to contain 4 percent by weight of holmium oxide (H02O3) in 10 percent

(V/V) perchloric acid (HCIO4). The solution is flame-sealed in a non-fluorescent, fused-silica cuvette of optical

quality by fusion of the tubular end of the cuvette. The square cuvette has a nominal 10 mm pathlength so that it

fits in the sample compartment of most conventional absorption spectrometers (see Note 1 under "Instructions for

Use").

Certification: The certified wavelengths are given in Table 1 of this Certificate and apply to Series No. 94 as well

as to all preceding series.

Intrinsic Wavelength Stability and Storage: The certified wavelengths listed in Table 1 in this Certificate are expected to be valid as long as the SRM 2034 solution is chemically stable. Wavelength verification measurements

have been performed in the NIST Inorganic Analytical Research Division on original SRM 2034 solutions

maintained under typical laboratory conditions. The results of these wavelength verification studies indicate that the SRM 2034 solutions are stable for at least 8 years. When not in use, SRM 2034 should be stored in the

container provided, and at a temperature between 20 - 30 °C.

CAUTION: SRM 2034 contains perchloric acid as well as holmium oxide which are considered hazardous.

Caution should be exercised when handling this SRM (see section on Safety Precautions).

The research, development, and initial production and certification of SRM 2034, including the investigations on

the various chemical and physical parameters that might influence the certified measurements and results, were

performed by V.R. Weidner, NIST Radiometric Physics Division, and R. Mavrodineanu, NIST Inorganic Analytical

Research Division.

The production and certification renewal of SRM 2034 were performed by J.C. Travis, M.V. Smith, and N.K.

Winchester, NIST Inorganic Analytical Research Division. The technical leadership for the production and

measurements leading to the certification of this SRM was provided by J.C. Travis.

The overall direction and coordination of technical measurements leading to certification was performed in the

NIST Inorganic Analytical Research Division by J.C. Travis and R.L. Waiters, Jr.

The support aspects involved in the issuance of this SRM were coordinated through the Standard Reference

Materials Program by J.C. Colbert.

Gaithersburg, MD 20899 Thomas E. Gills, Acting Chief

December 2, 1993 Standard Reference Materials Program

(Revision of certificate dated 2-3-92)

(over)
Certification Values

The transmittance spectrum of SRM 2034, referenced to air, for a 0. 1 nm spectral bandwidth, is illustrated in Figure

1 in this Certificate. The certified wavelengths of minimum transmittance, expressed in nanometers (nm), for 14

bands from 240 to 650 nm and for six spectral bandwidths from 0.1 to 3.0 nm are given below in Table 1. Wavelengths for band No. 10 for the three narrowest spectral bandwidths are not given because this band splits into

two transmittance minima for nominal spectral bandwidths less than 1 nm.

Table 1. Certified Wavelengths (nm) of Minimum Transmittance

of 14 Bands for SRM 2034 at Six Spectral Bandwidths, Referenced to Air

SRM 2034

Band Mo.

Spectral Bandwidth (nm)

0.1 0.25 0.5 1 2 3

1 240.99 240.97 241.01 241.13 241.08 240.90

2 249.83 249.78 249.79 249.87 249.98 249.92

3 278.15 278.14 278.13 278.10 278.03 278.03

4 287.01 287.00 287.01 287.18 287.47 287.47

5 333.47 333.44 333.43 333.44 333.40 333.32

6 1 345.55 345.55 345.52 345.47 345.49 345.49

7 361.36 361.35 361.33 361.31 361.16 361.04

8 * 385.45 385.42 385.50 385.66 385.86 386.01

9 416.07 416.07 416.09 416.28 416.62 416.84

10 451.30 451.30 451.24

11 467.82 467.82 467.80 467.83 467.94 468.07

12 485.28 485.28 485.27 485.29 485.33 485.21

13 536.54 536.53 536.54 536.64 536.97 537.19

14 640.51 640.49 640.49 640.52 640.84 641.05

Certification Uncertainty

The expanded uncertainty (U) for all of the wavelength values given in Table 1 is U = ±0.1 nm, determined from

the root-mean-square combination of component standard uncertainties (i.e., estimated standard deviations) and a "coverage factor" (k) of k=2. The coverage factor is based on the student's t-distribution for > 30 effective degrees

of freedom, to define an interval within which the unknown value of the band-minimum wavelength can be asserted

to lie with a level of confidence of approximately 95%. This uncertainty includes "Type A" components of

uncertainty, which are evaluated by statistical methods, and "Type B" components of uncertainty, which are evaluated by other means. Component standard uncertainties for SRM 2034 are uA = ± 0.025 nm for the

calibration of the NIST spectrophotometer wavelength scale against atomic spectral lines, UB1 = ± 0.03 for the

estimation of absorption band minima, and uB2 = ± 0.03 nm for possible wavelength shifts with temperature and

concentration of the solution. The methods used to combine standard uncertainties are described in NIST Technical

