Standard Reference Materials: A/BS Special Pat>licaf/on ^60

Accuracy in Analytical Spectrophotometry

R. W. Burke and R. Mavrodineanu'

Center for Analytical Chemistry

National Bureau of Standards

Washington, DC 20234

°.f„ S,T*.N,P, & TECH R.I.C. Illllfl "WHENCE

OS 03703b

MIST PUBLICATIONS

•Retired, present address: 227 Almena Road. West Palm Beach, FL 33405

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS. Ernest Ambler. Director Issued April 1983

Library of Congress Catalog Card Number: 83-600512

National Bureau of Standards Special Publication 260-81

Natl. Bur. Stand. (U.S.), Spec. Publ. 260-81,140 pages (April 1983)

CODEN: XNBSAV

Disclaimer: Certain trade names and company products are mentioned in the text or identified in an illustration in order to adequately specify the experimental

procedure and equipment used. In no case does such

identification imply recommendation or endorsement

by the National Bureau of Standards, nor does it imply

that the products are necessarily the best available for the purpose.
PREFACE

As mandated in 1901 by an act of Congress, a prime func- tion of the National Bureau of Standards (NBS) is to develop

and maintain the capability for making accurate physical and chemical measurements on the various materials used in science and industry. One of the principal ways in which this mandate has been fulfilled is through the issuance of certified Standard Reference Materials (SRM's). These standards are used to transfer the accurate measurements made

at NBS to the user and are frequently issued with detailed publications describing how these SRM's were produced and should be utilized. An NBS series of papers, of which this publication is a member, called the NBS Special Publication -

260 Series , is used for this purpose. Because of a demonstrated need for more practical trans- mittance standards, particularly for use in clinical chemistry, a program was initiated in 1969 to study the fundamental principles of optical transmittance and to develop the instrumentation necessary to insure its accurate measurement. This publication is a summary of the work undertaken during the last decade for achieving this objective. It contains a description of the instrumentation

that was developed in the NBS Center for Analytical Chemistry

for providing accurate transmittance measurements on solid and liquid samples, a summary of the SRM's that were produced

and, in addition, brief descriptions of techniques and methodologies that were developed for the accurate chemical characterization of various materials by molecular absorption

spectrometry. The scientific papers published in this field

are also reviewed together with considerations for future developments. Inquiries concerning the technical content of this paper should be directed to the authors. Other questions concerned with the availability, delivery, price, and so forth will receive prompt attention from: Office of Standard Reference Materials

National Bureau of Standards Washington, D.C. 20234

George A. Uriano, Chief Office of Standard Reference Materials

Ernest L. Garner, Chief Inorqanic Analytical Research Division Center for Analytical Chemistry
OTHER NBS PUBLICATIONS IN THIS SERIES

Catalog of NBS Standard Reference Materials (1981-83 edition), R. W. Seward, ed., NBS

Spec. Publ. 260 (November 1981). 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-I 1061"

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 LowAlloy Steel Standard. NBS Misc. Publ. 260-3 (October 1964). COM74-1 1060** Hague. J. L. Mears. T. W.. and Michaelis. R. E.. Standard Reference Materials: Sources of Information, NBS Misc. 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 (March 1965). PBI68068"

Shultz, J. I.. Standard Reference Materials: Methods for the Chemical Analysis of White

Cast Iron Standards. NBS Misc. Publ. 260-6

(July 1975). COM74-1I068"

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-

1 1067"

Richmond, M.S., Standard Reference Materials: Analysis of Uranium Concentrates at the

National Bureau of Standards. NBS Misc. Publ. 260-8 (December 1965). COM74-1 1066** Anspach, S. C. Cavallo. L. M. Garfinkel, S. B. Hutchinson, J. M. R., and Smith. C. N., 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). COM74-1 1065"

I

I

Yakowitz, H., Victh, D. L.. Heinrich, K. F. J., and

Michaelis, R. E., Standard Reference Materials: Homogeneity Characterization on NBS Spectrometric Standards II: Cartridge Brass and LowAlloy Steel, NBS Misc. Publ. 260-10 (December

1965). COM74-II064"

Napolitano, A., and Hawkins, E. G„ Standard Reference Materials: Viscosity of Standard

Lead-Silica Glass, NBS Misc. Publ. 260-11 (November 1966). NBS Misc. Publ. 260-11"

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). NBS Misc. Publ. 260-12"

Spijkerman, J. L.. Snediker, D. K... Ruegg, F. C,

and DeVoe. J. R., Standard Reference Materials: Mossbauer Spectroscopy Standard for the Chemical Shift of Iron Compounds, NBS Misc. Publ. 260-13 (July 1967). NBS Misc. Publ. 260-13"

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

Materials: Determination of Oxygen in Ferrous

Materials - SRM 1090, 1091. and 1092, NBS

Misc. Publ. 260-14 (September 1966). NBS Misc.

Publ. 260-14"

Passaglia, E., and Shouse. P. J. Standard Refer- ence Materials: Recommended Method of Use

of Standard Light-Sensitive Paper for Calibrat- ing Carbon Arcs Used in Testing Textiles for Colorfastness to Light, NBS Misc. Publ. 260-15 (June 1967). (Replaced by NBS Spec. Publ.

260-41.) Yakowitz, H.. Michaelis. R. E., and Vieth, D. L, Standard Reference Materials: Homogeneity

Characterization of NBS Spectrometric Standards IV: Preparation and Microprobc Characterization of W-20% MO Alloy Fabricated by Powder Metallurgical Methods. NBS Spec.

Publ. 260-16 (January 1969). COM74-1 1062"

Catanzaro. E. J.. Champion. C. E.. Garner, E. L., Mannenko. G„ Sappenficld. K. M.. and Shields, W. R. Standard Reference Materials: Boric

Acid: Isotopic and Assay Standard Reference

Materials. NBS Spec. Publ. 260-17 (February

1970). Out of Print

i v
Geller, S. B., Mantek, P.A.. and Cleveland. N. G., Standard Reference Materials: Calibration of NBS Secondary Standard Magnetic Tape (Computer Amplitude Reference) Using the Reference Tape Amplitude Measurement

"Process A, "NBS Spec. Publ. 260-18 (November

1969). (See NBS Spec. Publ. 260-29.) Paule, R. C, and Mandel, J., Standard Reference

Materials: Analysis of Intcrlaboratory Measure- ments on the Vapor Pressure of Gold (Certifica- tion of Standard Reference Material 745). NBS

Spec. Publ. 260-19 (January 1970). PBI9007I"

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

Materials: Analysis of Interlaboratory Measure- ments on the Vapor Pressures of Cadmium and

Silver, NBS Spec. Publ. 260-21 (January 1971). COM74- 1 1359"

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). COM74-1 1357** Napolitano, A., and Hawkins, E. G.. Standard

Reference Materials: Viscosity of a Standard

Borosilicate Glass, NBS Spec. Publ. 260-23 (December 1970). COM7I-00157**

Sappenfield, K. M., Marineko, 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.. and

Michaelis. R. E„ Standard Reference Materials: A Standard Reference Material Containing

Nominally Four Percent Austenite, NBS Spec.

Publ. 260-25 (February 1971). COM74-1 1356** Martin. J. F., Standard Reference Materials:

National Bureau of Standards-US Steel Corportion Joint Program for Determining Oxygen and Nitrogen in Steel, NBS Spec. Publ. 260-26

(February 1971). 85 cents* PB 81 176620 Garner, E. L., Machlan, L. A., and Shields, W. R., Standard Reference Materials: Uranium

lsotopic Standard Reference Materials, NBS

Spec. Publ. 260-27 (April 1971). COM74-

11358'*

Heinrich. K. F. J., Myklebust, R. L.. Rasberry, S. D.. and Michaelis, R. E., Standard Reference Materials: Preparation and Evaluation of SRM's 481 and 482 Gold-Silver and GoldCopper Alloys for Microanalysis, NBS Spec.

Publ. 260-28 (August 1971). COM7 1-50365**

Geller, S. B., Standard Reference Materials: Cali- bration 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). COM71-50282

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). COM7I-50563** % Mavrodineanu, R., and Lazar, J. W., Standard

Reference Materials: Standard Quartz Cuvettes,

for High Accuracy Spectrophotometry, NBS

Spec. Publ. 260-32 (December 1973). 55 cents* SN0O3-O03-0 1213-1 Wagner, H. L., Standard Reference Materials: Comparison of Original and Supplemental SRM 705, Narrow Molecular Weight Distri- bution Polystyrene. NBS Spec. Publ. 260-33 (May 1972). COM72-50526**

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

Materials: Thermoelectric Voltage, NBS Spec.

Publ. 260-34, (April 1972). COM72-50371**

Sparks, L. L., and Hust. J. G.. Standard Refer- ence Materials: Thermal Conductivity of

Austenitic Stainless Steel. SRM 735 from 5

to 280 K, NBS Spec. Publ. 260-35 (April 1971)

35 cents* COM72-50368**

Cali, J. P., Mandel, J., Moore, L. J., and Young, D.

S., Standard Reference Materials: A Referee 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, and Menis.

O., Standard Reference Materials: Methods of Analysis of NBS Clay Standards, NBS Spec.

Publ. 260-37 (June 1972). COM72-50692**

Richmond, J. C, and Hsia, J. J., Standard Refer- ence 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.. and

Powell. R. L., Standard Reference Materials The Eddy Current Decay Method for Resistivity Characterization of High-Purity Metals. NBS

Spec. Publ. 260-39 (May 1972). COM72-50529**

v
McAdie, H. G., Gam, P.D., and Menif, O., Standard Reference Materials: Selection of Thermal

Analysis Temperature Standards Through a Cooperative Study (SRM 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 LightSensitive 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**

Greifer, B., Maienthal, E. J. Rains, T. C, and

Rasbcrry, S. D., Standard Reference Materials: Powdered Lead-Based Paint, SRM 1579, NBS

Spec. Publ. 260-45 (March 1973). COM73-

50226**

Hust, J. G., and Giarratano, P. J., Standard Refer- ence Materials: Thermal Conductivity and

Electrical Resistivity Standard Reference Materials: Austenitic Stainless Steel, SRM's 735 and

798, from 4 to 1200 k, NBS Spec. Publ. 260-46 (March 1975). SNOO3-OO3-0I278-5

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). -COM74-50I76** Mangum, B. W., and Wise, J. A., Standard Refer- ence Materials: Description and Use of Precision Thermometers for the Clinical Laboratory, SRM 933 and SRM 934, NBS Spec. Publ. 260-48 (May 1974). 60 cents* SN003-003-0 1 278-5 Carpenter, B. S., and Reimer, G. M., Standard Reference Materials Calibrated Glass Standards for Fission Track Use. NBS Spec. Publ. 260-49 (November 1974). COM74-5I185

Hust, J. G., and Giarratano. P. J.. Standard Refer- ence Materials: Thermal Conductivity and Electrical Resistivity Standard Reference Materials:

Electrolytic Iron, SRM's 734 and 797 from 4 to 1000 K. NBS Spec. Publ. 260-50 (June 1975). $1.00* SN003-003-0 1425-7 Mavrodineanu, R., and Baldwin, J. R., Standard

Reference Materials: Glass Filters As a Standard Reference Material for Spectrophotometry;

Selection; Preparation; Certification; Use-SRM

930. NBS Spec. Publ. 260-51 (November 1975). $1.90* SN003-003-0148I-8

Hust. J. G.. and Giarratano. P. J., Standard Refer- ence Materials: Thermal Conductivity and Electrical Resistivity Standard Reference Materials 730 and 799. from 4 to 3000 K. NBS Spec. Publ. 260-52 (September 1975). $1.05* SNOO3-O03-

01464-8 Durst, R. A., Standard Reference Materials: Standardization of pH Measurements, NBS

Spec. Publ. 260-53 (December 1975, Revised). $1.05 SN0O3-OO3-OI551-2

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

Reference Materials: Certification and Use of Acidic Potassium Dichromaie Solutions as an

Ultraviolet Absorbance Standard, NBS Spec.

Publ. 260-54 (August 1977). $3.00* SN003-OO3-

01828-7

Ditmars. D. A., Cezairliyan, A.. Ishihara, S., and

Douglas, T. B., Standard Reference Materials: Enthalpy and Heat Capacity; Molybdenum SRM 781. from 273 to 2800 K, NBS Spec. Publ. 260-55 (September 1977). $2.20* SNOO3-O03-

01836-8

Powell. R. L.. Sparks, L. L., and Hust, J. G., Standard Reference Materials: Standard

Thermocouple Materials, Pi. 67: SRM 1967, NBS

Spec. Publ. 260-56 (February 1978). $2.20* SN003-003-OI8864

Cali, J. P. and Plebanski. T, Guide to United

States Reference Materials. NBS Spec. Publ. 260-57 (February 1978). $120* PB 277173-

Barnes. J. D., and Martin. G. M.. Standard Refer- ence Materials: Polyester Film for Oxygen Gas

Transmission Measurements SRM 1470. NBS

Spec. Publ. 260-58 (June 1979) S2.00* SN003-

003-02077 Chang, T., and Kahn, A. H. Standard Reference

Materials: Electron Paramagnetic Resonance

Intensity Standard; SRM 2601. NBS Spec.

Publ. 260-59 (August 1978) $2.30* SN003-

003-01975-5

vi
Velapoidi, R. A., Paule, R. C, Schaffer, R. Mandel, J., and Moody, J. R., Standard Refer ence Materials: A Reference Method for the Determination of Sodium in Serum, NBS Spec. Publ. 260-60 (August 1978). $3.00» SN003-003

01 978-0

Verdier, P. H., and Wagner. H. L. Standard Refer- ence Materials: The Characterization of Linear Polyethylene (SRM 1482, 1483, 1484), NBS

Spec. Publ. 260-61 (December 1978). $1.70* SN003-003-02006-I

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). $2.30* SN003-O03-

02047-8 Velapoidi, R. A., Paule, R. C, Schaffer, R. Mandel, J., Machlan. L. A., and Gramlich, J. W., Standard Reference Materials: A Reference Method for the Determination of Potassium in Serum. NBS Spec. Publ. 260-63 (May 1979). $3.75* SN003-003-02068

Velapoidi, R. A., and Mielenz. K. D., Standard

Reference Materials: A Fluorescence Standard

Reference Material Quinine Sulfate Dihydrate (SRM 936), NBS Spec. Publ. 260-64 (January

1980). $4.25' SN003-003-02 148-2 Manncnko. R. B.. Heinrich. K.. F. J., and Ruegg,

F. C, Standard Reference Materials: Micro- Homogeneity Studies of NBS Standard Reference

Materials, NBS Research Materials, and Other Related Samples. NBS Spec. Publ. 260-65

(September 1979). $3.50* SN003-003-021 14-1 Venable. W. H., Jr.. and Eckerle. K.. L. Standard

Reference Materials: Didymium Glass Filters for Calibrating the Wavelength Scale of Spectro- photometers (SRM 2009.2010.2013). NBS Spec.

Publ. 260-66 (October 1979). $3.50* SN003-003-

02127-0

Velapoidi. R. A., Paule, R. C. Schaffer, R., Mandel.

J.. Murphy, T. J., and Gramlich. J. W.. Standard

Reference Materials: A Reference Method for the Determination of Chloride in Serum, NBS Spec. Publ. 260-67 (November 1979). S3.75* SN003-

003-02136-9 Mavrodineanu, R. and Baldwin, J. R.. Standard

Reference Materials: Metal-On-Quartz Filters as a Standard Reference Material lor Spectrophoto- metry-SRM 2031. NBS Spec. Publ. 260-88

(April 1980). $4.25» SN003-003-O2I67-9

Velapoidi, R. A., Paule, R. C, Schaffer, R., Mandel, J., Machlan. L. A., Garner, E. L., and

Rains, T. C, Standard Reference Materials: A

Reference Method for the Determination of Lithium in Serum, NBS Spec. Publ. 260-69 (July)

1980). $4.25* SN003-O03-022I4-4

Marincnko. R. B., Biancaniello, F., Boyer, P. A., Ruff, A. W., DeRobertis. L.. Standard Reference

Materials: Preparation and Characterization of an Iron-Chromium-Nickel Alloy for Microanalysis, NBS Spec. Publ. 260-70 (May 1981). $2.50* SN0O3-003-O2328-I

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). $6.50* SN0O3-O03-0238 1-7 Reeder, D.J., Coxon, B., Enagonio, D., Christensen,

R. G., Schaffer, R., Howell. B. F., Paule, R. C,

Mandel, J., Standard Reference Materials: SRM

900, Anticpilepsy Drug Level Assay Standard, NBS Spec. Publ. 260-72 (June 1981). $4.25* SN003-OO3-02329-9

lnterrante, C. G.. and Hicho, G. E.. Standard Refer- ence Materials: A Standard Reference Material Containing Nominally Fifteen Percent Austenite (SRM 486), NBS Spec. Publ. 260-73 (January

1982). $2.75» SN003-003-02386-8 Marinenko. R. B., Standard Reference Materials: Preparation and Characterization of K-4II and

K-414 Mineral Glasses for Microanalysis: SRM

470. NBS Spec. Publ. 260-74 (April 1982). $3.50 SN003-003-023-95-7

Weidner, V. R.. 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). $3.75 SN 003-003-023-99-0

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). $3.50 SN003-003-024-33-3 Furukawa. G. T., Riddle. J. L.. Bigge. W. G.. and

Pfieffer, E. R., Standard Reference Materials: Application of Some Metal SRM's as Thermometric Fixed Points, NBS Spec. Publ. 260-77 (August 1982). S6.00 SN003-003-024-34-1
Hicho. G. E. and Eaton. E. E.. Standard Reference

Materials: Standard Relerence Material Containing Nominally Thirty Percent Austenue (SRM

487). NBS Spec. Publ. 260-78 (September 1982). $3.75 SN003-003-024-35-0 Richmond. J. C. Hsia. J. J.. Weidner. V. R.. and Wilmenng. D. B.. Standard Relerence Materials: Second Surtace Mirror Standards of Specular

Spectral Reflectance (SRM's 2023. 2024. 2025). NBS Spec. Publ. 260-79 (October 1982). S4.50 SN003-O03-024-47-3

Schaller. R.. Mandel. J.. Sun. T.. Cohen. A., and

Hertz. H. S.. Standard Relerence Materials; Evaluation by an ID/ MS Method of the AACC

Reference Method lor Serum Glucose, NBS

Spec. Publ. 260-80 (October 1982). $4.75 SN003-

003-024-43-1 Burke. R. W.. Mavrodineanu. R. (NBS retired). Standard Reference Materials: Accuracy in Analytical Spectrophotometry. NBS Spec. Publ. 260-81 (In Press). Weidner. V. R.. Standard Relerence Materials: White Opal Glass Diffuse Spectral Reflectance Standards lor the Visible Spectrum (SRM's 2015 and 2016). NBS Spec. Publ. 260-82 (In Press).

* Send order with remittance to Superintendent

of Documents. US Government Printing Office Washington. DC 20402. Remittance Irom

foreign countries should include an additional one-fourth ot the purchase price for postage.

•• May be ordered from: National Technical Information Services (NTIS). Springfield

Virginia 22 151.
TABLE OF CONTENTS

Pane Preface iii Other MBS Publications in This Series iv Foreword xii Abstract xiii Disclaimer xiii PART ONE

1 . General 1 2. Experimental 1 2.1 DescriDtion of the Hi qh-Accuracy SoectroDhotometer 1 2.2 Standard Reference Materials 7 2.2.1 SRM 930 7 2.2.2 SRM 2030 17 2.2.3 SRM 931 21 2.2.4 SRM 932 21 2.2.5 SRM 935 22 2.2.6 Limitations of SRM's 930, 931 , and 935 25 2.2.7 SRM 2031 25 2.2.8 SRM's 2009, 2010, 2013, and 2014 29 2.2.9 SRM's 2032, and 2033 29

3. Future Developments in the Field of SRM's for Spectrophotometry 32

4. Transmittance Transfer Standards from the National Physical Laboratory in Great Britain . . 32 PART TWO

5. Soectroohotometric Applications 33

6. ExDerimental 33 6.1 Cuvettes 33 6.2 Temperature Control 34

7. Selected Methodologies 35

7.1 Antimony and Arsenic 35 7.2 Nickel

.