Note 1297. [1]

The certified wavelengths listed in Table 1 in this Certificate are not valid if SRM 2034 is used outside the range

of experimental conditions investigated. For example, the wavelengths of minimum transmittance of SRM 2034

for spectral bandwidths greater than 3 nm have not been evaluated.
Production and Certification Procedure

Specific details concerning the materials, instrumentation, and method used in the certification of SRM 2034, are given elsewhere. [2,3] NBS Special Publication 260-102 [2] discusses the influence of temperature as well as the

purity and concentration of the holmium oxide solution on the certified wavelengths. The procedures used for the

assessment of the wavelengths of minimum transmittance and the establishment of the accuracy of the wavelength

scale of the reference spectrometer used for the certification are also described in Special Publication 260-102.

Instructions for Use

To maintain the integrity of SRM 2034, the cuvette should be handled only by the tubular end or by its opposing

frosted sides. While not in use, SRM 2034 should be stored in the container provided.

SRM 2034 is to be carefully inserted in the cuvette holder of the sample beam within a lightproof sample

compartment of the spectrometer being tested (see Note 1 below). The desired bands are scanned to measure their

locations of minimum transmittance for known spectral bandwidth conditions. The transmittance spectrum is to be

acquired at a laboratory temperature of 25 ±5 °C, and is referred to air, i.e., no cuvette or blank is to be placed

in the reference position. The measured wavelength of the minimum transmittance of a specific band is compared

to the certified wavelength in Table 1 for that band; the certified wavelength for the spectral bandwidth that is most

representative of the spectral bandwidth of the spectrometer being tested is used as the reference value. Taking into

account the certification uncertainty of ± 0.1 nm for SRM 2034, any significant differences between the measured

and the certified wavelengths for the peaks measured may then be used to recalibrate the wavelength scale.

NOTE 1. If the cuvette is too tall for the sample compartment of the absorption spectrometer being tested, a piece

of black opaque cloth placed over the sample compartment can be used to provide a temporary lightproof enclosure

during the wavelength verification procedure.

Wavelength Verification

Good laboratory practice in the handling and storage of SRM 2034 is highly recommended to help maintain the

integrity of the certified wavelength values given in Table I in this Certificate. If the user determines at any time

that this SRM has been exposed to adverse conditions that could affect the chemical stability of the solution, and

perhaps invalidate the certified wavelength values, this SRM may be returned to the NIST Inorganic Analytical

Research Division for wavelength verification. Prior to return shipment of this SRM, however, the NIST Inorganic

Analytical Research Division should be contacted directly by telephone at (301) 975-4117 to obtain the necessary

information regarding recommended shipping instructions and verification measurement fees.

Safety Precautions

SRM 2034 contains perchloric acid as well as holmium oxide in cuvettes that have been individually vacuum-tested

for leaks. Should a leak in the cuvette subsequently develop, or if the cuvette is accidentally broken, the general

recommendation is to carefully treat the perchloric acid spill immediately with copious amounts of water. [4-6]

Specifically, the following remedial action should be taken:

"Perchloric acid spills should be diluted immediately with water, taken up with swabs (preferably

wool), and then washed with generous amounts of water. The swabs should also be washed with

water before discarding. Swabs of cotton or other cellulose material contaminated with perchloric

acid should be regarded as fire or explosion hazards if not washed thoroughly with large amounts

of water. "[5]
Acknowledgments

The initial research at NIST concerning the potential use of the H02O3-HCIO4 solution as a wavelength standard was performed by K.D. Mielenz of the NIST Radiometric Physics Division and R.A. Velapoldi of the NIST Surface

and Microanalysis Science Division.

The vacuum-testing and flame-sealing of the fused-silica cuvettes for this SRM were performed by J. Anderson,

of the NIST Fabrication Technology Division.

REFERENCES

[1] Barry N. Taylor and Chris E. Kuyatt, "Guidelines for Evaluating and Expressing the Uncertainty of NIST

Measurement Results," NIST Technical Note 1297, (1993).

[2] V.R. Weidner, R. Mavrodineanu, K.D. Mielenz, R.A. Velapoldi, K.L. Eckerle, and B. Adams, Holmium

Oxide Solution Wavelength Standard from 240 to 640 nm - SRM 2034, NBS Special Publication 260-102,

(1986).

[3] V.R. Weidner, R. Mavrodineanu, K.D. Mielenz, R.A. Velapoldi, K.L. Eckerle, and B. Adams, Spectral

Transmittance Characteristics of Holmium Oxide in Perchloric Acid, J. Res. Natl. Bur. Stds., Vol. 90,

No. 2, 115-125 (1985).

[4] J.C. Schumacher, Ed., Perchlorates: Their Properties, Manufacture and Uses, Reinhold Publishing Corp.,

New York, NY, p. 180 (1960).