37 7.3 Simultaneous Determination of Cobalt, Nickel, and Copper 39 7.4 Cooper 39 7.5 Other Elements 40

8. Future DeveloDments in New Soectrophotometri c Methodologies 40

9. References 42 Acknowledgements 43 Apnendices. 43

ix
LIST OF FIGURES

Figure No. Page

1 Schematic description of the single beam high-accuracy spectrophotometer 2

2 Arrangement of the two radiation sources used with the high-accuracy

spectrophotometer 3

3 Circular platform carrying seven cuvette or filter holders 4 4 Console containing the power supply for the photomultipl ier tube, the null detector, the current-to-voltage converter, the digital voltameter, the command panel for the microcomputer operation, and the command oanel for the pneumatic controls 5

5 Front view of the double-aperture unit 6 6 Illustration of SRM 930, Glass Filters for Spectrophotometry, showing one of the three glass filters, the metal holder with its two plastic shutters, and the retaining spring with its nylon screw and washer 14

7 Details of the filter holder 14

8 Instrumentation used to examine, clean, and mount the glass filters that constitute SRM 930 15

9 Soxhlet extractors used for the final cleaning of the glass filters, using

isopropyl alcohol of electronic grade 16 10 Illustration'of SRM 2030 consisting of one glass filter Schott NG-5 in its metal holder provided with the removable front and rear shutters, and an empty

filter holder, in the storage box 17

11 Control chart for a precision spectrophotometer showing the variation of transmittance with time for a Schott NG-5 neutral glass filter at 635.0 nm (a) and 590.0 nm (b) 18 12 Illustration of SRM 931, Liguid Absorbance Standard; SRM 935, Crystalline

Potassium Dichromate Ultraviolet Absorbance Standard; and SRM 932, Quartz Cuvette for Spectrophotometry 21 13 Illustration of SRM 932 Ouartz Cuvettes for Spectrophotometry, showing the fused silica material and parts used for constructing the cuvettes for high-accuracy measurements 22

14 Instrument for measuring the thickness of window plates and length of fused

silica segments 23

15 General view of the standard instrument used to measure the radiation Dathlength of assembled cuvettes 24

16 Illustration of SRM 2031, Metal -on-Ouartz Filters for Soectroohotometry 26

17 Transmittance of a didymium glass filter recorded at the Corning Glass Works 30 18 Illustration of the radiation attenuator consisting of a metal filter holder provided with a filter made of a semi transparent evaporated metal-on-fused silica plate 31 19 Outline of the spectrophotometric procedures used for determining antimony and arsenic in NBS iron and copper base alloys 35

x
LIST OF FIGURES (continued) Figure No. Page

20 The effect of various oxidation procedures on the absorbance spectra of the brilliant green-hexachloroantimonate complex 36

21 Comparison of spectrophotometri c and IDMS results for nickel 1n NBS Trace-Elements-in-Glass SRM 612 38 22 High-performance liquid chromatographic separation and spectrophotometri

c

detection of a mixture of copper, cobalt, and nickel

d1 benzyl dithiocarbamate chelates 41

LIST OF TABLES

Table No. Page

1 Summary of Functioning Characteristics of the High-Accuracy Spectrophotometer .... 7

2 Standard Reference Materials for Spectrophotometry Provided by the National Bureau of Standards 8

3 Stability of Transmittance (T) as a Function of Time, Measured on SRM 930 19

4 Transmittance of a Set of SRM 930 at Two Wavelengths Obtained Using the Center

for Analytical Chemistry (CAC) and the Center for Radiation Research (CRR) High-Accuracy Spectrophotometers 20

5 Comparison Between the Percent Transmittances (%T) Measured on Three Schott NG-4 Glass Filters at NPL and NBS 20

6 Transmittance of Some Inconel-on-Fused-Silica Filters Obtained Using the Center

for Analytical Chemistry (CAC) and the Center for Radiation Research (CRR) High-Accuracy Spectrophotometers 27

7 Measurement of Scatter, Homogeneity, and Polarization on Three Inconel -on-FusedSilica Filters 28

8 Comparative Measurements of Transmittance on Inconel -on-Fused-Si 1 ica Filters in the Ultraviolet and Visible from 250 nm to 635 nm at NBS and at Naval Weapons Center (NWC) 29

9 Reproducibility of Absorbance Measurements at 235 nm for a Cycle of Solvent and Sample Runs r 34

10 Absorbances of a Set of 0.001 N Perchloric Acid Solutions of SRM 935 K2 Cr2 07 at Time of Preparation and After 20 Months Storage 34

11 Temperature Differences Between Thermostating Bath and Cuvette Solution; Ambient T = 24.2 °C 34

12 Antimony and Arsenic in MBS Copper Benchmark SRM's 37 13 Comparative Determinations of Nickel in Various SRM's by Gravimetry and by the Persulfate-Dimethylglyoxime Method [28] 38

14 Simultaneous Determination of Cobalt, Nickel, and Copper in NBS SRM's 39 15 Comparison of Spectrophotometri c and IDMS Results for Copper in NBS Botanical SRM's 40

xi
FOREWORD

This work presents a discussion concerning the use of spectrophotometry 1 techniques for the assessment of the optical transmittance of solid or liquid materials, and for the determination of several selected chemical species in a number of matrices. Particular emphasis is placed on the accuracy of the measurements obtained by using the instrumentation, the Standard Reference Materials, and

the analytical procedures developed during the last decade in the Center for Analytical Chemistry of the National Bureau of Standards (NBS). This publication

is divided into two parts: the first part contains a concise description of the high-accuracy spectrophotometer constructed at the NBS Center for Analytical

Chemistry, together with a review of the Standard Reference Materials (SRM's) produced at NBS in the field of spectrophotometry. The second part is concerned

with the description of several selected analytical applications of spectrophotometric techniques to the determination of antimony, arsenic, nickel,

cobalt, and copper, in various matrices and with a known accuracy. Future developments in spectrophotometry are considered in the concluding

remarks to this work. It includes the potential development of new analytical methods based on the formation of ternary complexes and the use of high performance liquid chromatography as a separation technique for simultaneous multielement analysis.

For further information and convenience, a number of selected publications, directly related to the subjects discussed in this work, are reproduced in the Appendix, together with copies of the certificates for the Standard Reference

Materials produced by the National Bureau of Standards in the field of spectrophotometry.

The material used in this work was taken from two lectures delivered by the authors, one at the Society for Applied Spectroscopy meeting in Chicago, Illinois

on October 21-22, 1981, with the title "Standard Reference Materials for Spectrophotometry" and the other at the American Society for Testing and Materials

Symposium, Boston, Massachusetts, June 27, 1978. This last lecture is published

in an ASTM Special Technical Publication 708, pages 45-65, 1980, with the title "Assessment of the Accuracy of Spectrophotometric Measurements and Methodologies".

Note: the terminology used throughout this publication follows the nomenclature defined by

K. D. Mielenz [l] 2

. figures in brackets indicate the literature references at the end of this paper.

x11
ABSTRACT

This publication describes activities undertaken since

1969 within the Center for Analytical Chemistry of the National Bureau of Standards (NBS) in the field of high- accuracy spectrophotometry. The first part of this work presents a summary of the Standard Reference Materials (SRM's) that have been developed for checking the proper functioning

of ultraviolet and visible spectrophotometers and includes a description of the high-accuracy spectrophotometer specially constructed in the Center for Analytical Chemistry and subsequently used for performing all of the transmittance

measurements. The second part of this publication is devoted

to a critical discussion of the analytical factors that can affect the accuracy of selected spectrophotometric procedures

that have been widely used at NBS in the characterization of various SRM' s. Key Words: absorbance; accuracy; antimony; arsenic; cobalt; comparative measurements; copper; high-accuracy spectrophotometry; nickel; polarization; scatter; spectral bandpass; spectrophotometric determinations; Standard Reference Materials; stray light; transmittance; transmittance stability; wavelength

accuracy.

DISCLAIMER

In order to describe materials and experimental procedures adequately, it was occasionally necessary to identify commerical products by the manufacturer's name or

label. In no instances does such identification imply endorsement by the National Bureau of Standards nor does it imply that the particular products or equipment are necessarily the best available for that purpose.

xiti

PART ONE

1. General Ultraviolet and visible spectrophotometry is a measuring technique widely used in science and technology for the determination of numerous chemical elements in a large variety of materials. In the field of health alone, over 95 percent of all quantitative analyses are performed by spectro- photometry and this percentage represents over 3,000,000 daily tests carried out in the chemical laboratories in this country. The need for precision, and particularly for accuracy is of vital importance to these measurements. All determinations that use spectrophotometry are based on the measurement of the optical transmittance of the analytical sample, and the accurate measurement of this parameter is an essential requirement, especially when used not only to establish the optical characteristics of the material, such as the molar absorptivity, but also in enzyme studies, and dosimetry. Transmittance accuracy is also indispensable for inter laboratory exchange and comparison

of analytical data. As indicated by Royden Rand in a comprehensive review [2], numerous i nterl aboratory studies, using various conventional instruments and various standard solutions, have proven that the accuracy

of wavelength and transmittance measurements may be "exceedingly poor". As a result of this situa- tion and following the numerous requests from the users of spectrophotometers, and in particular,

from the clinical chemists, the National Bureau of Standards 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 measure- ments [3]. The sources of variance that are responsible for the uncertainty of spectrophotometric measure- ments were identified as wavelength accuracy, spectral bandpass, radiation pathlength, stray radiant

energy, and transmittance scale accuracy. 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 [4]. This research instrument was completed and tested in 1970 and was used, since then, for the certification

of all the Standard Reference Materials for transmittance and stray radiant energy produced to date

at NBS. A condensed description of this instrument is given in the following section.

2. Experimental

2.1. Description of the High-Accuracy Spectrophotometer

The high-accuracy spectrophotometer developed at NBS is a single beam instrument and is illus- trated 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-photomultipl ier unit connected to a digital voltmeter; and

(f) a data acquisition and display system. A circular neutral wedge is located between the radia- tion source and the entrance slit of the monochromator and it is used to select various levels of radiation flux intensities required by the measurements. Adequate quartz- 1 ithium fluoride achromats are used as objectives to produce the necessary beam geometries.

The radiation source for the spectral range 400-1000 nm is a ribbon tungsten filament lamp supplied by a constant current source. The stability of this current is monitored with a potentio- meter capable of detecting changes of 2-3 parts in 10 6

. 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 on the entrance slit of the monochromator, using a 45° mirror. The monochromator is a 1 m Czerny-Turner grating instrument provided with a quartz prism pre- disperser, 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 through a microcomputer. The integrating sphere-photomultipl ier unit is placed at the end of the optical path and is made from a hollow aluminum sphere provided with a target coated on the inside with BaS0 4

. The image of the exit slit of the monochromator is projected on the target (Figure 3).

1
in mill mi

ini-iceiitei

:ricTiiriiTiiiTit

r|r*a

l Grating Monochromator iCiamy-Tumar

)

Pracraion Optical Bench

NPl Apartura tor Linearity Control

(Aftar F.J.J. Clarke

•

y

(A) + (B)=(A + B)

•

Intagrating Sphere

Preeiaion Optical Banch in Lightproof Encloaura

Figure 1. Schematic description of the single beam high-accuracy spectrophotometer. The double aperture unit is placed on the optical bench only when linearity measure- ments are performed [3,4]. The data acquisition and presentation system consists of a digital voltmeter capable of taking

reference beam, passing through air (I

Q a^ r

) or the blank sample (I

Q blank ) are used by the micro- computer to calculate transmittance T f -j—

j or internal transmittance T. ( j- J, which can be

\ o air/ ' o blank/ expressed as transmittance density (-log-jg T) or absorbance (-log^g ). The microcomputer is also programed to operate all of the measuring sequences, including wavelength settings, light source intensity adjustments, sample changings, and transmittance measurements. These parts are shown in Figure 4. An essential element of this instrumentation is the system for verifying the accuracy of the transmittance measurements. As mentioned previously, the radiation intensities passing through the sample and reference channels generate corresponding photocurrents i and i Q at the photomultipl ier. These photocurrents are substituted for I and I Q in the above transmittance expression and their

ratio is the optical transmittance value for the material measured. If these photocurrents can be measured accurately, it follows implicitly that the optical transmittance value will be accurate provided wavelength, radiation pathlength accuracy, adequate spectral bandpass, and freedom of stray radiant energy have been achieved.

A necessary relationship that is implicit for the accurate measurement of photocurrents is that

a linear relation exists between the radiation flux intensity and the corresponding photocurrent generated by the photodetector. 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. The two aperture system, illustrated in Figure 5, consists of a metal plate provided

with two rectangular windows A and B. Each window can be closed by shutters operated remotely through pneumatic controls [5]. This aperture system is placed following the exit slit of the mono- chromator 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 separately with aperture A and B should be equal to the photocurrent obtained when both apertures A and B are open. If this is not the case, the system shows a nonl inearity which is pro- portional to the amount by which the sum of (A) + (B) differs from (A + B). This difference is then used to correct the transmittance measurements. During the individual measurement of (A), (B), and the combined (A + B), experimental conditions should remain identical.

2
0) J= E 3 J3 |

S- S- 3 <1) U T3 ! 11 0J£ c ac < > T3 •»-> 0) O 10 3
</> o

4
Figure 4. Console containing the power supply for the photomul tipl ier tube, the null detector, the current-to-voltage converter, the digital voltmeter, the command

panel for the microcomputer operation, and the command panel for the pneumatic

controls. 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 the bottom left: teletype for data presentation. Right: light panel indicating the position of the sample holder. On top: microcomputer.

5
Figure 5. Front view of the double-aperture

unit. The two pneumatically con- trolled shutters are located on the back of the plate.

A critical study by K. D. Mielenz and K. L. Eckerle [6] has concluded that the possible causes

of errors generated by optical diffraction and interferences occurring at the apertures system will not affect the accuracy of the measurements at 1 part in 10 s

. The stability of the various elec- tronic and optical parts 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 can be determined with an uncertainty of 1 part in 10 4

. Since the same uncertainty is obtained when the linearity of the photocurrent is measured, the accuracy of the transmittance is also assessed with an uncertainty of

1 part in 10 4

. A similar high-accuracy spectrophotometer, designed by K. D. Mielenz eta].. [7], was constructed

and is used in the NBS Center for Radiation Research for the study of various physical parameters in spectrophotometry. This instrument differs from the one described here in that it uses reflecting

rather than refracting optics. Some of the parameters studied on this research instrument are: interref lection phenomena, polarization, homogeneity of the photomultipl ier sensitive surface, effectiveness of integrating spheres and diffusers, scatter and homogeneity of transparent solid

optical filters, wavelength calibration of glass filters, etc., [8].

6
Table 1. Summary of Functioning Characteristics of the High-Accuracy Spectrophotometer.

- Stability of the electronic system 1 to 2 parts in 10

5 - Stability of the electronics and PM tube (dark) 4 parts in 10

5 - Stability of electronics, PM tube, and radiof 1 uorescent source 7 parts in 10

5 - Stability of electronics, PM tube, and tungsten ribbon filament 2.2 parts in 10

4 - Reproducibility of transmittance measurements 1 part in 10^

2.2. Standard Reference Materials

The high-accuracy spectrophotometer housed in the Center for Analytical Chemistry is described

in this paper because it represents the primary transmittance standard. The only way known to us by which the accurate transmittance values determined with this instrument, on solid and liquid materials in the spectral range from 200-800 nm, can be transmitted to the users of spectrophotometers is with the help of Standard Reference Materials, or SRM's. These SRM's are defined as "wel 1 -characterized materials produced in quantity and certified for one or more physical or chemical properties", and their objective is to assure the accuracy and compatibility of measurements through- out the nation, by transmitting to the user the accurate values obtained at NBS.

A number of SRM's have been prepared at NBS for the verification of the accuracy of the wave- length and transmittance scales of conventional spectrophotometers, and for assessing the stray radiant energy. Cuvettes having a known radiation pathlength and a fluorescent SRM have also been

issued. These SRM's are described in Table 2 which presents a summary of their major characteristics together with information on the publications prepared at NBS that describe in detail the selection, characteristics, preparation, calibration procedures, and uses of these materials.

2.2.1. SRM 930 The first SRM for spectrophotometry, issued by NBS in March 1971, is the transmittance standard illustrated in Figure 6. It consists of three glass filters made of Schott NG-4 and NG-5 optically

neutral glasses cut to fit a metal holder that can be placed in the cuvette compartment of conventional spectrophotometers. Their nominal transmittances, in the visible spectral range from 400-635

nm, are 10, 20, and 30 percent. Each transmittance is certified with an uncertainty of 0.5 percent,

at the time of certification, at wavelengths of 440, 465, 546, 590, and 635 nm and with spectral bandpasses of 2.2-6.0 nm according to the wavelength.

Each metal holder is provided with two shutters for the protection of the glass, and a fourth empty holder is provided, as shown in Figure 7, for use in the reference beam. The filters are placed in a black anodized aluminum holder for storage and transportation. Before certification,

each glass filter is examinea using a stereomicroscope (x30) for surface defects. It is then numbered for identification, cleaned in an ultrasonic bath, placed in a stainless steel holder, and finally cleaned in a Soxhlet extractor with isopropyl alcohol. The clean filters are then mounted

in the metal holders. The equipment is shown in Figure 8, and the extractors are illustrated in Figure 9. All operations are performed in a dust-free polypropylene hood with vertical laminar air flow. The cleaning process described here is satisfactory for the purpose of this work. A detailed bibliography of the numerous works on this problem is given by P. Bruce Adams [9]. In addition a chapter on "The Cleaning of Glass" is part of the book by L. Holland "The Properties of Glass Surfaces" [10], where this subject is discussed in depth. Transmittance measurements are first performed on these filters after waiting several weeks for their freshly-cleaned glass surfaces to "stabilize" and then also six months later to verify the optical stability of the material. The filters are issued when these two sets of measurements agree within 0.3 percent. As discussed later, this aging period is an essential condition to insure that SRM 930 has the transmittance stability required.

7
Table 2. Standard Reference Materials for Spectrophotometry Provided by the National Bureau of Standards.

SRM Material Composition

Solid Filters

930 Glass Filters for Spectrophotometry

Solid filters made of Schott

NG 4 and NG 5 optically

neutral glass. Obtained from

the Schott-Jenaer Glaswerk,

Mainz, Germany

Three filters with nominal transmittances of 10, 20, 30%. Each filter is mounted in a black anodized aluminum holder provided with front and rear

shutters. The filters are stored in a cylindrical aluminum container.

2030

Glass Filter for Transmittance

Measurements

Solid filter made of Schott

NG 5 optically neutral

glass. Obtained from the Schott-Jenaer Glaswerk,

Mainz, Germany,

One glass filter with a nominal transmittance of 30% in a black anodized aluminum holder with

shutters. The filter is stored

in a wood-like plastic box.

2031 Metal -on-Quartz

Filters for Spectrophotometry

Solid filters made of semi- transparent evaporated chromium-on-fused silica (non-fluorescent) plates.

Three filters with nominal transmittances of 10, 30, 90%. Each filter is made from two fused silica plates assembled

by optical contact. The 90% filter is made by assembling 2 clear plates. The 10% and 30% filters are made by assembling a"fused silica plate which

carries a film of semitransparent chromium to produce the desired transmittance, and a clear plate. Each filter assembly is placed in a metal holder provided with shutters.