[5] A. A. Schilt, Perchloric Acid and Perchlorates, The G. Frederick Smith Chemical Co., Publisher,

Columbus, OH 43223, p. 157 (1979).

[6] GFS Chemicals 90/91 Catalog, GFS Chemicals, Powell, OH 43605, p. 458.
U.S. Department of Commerce

Juanita M. Kreps

feCKtary

Appendix 18

National Bureau of Standards

Ernest Ambler, Director

^National ^Bureau of tandartte

Certificate

Standard Reference Material 936

Quinine Sulfate Dihydrate

R. A. Velapoldi and K. D. Mielenz

This Standard Reference Material is intended for use in the evaluation of methods and the calibration of

fluorescence spectrometers. It is certified for the relative molecular emission spectrum, E(X), in radiometric

units for a solution of 1.28 x 10 mol/L quinine sulfate dihydrate in 0.105 mol/L perchloric acid using an

excitation wavelength of 347.5 nm. The certified values of the molecular emission spectrum at 5 nm wavelength intervals from 375 to 675 nm are given in table 1. These values have been corrected for instrument and

sample parameters, including the spectral responsivity of the detection system, monochromator bandwidth,

photomultiplier tube nonlinearity, monochromator wavelength error, solvent refractive index, and cell window

transmittance. The relative standard error in E(X), RSE [E(X)], is given in table 1. The estimate of the

relative systematic error limits in the molecular emission spectrum, RSEL [E(X)], is also given in table 1 and

was determined by the addition of the absolute values of the estimated systematic errors. These relative error

limits include uncertainties in the calibration values for the spectral responsivity, the wavelength position of the

emission peak maximum, and in the corrections applied for instrument and sample parameters.

From the certified values of E(X), values may be calculated for the molecular emission spectrum in the various

photon, radiometric, wavelength, and wavenumber units using the following equation: [1,2]

These values have been calculated and are given in NBS Special Publication 260-64.

The technical emission spectrum, E

T

(X), i.e., the emission spectrum corrected for instrument parameters only,

is also given in SP 260-64. The quinine sulfate dihydrate used for SRM 936 was a special lot of material

obtained from the J. T. Baker Chemical Co., Phillipsburg, N.J.

The technical and support aspects concerning the preparation, certification, and issuance of this Standard

Reference Material were coordinated through the Office of Standard Reference Materials by T. W. Mears and

R. W. Seward.

Washington, D.C. 20234 George A. Uriano, Chief

April I, 1979 Office of Standard Reference Materials

E(X) =

Ep(X) E(V) Ep(?r)

X X

2 X3

(over)
Table 1. The Molecular Emission Spectrum, E(X), of Quinine Sulfate Dihydrate in 0.105 mol/L HCIO4, the

Relative Standard Error, RSE, and the Estimated Relative Systematic Error Limits, RSEL, in the

E(X) Values.

X,nm E(X) RSE rE(X)l RSEL rEfXVI

375.0 0.005 0.019 0.087

380.0 .012 .006 .078

385.0 .028 .003 .071

390.0 .057 .003 .064

395.0 .103 .002 .059

400.0 .170 .002 .054

405.0 .257 .003 .049

410.0 .359 .003 .045

415.0 .471 .003 .041

420.0 .586 .003 .037

425.0 .694 .003 .034

430.0 .792 .002 .031

435.0 .874 .002 .028

440.0 .940 .001 .026

445.0 .984 .001 .024

450.0 .999 .001 .023

455.0 .997 .001 .023

460.0 .982 .001 .024

465.0 .947 .001 .024

470.0 .897 .001 .025

475.0 .838 .002 .026

480.0 .782 .002 .027

485.0 .719 .002 .027

490.0 .657 .002 .027

495.0 .595 .003 .027

500.0 .541 .002 .027

505.0 .486 .001 .028

510.0 .434 .003 .028

515.0 .386 .003 .028

520.0 .342 .002 .028

SRM 936

Page 2

X,nm E(X) RSE ****** rE(X)l L V /J RSEL [E(X)]

525.0 0.302 0.001 0.029

530.0 .264 .003 .029

535.0 .231 .003 .029

540.0 .201 .002 .029

545.0 .175 .002 .029

550.0 .153 .001 .029

555.0 .132 .001 .029

560.0 .116 .001 .029

565.0 .101 .002 .029

570.0 .088 .002 .029

575.0 .076 .003 .029

580.0 .065 .003 .029

585.0 .057 .001 .029

590.0 .050 .003 .030

595.0 .043 .004 .030

600.0 .037 .006 .030

605.0 .032 .002 .030

610.0 .028 .006 .030

615.0 .024 .003 .030

620.0 .021 .011 .030

625.0 .018 .003 .030

630.0 .016 .015 .030

635.0 .014 .014 .030

640.0 .011 .037 .030

645.0 .010 .015 .030

650.0 .009 .027 .030

655.0 .008 .035 .031

660.0 .007 .073 .031

665.0 .006 .046 .032

670.0 .005 .053 .032

675.0 .004 .065 .033
SUPPLEMENTARY DATA

The following data for the specific molar absorbances, water content, photon yields, and fluorescence lifetimes are considered to be supplementary and are not to be considered certified values.