Liquids and Cuvettes

931b Liquid Absorbance

Standards for Ultraviolet

and Visible Spectrophotometry

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 T. Prepared in the NBS Center for Analytical Chemistry.

Three solutions and a blank are delivered in sealed ampoules

and have nominal absorbances

of 0.1 to 0.9.

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, New Jersey.

The crystalline material of reagent grade purity is offered in glass bottles provided with plastic screw

caps.

8
Certification Use Remarks

The transmittance of each filter is measured with the» high-accuracy spectrophotometer at X 440, 465, 546.1 , 590, 635 nm, using spectral bandpasses of 2.2, 2.7, 6.5,

5.4, 6.0 nm respectively.

This SRM is intended as a reference source for the verification of the trans- mittance and absorbance

scales of conventional spectrophotometers

.

The use of this SRM is limited

to the visible spectrum and requires narrow spectral bandpasses. For further details, consult the Certificate included in the Appendix; see also NBS Special

Publ ication 260-51

.

The transmittance of the filter

is certified as for SRM 930

but only for x 465 nm and for

a nominal transmittance of 30%.

The transmittance of each filter is measured with the high-accuracy spectropho- meter at x 250, 280, 340,

360, 400, 465, 500, 546.1,

590, 635 nm.

This SRM is intended as a reference source for one-point

verification. of the transmittance or absorbance scales of spectrophotometers

.

This SRM is intended as a reference source for the verification of the trans- mittance and absorbance

scales of conventional spectrophotometers in the ultraviolet and visible

spectral region.

Same as for SRM 930. SRM 2030

is particularly useful in connection with the chemical analysis of spices. A copy of

the Certificate is included in the Appendix. SRM 2031 has good optical neutrality and can be used to calibrate spectrophotometers

with wide spectral bandpasses from about 200 to 800 nm (with possible extension to x=3 um). For further details, consult

the Certificate in the Appendix; see, also NBS Special Publication 260-68.

The absorbance of each solution was measured at x 302, 395, 512, 678 nm using

spectral bandpasses of 1.0, 1.7, 2.0, 6.5 nm respectively.

The certification is made with

an uncertainty of about ±1.0%.

This SRM is primarily

intended for the verification

of the accuracy of transmittance or absorbance scales

of spectrophotometers.

Requires the use of SRM 932

and of narrow spectral bandpasses. Provides only one certified value in the ultraviolet. For further details consult the Certificate

included in the Appendix. The apparent specific absorbance is certified for five concentrations at x 235,

257, 313, 345, 350 nm using

spectral bandpasses of 1.2, 0.8, 0.8, 0.8, 0.8 nm respectively, using SRM 932.

This SRM is intended to be used as a reference standard

for the verification of the accuracy and linearity of the absorbance scale of spectrophotometers.

Requires accurate preparation

of solutions of potassium dichromate in 0.001 N perchloric acid. Requires the use of SRM 932 and of narrow spectral bandpasses. Can be used only

in the ultraviolet. For further details, consult the Certificate included in the Appendix; see also NBS Special Publication 260-54.

9
Table 2. continued.

SRM Material Composition

932 Quartz Cuvette for Spectrophotometry

All -quartz cuvette made of non-fluorescent fused silica

of optical quality.

Each cuvette is made of fused silica and is provided with a Teflon stopper. The trans- parent windows are attached

to the body of the cuvette by direct fusion. The cuvettes

are stress-relieved by proper

annealing. The cuvette is stored in a transparent

plastic container.

Stray Light

2032 Crystalline potassium The reagent-grade, crystalline Potassium Iodide for use iodide of established KI is 99.8% pure as indicated

as a Stray Light Standard purity, obtained from the by the manufacturer. Moisture

J. T. Baker Chemical Co., is 0.007%. Homogeneity was Phillipsburg, New Jersey. determined by absorbance measurements at 265, 267, and 270 nm, and was found adequate.

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 holder

provided with shutters. The

KI was obtained from the

J. T. Baker Chemical Co., Phillipsburg, New Jersey. The filters and the holder were

made by the NBS Instrument Shop.

Potassium iodide as for

SRM 2032. Two semi- transparent evaporated chromium-on-fused silica (non-fluorescent) filters of optical quality.

Wavelength

2009 Didymium Glass Filter for Checking the Wavelength

Scale of Spectrophotometers

The didymium glass was prepared by Corning Glass Works, Corning, New York,

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 holder which fits

into the cuvette holder of the spectrophotometer.

2010 Same as SRM 2009. Kare earth oxides in a glass Didymium Glass Filter for matrix. This filter is Checking the Wavelength 5.1 cm square and 3 mm

Scale of Spectrophotometers thick. 2013 Same as SRM 2009. Same as SRM 2009. Didymium Glass Filter for Checking the Wavelength

Scale of Spectrophotometers

10
Certification Use Remarks The nominal inner pathlength

is 10 mm and it is certified

at 10 points along the height

of the transparent windows with an uncertainty of ±0.0005 mm. The cuvettes are

12.5 mm square and 48 mm high.

To be used when accurate transmittance or absorbance measurements are made on liquid samples. Should be used in conjunction with

SRM's 931 , 935, 2032, and 2033.

For further details, consult

the Certificate included in the Appendix; see also NBS Special Publication 260-32.

The specific absorbances were measured with the high- accuracy spectrophotometer at 240, 245, 250, 255, 265, 270, and 275 nm and 23.5 °C using a spectral bandpass of 0.2 nm. The measurements were performed using SRM 932 quartz cuvettes. Same as for SRM 2032. The transmittance of the evaporated metal -on-fused silica filters was measured

at 255 nm with the high- accuracy spectrophotometer.

The nominal value is 10% for each filter, and a combined value of 1%.

SRM 2032 is to be used to assess heterochromatic stray light in the ultraviolet

region below 260 nm, in absorption spectrophotometers. It is recommended

that SRM 932 be used in the measurements.

SRM 2033 is to be used to assess heterochromatic and isochromatic stray light 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.

Same as for SRM 2032. The

two semi-transparent

evaporated metal -on-fused silica filters in the metal holder should be stored in the plastic container

provided with SRM 2033.

The wavelengths of maximum absorption were determined with a high-precision spec- trophotometer 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 wave- length 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 2009 was not measured

individually. It is recommended for most

applications. For further

details, consult NBS Special Publication 260-66.

Same as SRM 2009. Same as SRM 2009. Same as SRM 2009.

Each filter was measured separately ; hence the data provide a more accurate representation of the optical properties of the individual filter.

Same as SRM 2009 SRM 2013 was measured individ- ually and should be used when assurance in the evaluation of

an instrument's wavelength

scale is highly critical . For further details, consult NBS Special Publication 260-66.

n
Table 2. continued.

SRM Material Composition

2014 Same as SRM 2009 Same as SRM 2009. Didymium Glass Filter for Checking the Wavelength

Scale of Spectrophotometers

2034 Holmium Oxide for use as Wavelength Standard in Spectrophotometry and Fluorescence Spectrometry.

(Planned SRM)

Holmium oxide of established purity dissolved in perchloric

acid.

Holmium oxide is to be offered in solutions in sealed 10 mm quartz

cuvettes.

Fl uorescence

936 Quinine Sulfate

Di hydrate

The quinine sulfate dihydrate

was a special lot of material obtained from J. T. Baker

Chemical Co., Phillipsburg,

New Jersey.

SRM 936 contains 1 .7% impurities determined by liquid chromatoaraphy 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. Theoretical

value is 4.60%.

NOTE : For further scientific information concerning the following Standard Reference Materials, write or call at the National Bureau of Standards, Washington, D. C. 20234: SRM's 930 ; 931 ; 932 ; 935 ; 2030 ; 2031 ; 2032 ; and 2033 : Dr. R. W. Burke, Inorganic Analytical Research Division, Chemistry Building, Room B-216. Phone: 301-921-2141. SRM 936 : Dr. R. A. Velapoldi, Deputy Director, Center for Analytical Chemistry, Chemistry Building, Room A-311. Phone: 301-921-2852. SRM's 2009 ; 2010 ; 2013 ; and 2014 : Mr. K. L. Eckerle, Radiometric Physics Division, Center for Radiation Research, Metrology Building, Room

A- 321. Phone: 301-921-2791; for Scientific information concerning physical parameters

involved in spectrophotometry and luminescence measurements Dr. K. D. Mielenz, Chief, Radiometric Physics Division, Center for Radiation Research, Metrology Building, Room

B-304. Phone: 301-921-3864.

For information concerning the availability and purchase of the SRM's discussed in this paper contact: Mr. R. W. Seward, Technical Representative, Office of Standard Reference Materials, Chemistry Building, Room B-313. Phone: 301-921-2045. The NBS Publication discussed in this paper can be purchased from the: Superintendent of Documents, U.S. Government Printing Office, Washington, D. C. 20034. Please write for information concerning availability, delivery, and price.

12
Certification Use Remarks

Same as SRM 2013. Same as SRM 2009. Same as SRM 2013.

The wavelengths of maximum

absorption of holmium oxide

solutions in perchloric

acid will be determined for various spectral bandpasses

at the nominal wavelengths;

279, 288, 338, 361, 386, 418, 453, 536, 637 nm.

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

of the wavelength scale of absorption and fluorescence

spectrometers, in the ultra- violet 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. The use of solutions in conjunction with a diffusing screen

(Teflon) placed at 45° in the

10 mm cuvette permits the use of SRM 2034 for the wavelength calibration of fluorescence spectrometers. The issuance

of SRM 2034 is projected for the near future.

The material is certified for

the relative molecular emission spectrum E(x), in radio- metric units for a solution of

1 .28 x 10" 6 mol/L io 0.105

mol/L perchloric acid, using

an excitation wavelength of 347.5 nm. The certified

values of the molecular emis- sion spectrum at 5 nm intervals from 375 to 675 nm are given. This certification

was made with the NBS reference fluorescence spectrometer.

For evaluation of methods and the calibration of fluores- cence spectrometers. A solution of 0.1 mg/mL in 1000 mL 0.105 mol/L perchloric chloric 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 original 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.

13
Figure 6. Illustration of SRM 930, Glass Filters for Spectrophotometry, showing one of the three glass filters, the metal holder with its two plastic shutters, and the retaining spring with its nylon screw and washer. A fourth empty filter holder

is provided, and the four holders are stored in the cylindrical metal container.

Figure 7. Details of the filter holder. From left to right: front and rear view of the holder body; retaining spring and nylon screw with washer; glass filter; plastic

shutters; the holder with the front and rear shutters on; and the filter with

both shutters closed.

14
15
16
2.2.2. SRM 2030 Another glass filter standard is SRM 2030 which is derived from SRM 930. This material, illustrated in Figure 10, is made of only one glass filter calibrated at only one wavelength, namely

465 nm (SBP 2.7 nm). SRM 2030 was issued at the request of the spice industry which uses it as a reference standard in the methods used for determining the quality of various spice extracts. The use of this SRM has decreased the uncertainty of the spectrophotometry analyses from about 10 percent to 2 to 3 percent.

Figure 10. Illustration of SRM 2030 consisting of one glass filter Schott NG-5 in its metal holder provided with the removable front and rear shutters, and an empty filter

holder, in the storage box.

2.2.2.1. Monitoring the Stability of Spectrophotometers

The short and long term stability of a spectrophotometer is directly related to its overall design and construction. For many modern commercial instruments, the short term (day-to-day) variation in transmittance (T) does not exceed 5 x 10-4

. This variability corresponds to an absor- bance (A) variability of 2 x 10~3 at T = 0.1 (A = 1) and to a AA of 3 x 10~4 at T = 0.8 (A = 0.1). For a longer term, namely, month-to-month, the spectrophotometer may exhibit results that indicate a gradual shift in performance outside of these limits. This shift is indicative of aging, wear, and deterioration of the instrument's various optical, mechanical, and electronic components and should

be monitored periodically. Any abrupt change in the spectrometer's performance indicates a malfunc- tion and corrective action should be taken immediately. Corrective action- should also be taken whenever cummulative gradual changes are sufficient that the performance of the instrument is outside

of the manufacturers' specifications.

The recommended way to monitor the short and long term stability of a spectrophotometer is by construction of a control chart. The essential requirement for such a control chart is the avail- ability of a stable standard. Of the several types, the most convenient are glass filters, specially selected for their stability. Of the several kinds of glasses that can be used, the SRM 930 type is the most suitable for general purposes. The chart is constructed by making transmittance measure- ments at several well-spaced wavelengths and plotting the results on the ordinate and the date on

17
which the measurements were made on the abscissa. Measurements should be performed daily. Additional data which should be recorded at the time of measurement are the slit width and the temperature.

A control chart for one of the precision spectrophotometers in the Center for Analytical Chem- istry at NBS is shown in Figure 11. The upper plot (a) was obtained from measurements taken at 635.0 nm and the lower plot (b) from measurements taken at 590.0 nm. An examination of these data

shows that the stability of this instrument varied during the period of October 22 to February 5 from 32.17 to 32.28 percent T with an average of 32.23 percent T at 635.0 nm. At 590.0 nm the corresponding values were 30.79 and 30.90 percent T with an average of 30.85 percent T. For each wavelength,

the spread is 0.11 percent T. This spread results from the inherent instability of the spectro- photometer and of the glass filters used. The performance of this spectrophotometer changed markedly on February 8 when a transmittance of 31.94 and 30.59 were obtained at 635.0 and 590.0 nm, respectively. Since these values were well outside the spread for this instrument, the manufacturer's

service representative was consulted. An examination of the spectrophotometer revealed that a potentiometer was malfunctioning and this part was replaced. The spectrophotometer was checked again

on February 18 and the transmittance values, measured on the same glass filters, were within the spread previously established for the instrument. The periodic verification of the stability of spectrophotometers as described above and illustrated in Figure 11 should be used in analytical laboratories on a routine basis. A control chart, in conjunction with corresponding slit width data, provides an excellent diagnostic check when an instrument should be serviced or a particular

component replaced.

AT WAVELENGTH 635.0 nm AND 24.0'C

AT WAVELENGTH 590-0 nm

Figure 11. Control chart for a precision spectrophotometer showing the variation of trans- mittance with time for a Schott NG-5 neutral glass filter at 635.0 nm (a) and 590.0 nm (b).

18
2.2.2.2. Stability of Glass Filters

Of the many characteristics of these glass filters, only two will be mentioned here. Briefly, these are the stability of the transmittance and the capability of these transmittance measurements

to be transmitted to other laboratories. These properties are obviously of major significance not only in relation to these standards, but for any type of transfer standards.

Several sets of SRM 930 were tested over a period of 10 years, during which transmittance measurements were performed at selected time intervals and at four to five wavelengths in the visible

spectral range. The data from Table 3 illustrate the results obtained over a 5 year time span, indicating good stability, with a variation in transmittance not exceeding 0.3 percent. Similar

results have been observed during the 10 years that such measurements have been made. Table 3. Stability of Transmittance (T) as a Function of Time, Measured on SRM 930. ------- Percent T Wavelength _J 2 3 Date

32.87 19.80 11.59 5-18-71 32.88 19.83 11.62 7-12-72

32.91 19.81 11.59 8-30-73

32.96 19.84 11.61 11-16-73 32.98 19.84 11.62 1- 9-74 32.94 19.84 11.61 8- 1-74 32.95 19.84 11.62 1-13-75

35 53 22.59 13.56 5-18-71

35 54 22.62 13.59 7-12-72

35 54 22.62 13.59 8-30-73

35 62 22.62 13.58 11-16-73

35 63 22.63 13.58 1- 9-74

35 60 22.62 13.57 8- 1-74

35 60 22.62 13.58 1-13-75

31 13 19.16 10.37 5-18-71

31 14 19.20 10.41 7-12-72

31 14 19.20 10.41 8-30-73

31 21 19.19 10.40 11-16-73

31 21 19.20 10.41 1- 9-74

31 18 19.20 10.40 8- 1-74

31 19 19.20 10.41 1-13-75

32.55 20.60 11 37 5-18-71 32.59 20.65 11 39 7-12-72

32.59 20.65 11 39 8-30-73

32.66 20.65 11 40 11-16-73

32.68 20.67 n 41 1- 9-74 32.63 20.66 n 41 8- 1-74 32.64 20.66 n 40 1-13-75

Interlaboratory comparisons of the transmittance of SRM 930 were made on the two high-accuracy spectrophotometers at NBS. The results given in Table 4 show that such measurements can be performed within the uncertainty of the instrumental measuring capability. Similarly, measurements made by NBS and by the NPL exhibited a high degree of agreement as illustrated in Table 5.

19
Table 4. Transmittance of a Set of SRM 930 at Two Wavelengths Obtained Using the Center for Analytical Chemistry (CAC) and the Center for Radiation Research (CRR) High-Accuracy

Spectrophotometers. The difference, A(CRR), is the difference in transmittance measured by illuminating the filters with polarized radiation having the electric vector in either

the horizontal or vertical directions.

Nominal 30% Filter

465

0.3427

0.3430 -0.00002

546.1

0.3280 0.3284 0.00007

- Transmittance - - Nominal 20% Filter

465

0.1980

0.1983

•0.00008

546.1

0.1840

0. 1844

•0.00005

Nominal 10% Filter

465

0.1120

0.1121 -0.00001

546.1

0. 1014

0. 1015 0.00002

Table 5. Comparison Between the Percent Transmi ttances (%T) Measured

on Three Schott NG-4 Glass Filters at NPL and NBS.

Wavelength

nm NBS %T NPL, %T

Relative Difference

NBS - NPL, %

440.0 12 92 12.93 -0.07 465.0 14 97 15.01 -0.27 590.0 11 67 11.67 0.0 635.0 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 0.0 635.0 20 '41 20.47 -0.29 440.0 32 88 32.98 -0.12 465.0 35 53 35.66 -0.36 590.0 31 13 31.21 -0.26 635.0 32 54 32.62 -0.25 Average difference between NBS and NPL percent T values = -0.19 percent.

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. However, we would like to mention, briefly, a case where the glass material exhibited a greater instability

[11]. This incident 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 operation of the glass material by the manufacturer. The problem was eliminated by

fine grinding and polishing, and by aging the material in our laboratory for at least six months before its issuance as an SRM. Further information concerning the stability of the optical transmittance of glass filters will be found in a paper by W. R. Blevin [12]. The influence of temperature on the transmittance of glass filters is discussed by A. T. Young [13]. The usefulness of glass filters has beery 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. To date, about 1000 sets of SRM 930 are in use in this country and abroad.

20
2.2.3. SRM 931 The limitations of SRM 930 are due to the nature of the glass material which can be used only

in the visible spectral range from about 440-700 nm. The need for a material that can be used in the ultraviolet led to the development of the new SRM [14] described below. SRM 931, illustrated in Figure 12, is a liquid filter consisting of three vials containing a solution of nickel, cobalt, and nitrate ion in dilute perchloric acid and a fourth vial containing

only dilute perchloric acid for use as the blank solution. The contents of these vials are trans- ferred to quartz cuvettes with a known radiation pathlength and the transmittances are measured for the three concentrations at 302, 395, 512, and 678 nm using the spectral bandpasses indicated in the certificate. These transmittance values are certified with an uncertainty of 1 percent. SRM 931 has been available from NBS since late 1971, and to date, about 1,050 sets have been sold. Since the use of this liquid SRM requires the availability of cuvettes with a known radiation pathlength, NBS issued a reference cuvette standard (SRM 932) in 1973.

Figure 12. Illustration of SRM 931, Liquid Absorbance Standard; SRM 935, Crystalline Potas- sium Dichromate Ultraviolet Absorbance Standard; and SRM 932, Quartz Cuvette for Spectrophotometry.