The quinine sulfate dihydrate (QSD) used for SRM 936 was found to be homogeneous to better than 0.5% by

thin-layer chromatography with development by two solvent systems and the determination of specific molar

absorbances, e, at three different wavelengths. The SRM contains approximately 1.7% of an impurity as determined by high performance liquid chromatography using absorbance and fluorescence detection. This

impurity is believed to be dihydroquinine sulfate dihydrate, which has optical characteristics that are similar to those of the quinine sulfate dihydrate. The ultraviolet absorption spectrum of SRM 936 in 0. 105mol/L HC104 exhibits the following absorption maxima:

250.0 nm, ema* = 56,990 ± 90 Lmof'cm" 1

and, on the side of a peak:

347.5 nm, emax = 10,810 ± 20 Lmof' cm"

1

365.0 nm, Cobs = 6,920 ± 10 L mol cm"

The water content of this material was measured by two methods. The average of six determinations by the

Karl-Fischer method gave a value of (4.74 ± 0.05%), while the average of four determinations by a weight loss

procedure gave a value of (4.57 ± 0.04%). The theoretical value for water in quinine sulfate dihydrate is 4.60%.

The photon yield, Q, and the fluorescence lifetime, r, of SRM 936 were compared to values obtained for a sample of purified quinine sulfate dihydrate and are summarized below:

Q t, ns

0.5 mol/L H2 S04 0.5 mol/L H2SO4

SRM 936, QSD 0.544 ± 0.03 19. 1 ± 0.

1

Purified QSD 0.546* 19.2 ±0.1

a Melhuish, W. H., J. Phys. Chem. 65, 229 (1961); ibid, New Zealand J. Sci. Tech. 37, 142

(1955).

SRM 936

Page 3

(over)
PREPARATION AND USE OF SRM 936

This Standard Reference Material is for "in vitro" diagnostic use as a clinical laboratory standard. A "stock"

standard solution containing 0. 1 mg/mLof quinine sulfate may be prepared as follows: Weigh 0. 100 g of SRM

936 to the nearest one-tenth milligram and quantitatively transfer it to a 1000-mL volumetric flask. Dilute to the

calibrated volume with 0.1 05 mol/L HC104,to give a solution that is 1.28 x 10

4 mol/ L (0. 1 mg/mL) in quinine

sulfate. Store this solution in the dark in a well-stoppered, glass bottle. A "working" standard solution

containing 1 ng/mL may be prepared by transferring 10 raL of the above "stock" standard solution to a 1000-mL vojumetric flask and diluting to the calibrated volume with 0. 105 mol/LH004 to give a solution that

is 1.28x 10 mol/L (1 ng/mL) in quinine sulfate. Store this solution in the same manner as the above "stock"

standard solution.

Several opinions regarding the stability of quinine sulfate solutions have appeared in the literature [3]. NBS

considers the 0.1 mg/mL "stock" standard solution prepared from SRM 936 to be stable for 3 months when

stored as specified; and the 1 tig/mL "working" standard solution to be stable for 1 month when so stored.

SRM 936 should be kept in its original bottle and stored in the dark at room temperature (30 °C or less). It should not be subjected to heat or direct sunlight during storage. Experience at NBS indicates that under proper

storage this material is stable for at least 3 years. If this material degrades beyond the limits certified, purchasers

will be notified by NBS. It is recommended that the material not be used after 3 years from the date of purchase.

References:

[1] Ejder, E. J., J. Opt. Soc. Amer. 59, 223 (1969).

[2] Melhuish, W. H., J. Res. Nat. Bur. Stand. (U.S.) 76A, No. 6, 547 (1972).

[3] Melhuish, W. H., J. Phys. Chem. 65, 229(1961);Gill,J.E.,Photochem.andPhotobiol.9,313(1969);Birks,

J. B., J. Res. Nat. Bur. Stand. (U.S.) 80A, 389 (1976); Heller, C. A., Henry, R. A., McLaughlin, B. A., and

Bless, D. E., J. Chem. Eng. Data 19, 2 14 ( 1974); West, M. A., and Kemp, D. R., Int'l. Lab., p. 27 (May/June

1976); and White, J. U., Pittsburgh Conf. Abstracts, Paper 488 (1977).

This Standard Reference Material has been measured and certified at the laboratories of the National Bureau

of Standards, Gaithersburg, Maryland. All inquiries should be addressed to:

Office of Standard Reference Materials

Room B311, Chemistry Building

National Bureau of Standards

Washington, D.C. 20234

The date of issuance and certification of SRM 936 was April 1, 1979

SRM 936

Page 4
Appendix 19

National SttBtttute nf Staniiariifi & (Sectinnlogg

derttficate

Standard Reference Material 1931

Fluorescence Emission Standards for the Visible Region

This Standard Reference Material (SRM) is intended for use in the evaluation and calibration of the relative spectral response

of fluorescence spectrometers. It consists of four fluorescence standards and a "blank" specimen mounted in anodized

aluminum cuvette-sized holders. The fluorescence standards are certified for the relative corrected emission spectrum, E(X) in energy/wavelength units. The certified values of the blue, green, yellow and orange emission spectra at 2 nm wavelength

intervals are given in Tables I, II, in and IV, respectively. The fluorescence emission standards are:

Fluorescence Excitation Emission

Emission Standard Wavelength, nm* Wavelength Range, nm*

B (Blue) 340 400 - 550

G (Green) 280 490 - 600

Y (Yellow) 410 490 - 740

O (Orange) 235 530 - 740

"Excitation and emission monochromator bandpass set at 5.3 nm, temperature was 25'C

The fluorescence emission standards are composed of inorganic phosphors (10% by weight) and polytetrafluoroethylene

(FI FE) which were pressed into wafers and sintered. The phosphors selected emit in the blue, green, yellow or orange

wavelength regions (Figure 1). The PTFE wafer is mounted in an anodized aluminum cuvette-sized holder. The fluorescence

excitation beam angle of incidence is 60 degrees from the normal to plane of the sintered wafer. SRM 193 1 L and R are oriented for left and right-handed spectrofluorimeters, respectively, as defined by viewing the fluorescent sample along the

excitation beam. If the first emission optical component is located to the right of the sample then this orientation is designated R and if it is to the left then it is designated L. The anodized holder is engraved with the first letter of the color of

the emission (B, G, Y or O) over the excitation beam aperture. A PTFE blank in a non-engraved holder is also included in the set for diagnostic purposes.

The technical measurements leading to certification of spectral emission were performed by A. Thompson and K.L. Eckerle of

the Radiometric Physics Division.

The overall direction and coordination of this project was provided by K.D. Mielenz and JJ. Hsia, NIST Radiometric Physics

Division.

The technical and support aspects involved in the issuance of this Standard Reference Material were coordinated through the

Office of Standard Reference Materials by RX. McKenzie.

Gaithersburg, MD 20899 Stanley D. Rasberry, Chief

August 15,1 989 Office of Standard Reference Materials

(Over)
CALIBRATION MEASUREMENTS: Calibration measurements were done with s emission polarization (perpendicular to the plane of incidence at the monochromator grating) to minimize polarization errors. Ten spectra were measured for each

phosphor standard in the master set, except for the yellow phosphor for which only nine spectra were measured. In an

individual emission spectrum, the fluorescence value at each wavelength was the mean net signal from 100 measurements of

the fluorescence signal and ten dark measurements. These values have been corrected for the following instrument parameters:

spectral responsivity of the detection system, photomultiplier tube nonlinearity, and monochromator wavelength error. The

corrected values are used to calculate the relative emission spectrum, E(X), which is presented in Tables I, II, III, and IV. The

total standard error , TSE[E(X)], (3 sigma) of E(X) is given in Tables I, II, III, and IV. These error limits include quadrature sums of random and systematic uncertainties for instrument parameters, the measurement of the emission spectra, and the

measurement of the calibration values for the spectral responsivity. In addition to the master set calibration, commercial

spectrofluorimeter measurements were carried out on 18 sets out of SO sets to estimate the uniformity of the SRM 1931 batch

preparation. Variation in the standards emission spectra due to: UV instability, long-term stability, sample nonuniformity,

sample to sample variation, temperature, excitation wavelength and excitation and emission polarization were estimated and

their values will be given in an NIST 260-series Special Publication (currently in preparation). Values for the emission

spectra in photon, wavelength and wavenumber units will also be presented in this 260-series Special Publication.

STORAGE AND USE OF SRM 1931 : SRM 1931 should be stored in the aluminum container supplied in a location with

stable temperature and humidity. The PTFE wafer will readily adsorb tobacco smoke and other organic aerosols. To avoid the adsorption of fluorescent contaminants SRM 1931 should not be exposed to these environments. The left or right-handed

orientation of the wafer should not be changed by the user, if reorientation is necessary the standards should be returned to the

NIST Radiometric Physics Division (534). The measured corrected emission spectra of SRM 1931 is certified at a bandpass

of 5.3 nm. A procedure to correct these spectra for bandpass error is used in the 260-series Special Publication (in

preparation) to calculate the emission spectra at zero bandpass.