2.2.4. SRM 932

This SRM is illustrated in Figure 13 where it can be seen that the all-fused silica cuvette is made from a rectangular fused silica structure from which segments were cut, ground, and polished as close as possible to 10 mm. Each segment was measured with an uncertainty of 0.5 pm with the instru- ment shown in Figure 14, and to each face a fused silica window was attached by fusion. Each window

is 1.5 mm thick and is flat to 2-3 fringes of mercury green line (546.1 nm) and parallel to 0.002 mm. Each cuvette is provided with a Teflon stopper. The fused silica material is of optical quality and nonf luorescent [15].

21
Figure 13. Illustration of SRM 932 Quartz Cuvettes for Spectrophotometry,

showing the fused silica material and parts used for constructing the cuvettes for high-accuracy measurements.

The radiation pathlength is measured at 10 positions along the cuvette with a mechanical instru- ment provided with an electronic lever-type gauge having a resolution of 0.123 urn. The actual usable resolution, however, is 0.312 urn. Figure 15 illustrates this measuring instrument. At the time of calibration, the cuvette pathlengths are certified with an uncertainty of 0.5 um. The first 100 cuvettes were made in the Glass Shop at NBS. Subsequently, cuvettes were made by a commercial manufacturer following the procedures and specifications established at NBS. All are certified by NBS from measurements made in the Mechanical Processes Division in the Metrology Center.

2.2.5. SRM 935 The use of SRM 931 as an ultraviolet standard is limited to a wavelength of 302 nm. Since numerous measurements below this wavelength are performed in many analytical laboratories, an additional material was selected, studied, and finally issued as an SRM for ultraviolet spectro- photometry. This material is crystalline potassium dichromate dissolved in 0.001 N perchloric

acid, and is issued as SRM 935. This material provides the user with five certified absorbance values in the ultraviolet, and

can be used for absorbance linearity verifications at wavelengths near two isosbestic wavelengths, namely 322-323 nm and 345-346 nm. At these points the two chemical species, HCrO; and its dimer

Cr2 0y, present in the dilute perchloric acid solutions of K2 Cr2 0 7 , exhibit the same absorptivity. The wavelength recommended for absorbance linearity measurements is 345 nm where the apparent specific absorbances of the material are sufficiently independent of concentration that absorbance measurements at this wavelength can be used for verifying the absorbance linearity to about one part

in a thousand [16].

It is appropriate to remind spectrophotometri sts that absorbance linearity should not be equated with transmi ttance accuracy. Indeed, as discussed by F. J. J. Clarke "the absorbance linearity or obedience to Beer's law, establishes only that the photoelectronic system obeys a simple power law, not that the power (exponent) is exactly unity" [5]. It must also be kept in mind that, theoretically, Beer's law is valid only at infinite dilutions.

22
23
Figure 15. General view of the standard instrument used to measure the radiation pathlength of assembled cuvettes.

24
The crystalline potassium dichromate, of known purity and homogeneity, is issued in a non- actinic brown glass bottle where it should be kept at room temperature at all times. This SRM is illustrated in Figure 12. In certifying this material, solutions were prepared in 0.001 N perchloric

acid and certified for their apparent specific absorbance at five concentrations between 0.020 to 0.100 g

• kg"1 and at wavelengths of 235, 257, 313, 345, and 350 nm. Fused silica cuvettes with a nominal radiation pathlength of 10 mm and known to 0.5 urn were used in all measurements. The term "apparent" is used throughout both the certificate and the NBS SP260-54 publication to indicate that

the values are not corrected for the effect of internal multiple reflections within the cuvette, nor have the weights been corrected to vacuum. The accurate specific absorbances of acidic potassium

dichromate solutions, corrected for these two parameters are given in Reference 16. Since the radiation pathlength of the cuvettes is a critical factor in the measurements, it is recommended

that the NBS certified fused silica cuvettes SRM 932 be used with this SRM 935. The uncertainty

in the apparent specific absorbance ranged from 0.034 g

• kg-1 at the lowest concentration to 0.010 g

• kg -1 at the higher concentration.

2.2.6. Limitations of SRM's 930, 931, and 935 While each of the SRM's described thus far are useful and have positive qualities, they suffer

from certain inherent limitations. The glass filters, SRM 930, can be used only in the visible

spectral range; liquid standard SRM 931 provides only one calibration point in the ultraviolet; and

the potassium dichromate, SRM 935, requires that accurate solutions be prepared, and that cuvettes with accurate radiation pathlength be available. Furthermore, this material is useful for providing calibration data only in the ultraviolet. In addition, a common limitation of these SRM's, is that

they require the use of spectrophotometers capable of providing narrow spectral bandpasses.

2.2.7. SRM 2031

To eliminate these limitations, a number of materials were studied to determine their usefulness

as transmittance SRM's that could be used in the ultraviolet and visible spectral regions with little dependence on spectral bandpass, and could be used directly without any laboratory preparation. The material finally selected was the semi transparent evaporated metal-on-fused silica filters. Although

this material was available from the early stage of the project, the problem of protecting the exposed surface of the metal layer from deleterious environmental action had to be solved before further work could be undertaken to determine the suitability of this material as a transmittance

SRM [17]. The protection of the exposed metal semi transparent layer was achieved by covering the surface with a clear fused silica plate applied by optical contact. Under these circumstances the fused silica plate carrying the evaporated metal layer with the clear plate behave optically as if they were a single solid semi transparent block. A detailed study was undertaken to assess the optical transmittance properties such as: spectral transmittance, stability, scatter, polarization, uniformity, position dependence, aging, capability of interlaboratory exchange of measurements. Based on the results obtained in this study, the material was issued as an SRM for ultraviolet and visible spectrophotometry, and is illustrated in Figure 16. The standard consists of three filters, all made from nonfluorescent, optical grade fused silica material. Two of the filters are made from a sandwich of two fused silica plates, each 1.5 mm thick, one plate carrying the semi transparent evaporated metal layer, while the other plate is transparent. The plates are ground and polished to achieve optical contact. The third filter is made from a single plate of transparent material 3 mm thick. The filters have a nominal transmit- tance of 10, 30, and 90 percent. They are mounted in metal holders provided with plastic shutters

for mechanical protection. A metal cylindrical box is used to store the filters, and an additional empty filter holder is provided to be used in the reference beam of the spectrophotometer, to insure that identical measurement conditions are achieved. This SRM is usually certified at the following 10 wavelengths: 250, 280, 340, 360, 400, 465,

500, 546.1, 590, and 635 nm. Transmissions at additional or different wavelengths can be provided

on request. The uncertainty of the certified transmittance is 1.0 percent.

25
T .0, -I. -V; -* 'V

Figure 16. Illustration of SRM 2031 , Metal -on-Quartz Filters for Spectrophotometry. Top: Cylindrical container with screw cap, both made of black anodized aluminum alloy. Four filter holders can be stored in the container. Bottom: (from left to right): Aluminum alloy filter holder; metal-on-fused silica filter; retaining

spring with nylon screw and washer; assembled unit and two plastic shutters. All metal and plastic parts are flat black.

Of the numerous tests performed on this SRM, only the measurements performed by K. Eckerle at NBS using the high-accuracy spectrophotometer in the Center for Radiation Research will be described. From the data in Tables 6 and 7 it can be concluded that, within the experimental measuring conditions, there is no significant change of transmittance due to scatter. The material is homogenous

and no polarization effect could be detected. Results of inter laboratory measurements on the same filters by NBS and the Naval Weapons Center are illustrated in Table 8 which shows that the comparative transmittance could be achieved with an uncertainty similar to that of the performance

of the instruments themselves.

26
to i- 10 SC0 00 CO o *r Ln in o cri crt crt M N M O

O O O O O- LT) LD

CT» cn CTi CM CM CM O

VO ID IS

CTi O CO o VO IT) O eft en o> o CM C\J CM o

en o o o cn o o o csj co co

r*. in vo

i— CM CM o o o

cr> o

CO Cji Oi o Cn CTi Cn O CM CM CM

f-» <7l O .—

LD U3 lO — \ \ 0"i 00 O —»

CO CTI r— O t-- r-^ co o r-.

ID CO O

o r-~ r» o i— r- o

co co co

O CM CM § co co i— i— o

CO

«a- un in o CM CM CM O

co co co o

•a- in m

CO CO CO O CM CM CM O

o o o o CM CM O

.— r— r- O .— f— i— CM CM CM O

l-» <T> i— O CM CM CO O r— i— — O CM CM CM

LT> <d VO

(Jl CO o

O O O-i o co co cm

CT» CTI CT\ O

CTt CO o

27
Table 7. Measurement of Scatter, Homogeneity, and Polarization on Three Inconel-on-FusedSilica Filters 3

.

1.x, y; n i ; AT (1)°}

R

} AT (90°1

R 1( AVF"i AT ( AVF1

R #20 Filter 1

( + 1 ,0) 0. 0045 0. 10807 0.000018

(-1,0) 0 0045 0. 10798 0. 000009

(0,0) 0 0045 0. 10805 0.000013 0. 10841 0. 000020 0. 10823 0. 000012

(0,0) 0 0012 0. 10801 0. 000027 0. 10840 0. 000020 0. 10821 0. 000017

(0,0) 0 014 0. 10799 0.000010 0. 10831 0.000013 0.10815 0.000008

#20 Filter 2

( + 1 ,0) 0 0045 0. 32089 0. 000035

(-1 ,0) 0 0045 0. 32050 0. 000037

(0,0) 0 0045 0. 32056 0. 000046 0. 3Z09Z 0. UU0030 0. 32081 0. 000027

(0,0) 0 0012 0. 32069 0. 000039 0. 320/9 U. 00004b 0. 32073 0. 000030

(0,0) 0 0014 0.32045 0.000047 0.32050 0.000057 0.32047 0.000037

#20 Filter 3

(+1,0) 0 0045 0.93139 0.000118

(-1,0) 0 0045 0.93160 0.000096

(0,0) 0 0045 0.93131 0.000067 0.93125 0.000064 0.93128 0.000046

(0,0) 0 0012 0.93107 0.000122 0.93095 0.000039 0.93101 0.000064

(0,0) 0 014 0.93089 0.000041 0.93076 0.000080 0.93083 0.000045

NOTE: All measurements were performed at A.546.1 nm. 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 illumi- nating radiation horizontal, vertical, and average of horizontal and vertical, respectively, measured

at A.546.1 nm. Their respective standard errors are A.T R (0°), AT

R (90°), and AT

R (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 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. The transmittance measurements to assess the uniformity of the filters were made at a constant angle of 0.0045 steradians.

28
Table 8. Comparative Measurements of Transmi ttance on Inconel-on-Fused Silica Filters in the Ultraviolet and Visible from 250 nm to 635 nm at NBS and at Naval Weapons Center (NWC). Transmi ttance, Percent Filter

No. Lab. 250 300 340 400 440 465 500 546.1 590 635 NBS 1.492 1.844 1.899 1.973 2.083 2. 171 2.318 2.540 2.750 2.956

45 NWC 1.501 1.837 1.884 1.953 2.070 2.158 2.317 2.552 2.755 2.958

NBS 21.49 21.39 20.46 19.05 18.45 18.20 18.01 17.94 17.98 18.07

46 NWC 21.50 21.37 20.39 19.04 18.38 18.19 18.06 17.99 17.923

18. 10 NBS 28.73 29.96 30.24 29.99 29.70 29.56 29.44 29.41 29.45 29.55

47 NWC 28.80 29.89 30.23 29.95 29.68 29.55 29.45 29.42 29.43 29.48

NBS 91.25 92.28 92.64 92.91 93.00 93.04 93.10 93.15 93.19 93.22

48

NWC 91.21 92.22 92.59 92.90 92.98 93.01 93.14 93.19 93.21 93.24

aValue corrected as indicated in J. M. Bennett communication.

2.2.8. SRM's 2009, 2010, 2013, and 2014

The SRM's discussed thus far are intended to verify the accuracy of transmi ttance measurements

and the radiation lightpath. Certified materials to verify the wavelength accuracy and to assess

the stray radiant energy are also available from NBS. The SRM's for verifying the wavelength accuracy of conventional spectrophotometers are SRM's

2009, 2010, 2013, and 2014. They were calibrated by Venable and Eckerle of the NBS Center for Radiation Research [18]. Each of these standards consist of a didymium glass filter having a nominal spectral transmittance shown in Figure 17. These filters are produced in two nominal

sizes: 51 mm x 51 mm x 3 mm thick and 10 mm x 30 mm x 3 mm thick. They are mounted in a metal holder that fits into the cuvette holder of the spectrophotometer. They are individually calibrated

(SRM's 2013 and 2014) or batch calibrated (SRM's 2009 and 2010), and, depending on the accuracy

desired, one or the other SRM should be used. The spectral bandpass-wavelength dependence is described in detail 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.

2.2.9. SRM's 2032 and 2033

An instrumental parameter that can measurably affect the accuracy of transmittance is the stray radiant energy (SRE) or stray light. This can be defined generally as unwanted radiation that is measured by the photodetector, and contributes to the degradation of optical transmittance measurements, and to the reduction of specificity, sensitivity, and linearity of absorbance measurements. Two major types of SRE can be identified: heterochromatic and i sochromatic. 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. From these simple definitions it might be concluded that both stray lights 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. This situation is discussed in some detail in an NBS Special Publication, "Stray Radiant Energy in Ultraviolet Molecular Absorption Spectrophotometry, SRM's 2032 and 2033", R. Mavrodineanu and R. W. Burke (in press).

29
350 360 370 380 390 400 420 440 460 480 500 WAVELENGTH. NANOMETERS

550 600 650

1.00

80U

8.4 nm

L/8.7

9.7 nm

3.8 nm

.56 0.60 0.70 0.1 00 1.20 1.40 1.60 1.80 2.00 WAVELENGTH MICROMETERS2.20 2.40

Figure 17. Transmittance of a didymium glass filter recorded at the Corning Glass Works. The spectral bandpass used is marked at the corresponding transmittance minimum.

At this time, two SRM's (SRM's 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 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 specific absorbances at eight wavelengths from 240-275 nm in increments of 5 nm with an uncertainty of 5 percent. The measurement

consists of setting the spectrometer at a wavelength below 260 nm, say 255 nm, and any appreciable amount of light detected is heterochromatic stray light. Isochromatic stray light can be assessed with the help of SRM 2033. This material consists of two filters made from a semitransparent evaporated metal on a fused silica substrate each of which

has a nominal transmittance of 10 percent. One filter is mounted in a metal holder that can be

30
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, as illustrated in Figure 18. This system is used to assess the maximum isochromatic stray light in the sample com- partment 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.

II 2i 3| V si 61 f\ a-. % \0\

Figure 18. Illustration of the radiation attenuator consisting of a metal filter holder provided with a filter made of a semitransparent evaporated metal-on-fused silica

plate. Two plastic shutters are also shown, one is provided with a second filter consisting of a semitransparent evaporated metal-on-fused silica plate. Each filter has a nominal transmittance of 10 percent. The other shutter is opaque. The unit is stored in the plastic container shown in the figure.

31
The 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. This signal is the isochromatic stray light. Before concluding this summary description of the SRM's developed at NBS in the field of spec- trophotometry, we would like- to mention the availability of an SRM for fluorescence measurements.

This material, SRM 936, quinine sulfate dihydrate is certified for the relative molecular emission

spectrum in radiometric units from 375-675 nm at 5 nm intervals. The certification was made by

R. A. Velapoldi and K. D. Mielenz using a specially developed reference fluorescence spectrometer

built by K. D. Mielenz [19,20].

3. Future Developments in the Field of SRM's for Spectrophotometry

The program at NBS for the continuation of basic research in the field of high-accuracy spectro- photometry and for production of other SRM's in this field is active, and the following projects are presently being considered: - Production of a wavelength standard having reference values in the ultraviolet and having sharper absorption peaks, such as those exhibited by holmium oxide solutions in perchloric

acid. - Basic study on the nature and measurements of stray radiant energy, and development of addi- tional SRM's in this field, extending the assessment of this parameter closer to the visible,

i.e., 400 nm. - Extending the certification of solid or liquid transmittance standards into the near-infrared

spectral region. This will require the extension of the measuring capabilities of the NBS high-accuracy spectrophotometers into the near-infrared to 3 urn. The implementation of these and other projects in this field depends on the availability of resources and the demand from the users of spectrophotometers.

4. Transmittance Transfer Standards from the National Physical Laboratory in Great Britain

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 range. The metal used is a specially selected nichrome alloy. These filters are certified at any selected densities over this spectral range as required by the user. The exposed metal surface of these filters is not protected. 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. To preserve the integrity of the transmittance values certified for the metal -on-fused silica

filters, NPL recommends recertifi cation on an annual basis, and every 2 or 3 years for the neutral glass filters. This information was transmitted to us through a personal letter from

F. J. J. Clarke, and through a paper published by F. J. J. Clarke, M. J. Downs, and W. McGivern

[21],

32
PART TWO

5. Spectrometric Appl ications Ultraviolet and visible spectrophotometry continues to be used extensively for the chemical characterization of numerous types of materials. Today, as in the past, practically all of the measurements used for chemical analyses are made on liquid samples, for these measurements to be meaningful and the compositions subsequently calculated to be accurate, proper attention must be given in general to the operational aspects of the technique and, specifically, to the selectivity

of the method used. Recommendations involving the cuvette and the importance of maintaining good temperature control of the sample during measurement are presented in the two sections that immedi- ately follow. These discussions are followed by concise descriptions of several spectrophotometry

methods that have been in use at NBS for a number of years and are cited here because of their high selectivity.

6. Experimental

6.1. Cuvettes

The proper selection, care, use, and manipulation of cuvettes is one of the most overlooked operations in the spectrophotometric measurement process. Improper handling of the sample cuvette

has, in many instances, undoubtedly contributed to the uncertainty of the results. A recent paper

by Rothman et al., [22] has shown that the imprecision of cuvette positioning can be the limiting factor in making precise measurements.

The manufacturers have done a commendable job in making available quality cuvettes at a modest

cost. It is the users' responsibility to avoid any operations that might degrade this quality. The rectangular, nonf luorescent, fused quartz cuvette with fused edges and a tightly-fitting circular

stopper, preferably made from tetraf 1 uoroethylene polymer, is the most desirable and useful type. Upon purchase of a new cuvette, the user should immediately engrave a permanent mark on its side near the top so that measurements are always made with the same orientation of the cuvette in its holder.Numerous procedures have been used for cleaning cuvettes. These include dilute hydrochloric

acid, mild detergent in water, water and alcohol, ethanol-nitric acid, sulfuric acid, and sulfuric acid containing potassium dichromate or potassium permanganate. For most applications, our experi- ence has been that the use of distilled water and ethanol is generally adequate, especially if the cuvette is emptied and cleaned immediately after use. If a stronger cleaning measure is required, immersing the cuvette in either concentrated or, on occasion, fuming sulfuric acid is preferred. Recently, Goddard [23] has recommended a hot solution of tri sodium orthophosphate as providing the most satisfactory and practical cleaning of cuvettes for use in the ultraviolet (see also reference

10). As regards cuvette positioning, highest precision is obtained when the cuvette is placed in the sample holder within the sample compartment of the spectrophotometer and left in that holder until a set of absorbance measurements on a series of standards and sample solutions has been completed. For transferring solutions to and from the cuvette, the Pasteur-type, disposable pipettes have been found to be most useful. Both soft and borosilicate glass pipettes are widely available. In general, the soft glass type should not be used since their surface is sufficiently alkaline to affect the absorbance of pH-sensitive solutions [24]. All measurements are made relative to air and the absorbance of the sample is determined by subtracting the apparent absorbance of the blank

from the solution value. The data in Table 9 illustrate the typical sequence of measurements and the level of precision obtainable on the NBS high-accuracy spectrophotometer. The measurements were made on a set of acidic potassium dichromate solutions used in the certification of the crystalline salt as an SRM [16].

I

33
Table 9. Reproducibility of Absorbance Measurements at 235 Sample Runs. nm for a Cycle of Solvent and

Cuvette No. 14

16

1825

26 Reference filter3

a Inconel on quartz.

Solvent, after 5 rinses 0.05198

0.05099 0.05310

0.05002 0.05064

0.55913

Sample, after 5 rinses 0.30128

0.55052

0.80066

1 . 04863

1.30351 0.55925

Sample, after 7 rinses

0.30141 0.55057

0.80074

1 . 04855

1.30333 0.55936

Solvent, after 5 rinses 0.05213

0.05093

0.05321 0.05022

0.05065

0.55922

The agreement between absorbance measurements made on a set of acidic potassium dichromate

solutions in October 1975 and again in June 1977 (Table 10) illustrates, rather dramatically, the quality of results that can be obtained with accurate instrumentation, stable samples, calibrated

cuvettes, and good technique. Table 10. Absorbances of a Set of 0.001 N Perchloric Acid Solutions of SRM 935

K2 Cr2 0 7 at Time of Preparation and After 20 Months Storage. Nominal 257 n T) (max) 350 n m (max)

K2 Cr2 0 7 Concentration 1975 1977 1975 1977

20 0.2872 0.2871 0.2145 0.2150

40 0.5773 0.5760 0.4305 0.4307

60 0.8674 0.8677 0.6456 0.6450

80 1. 1623 1.1608 0.8612 0.8616

100 1.4559 1.4565 1.0773 1.0755

6.2. TemDerature Control

The importance of controlling temperature, particularly during the spectrophotometric analysis of liquid samples, is well established. Temperature coefficients of absorbance of 0.1 to 0.2 percent per degree Celsius are not unusual. As a rule, at least 2 minutes equilibration time should be allowed

for every degree Celsius above or below ambient that the measurement is performed. In addition, if measurements are made more than ±5 °C from ambient, the analyst must be aware of potential differences

in bath and sample compartment temperatures, even at equilibrium, because of losses from the supply

lines. Some data for a typical thermostating assembly are given in Table 11. The temperatures were measured by placing one end of an i ron-constantan thermocouple in the cuvette solution and the other

end in the constant temperature bath. Table 11. Temperature Differences Between Thermostating Bath

and Cuvette Solution: Ambient T = 24.2 °C. Bath Temperature, °C Cuvette Solution, °C Temperature Differences, °C 15.0 17.4 +2.4

20.0 21.4 +1.4

25.0 24.9 -0.1 30.0 29.1 -0.9

35.0 32.9 -2.1 40.0 37.0 -3.0

34
7. Selected Methodologies

This section describes some of our experiences in applying spectrophotometri c measurements to materials characterization, particularly for the trace inorganic characterization of NBS SRM's. A comparison of the spectrophotometri c values with those obtained by isotope dilution mass spectrometry

(IDMS), as well as by other trace instrumental techniques, has been used to estimate the bias of the spectrophotometri c data.

7.1. Antimony and Arsenic

The spectrophotometri c determination of antimony with brilliant green (BG) and of arsenic with silver diethyldithiocarbamate (AgDEDC) are probably the best wet chemical techniques for the deter- mination of trace amounts of these elements. These methods, as employed for the analysis of both

iron and copper-base alloys, require that the sample be dissolved under conditions such that antimony

and arsenic are obtained in their +3 oxidation states. This is achieved by dissolving the iron or copper alloy in concentrated hydrochloric acid in sealed borosilicate ampoules at a temperature of 200 to 250 °C. The principal steps in the two procedures are summarized in Figure 19. Sealed Tube Dissolution (12N HC1)

Sb +3 (6N HC1)

NaNO,

Sb +3

J As +3 Benzene or

(9N HC1) Chloroform AsCl 30.5N H ? S0l

SbCl

Sat. Na u P 2 0 7 %1 .5N HC1

Zn, HC1 +3 br 2

, A AsH, As AsOl

Ethanol ic BYi 1 1 iant Green

[BG

+ ][SbCl 6 "]/Benzene

(640 nm)

AgDEDC/ Pyridine

Red Complex

(535 nm)

(NH u ) 2 MoO,

Molybdenum Blue

(850 nm) Figure 19. Outline of the spectrophotometri c procedures used for determining antimony

and arsenic in NBS iron and copper base alloys.

The need for certifying antimony in iron base NBS SRM's was initiated by the general opinion among metallurgists in the early 1960's that trace amounts of this element were responsible for the cracking failures of rotor steels. Although this theory for the most part was subsequently disproven,

the original procedure [25] has been used unchanged except for one modification namely preparing the stock solution of brilliant green in ethanol rather than in water. Two important advantages were gained by this change. In the first place, the stability of the dye solution was increased from one

or two days to several months. Secondly, the use of ethanol eliminated the hypsochromic shift and decreased intensity of the absorbance maximum that had been observed previously when sodium nitrite

was used as the oxidant (Figure 20). Nitrite was, and continues to be, the preferred oxidant because close control of its concentration is not as critical as it is for cerium (IV) or hydrogen peroxide. Experiments performed to understand the role of ethanol indicate that, in its absence, nitrite interacts with the dye, most probably by addition of a nitro group to one or more of the ring systems. If ethanol is present, however, when the dye is added, or if the dye is added as an ethanol ic solution, then the excess nitrite remaining after oxidation of Sb(III) to Sb(V) is apparently converted into a nonreactive alkyl derivative (CH 3 CH2 0N0).

35
0.70

Fiqure 20. The effect of various oxidation

procedures on the absorbance

SDectra of the brilliant qreen- hexachloroantimonate complex.

500 600 700 WAVELENGTH, nn

The spectrophotometry determination of arsenic based on the color reaction between arsine and silver diethyldithiocarbamate is presently being used extensively in the analysis and certification

of NBS biological and botanical SRM's. Although the exact composition of the colored product is not

known, the method is straightforward and simple to run. The minimum amount of arsenic that can be determined is about 0.05 ug. Stibine and phosphine interfere by also giving colored products with

AgOEDC. However, their spectra are sufficiently different that their presence may be checked by making absorbance measurements at two wavelengths. If only arsenic is present, the ratio of the absorbance at 505 nm to that obtained at 535 nm (*max

for arsenic complex) should not exceed 0.82. The decision to use the arsi ne-AgDEDC procedure or the molybdenum blue method (Figure 19) is dictated

by the amount of arsenic present. For more than 10 |jg, the latter method is preferred.

In a subsequent publication [26], the possibility of coupling the selective extraction of arsenic (III) with the arsine-AgDEDC method for doing speciation measurements of As(III) and As(V) in organic materials was explored. Although the method works well for aqueous mixtures and may be useful for speciation measurements on natural water samples, it is presently not applicable to the analysis of organic materials until a digestion medium is found in which the original valence states

of arsenic are not altered. Using several mild, nonoxidizing acid media, some As(V) was invariably reduced to As(III), apparently by other solubilized constituents in the samples.

An important application of the antimony and arsenic procedures has been the determination of these elements in the NBS copper benchmark SRM's. The issuance of this series of unalloyed copper

standards will be the culmination of one of the largest undertakings of the NBS Standard Reference Materials Program. More than 70 participants cooperated in the planning of these SRM's, 16 companies contributed base materials, and 22 laboratories have performed cooperative analyses. Currently

11 copper SRM's containing varying levels of trace impurities are available from the NBS Office of Standard Reference Materials.

The NBS values for antimony and arsenic in SRM's 394, 395, and 396 are listed in Table 12 together with the average values obtained from cooperating laboratories.

36
Table 12. Antimony and Arsenic in NBS Copper Benchmark SRM's. ---- Values by Weight, ppm

NBS Values Average of Cooperating Labs. SRM No. Sb As Sb As 394 (Cu I) 4.8a 2.6

a 5.6 3.0

395 (Cu II) 7.9 (8.3)

b 1.8 (2.4)

b 7.0 2.2

396 (Cu III) <0.5 (0.16) <0.5 (<0.03) 2.0 1.5

a Spectrophotometric values. ^Values in parentheses were obtained by spark source mass spectrometry.

The spark source results were calculated on the assumption that the spectrophotometry values

on 394 are accurate. Paulsen [27] has since reported a value of 4.5 ppm for Sb in 394 by isotope dilution-spark source mass spectrometry. Antimony has not been determined to date in 395 or 396 by isotope dilution and arsenic cannot be determined by isotope dilution because it is monoatomic.

The data reported by the cooperating laboratories are based on optical emission methods. Their average values are in good agreement with the NBS results for 394 and 395. However, for 396, their

average value for both antimony and arsenic appear to be at least an order of magnitude high. This discrepancy may be caused by the optical emission methods being used at or near their detection limits

or the standards used containing less antimony and arsenic than assumed.

7.2. Nickel The spectrophotometry determination of nickel by the persul fate-dimethylglyoxime procedure has been studied extensively by Deardorff and Menis [28] and they have applied it exhaustively to the analysis of a wide variety of NBS SRM's. Although the exact nature of the colored complex is not

known, present evidence suggests that nickel is oxidized to the +4 oxidation state. The red complex

forms almost instantaneously when ammonium persulfate is added to a nickel solution containing

citrate ion, the pH is adjusted to 13, and alcoholic dimethylglyoxime is added. The presence of citrate is important not only for preventing hydrolysis but also because it enhances the rate of color formation. Ammonia also appears to play a role in the reaction rate and, in addition, it reduces the interference from copper. Cobalt is the most serious interference because its reaction with dimethylglyoxime is relatively slow under the experimental conditions used and, if significant

amounts are present, the absorbance at 465 nm slowly increases with time. Amounts of cobalt less than 200 ug do not interfere in the determination of as little as 20 ug of nickel if the absorbance

is measured within 15 minutes. When larger amounts of cobalt are present, the absorbance is measured

at 465 and 410 nm and an appropriate correction is applied to the nickel results. The interference

from copper is corrected for in a similar fashion. By performing absorbance measurements at these

two wavelengths and utilizing the fact that formation of the cobalt complex is slow, appropriate corrections can be determined for both copper and cobalt. Conversely, these corrections can and

have been used to estimate any copper, cobalt, or copper plus cobalt that may be present.

Iron and manganese in amounts greater than 1000 and 250 ug, respectively, in a final volume of

50 mL interfere by formation of precipitates. The volume of test solution must be sufficiently

small to keep manganese below this level. The interferences of large amounts of iron is eliminated

by extraction with a 1:1 mixture of methyl isobutyl ketone and amyl acetate.

To date, the persul fate-dimethylglyoxime procedure has been used for the spectrophotometry

determination of nickel in more than 50 SRM's. Concentrations measured have ranged from 0.005 to 80 percent. The scope of the method and the agreement of the spectrophotometry results with the classical gravimetric values are illustrated by the data given in Table 13.

37
Table 13. Comparative Determinations of Nickel in Various SRM's by Gravimetry and by the Persulfate-Dimethylglyoxime Method [28].

SRM Mat I la pHI I ala I Grav i metri c Spectrometri c Di f f erence Cast i ron 0.018 0 016 -0. 002 153a Co8-Mo9-W2-Cr4-V2 steel

3 0.168 0.160 -0.008

37e Sheet brass 0.53 0.52 -0.01 82b Nickel-chromium cast iron 3 1.22 1.22 0.00

346 Valve steel (Cr22-Ni4-Mn9) a 3.94 3.94 0.00

167 Cobalt-base alloy Co43-Mo4-Nb3-W4 20.65 20.57 -0.08

1193 High temperature alloy W 545 23.38 28.33 -0.05

Permal loy 79.62 79.75 +0.13

a Iron preextracted.

To improve precision and accuracy, differential measurements are recommended when more than 200 ug of nickel are present. Successful analyses have been performed using as much as 800 ug of nickel in a final volume of 50 mL (A S 4) for the reference solution. Unfortunately, IDMS values are not available for nickel in any of the above SRM's and the accuracy of the spectrophotometric results cannot be established unequivocally. Although the agree- ment between the gravimetric and spectrometric data are excellent, the possibility exists, however

remote, that both techniques have the same bias. One set of analyses in which this situation occurred

involved the determination of nickel in the 500 and 50-ppm trace elements in Glass SRM's (SRM 610 and

612, respectively). For the 500-ppm glass, the following average values were obtained: spectro- photometry, 451 ppm; polarography , 450 ppm, and IDMS, 458.7 ppm. Similarly, for the 50-ppm glass,

the average values were 37.6, 37.8, and 38.8 ppm, respectively. A comparative plot of the spectro- photometric and IDMS results is shown in Figure 21.

ROD NUMBER

Figure 21. Comparison of spectrophotometric and IDMS results for nickel in NBS Trace- Elements in-Glass SRM 612.

38
Although the spectrophotometry and polarographic values were in agreement, both exhibited

negative biases of almost two percent for the 500-ppm glass and 3 percent for the 50-ppm glass. In analyzing the possible sources of these biases, it is highly unlikely that the two techniques had inherent errors of these magnitudes in the measurement steps. Consequently, the low results can best

be explained by the loss of nickel during sample dissolution. The base glass from which these SRM's were prepared -contained approximately 12 percent CaO and included in the 61 doping elements were all of the rare earths. Both calcium and the rare earth elements form relatively insoluble fluoride

salts. Since hydrofluoric acid was used for sample dissolution, it is highly possible that the losses resulted from some coprecipitation or adsorption of nickel on the insoluble fluoride residues.

7.3. Simultaneous Determination of Cobalt, Nickel, and CoDoer

One of the few spectrophotometric reagents that we have used successfully for simultaneous multielement analysis is 2,3-quinoxal i nedi thiol . In aqueous ethanol , cobalt (II), nickel (II), and copper (I) react instantaneously with this reagent at pH 2 to form intensely colored complexes with absorbance maxima at 510, 606 and 665, and 625 nm, respectively. At pH 6, the reaction of copper is virtually eliminated while the cobalt and nickel reactions exhibit only slight decreases in sensitivity. The absorbances of these complexes are additive for all combinations of these elements in which the individual concentrations are varied from 0.1 to 1 ppm. The wavelength differences in their absorbance maxima in conjunction with the sensitivity of the copper complex to pH and the additivity of the absorbances of the three complexes has been made the basis for the simultaneous

method. The details of this method have been described previously [29]. In practice, two sample aliquots are required for analysis, one for color development at pH 2 and the other for color development at pH 6. Iron, platinum, and palladium are serious interferences. For analysis of iron-base alloys, the iron is readily extracted with a 1:1 mixture of methyl isobutyl ketone and amyl acetate. Two extrac- tions from 6 to 8 N hydrochloric acid are generally sufficient. The results obtainable for some typical SRM's is shown in Table 14.

Table 14. Simultaneous Determination of Cobalt, Nickel, and Copper in NBS SRM's. Certified Value,

SRM No. Type wei qht % Found, wei qht %

462 Low alloy steel Co 0.11 0.106

Ni 0.70 0.687

Cu 0.20 0.207 466 Ingot iron Co 0.046 0.046

Ni 0.051 0.051 Cu 0.033 0.033

55e Open-hearth iron Co 0.007 0.0065

Ni 0.038 0.037

Cu 0.065 0.062

87a Aluminum-silicon alloy Co <0.01

Ni CL57 0.568

Cu 0.30 0.301

7.4. CoDDer

The simplest spectrophotometric analysis that we perform is the determination of copper as the dibenzyldithiocarbamate complex. The yellow-brown copper complex i^max

= 435 nm) is formed and simultaneously extracted by equilibrating the sample solution for 1 to 2 minutes with a 0.01 percent solution of zinc dibenzyldithiocarbamate in either chloroform or carbon tetrachloride. The use of the colorless zinc chelate as the chromogenic reagent increases the selectivity of the method many common elements such as iron, chromium, or manganese form less stable complexes than zinc and do not undergo the exchange reaction. Selectivity is enhanced further by performing the extractions from

1 to 10 N hydrochloric or sulfuric acid. Under these conditions, bismuth and palladium are the only elements that might interfere by formation of similarly colored complexes. The method has been used most recently for the determination of copper in NBS botanical SRM's. A comparison of the spectro- photometric values and the isotope dilution-spark source mass spectrometric results is shown in

39
Table 15. The differences in the results obtained for copper by these two techniques are believed

not to be significant.

Table 15. Comparison of Spectrophotometric and IDMS Results

for Copper in NBS Botanical SRM's.

SRM No. 1570 Spinach

Material

- - - - - Copper, ppm Spectrophotometr i c 12.0 ± 0.2

a 11.5 ± 0.6

a IDMS

1573 Tomato leaves 10.8 ± 0.2 10.2 ± 0.6

1575 Pine needles 2.9 ± 0.2 2.9 ± 0.4

95% confidence interval

7.5. Other Elements

Antimony, arsenic, nickel, cobalt, and copper are not the only elements that can be conveniently

and accurately determined in a wide variety of matrices spectrophotometrically. The examples cited

were selected because of the frequency of requests that we have for their determination and because

of the long-term experience that we have had in applying these methods to the analysis of NBS SRM's.

As the SRM program continues to expand from the traditional metal standards into such newer areas as clinical, environmental, and biological standards, the analytical needs also continue to change. If this paper were being written three years hence, the elements cited might more appropriately include

selenium, lead, beryllium, molybdenum, iron, chromium, boron, titanium, zirconium, and zinc.

Since Tschugaeff first described the synthesis and use of dimethylglyoxime for the detection of nickel in 1905, analysts have optimistically sought new and more sensitive and selective organic

reagents for use in inorganic analysis. While many useful reagents and methods have evolved there

have been no dramatic breakthroughs. Recently, perhaps the most outstanding approach of note for improving concomitantly sensitivity and selectivity has been the development of methods based on the formation of ternary complexes. Although the molar absorpti vities of complexes used in inorganic analysis are typically in the range of 10,000 to 30,000 litre mol -1 cm

_1

, Braude [30] has calculated, using molecular dimensions and absorption probability, that the theoretical maximum value of molar absorptivity is about 100,000 litre mol" 1 cm"" 1

. In methods utilizing ternary complex formation, the element of interest reacts not with one ligand, but rather with two, that is, the brilliant green- hexachloroantimonate complex discussed previously. In this way it is possible for a larger and more strongly absorbing organic envelope to be placed around the element than is normally possible and consequently the sensitivity of the resulting ternary system is frequently superior. What is even more important is that the selectivity of ternary complex formation is likely to be much superior to

its binary counterpart. West [31,32] and coworkers have developed a number of useful methods based upon formation of highly colored and selective ternary complexes. The same workers have also advocated overcoming

the sensitivity barrier through use of amplification procedures based upon allowing the element determined to react in a catalytic fashion or by exploiting a favorable stoichiometric relation between the element measured and the element sought. Because of their favorable characteristics and

the fact that they lend themselves well to systematic study and application, the areas involving ternary complex formation and amplification techniques appear to merit considerable future attention. Probably the most exciting and challenging development at the moment in molecular absorption spectrometry is the possibility of using high performance liquid chromatography (HPLC) to separate mixtures of metal chelates prior to their subsequent quantitation through dynamic coupling of the chromatograph to a variable wavelength, ultraviolet-visible spectrophotometer. While HPLC has experienced a tremendous growth in the last seven or eight years, almost all of the work has been in organic rather than inorganic analysis. The worldwide use of this technique for separating complex mixtures of polynuclear aromatic hydrocarbons, drugs, pesticide residues, and the like is now commonplace. Only a few papers have appeared dealing with the HPLC separation of metal chelates and none

have been used for practical analyses. Included among these is the paper by Huber et a!., [33], the

8. Future Developments in New Spectrophotometric Methodologies

40
several publications by Bal 1 schmi ter et al., [34-36], the paper by Uden and Bigley [37], and, most

recently, a paper by Schwedt [38]. Our current research interests are centered on the use of nonselective chelating agents for group solvent extractions of the transition and heavy metals and their subsequent HPLC separation

and quantitation by spectrophotometry detection. Reagents under study include a-pyridylazonaphthol

,

diethyldithiocarbamate, and dibenzyldithiocarbamate. The separation of mixtures of metal complexes with each of these reagents is being examined on several types of column packings and for various compositions of the mobile phase. In general, separations on silica have been superior to those obtained on bonded-phase -NH2 , -CN, and -C 18 packings. A typical chromatogram of a mixture of copper, cobalt, and nickel dibenzyldithiocarbamates is shown in Figure 22. Although data are shown only for three elements, it is apparent that half a dozen or more elements can be separated with baseline resolution and determined in a 30-minute run provided their complexes have appropriate retention times. Presently, the major problem is not obtaining the desired separations but rather preventing on-column degradation of the metal complexes. Silica exhibits quite anomalous behavior

in this respect. For example, the degradation of nickel diethyldithiocarbamate on 5 to 10 urn irregular silica is quite extensive, whereas, on spherical silica of the same particle size, degradation is drastically reduced [39]. Obviously much work remains to be done to understand and control

the various interactions occurring on the column. Provided the problem of degradation can be surmounted, it is refreshing to believe that high performance liquid chromatography in conjunction with spectrophotometry detection can become a very practical and inexpensive multielement

technique. The traditional requirement that an element can be determined accurately in a complex matrix by absorption spectrophotometry only when it has been separated from all interfering elements

is still present in the proposed technique. The difference is that the HPLC operation provides quick and efficient separations i nstrumentally. Our optimism about the future of the technique is augmented further by its potential use in metal lo-organic speciation studies and by the fact that

the principles of isotope dilution can conceivably be applied for the determination of those elements having sufficient differences in isotopic mass and abundance.