The measurement of SRM 1931 is a front surface fluorescence measurement, Therefore the proper alignment of the excitation

and emission optical paths is essential, otherwise the emission monochromator may view only the fringe of the emission

spot or a non-emitting portion of the wafer. The cuvette holders of commercial spectrofluorimeters have a degree of

positional uncertainty in the placement of the cuvette. This allows one to place the standard in the holder and to adjust the

position of the cuvette in the excitation beam until the fluorescence signal is maximized. Typically this is done with the

emission monochromator near the fluorescence maximum. It is also necessary to filter out the excitation light from the

emission monochromator. A simple UV glass filter is sufficient for this purpose and prevents first and second order scattered

excitation radiation from distorting the emission spectra. This filter must then remain in the instrument for all subsequent

fluorescence measurements for which the calibrations generated using these standards are to be used.

The calibration of the yellow master standard required over 80 hours of measurement time. After this long measurement time

the standard was observed to have darkened in the region of the excitation spot accompanied by a diminution of the

fluorescence signal intensity and a slight change in the shape of the emission spectrum for wavelengths less than 570 nm.

Therefore as a conservative precaution, the certified values for the yellow fluorescence standard relative emission spectra

encompass the wavelength range from 570 to 740 nm with the values less than 570 nm included for information only. For

high accuracy calibrations it is recommended to use the certified values of the green and the orange standards to cover the

excluded wavelength region of die yellow fluorescence standard. In typical calibrations the average measurement time will be on the order of several minutes and the small changes observed in the emission spectrum of the yellow master would entail

little problems in accuracy. A more detailed treatment of the errors involved will be found in the 260-series Special

Publication. For optimum accuracy in the calibration of a spectrofluorimeter, it is recommended to use the standards at wavelengths where the relative emission is greater than 0. 10 whenever possible.
Table I. The relative corrected emission spectrum, E(X), of the blue phosphor, the total standard error in E(X), TSE[E(X)] (3

sigma), these error limits include random and systematic uncertainties added in quadrature.

X. nm EI2Q TSErm)i

400 0.04878 0.00123

402 0.05966. 0.00127

404 0.07666 0.00146

406 0.09738 0.00092

408 0.12213 0.00092

410 0.15203 0.00098

412 0.18512 0.00112

414 0.22375 0.00122

416 0.26715 0.00194

418 0.31380 0.00125

420 0.36546 0.00143

422 0.41954 0.00226

424 0.47674 0.00202

426 0.53385 0.00206

428 0.59184 0.00144

430 0.65132 0.00149

432 0.70840 0.00170

434 0.76155 0.00164

436 0.81095 0.00212

438 0.85735 0.00261

440 0.89676 0.00169

442 0.93083 0.00266

444 0.95797 0.00248

446 0.97772 0.00250

448 0.99213 0.00318

450 0.99757 0.00215

452 1.00000 0.00242

454 0.99535 0.00378

456 0.98189 0.00356

458 0.96785 0.00471

460 0.94216 0.00347

462 0.91896 0.00475

464 0.88925 0.00378

466 0.85539 0.00433

468 0.82415 0.00435

470 0.78743 0.00421

472 0.75231 0.00490

474 0.71694 0.00511

X. nm E&l

476 0.67794 0.00506

478 0.64256 0.00477

480 0.60298 0.00446

482 0.56859 0.00455

484 0.53476 0.00486

486 0.49894 0.00408

488 0.46922 0.00440

490 0.43649 0.00373

492 0.40798 0.00370

194 0.38092 0.00403

4% 0.35374 0.00372

498 0.32907 0.00324

500 0.30583 0.00330

502 0.28345 0.00332

504 0.26244 0.00235

506 0.24259 0.00229

508 0.22577 0.00215

510 0.20872 0.00209

512 0.19354 0.00206

514 0.17914 0.00189

516 0.16503 0.00173

518 0.15332 0.00174

520 0.14130 0.00155

522 0.13065 0.00141

524 0.12070 0.00133

526 0.11112 0.00115

528 0.10303 0.00108

530 0.09498 0.00102

532 0.08794 0.00098

534 0.08111 0.00093

536 0.07475 0.00084

538 0.06920 0.00082

540 0.06366 0.00069

542 0.05892 0.00069

544 0.05433 0.00064

546 0.04996 0.00055

548 0.04629 0.00054

550 0.04254 0.00051

3
Table II. The relative corrected emission spectrum, E(X), of the green phosphor, the total standard error in E(X), TSE[E(X)] (3

sigma), these error limits include random and systematic uncertainties added in quadrature.