0.10 -

0 5 10 TIME, MINUTES

Fiqure 22. Hiqh-performance liquid chromatograDhic separation and spectrophotometry detecti of a mixture of copper, cobalt, and nickel dibenzyldithiocarbamate chelates.

41
9. References

[1] Mielenz, K. D. Anal. Chem. 48: 1093-1094; 1976, reproduced in the Appendix.

[2] Rand, R. N. Clin. Chem. 15: 839-863; 1969. See also J. Res. Nat. Bur. Stand. (U.S.) 76A(5): 499-507 (Phys. and Chem.); 1972 September-October.

[3] See "Spectrophotometry", Sections in NBS Tech. Note 544; 1969 June - 1970 June. pp. 3-41, and

ibid. 584: 1-52; 1970 July - 1971 June, containing the Summaries of Activities of the Analytical Coordination Chemistry Section edited by 0. Menis and J. I. Shultz.

[4] Mavrodineanu, R. See reference 3 and J. Res. Nat. Bur. Stand. (U.S.) 76A(5): 405-425 (Phys. and Chem.); 1972 September-October, reproduced in the Appendix. Also NBS Spec. Publ. 260-68;

1980. pp. 12-16, for further improvements.

[5] Clarke, F. J. J. J. Res. Nat. Bur. Stand. (U.S.) 76A(5): 375-403 (Phys. and Chem.); 1972 September-October.

[6] Mielenz, K. D. ; Eckerle, K. L. Appl. Optics 11(10): 2294-2303; 1972. [7] Mielenz, K. D. ; Eckerle, K. L. ; Madden, R. P.; Reader, J. Appl. Optics 12(7): 1630-1641;

1973. See also NBS Tech. Note 729; 1972 and 913; 1976.

[8] Mielenz, K. D. J. Res. Nat. Bur. Stand. (U.S.) 76A(5): 455-467 (Phys. and Chem.); 1972 September-October.

[9] Adams, B. P. Standardization News 5(1): 26-57; 1977 of the American Society for Testing and Material s. [10] Holland, L. The Properties of Glass Surfaces. London: Chapman and Hall; 1964. 556 pp. See also Putner, T. British J. Appl. Phys. 10: 332-336; 1959 July, and Butler, E. B. ; Johnston,

W. H. Science 120: 543-544; 1954. [11] Mavrodineanu, R. ; Drews, U. W. Anal. Chem. 48; 1976 in Editor's Column.

[12] Blevin, W. R. Intern. J. Optica Acta 6: 99-101; 1959. [13] Young, 'A. T. Mon. Not. R. Astr. Soc. 135: 175-180; 1967. [14] Burke, R. W. ; Deardorff, E. R. ; Menis, 0. J. Res. Nat. Bur. Stand. (U.S.) 76A(5): 469-489

(Phys. and Chem.)-; 1972, reproduced in the Appendix. See also NBS Tech. Notes 544 and 584; 1969 July -1970 June and 1970 July - 1971 June. [15] Mavrodineanu, R. Lazar, J. W. Clin. Chem. 19(9): 1053-1057; 1973. See also NBS Spec. Publ. 260-32; 1973. [16] Burke, R. W. ; Mavrodineanu, R. J. Res. Nat. Bur. Stand. (U.S.) 80A(4): 631-636 (Phys. and Chem.); 1976 July-August, reproduced in the Appendix. See also NBS Spec. Publ. 260-54; 1977. [17] Mavrodineanu, R. J. Res. Nat. Bur. Stand. (U.S.) 80A(4): 637-641 (Phys. and Chem.); 1976 July-August. See also NBS Spec. Publ. 260-68; 1980. [18] Venable, W. H. ; Eckerle, K. L. NBS Spec. Publ. 260-66; 1979. [19] Velapoldi, R. A.; Mielenz, K. D. NBS Spec. Publ. 260-64; 1980. [20] Mielenz, K. D. NBS Spec. Publ. 260, to be published.

[21] Clarke, F. J. J.; Downs, M. J.; McGivern, W. UV Spectrometry Group Bull. 5: 104-108; 1977 December.

[22] Rothman, L. D. ; Crouch, S. R. ; Ingle, J. D. , Jr. Anal. Chem. 47(8): 1226-1233; 1975. [23] Goddard, D. A. UV Spectrometry Group Bull. 4: 19-25; 1976. [24] McCormick, P. G. ; Burke, R. W. ; Doumas, B. T. Clin. Chem. 18(8): 854-856; 1972.

42
[25] Burke, R. W. ; Menis, 0. Anal. Chem. 38(10): 1719-1722; 1966. [26] Diamond stone, B. I.; Burke, R. W. Analyst 102: 613-614; 1977. [27] Paulsen, P. J. personal communication, 1978 May. [28] Deardorff, E. R. ; Menis, 0. NBS Tech. Note 402: 44-56; 1967 July, U.S. Government Printing Office, Washington, DC. [29] Burke, R. W. ; Deardorff, E. R. Talanta 17: 255-264; 1970. [30] Braude, E. A. J. Chem. Soc. 1950: 379-384.

[31] West, T. S. Analyst 91: 69-77; 1966. [32] West, T. S. Trace Characterization - Chemical and Physical. W. W. Meinke and B. F. Scribner,

eds. Nat. Bur. Stand. (U.S.) Monogr. 100; 1967. pp. 215-237.

[33] Huber, J. F. K. ; Kraak, J. C. ; Veening, H. Anal. Chem. 44(9): 1554-1559; 1972. [34] Heizmann, P.; Bal 1 schmiter , K. Z. Anal. Chemie. 266(3): 206-207; 1973. [35] Heizmann, P.; Bal lschmiter, K. J. Chromatography 137: 153-163; 1977. [36] Lohmuller, M. ; Heizmann, P.; Ballschmiter, K. J. Chromatography 137: 165-170; 1977. [37] Uden, P. C. ; Bigley. Anal. Chimica Acta 94: 29-34; 1977. [38] Schwedt, G. Chromatographic 11(3): 145-148; 1978. [39] Burke, R. W. ; Diamondstone, B. I., unpublished results.

The authors gratefully acknowledge the assistance of the following organizations: The National Institute of General Medical Sciences for supporting, in part, the work reported in this publication;

and the Center for Analytical Chemistry Text Editing Facility for typing the manuscript.

APPENDICES

These appendices contain reproductions of several publi- cations which should provide further and more detailed information on the subjects discussed in this work. Copies of the certificates for the SRM's described in this publication

are also included in these appendices.

43
Rapriaud from ANALYTICAL CHEMISTRY. Vol 48. Pan 1093. June 1976

Comments on Spectrometry Nomenclature

Sir As an optical physicist who collaborates with analytical chemist* 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 point- ed out before (/) it seems worthwhile to familiarize the

readers of this journal with the currently prevailing termi- nology of physical optics. Perhaps the following comments may be helpful to editors, authors, and nomenclature committees in establishing a common and consistent terminol- ogy 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/d V. Energy per unit volume.

Flux, dQ/dt. Time rate of energy flow. Flux density, W d<i>/dA. Flux per unit area. Intensity, I = d*/dJl. Flux per unit solid angle. Sterance, L = d

24>/d(Acos6)dn. Flux per unit projected area and unit solid angle. (A number of alternative and additional terms have been

proposed. For example: pointanee 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 fluejice 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 includ- ed 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, u,|J m~J

]. Luminous intensity, / y (lm sr" l |.

Photon sterance, LP [E m~2 sr_l s-1

]. (The basic unit for photon energy used here is the einstetn

|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, u«,» AuJdX [J m~4

]. Radiant energy density per unit wavelength interval. Spectral photon intensity, /„., =» d/p/d<r |£ sr"' s~ l 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 orp 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 or(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 ve or photon yield nP (not energy yield or quantum yield). In this case, spectral radiant yield, = dy^/do, and spectral photon yield, tj Pi „ = dtjp/dff, 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 indi- cated by a subscript (»;„.,) when the spectrai distribution is defined as a derivative, but in functional form |or(X)j 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 ot 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 def44
mitioa of abaorbanca. The International Lighting Vocabulary suggests trarumiMMion density and internal transmission density instead of optical density and absorbance. This is mora 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. 1 believe that all confusion would be avoided by defining: Transmittance, r. Ratio of the flux transmitted by a sample to the flux incident upon it. Internal transmittance, r;. Transmittance exclusive of

losses at boundary surfaces and effects of interre flec- tions between them. Transmittance density, D « —logior. Negative loga- rithm to base ten of transmittance. Absorbance (Internal transmission density), A

-logtoTj. 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 (Aloe), whereas

the International Lighting Vocabulary (2) defines it as in- ternal absorptance per unit path length (dajdb). Similarly,

it specifies transmissiuity 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 Ibe 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 » dajdb (m_l

). Internal absorptance per

unit path length. Specific absorbance, t = Albc (kg-1 m2

]. Absorbance per

unit path length and unit concentration.

Specific molar absorbance, < m = Albcm (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 defi- nitions are: Beer'* 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 angstrim unit in terms of the

red line of cadmium haa been abrogated several years ago.

It is now defined as 10" 10 m. exactly. However, the Ang- strom 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 spec- trometer. 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 spectrofluorimeter 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) a A. W. Rutgers, in "Accuracy in Spactropnotornatry and Lumnaacance Manuramams." Mar. Bur Stand. {U.S.). Spac. Publ. 371. S3 ( 1973).

(2) "mtamauonal Lighting Vocabulary". 3rd ad.. Publication CC No. 17. Parts. 1S70.

(3) Editor's Page. J. Opt. Soc Am.. S7. 854 (1967).

(4) J. Gatst and E. Zalewiki. Appl. Opt.. 12. 435 (1973V

(5) J. J. Murray. F. E. Nicodamua. and L Wundarman. Appt. Opt.. 10, 1405

(1971).

(6) "SpacfromeVy Nomenclature". Ana.. Cham.. 47. 2528 (19751.

(7) The International System of Units (SI)". Hat. fiur. Stand. (US ). Spac

PuM. 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 De- cember 23, 1975.

45
JOURNAL OF RESEARCH of th« National 8ur*au of Standards —A Phytic* and Chemistry Vol 76A. No. S, Soptombor-Octobor 1973 An Accurate Spectrophotometer for Measuring the

Transmittance of Solid and Liquid Materials

R. Mavrodineanu

Anaiyticai Chemistry Division, National Bureau of Standards, Washington, O.C 20234

(May 31, 1971) The optical transmittance of solids and Liquids as well aa 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 perf orm such measurements, a high accuracy spectrophotometer was designed and

assembled at NBS, Analytical Chemistry Division, and will be described in this paper. This single- beam instrument is composed of a constant radiation source, a monocbromator, a sample carnage, an

integrating sphere-photomuitipiier assembly followed by appropriate electronics, and « 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 mode*, will be pre- sented together with its application to the certification of solid and liquid Standard Reference Material*

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 i tensities. It is therefore necessary that the photometr

scale of the spectrophotometer used to perform th measurements be accurate. The transmittance of a pa

ticular material is also a function of wavelength: henc

the wavelength scale of the monochromator should ak

be accurate, and appropriate spectral bandpasse

should be used. The measurements should be maa

using collimated radiations. Such radiations define ur ambiguously the actual path length through the trans mining medium, the reflection losses, and eliminat the effects of polarized radiations that are produced a the surface of the sample. Other important factor which must be considered are: homogeneity and sts bility of the sample, radiation scatter inside the sample

interference phenomena, stray radiation, polarization fluorescence, temperature, particulate matter, and sur face conditions. Since transmittance measurement

depend on a diversity of factors, meaningful values cai be obtained only by defining the experimental condi

tions for obtaining transmittance data [1, 2]l s Spectro photometers are used to perform two types of meas urements:

(1) Quantitative determination of chemical species using the relation between- optical transmission of the material, and the concentration as a measuring param

eter. Under these circumstances, the photometric scale

1 Ftfure* oi bracket* indicate the literature refereacea at the end of thia 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 distort the data.1

46
of the spectrophotometer is calibrated in meaningful

units, 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 solely

to the accuracy with which the concentration of the reference solutions is known and to the precision (sta- bility, sensitivity, reproducibility) of the spectrophoto- metric 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 meas- urements 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 pro- vide 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 deter- mine 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 Phvsical Laboratory (NPL). Teddington. England, where a loni: tradition of high accuracy spectrophotometry exists. The instrument described in this work uerforms 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 inter- ferences 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 10* 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 instru- ment was given earlier in reference [S\

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-beani type, it is essentia! that the radiation source be constani and homogeneous. Additional

desirable conditions are: capability of monitoring the current supDiied to the source and radiation similar

to that trom a Planckian radiator. The source is similar in design to that developed and used al NES

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 1. 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 0.1ft 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

' Tat commeraal umrannu nd pan* mmd u tht

etcr lit Kftentibed id the UttMum.

47
Prwnaon Optul Bitch

FIGURE t. I'nncipie ot the sinttle beam nigh accuracy spectrophotometer. The aperture unit is placed on the optical bench oniv when linear measurements are pertormeil.

istic 01 this function is the ability to automatically change its output voltage to maintain a constant

current ;o the load resistor, which, n our case, is the lamD source. The nominal current regulation obtainea is setter than 0.01 percent, ind the staoility over an 3 hour period, n constant ioad fmperature.

is better than 0.02 percent. The stabiiitv ot tne rurrent

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 po- tentiometer is connected to the current source across

a resistor (O.Olfi and 100 A) placed in series with the lamp. The demagnihed '2 to 1) image ot the nbbon rila- ment is projected on the entrance slit of the pre- disperser by a fused quartz (nonfluorescent SiOi)

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 diami. 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 preaispersu attachment to reduce the strav light (fig. 3). This pr

Figure 2. General views ot the spectrophotometer. Rear: optica bench carrying the tungsten-halogen radiation source used fo checking the alignment of optical components, followed bv a quart

lens, the circular quartz neutral wedge, and a flat mirror. Lett the 1-m Czerny-Turner grating monochromator (the predisperser

is not illustrated heret. Front: optical bench carrying a quart:

lens, the single-sample and biank carnage, a second quartz lens and the integrating sphere with the photomultiplier housinc

470-063 OL - 72 - 3 48
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 bv an enclosure with black

wall* (50 cm x 50 cm x 70 cm hieh). Rear: enclosure containing thr

optical units illustrated in figure 2. disperser is a small quartz prism monochromator

connected to the scanning system of the 1-m instru- ment. 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 optica] components arc placed on precision lathe bed type optical benches whtcr, are 160 and 120 cm

long, and are equipped with appropriate earners

provided with x-y-z adjustments.

c. Sample Carrying Systems. The spectro- photometer is provided with two sample carrying sys- tems. 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 plat- form provided with two vertical holders which can accept i-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 dis- tance 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 lus«d silica lens was removed to show the fused

silica disc with the evaporated metal iaver.

and is located between the two quartz lenses. The

sample holder is designed to accept conventional

solid or liquid filter holders which fit most spectro- photometers. 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 hori- zontally on an appropriate earner 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).

49
FICURE 5. Single coil tungsten halogen lamp j„ the adjus.aole

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 auto- matic computer operating mode is used.

d. System to Check the Accuracy of the Photometric Reading. Since the high accuracy

spectrophotometer is single beam, accurate photo- metric 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 muhple apertures [11, 12, 14, 16. 21. 23. 25, 26. II, 3U|. A novel approach to the problem of accurate

i !f- Pl,tf0rm Whlch cames ,he la">P « 22 "f^" • lx

, ,cr«ws around the ,dEes of feS^ST*- Thr" screws push the *

photometric measurements was described bv 0 Jones and F. j. J. Clarke [24, 29] and by F. DesvVgn TlJ;

^ m A Critical discussio" of soi

fn »n MRQ

acCUrate

. m

spectrophotometry will be fou, an NBS manuscript by Gibson and associates [2< with nn

ad

1?

tI0;. Prindple - Usin« two aP«rtur, with one source of radiation, was chosen for our wor The aperture method for checking the lineantv photometric data was in use at the National Phvs.c Laboratory from about 1930 onwards, and one form of was described by Preston and Cuckow [ll]Tn comun- tion with a single beam spectrophotome er, us"ng KmT^A 9CreK^ °!le yT W< Buchmuller a'n NBS SSL

^"i USCd a tWO aPerture * 5

r^

r°W [14] used a 10 aperture arraneemen while Harding [16] and Cordle and HabeU [25] at \P described a two aperture system. Multiapertures wer and

used by Hoppmann [21], Bischoff [23], Sanders \% Nonaka and Kashima [27]. Finally, Clarke 3

50
Figure 7. Same as figure 2 except tor the sample noider which

in this case is capable of rotating the sample 360° and to cusDiace

it in the x-y direction through the micrometer screws.

Figure 8. Circular platform carrying seven filler holders. The lb position 9 witches |7 sample positions and 8 blank position? i are

visible alonp with the two quartz lenses. The exit slit of the monochromator is at left. discussed in detail the use of a two aperture svstem 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 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 10. Samr as figure 2. In this case the dual-aperture unit

for linearity control is located on the optical bencn 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 piate 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 pro- jected 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 arrange- ment is illustrated in figure 10. No optical element

51
One .penure*!. o^XZZ ^LT"" *" ' W° ' hunmshouid^ be placed between the apenure plate and the monochromator. The linearity check consists f TZTn^ f

0t°/ A .a open Urrem P^edwhSI enue then closed, and then aperture B is ooen ,,13 then closed. The value o{(A)+<R\iJ7 ?

9

, system

values obtained

is with]bo* ate«£e (A^t^ut linear these iwo'^T'.^bMc^

which Is n°rnn^

CaS P'6 "1 shoW9 nonlinearity ^-^^a—- ~d

Ai^^e^n/n

fy1 Sphf

rC and pho*omultipUer

sforf u sut 6 radiatlons «TMtted from the exit of the monochromator and passing through thl aperture or the filter are received'on thl target tt 5r2ng

7

S fc ,0^ °J

hl

TC is ^ated in

FlCURE 12. Front view, n,ot rhthe J i duaJ-apertiire , Jru t. with c apertures open. "e^Tf T be dM,!d b ' remotely

a sh "»" which is opera- by a pneumatic system A 50 mm J- opening

the

is a. the oppos.te end tp°wWch the hotma photcmtiitipher is attached bv an "O" JZ f

? Ssnde a Bgh-tigh, jotnt. The fnside of'°he spnere'

a suspens, °" 01 Baso- •*« .s:

Under these c.rcumstances the sensitive surface theT. raw ,

' he ">°"»chromator onfv ai re?,:.ip„l

at,0n5 haVe U"derg°ne « '« diffto IsoTn'm Tt"

ge °

f ,Uie iS from W»S. ? Spj&^.Ery :„ppt:d

us

r„d ;

850 v T

" l~*%riT e~V5 «i opereLnram';

52
Figure 13. Schematic of the current -to-voltage circuitry. Courtesy of K. W. Yee. The optical component* located after the exit slit of the monochromtitor. 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 aperture's, sample carriage and integrating sphere shuner, 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 monochromator are secured by stop* which are attached to the table surface. The align- ment of the optical parts is made and checked periodi- cally with a low-power laser shown in figure 3 (CW gas

laser, output power 2 mW, X 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 passm? through the aperture system {A. B. A — B) or the samples (/) and

blanks (lo), 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

53
Fl 2iHfJ4

' p»ns°Von

,tainin S

the pow" r supp'y for th « photo- multiplier tube, the dc null detector, the currenfto-voltaee

converter, the digital voltmeter, the command panel for computer operation, and the command panel for the pneumat.c operation of the shutter, aperture svstem. and smele sample carnaee The

electric .witcties tor operating :he • .rcular neutral wedee an- -Uo located on this uanel. Middle n^nt: potentiometer for monitorina the dc current supplied to the tuniaten nbbon nlament lamp. At bottom left: teletvpe for data presentation seveflarK hTf'

' nd' Ca,eS ,he P°S '" on °' the ^tomafc seven sample holder. deviation, and sample position number to identify the measurement, ft hen transmittance measurements

are made on individual samples or when linearitv checks are performed, the reading are initiated manually for every position. When the seven sample

holder is used lor sequential measurements, the operation is performed automatically bv the computer. It is programmed to take a predetermined number of mdividual DVM readings (50), print the arithmetic average, followed bv the standard deviation, relative standard deviation, percent transmit- tance and sample position (fig. 15). At the conclusion

oi each measurement, the computer initiates a si-nal which rotates the holder to the next position. This is followed bv the data taking and sample chanansequence until the measurements are stopped manually or automatically bv a provision made in the computer program.

CAL I BKAT I ON OF SRM930 AT 440 NH SAMPLE NUMBER AND POSITIONl

!^ ?i '! 79 ,N 4; 2-79 ,N * ! REFERENCE 1"

•2001 700E 01 •A41344IE oo •200I230E 01 •A52834<SE 00 •200I040E 01 •4281 1 10E 00 •2000995E 01 •A560I42E 00 •2001402E 01 •23587*2E 00 •2001539E 01

• S739426E oo •200I205E 01 •238A761E 00 •2001 101 E 01 •2001622E 01 •64I2983E 00 •2002043E 01 •A52A482E 00 •20O1551E 01 •4227282E 00 •20O1S07E 01 •ASA20C4E 00 •2P0|S?9E r>, -^:59033E 00 •iOOUHE 01 I099E 00 ->001SH9F 0100 •P001S29F 01 «!P0?2I4F )1

• 64l347aE 00 •20018H2E 01

• *S2M77r oo •20017417 01 •4223418F 00

• 200207SE 01

• *s<, 2 083E 00

• 200I871F 0! 23S878*

>3E

00

• r 00 I ••0388354F 0C

• i.002478F 0 1

s S/AU

• 4388E-03 •2192E-03

• 32S6E-03 •S077E-03

•4320E-03 •2I59E-03 •4804E-03 •73S9E-03

-4254E-03 •2126E-03

•2293E-03 • S432E-03

• 304 IE -0 3 . IS20E-03 •3189E-03 .4861 E-03

.447ISE-03 •e236F-03

• 1425E-03 • '.039E-03

.41 14E-03 .2055E-03

• .0648E-O3 • 4226E-.)3 •-678E-03 • 2338E--J3

• I064E-03 •4456E-03

•-1S0E-03 • 2074E-'I3 •3I87E-03 • 1 S92E-J3

• 1321E-03 •2060E-03 •M39E-03 •30A7E-03

•2474E-03 •3790E-03

•475AE-03 •2376F-03

.2 1 74E-03 • 5149F-03

••435SE-03 .2l7*F-03

. 3344F -03 S095E-03

• J337E-03 •2A*7K-03

• l*8*E-03 • 7992E-03 •-S32E-03 • «!2ft4E-03 •I41MF-03 • c.0->0F-03 •~3<,8E-03 . J 182F-03

1*046-03 A72PF-01

• J3K8E-03 • IA93E-03

.^HJf.E-03 • I 4 I2E-.J3

J 5 1 8E-03 . S4HAF-03

38B2F-0.* • 1939E-03

• 251 3E-0 1 • 38<jlE-03

• 384 1 F -O 3 • 19I9F-03

• TOOOE-n^

•4892F-0j .244.36-03

• 13",5E-C3 •208IE-03

J708E-O.J . kwf-.i:

• 1 596F-03 .A7A7E-03

• :O3AE~03 • -07 I5E-03

• 1022F-..3 —«*i*F-j3

• 4545F--''3

• 1 70HF-.13 •

i 33E-03 •297^E-03 •I48SF-03 mputer data presentation. FlCURE 15. The programming of the entire computer opers was developed by J. Aronson. R. Freemire. an Wing. The computer-instrument interfacing penormed by F. Ruegg and R. Shideler of the

" Analytical Chemistry Division. Technical <er Group, under the supervision of J. DeV'oe.

III. Stability of the Electronic System

As a rule, before taking measurements with spectrophotometer, a warmup period of one hou

required. The room temperature is kept at 24*1

and the relative humidity is 35 percent. The pan

late matter is controlled through special filters wr

rates the room in the 100.000 class. The dark current of the photomultiplier tube measured by taking 15 replications each consign

the average of 50 individual digital voltmeter readir

1 hese measurements were made using 850 V at anode. The average dark current under these cumstances produced 0.000682 V with a relar standard deviation of 0.71 percent.

In all of our work, a dark current buck-out arran ment was used. A series of measurements were t formed to determine the stability of this dark curr compensation. To this effect. 15 consecutive me

urements. each representing the averase of 50 ir

54
vidual digital voltmeter readings, were made and the average dark current value was 0.000024 V with a relative standard deviation of 23.1 percent. Four tests were made to determine the stability of the electronic system and the radiation source using

the computer data acquisition mode.

a. Stability of the Current-to-Voltage Converter. A constant voltage was supplied to the con- verter using the dark current compensation provided on the unit. Fifty individual measurements were taken every 5 seconds and the average value was printed along with its percent standard deviation. The meas- urements were then repeated 15 times and an average

of the 15 values was calculated along with the cor- responding percent standard deviation. These meas- urements were then repeated three times. The results are summarized in table 1. This table also presents

the values for the first group and the average values and corresponding percent standard deviation for the two consecutive groups. It can be seen from the

stability of the current-to-voltage unit that measure- ments can be performed with a reproducibility of about 0.0012 (at the 67% confidence level) expressed

as percent standard deviation for a single determination. The time interval between the first and last 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 Lotion measurements: volts deviation

1 1.003494 0.0013.

2 1.003496 0.0012,

3 1.003482 0.0014,

4 1.003507 0.0011,

5 1.003515 0.00134 6 1.003508 0.0013, 1.003497 O.OOlh

1.003498 0.0013, 1.003505 0.0011,, 10 1.003510 0.0014,

11 1.003518 0.0013s 12 1.003521 0.0012,

13 1.003522 0.0013,

14 1.003507 0.0012,

15 1.003527 0.0013,

Average of replication Percent standard

deviation

Average oi replications Percent standard

deviation

1.003507

0.0012

1.003535

0.0015

First group

Second group

Average oi replications Percent standard

deviation

1.003545 Third group

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 0.0228 for single measurements with 2.00 V at the photomultiplier tube

anode.

55
Replication

123

4

Average of 50 individual measurements: voits 1.012329

1.012342

1.012322 1.012320

1.012394

1.012421 1.012404

1.012406 1.012365 1.012402

1.012465

1.012412

1.012451 1.012417

1.012481

in total dark;

Average of replications Percent standard

1.012395

deviation 0.0050 Average of replications Percent standard 1.012467

'deviation 0.0033 Average of repucat,ons Percent standard 1012510

deviation 0.0035

Percent standard

deviation

0.027, 0.048,

0.027,

0.033,

0.043,

0.015,

o.oia,

0.029, 0.019,

0.019, 0.025,

0.061,

0.023, 0.029.

0.024,

group

ribbon lamp was momWH ?1 l

'° the tUn*sten and the vanat.on J S P W"h tHe P"eTMometer.

Pan in 10» during a eries 0T15

' eSS tha" ° ne urements (5 -fnin) 13 consecutive rneas- a «~ earlier,

to determine the rl 5 me

Laf. uremei«s were made

measurement,. To Z effect

° f lTM*TM*TM

sample

neutral carrying

glass filters system ^^J^t ^t ^ and XTa automatic sample changing operattoV

a^»on and

maticaUv through 12 , Ponied auto- Previouslv, th sarnble e^" " ^ M me"tioned seven samples &

the nonattenuated St oTbeam *

au- and are marked / "hSe th. pass,nS thr°Ugn

respond to measurement thi J TM n Tbers cor" after passing through the ah!n h

radiations -rked , /he

^e^t ^^

j Third group

c"^P<>"*ng-t„ abou, 2.002C

, , Inefl for the nonattenuated beam / tu^ l

for U» 8^TM'^"'"'

/4 7* ^a 7f+7°

; e,c -

r4- IV. Wavelength Calibration

56
Table 3. Stability of the current-to-voltage convener and the photomultiplier tube at 850 V and exposed to the

radiation of a tritium activated fluorescent source

Perceni standard

plication measurements; volls deviation

1 1.536345 0.0275 2 1.536326 0.030,

3 1.536196 0.022. 4 1.536289 0.023,

5 1.536106 0.03O, 6 1.536117 0.031,

7 1.535916 0.025,

8 1.536065 0.023,

9 1.536179 0.029,

10 1.536003 0.023,

11 1.536083 0.021 5 12 1.535961 0.026,

13 1.536052 0.031,

14 1.536095 0.026,

15 1.536092 0.026,

Average of replications Percent standard

deviation

Average of replication? I>rceni standard

deviation

Average of replications Percent standard

deviation

1.536122

0.0082

1.535768

0.0095

1.535522

0.0054

First group

Second p-oup

Third group

accuracv with a low pressure mercury discharge lamp placed before the entrance slit of the monochromalor. 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 waveiengtn?

are needed, a Cd-Hg or a He-discharge lamp could be

used for calibration. The wavelength counter was

then checked usmii the procedure recommended by Gibson [2]. and a sht of 0.1 mm which is equivalent to an effective spectral bandpass of 0.08 nm. The deviation ol the wavelength counter from the true value was found to be less than z: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 deter- mine SRE in the blue and yellow spectral range. In this procedure, a solution of metnyiene blue, which has a strong absorption in the range from A 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 sht of the monochromator and the integrating sphere. The

measurements were performed using a sht of 1 mm by

placing a front surface mirror at the sample position, which reflects to the instrument all radiations received from the exit sht 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 A 577.3 nm. using a radiation flux intensity

five times greater than that used in routine trans- mittance 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

57
Tablk 4. Stability of the current-to-voltage converter, the photomulupli

filament lamp

er tube at 850 V, and the tungsten rib

Replication

12

34

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 Average of replications Percent standard

deviation Average of replications Percent standard

deviation Average of replications Percent standard

deviation Average of replications Percent standard

deviation

2.001441

0.026 1001517

•'.012

2.000826

0.025

2.001268

0.027

Percent standard

deviation 0.038.

0.022.

0.024r 0.026t 0.020T 0.028,

0.026, 0.023, 0.026,

0.021,

0.023, 0.0244 0.028. 0.023, 0.0244

First

Second group

Third group

Percent standard

deviation Q ounn group

Replication No.

2 4 6 33.327 21.711 12.236 33.325 21.710 12.237 33.321 21.711 12.241 33.320 21.708 12.240 33.323 21.710 12.239 33.325 21.710 12.238 33.32 21.710 12.238 0.0080 0.0051 0.0150

Average

Percent tr Average percent <r was opened and measurements were made aaain £no'sRE ; Id i°^

d

?-°37 mV- This mdic«ed

Percent transmittance

50.990

50.983

50.992 50.988

50.983

50.986

50.987

0.0072

10 12 14 33.377 20.906 13.473 33.377 20.903 13.471 33.383 20.900 13.474 33.375 20.901 13.470 33.379 20.901 13.474 33.377 20.904 13.470 33.378 20.902 13.472 0.0083 0.0108 0.0141 0.010 VI. Linearity Control The single-beam static optical svst^m rl««„> k j

<h.s work Permits the meqLitZz ts„:

58
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 rneasurements. 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-voltage 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 k 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 10* [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 Y was obtained when both apertures. A and

E. were open. A setting of 850 Y was used at the pnotomultiplier tube with a 30 Mfi, resistor at trie currentto-voltage converter. Fiftv individual DYM readings were taken and the average value for {A-rB) 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 iA — B) value. Identical measurements were made over a range of attenuation corresponding to 4 cascaded steps of 2

to 1 a.- illustrated in the actual example which follows: Step

Av.

Diff. °k Corr. Step 2

Av.

Diff. % Corr. Step 3

(A^B)

2.00147 2.0015« 2.0020€ 2.0021,

A B

1.01596 0.9864, 1.0159^ 0.9864n

1.0160, 0.9862^ 1.01598 J- 0.98635 = 2.00233

2.00181 2.00233 - 2.00181=- = -0.026

(A + B)

1.0004*

1.0007, 1.00066 1.0009o

A

0.5102o 0.50996 0.51 00<

0.00052 B

0.491

0

8 0.4909, 0.4911 5 0.51007 -r 0.49105=1.00112

1.00071 1.00112-1.00071 = -0.041

(A + B)

0.5006, A

0.2565,

0.00041

B

0.2443o

0.25673 0.2443* 0.25665 0.2443,

Av.

Diff. % Corr. Step 4

Av.

Diff. % Corr.

0.5005* 0.5007o

0.5006; 0.50063 0.50098 - 0.50063 = + 0.00035 = -0.069

0.25665 + 0.24433 = 0.50098

(A-rB)

0.2502o

0.2502*

0.2501 9 0.2502,

A

0.12872 0.12856 0.1285.

B

0.12175 0.1216. 0.1216e 0.12861 + 0.12170 = 0.25031 0.25023

0.25031 - 0.25023 = + 0.00008 = -0.031 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.

- -0.05P-

** t

| -o i o£- /'

s _ci5r/

c

tr U C s J_ VOLTS 3

0.50 1.00 IJO w

625 125 25 50 %T

75 100

FlGURF 10 Linrantv correction curve.

Voltage % T % Correction of % T

2.00 100 0.0 1.00 50 0.026

U.50 25 0.067

0.25 12.5 0.14 0.125 6.25 0.167 When transmittance measurement? 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 70 = 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 is then: 27.081-

f

27- 08^

0 072

) -27.06!.

59
Mielenz and Eckerie have studied recently the double aperture method for testing photometric hneanty, and have used a curve-fitting procedure

tor finding the nonlineantv correction rather than

the method described earlier (38). VII. Sample Position A series of measurements were performed to deter- mine 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 lis described in section II, paragraph c, was used. Trans- mittance 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) , measurec

three neutral glass filters 1.0; 1.5; and 2.0 thick at A = 440 nm, at three angle, incidence

Angle of incidence Normal incidence

1° 2°

3°

Filter 1-0 mm

niter FUt«

1.5 mm 2.0 rr 32.91, 19.83, H.6C

32.89, 19.83, 11.60 32.89T 19.81, 11.59 32.88, 19.81, u.5a

id.SnrTi

ar

/meaSUrement9 Were made 10 determine chTnaZ W ^K10"9 °n the ^even-sample autora. changer described in section II, paragraph c

this experiment, seven neutral glass filters A; B: SnTaT a8 deSCdbed by Mann,

T*.u , Evaluation of the identity of the seven stations of the automatic sample changer

Percent Transm.ttance i%T) at stat.on number , St. No.) 2; 4; 6- 8- 10- 12- and

14 for filter, A, B. C. D. £. F. and G a, . = 465.0 1 ' " Run ;\o. A B C D E F G %T

I St. 27.08 16.44 46.39 -'6.12 15.22 37.34 23.23

No. 2 I 6 3 10 12' 14 %T

II St. 27.07 16.40 46.39 26.11 15.21 37.34 23.23 No. 14 2 4 6 8 10 12 %T

III St. 27.08 16.43 46.39 26.11 15.21 37.33 23.23 No. 10 12 14 2 6 8 on St fiT Lmeasureme«s were then performed Zf ^ f°r the tHree different ^angements and the results are given ,n table 7. From these data it can

ahlJ

et

thaV

hC 9CVen StatIOns « interchange- able and will produce measurements which will not differ by more than one pan in one thousand.

VIII. Influence of Polarized Radiations on

Transmittance Measurements

This effect was determined by measuring the trans- -veTen^^"0" 2^ "Iter alTo" wavelengths using radiations emerging from the ore- disperser-monochromator unit, and by pro^c ,ng the tace of the filter with a convergent beam geometry

fretIrn 7 V

meas cements with a polariscope for show, ?h

,nteraai

L

teTM<>ns. Column one of table 8 shows the results obtained when transmittance measthe urements were made using the radiations product by spectrophotometer. Column two shows the results

obtained when a polarizing sheet, with the vibrate The

plane horizontal, was placed in front of the gla^The

in vertical measurements

pos.non, obtained with the vibrarion p£ are given in column three

? J irrrememS Sh°K tHat PO,arized radi at>on ;^ic fiif transmittance measurements of 90ii, used TWff^" n0"CoiIi

!!,auted beam geometry , mention^ U "> pre<5

cted by tHe Fresnel e<T^on

when colhmated radiations are used (1, pg. 100) XI. Comparison of Transmittance Measurements

tesT^o Zt

°f S0,idu ^st

nlters wer* "sed in a comparativ, to determine the reproducibility of transmittal made

measurements

from between two laboratories. OneTtTa

mg nominal

three neutral glass Schott NG-4 filters hav The second

percent transmittances of 10; 20 id 30 Three

set was made as described elsewhere ,4| evaporated metal (Inconel) on fused aUart (nonfluorescent. plates having nominal perc

e

60
Table 8. Effect of polarization on percent transmittance (%T) measured at four wavelengths on a Schott NG-4

glass filter

Wavelength nm

Produced by

spectro- photometer

Polarizer, plane of vibration horizontal

Polarizer, plane of vibration

vertical

% Difference

440.0

465.0

590.0

635.0

19.81, 22.59,

19.17,

20.61,

19.80o 22.600 19.17o

20.60: 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 slighti\ 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 measurement? were

19.78, 22.56,

19.09, 20.54,

-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 S pectro photometry

The need for providing means and materials U

check the proper functioning of a spectrophotometer was discussed in some detail in previous publications [3. 41. 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 transmittance? (%T\ measured on three Schott NG-4 glass filters at NPL and NBS

'avelenph NBS. %T NBS. kt NBS. %T NPL. %T % Diff nm March 12. Mav 18. average February NBS to Nl

1971 1971 1971

440.0 12.92 12.91 12.91, 12.93 -0.11

465.0 14.96s 14.98 14.973 15.03 - 0.25 590.0 11.70 11.64 11.67 11.67 0.0 635.0 12.72 12.68 12.70 12.72 -0.16

440.0 19.62s 19.58 19.603 19.62 -0.09

465.0 22.38, 22.35 22.36, 22.43 - 0.28 590.0 19.06 18.95 19.00, 19.01 -0.03

635.0 20.45, 20.37 20.41, 20.47 -0.23

440.0 32.89 32.86 32.87, 32.98 -0.32

465.0 35.52 35.54 35.55 35.66 -0.36

590.0 31.16, 31.10 31.13;, 31.21 -0.25

635.0 32.56, 32.52 32.54, 32.62 -0.24

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.