X. nm E(X) TSEfEfVH

490 0.07105 0.00066

492 0.09572 0 00090

494 0.12615 0 00136

496 0.16349 0\J • 00181 \J\J AO*

498 0 20628 0 00205

son 0 00252

502 0 11545 0 00309

J\rr 0 17757 0 001 54

0 44751 n 00440

508 0 52091 0 00606

510 0 60521 0 00607

512 0 68943 0\J*\J\J 00731 1 J 1 514 0.77264 0 00824

516 0 84898 0 00861

518 0 91209 0 01006

520 0.95862 0.01030

522 0.98784 0.01041

524 1.00000 0.01050

526 0.99402 0.01001

528 0.97492 0.00966

530 0.94302 0.00977

532 0.90401 0.00992

534 0.85611 0.00934

536 0.80295 0.00863

538 0.75121 0.00847

540 0.69238 0.00731

542 0.63746 0.00718

544 0.58267 0.00675

X. nm ECk\ Mm TSEfEflOl

546 0 52942 0 00568

548 0 48144 0 00542

550 0 43342 0 00500

552 0 30022 0 00488

554 0 15026 0 00412

J556JO 0 00170

558 0 28026 0 00178

560 0 00285

562 0 221 51 fl 00251

Jut 564 0 19602 0 00212

566 0 17425 0 00175

568 0 15547 0 00169

570 0 13734 0 00145

572 0 12177 0 00128

574 0 10801 0U.W001ill17

576 0.09537 0.00101

578 0.08408 0.00089

580 0.07485 0.00080

582 0.06643 0.00070

584 0.05896 0.00061

586 0.05208 0.00057

588 0.04663 0.00046

590 0.04122 0.00045

592 0.03666 0.00039

594 0.03284 0.00041

596 0.02912 0.00034

598 0.02603 0.00033

600 0.02330 0.00034
Table III. The relative corrected emission spectrum, E(X), of the yellow phosphor, the total standard error in E(X), TSE[E(X)1

(3 sigma), these error limits include random and systematic uncertainties added in quadrature. The values in italics (wavenlengths 490 to 568 run) are not certified, and are presented for purposes of Information Only.

X. nm mi TSEIE(X)]

4Q0 fyu 0 0^77 0 00300

4Q? 0 06141 0 00276

dQd u.u/ jyo o oo?qs

tyQ 0U.UOUJ OR/ISJ? o oo * tyo o innd? fi 0014 J son 0\j . 1

1 ilfiQQ uyy fi 00166

U.I JJUO fi fifidfi 1 S04 0 1S411 fi 00427

506 0 17543 0 00452

508 0.19787 0 00452

510 0.22300 0 00461

512 0.24977 0.00503

514 0.27833 0.00536

516 0.30845 0.00528

518 0.34007 0 00592

520 0 37325 0 00582

522 0.40713 0.00599

524 0.44221 0.00616

526 0.47887 0.00619

528 0.51535 0.00644

530 0.55208 0.00695

532 0.58959 0.00758

534 0.62623 0.00774

536 0.66228 0.00777

538 0.69798 0.00842

540 0.73159 0.00823

542 0.76552 0.00909

544 0.79626 0.00936

546 0.82586 0.00920

548 0.85557 0.00984

550 0.88183 0.01036

552 0.90526 0.01129

554 0.92653 0.01133

556 0.94435 0.01132

558 0.96078 0.01106

560 0.97309 0.01119

562 0.98352 0.01114

564 0.99064 0.01060

566 0.99277 0.00961

568 1.00000 0.01066

570 0.99841 0.01034

572 0.99498 0.01055

L_nm mi ISEmn

J"S74/* 0 Q01 1

5

U.77 1 i J fi 0 1 080

J /o fi 08701 fi fi1fi57

J578lo A 07^75 fi filfYJI con joU fi QAAAQ fi fiififi*; 587 fi AAQ78

Jot 584 U.7JJ lo fi 0D0Q8

Joo fi Q1 577 fi fifiORfi Joo 588 fi RQ850 fi fifi048 son n 87710 0 00935

592 0 85788 0 00869

594 0.83751 0 00906

5% 0 81368 0 00899

598 0.79210 0 00839

600 0 76586 0 00814

602 0 74265 0 00801

604 0 72051 0.00795

606 0.69391 0 00763

608 0.67077 0.00776

610 0.64501 0.00762

612 0.62066 0.00741

614 0.59752 0.00714

616 0.57128 0.00685

618 0.54897 0.00678

620 0.52502 0.00675

622 0.50147 0 00639

624 0.47867 0.00595

626 0.45644 0.00563

628 0.43651 0.00550

630 0.41521 0.00561

632 0.39552 0.00496

634 0.37593 0.00475

636 0.35701 0.00465

638 0.34006 0.00442

640 0.32249 0.00424

642 0.30667 0.00426

644 0.29031 0.00399

646 0.27481 0.00381

648 0.26146 0.00354

650 0.24707 0.00354

652 0.23405 0.00330

654 0.22145 0.00319

656 0.20956 0.00317

L nm TSErE(X)i

\J JO fi 10847 fi fifi?01 AAA fi 187SS fi fifi78ft

fi 177fifi fi fif)7 77

uw fi 168fiA fi wyxx

uw fi 1S8S4 fi fifi741

V7\JO fi 1 Sfi47 fi fifi774

67fi fi 14205 fi fifi216 672 fi 13436 fi\J .\J\J 00210 4* 1 V-/ 674 0.12702 0 00190

676 0 12001 0 00175

678 0.11394 0.00176

680 0.10745 0.00144

682 0 10166 0.00145

684 0.09645 0.00138

686 0.09115 0.00131

688 0 08665 0.00129

690 0 08218 0 00122

692 0.07789 0.00111

694 0.07417 0.00105

696 0.07037 0.00105

698 0.06704 0.00104

700 0.06385 0.00094

702 0.06064 0.00102

704 0.05762 0.00082

706 0.05502 0.00075

708 0.05264 0.00090

710 0.05011 0.00067

712 0.04795 0.00074

714 0.04574 0.00065

716 0.04369 0.00077

718 0.04205 0.00064

720 0.04028 0.00057

722 0.03854 0.00057

724 0.03698 0.00062

726 0.03504 0.00063

728 0.03372 0.00055

730 0.03265 0.00049

732 0.03122 0.00058

734 0.03003 0.00059

736 0.02896 0.00049

738 0.02809 0.00062

740 0.02681 0.00053

5
Table IV. The relative corrected emission spectrum, E(X), of the orange phosphor, the total standard error in E(X), TSE[E(X)]

(3 sigma). these error limits include random and systematic uncertainties added in quadrature.

X. nm Em TSErEfXIl

530 0.05955 0.00078

532 0.07052 0.00084

534 0.08308 0.00099

536 0.09717 0.00117

538 0.11198 0.00134

540 0.12838 0.00141

542 0.14553 0.00169

544 0.16366 0.00201

546 0.18253 0.00204

548 0.20208 0.00234

550 0.22286 0.00266

552 0.24360 0.00315

554 0.26466 0.00335

556 0.28691 0.00359

558 0.30904 0.00375

560 0.33159 0.00399

562 0.35557 0.00419

564 0.37951 0.00412

566 0.40434 0.00394

568 0.43125 0.00474

570 0.45850 0.00486

572 0.48691 0.00510

574 0.51608 0.00547

576 0.54674 0.00563

578 0.57758 0.00587

580 0.61010 0.00608

582 0.64235 0.00631

584 0.67579 0.00672

586 0.70836 0.00667

588 0.73973 0.00674

590 0.77222 0.00705

592 0.80306 0.00741

594 0.83334 0.00813

596 0.86123 0.00844

598 0.88814 0.00846

L nm E(k) TSErEmi

600 0.91150 a aar»/% ^ 0.00822

602 0.93258 A aaoa v 0.00891

/"A * 604 0.95190 0.00964

606 0.96739 A AAA 4 a

0.00940

608 A AOA A 1 0.98043 0.01011

610 0.98893 0.01014

612 0.99655 0.01053

614 1.00000 0.0 1 075

616 0.99764

/\ At AlA

0.01039

618 0.994% 0.01087

620 0.98860 0.01 108

622 0.97871 0.01075

624 0.96646 0.01063

626 0.95130 0.01006

628 0.93603

A A « A 4 A 0.01012

630 0.91689 0.01026

632 0.89531 A AAAHA 0.00979

634 0.8718

A AAAf

0.00955

636 0.84712 A AAA A A 0.00944

638 0.82353 A AAAC^: 0.00956

640 0.79665 A AAA 10.00917

642 0.77064 A AAA A ^ 0.00942

C A A 644 0.74 192 A AAA/V4 0.00904

646

a n't OA 0.71389 A AAOT

0.00871

648 0.68536 0.00826

650 0.65873 0.00851

652 0.62988 0.00797

654 0.60332 0.00786

656 0.57459 0.00766

658 0.54767 0.00723

660 0.52094 0.00693

662 0.49549 0.00683

664 0.46983 0.00622

666 0.44395 0.00593

668 0.42125 0.00584

670 0.39798 0.00561

\. nm E(X) TSErEMl

672 0.37641 0.00547

674 0.35497 0.00484

676 0.33378 0.00456

678 0.31525 0.00393

680 0.29606 0.00349

682 0.27753 0.00332

684 0.26075 0.00319

686 0.24390 0.00288

688 0.22961 0.00291

690 0.21502 0.00289

692 0.20139 0.00262

694 0.18875 0.00244

696 0.17650 0.00221

698 0.16517 0.00216

700 0.15420 0.00207

702 0.14413 0.00195

704 0.13512 0.00200

706 0.12585 0.00183

708 0.11756 0.00157

710 0.10966 0.00152

712 0.10238 0.00148

714 0.09560 0.00152

716 0.08861 0.00146

718 0.08284 0.00114

720 0.07723 0.00115

722 0.07140 0.00105

724 0.06721 0.00091

726 0.06202 0.00105

728 0.05810 0.00106

730 0.05434 0.00120

732 0.05009 0.00132

734 0.04674 0.00152

736 0.04361 0.00116

738 0.04029 0.00096

740 0.03806 0.00168

6
400 450 500 550 600

Wavelength, nm

650 700 750

Figure 1. Relative Corrected Emission Spectra of SRM 1931

*U.S. GOVERNMENT PRINTING OFFICE: 1994-300-568/03021

7

Ih
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