61
Table 10. Comparison between the percent trans nuttances <%T) me„ u NPL and NBS

meMured on three mconel-on-silica fil, Wavelength

450.0

550.0

650.0

450.0

550.0

650.0

450.0 550.0

650.0

NBS.%7"

1

24.87

23.78

23.38

49.35 47.60

+6.85

72.17

72.05 72.20

24.88

23.82 23.39

49.33 47.60

46.85

72.20

72.11 72.34 Average difference between NBS and NPL percent T value,

_ NBS. %T

average

24.87, 23.80

23.38,

49.34 47.60

46.85

72.18, 72.08

72.27

- 0.30 percent.

NPL, %T

24.93

23.86 23.46

49.56

47.81 47.14

72.30

72.20

72.33

% Diff. NBS to NPL

-0.18

-0.25 -0.32

-0.44* -0.44 -0.64

-0.16

-0.17

-0.08

SRM 930 developed in the Analytical Chemistry

thrVr°j;

and„

Jvaila^ stnce March 1971 consiTts of n^K^aS^

hltuerS

- Each fi,ter bears a" identification number, and the upper | eft corner has been removed ^ndicate correct orientation in the metai holder thJ

h

he,i

ranSmittanCe measurTMnts were made with

hfs o£er

aCZaCY ^"Photometer described in /

r

u Cemfied TM uncertainty of sum of he" ra°nH sum

the ^ This un«rtamty is'the ot the random errors of ±0.1 percent (2SD .nut, and of estimated biases wh.ch a? 4 per

errors ori^ ^ TM due l ° possib,e systematic errors ongmatmg pnnc.pally from the inherent mhomogene.ty and instability of the glass as wei made3m

at P

°24

t T

,ng

°i *» Measurements were Sees Celsius „V ^

Vamti°n9 SeveraJ de" Smiv »ffZ ,u H

u

S temPera»"re will not signifi- NG-4 £i! C

tft;ion of *e niters. The neSri

of Mainf r!ri

6 ?

tC?

Was Provided ^ Schott and n

3" 7 '"I

18 de««»««* as "Jena Colored

filter 15 Lm \l

S\ • N

o°ominaJ tran«niittance for a ffSh m

?o

tHlck 13 20 Dercent « 400 0 nm wave we?n the 32

, percenl: at 7°0.0 nm wavelen^h Beof Tile fikerAn^r ' fnUne 3nd the size and *hape

consideration, ^ T 9eleCted

< for P^tical standard ° ^i0TM t0 the dimensions of the

in mo?t f

CUVetteS f°r Which hoIde« are supplied

the^ertiSS I bandP«« <«ed to deter:

at M0 0 „m

V^

e9 WCre

««iuai lo or smaller than 2.1 annT'n

; Z'LT at 465 0 nm'- 5- 4 nm at 590 0 memt dr

at H

635

:° nn\

The transmittance nVeas

tt ,* hy ProducinS the image of the (about geometry

8 mm by 0.5 mm) using a convergent b« with an opening of f:10^1

reproduce the m" beam ge°metrv was Us^

EZa average experimental conditi ete»^SLlS?1 °i theD

conventional spectrophot e era avatlable today. Prior to the certification e ckaned IfThTM cleaned. If. defeCtS and *«TM«

filter through handling, the surface of with becomes contaminated, it may be clear a small soft brush attached to a rubber u Ss from

a

nVaCUUm S°UrCue ^

I^omamYna

before m?Lfin8erPnnts, they must be remo*

oTished K g measu1 rements. This may be acco it hJl rem°V,ng th/

fUter from it3 holder, b«a

b Ufted tith 1trI\

thC 8prin«joaded P^te shou

Under no circumstances should other ^L,

cat*, j, sh„u]d be forwarded ,oOTS Af£

62
Figure J" NBS Standard Keierence Materia! 930 and calibration certificate. recalibrated using the high accuracy spectrophotometer described in this work.

It was already stated that the accuracy of photo- metric 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 J. For instance,

a well-known Dtienomenon 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

63
phenomena ore 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 redistilled ethyl alcohol and pure water (thermally

distilled and deionized). Other cleaning procedures

are under consideration.. The use of isopropyl alcohol

in rapor or liquid form associated with mild ultra- sonic 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 semitrans parent 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 ad- hesion to the substrate. The substrate was nonfluo- rescent fused quartz. A series of filters were prepared by the optical shop at NBS according to the following

specifications: a number of non fluorescent 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 transnuttances 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. 13x13x57 mm) fitting the cuvette holder found in most spectrophotometers. The metal

holder was also marked at one side to permit posi- tioning 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:

Wavelength, nm 250 380 500 650

Before exposure 44.48 5135 48.90 47.41 After exposure 44.11 5134 48.92 47.47 The relative standard deviation for a single deter- mination 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 ab- sorbing. This can produce, in certain circumstances,

undesirable stray light 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 ob- tained 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.

470-063 OL - T2 - 4

64
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 equip- ment 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 ultra- violet: Atlas single coil halogen (Bromine) lamp,

type Pl/8, 30V, 250W: GTE Sylvama. 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 tungsten- halogen 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: Dvnasil Corporation of America, Berlin, New Jersey 08009. Neutral Density Attenuator and BaSO< 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 sampie

holder with micrometer control: Gaertner Scientific Corp., 1201 Wrightwood Ave.. Chicago. Illinois 60614. Lens holders: Ardel Instrument Co.. Inc.. P. 0. 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: Cary Instruments, 2724 South Peck Road,

Monrovia, California 91016. Pneumatic ratchet system: AUenair Corp., P. 0. 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 Divi- sion, 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: Fish- Schurmann 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., Schlumbeiger Co., 8001 Blooniinpton 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. Venabie (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 th» 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. Hofert and A. Reule (C. Zeiss, Oberkochen,

Germany), for valuable information on hneanry of

65
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. iSpnnger-Verlag, 1962).

[2] Gibson. K. S.. Spectrophotometry, Nat. Bur. Stand. (U.S.). Circ. 484. 48 pages ( 1949).

[3 1 Mavrodineanu. R.. Spectrophotometry, instrumental develop- ment, 0. Merus and J. I. Shultz. Eds.. -Nat. Bur. Stana.

fU.S.). Tech. Note 584. pp. 9-21 (197H

[4| Mavrodineanu. R.. Solid materials to check the photometric

scale of spectrophotometers, 0. Menis and J. I. Shultz. Eds.. Nat. Bur. Stand. lU.S.), Tech. Note 544, pp. 6-17 (1970).

[5] Elster. J. and Geitel. M.. Un tne comparison of light intensities by photoelectric methods. Ann. Physik. Chemie 48, 625-635

i 1893).

[6] Elster. J. and Geitel. H.. The photoelectric effect on potassium

at low light levels. Phys. Ztscfa. 13; 468-976 (1912).

[7] Koller. L. R-. and Breeding. H. A.. Characteristics ot 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. Phvs. Soc. (London) 46, 256-272

(1934).

[11] Preston. J. S.. and Cuckow. F. W., A photoelectric spectro- photometer 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 (1947-19491

[16] Harding, H. G. W., Precautions necessary for accurate measure- ments of optical density standards, Photoelectnc Spectrometry Group Bulletin 4, 79-86 ( 1951 )

.

[17] Kortiim. G., and Mater. H.. Dependence of photocurrent and

illuminating intensity in photocells and photomuitipliers, Z. Naturforschung 8A, 235-245 (1953).

[18] Hansen. G.. Check of photometric function of spectrophotometers. Mikrochimica Acta 1955, 410-415.

[19] Hermann. W.. Noise and linearity check on photomuitipliers. Z. Naturforschung 12A, 1006-1013 (1957).

[20] Reule. A., Checking the photometer scale of absorption instru- ments. Zeiss Mitt. 1, 283- 299 (1959).

[21] Hoppmann. H.. An instrument for checking the proportionality

of physical radiation receivers. Technisch-Wissenschaftliche Abhandlungen Der Osram-Gesellschaft 7 , 306- 312 (1958).

[22] Gibson. G. L.. Hammond. H. K.. III. Holford. W. L, and

Nimeroff. I.. Calibration of photometers. NBS Manuscript. Nov. 1, 1960.

[23| Bischoff, K., Measurement of proportionality of radiation re- ceivers over a large radiation intensity range. Z. Instr. 69,

143-147 (1961).

[24] Jones, O. C. and Clarke, F. J. J., A new photometric technique

using a variable shutter device. Nature 191, 1290 (1961).

[25] Cordle. L. C. and Habell. K. J.. Photometry of telescopes and

binoculars. National Physical Laboratory, Notes on Applied Science No. 14. 18 pp. (1962).

[26] Sanders. C. L.. \ photocell iineantv tester. AppL Optics 1, 207-271 (1962).

[27] Nonaica. M.. and Kashima. T.. Lineantv characteristics oi multiplier phototubes. Japanese Journal Applied Physics 2. 785-

791 (1963).

f 28 1 Hofert. H. J., and Loof. H.. Calibration of photometric icales of a reflection photometer. Z. Instrumentenkunde 72. 139-143

(1964).

[29| Clarke. F. J. J.. Time ratio photometry. Lecture to the Optical Group of the Institute of Physics and the Physical Societv (June 9. 1966).

[30] Clarke. F. J. J.. Hiuh accuracy spectrophotometry of three filters. NPL. Report No. 3042. 14 pp. (Not. 4. 19681

[31 1 Reule. A.. Testing spectrophotometer linearity, Appi. Optics 7, 1023-1028(1968).

[32] Desvignes. F.. and Ohnet. J.. Characteristics and measurement

of properties of radiation receivers. Techniques Philips N.6,

1-18 (1968).

[33 1 Kunz. H.. Representation of the temperature scale above 1337.58 K with photoelectric direct current pyrometers. Metrologja 5, 88-102 (1969).

[34] Kostkowski. H. J.. Personal communication (NBS 1970).

[35| ASTM Tentative Method of Estimating Strav Radiant Energy, \STM:E 587-69T.

[36] Clarke. F. J. J.. Personal communication (NPL, August 1970).

[37] Mielenz. K. D.. and Eckerle. K.. Accuracy of polarization attenuators (to be published). AppL Optics (1972V

[38] Mielenz. K. D., and Eckerle. K., Spectrophotometer linearity testing using the double-aperture method, AppL Opt 11, 2294 (1972).

[39] 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. 0J.S.).7OC (Eng. and Instr.). No 2. 53-63 ( 1966).

[40] Ediabury, J. R.. Practical Hints on Absorption Spectrophotometry (Plenum Press, New York, 1967).

[41] Holland. L, The Properties of Glass Surfaces (Chapman and

Hall, LondonJ9641

[42] Haas, G„ and Thun. R. E.. Editors, Physics of Thin Films (Academic Press. 1967). (Paper 76A5-729)

66
JOURNAL OF RESEARCH of th« Nationol Bureau of Standard! — A Phyiici and Chemutry

Vol. 76A, No. 5. Seotember-Odober 1 972 Liquid Absorbance Standards

R. W. Burke, E. R. Deardorff, and O. Menis

Institute for Materials Research, National Bureau of Standards, Washington, D.C. 20234

(JuiM 14, 1972)

Errors in the measurement of the absorbances of liquid filters result from instrumental and chemical uncertainties. This paper presents a systematic study of these variables on the absorbances of selected filters. Three types of liquid filters are discussed. These are (1) individual solutions of high

purity compounds, (2) composite mixtures and (3) aqueous solutions of organic dyes. The accuracy of the absorptivity data is established using NBS-calibrated glass filters. The magnitude of the errors

arising from spectral bandpass, beam geometry, stray light, internal multiple reflections, and refractive index are delineated. Finally, as a practical outgrowth of this study, the development and issuance

of NBS Standard Reference Material 931, Liquid Absorbance Standards for Ultraviolet and Visible Spectrophotometry, is described. Key words: Absorptivity data; accuracy; liquid absorbance standards; Standard Reference Materials.

I. Introduction

In the use of filters for checking the accuracy of the photometric scale of spectrophotometers, one

needs materials which exhibit absolute spectral neutrality. Such ideal filters are not available at present. The use of the materials which have been suggested is limited by instrumental and sample variables. For this reason, one must carefully specify conditions which

define the other sources of errors. The first part of this paper reviews several of the more important instrumental parameters and dis- cusses their effects on absorbance measurements. Examples from the literature are given to illustrate the magnitude of the errors arising from spectral bandpass, stray light, nonparallel radiation, and

multiple reflections. The second part presents some

experimental studies of several materials which may

serve as calibration "filters" or standards.

II. Instrumental Parameters

A. Wavelength Accuracy The absorbances of most samples are sufficiently wavelength dependent that even in the most favorable

regions of maxima and minima a wavelength error of 1—2 nm can produce absorbance errors of a few

tenths of a percent. When performing measurements

on the slopes of absorption peaks, the wavelength

setting is obviously much more critical and errors are

typically several percent per nanometer. Wavelength accuracy, therefore, becomes especially important when making absorbance measurements

at, for example, an isosbestic point.

A line source provides the most definitive means for establishing the accuracy of the wavelength scale. The best single source is a mercury lamp which may

be used throughout the range of 200-1000 nm. For

calibration in the visible region, helium lines are

also useful. Gibson [l] 1 has discussed the use of mercury and

helium sources and has listed those lines best suited

for wavelength calibration. Other sources which have been used include neon, cadmium, cesium, and sodium. The M.I.T. Wavelength Tables [2] summarizes the most

prominent emission lines of these elements. Not all wavelengths are given and care must be taken in using any of these lines in order not to confuse closely adjacent lines with the one being checked. For those spectrophotometers having a hydrogen or deuterium

source, the emission lines at 486.13 and 656.28 nm

(H) or 485.99 and 656.10 nm (D) may provide con- venient checks at these wavelengths.

Calibrated holmium oxide and didymium glasses may

be useful secondary standards, particularly for checking recording spectrophotometers in which a dynamic

check of the instrument is often desirable. The

apparent absorption maxima of these filters, however, may vary with spectral bandpass. Therefore, for the

highest accuracy, they should be calibrated for the

spectral bandpass at which they are to be used.

In general, prism instruments require more frequent

calibration than the grating type because their dis- persion is temperature dependent. Corrections in the

visible region are of the order of 0.1 nm per degree

Celsius. Hysteresis effects must also be considered

1 Figure* in bracket* indicate tbe ntereture reference* u the end of thi* paper.

67
and the wavelength settings should always be ap- proached from the same direction.

B. Finite Bandwidth

Spectral bandpass differences undoubtedly account

for a significant portion of the discrepancies in molar

absorptivity values reported in the literature. Some

have resulted from the employment of inadequate in- strumentation; others have resulted from improper use

of these instruments or complications arising from

solution equilibria. Increasing the spectral bandpass at an absorption maximum has two effects on apparent peak heights:

(1) the observed values are always less than the

true values and (2) the differences between the two

are proportionally greater at higher absorbances. Thus, for a finite bandpass, a plot of absorbance

versus concentration or path length will always have

a smaller slope than it does in monochromatic radiation and, in addition, will be concave to the concentration axis. Figure 1 illustrates these effects. The reverse behavior will be observed for measure- ments at an absorption minimum. Meehan [3] has

given a simple example which verifies these two

effects mathematically.

CONCENTRATION (PATH LENGTH)

FIGURE 1. Effect of spectral bandpass on absorbance: (a) Monochromatic radiation (b) Finite bandwidth. Consider that an absorbing system is illuminated by two monochromatic radiations of wavelengths

Xi and X2 and that the Lambert-Beer law is obeyed

at each wavelength with absorptivities a x and a2 . If the effective intensities of the two wavelengths are

Ii and / 2 , respectively, the trans mittance, T, is T= (7,-10- a «*c +h 10-«2»c )/(/, + /,) (1) where 6 = path length and c = concentration. If Xi

is the wavelength of maximum absorption so that ax > a2 , then

T= -

|

-10— + r.i0<«> (2) where r = /2//,. The absorbance, A, defined as — log FisA = aybc + log ( 1 + r) - log [1 + r-W*i—*»e

] (3) which may be differentiated with respect to 6c to give dAld(bc) = a, - [r(a, - a2 ) 10<"i—«»*]/

[I + r-10*«i-€«>*c

] (4) At low absorbances dA/dibcn^-j—

(5) whereas when be becomes large, 10a,6c ^lO"** and

the limiting slope isdAld(bc) »c _. = a«. (6) Thus, the limiting absorptivity at low absorbances

is a weighted average of a\ and a* while, at higher absorbances, it is equal to az, the smaller of the

individual absorptivities. The theory for the correction of spectral bandpass

errors has been developed by Hardy and Young [4], Eberhardt [5], and Broderson [6J. Rigorous calculation requires integration of the relation

f/xSx<fX ^0b8=l0g-7J (7) J/jySxlO-'dA where I\ is the incident intensity andSx is the spectral

sensitivity of the detector. Equation (7) emphasizes

the fact that the observed absorbance depends not only on the shape of the absorption curve, but also on

the wavelength distribution of the source and the detector response. If /*Sx is constant over the wavelength interval used, approximate corrections for spectral bandpass may be calculated by assuming a Gaussian shape for the absorption peak and a tri- angular slit function for the instrument. Some calculations are given in table 1 based on these assumptions. The tabulated values of AobJA agree closely with those observed experimentally for A up to 1. As

shown by Broderson [6] however, for values of RBW

larger than 0.5, A^JA also depends on A and the above treatment is no longer applicable.

C. Stray Light Stray light is defined as any light outside the spectral region isolated by the monochromator that reaches the

detector. It is produced by scatter from the optics and

68
Table 1. Dependence of Aot>t on spectral bandpass

at an absorption maximum under 1 conditions (see text) RBWa A„JA RBW a A aJA

0.0100 0.99995 0.0800 0.9970

.0200 .9998 .0900 .9962 .0300 .9995 .1000 .9954

.0400 .9992 .2000 .9819 .0500 .9988 .3000 .9604 .0600 .9983 .4000 .9321 .0700 .9977 .5000 .8987

" Relative bandwidth: Ratio of spectral band- width to half bandwidth of fully resolved peak. walls of the monochromator and is present in varying amounts in all spectrophotometers.

Stray light can lead to varied problems in spectro- photometry. Spurious absorption bands may arise in some cases. More frequently, however, deviations from the Lambert-Beer law are produced. These

deviations are positive if the stray light is absorbed and

negative if it is not. Extensive literature exists on the detection, measure- ment and mimization of stray light. Several of the more useful papers are those by Hogness, Zscheile and Sidwell [7], Perry [8], Slavin [9], and Poulson [10]

in which additional references may be found. The amount of stray light present is proportionally

large in those wavelength regions where the trans- mission of the monochromator, the source intensity or the detector sensitivity are relatively low. These

regions, which should first be checked, are 200-230 nm and 600-700 nm. If a tungsten lamp must be used

in the range 350-400 nm, a visible cutoff filter should always be employed. The quantitative assessment of stray light requires

the use of extremely sharp cutoff filters. Slavin [9] and Poulson [10] have described a number of liquid and solid filters that may be used. Aqueous solutions of the alkali halides, for example, are extremely useful

in the ultraviolet. Care must be taken, however, that

light leaks in the cuvet compartment are not contribut- ing to the observed results. The apparent stray light will not vary appreciably with slitwidth if only instru- mental stray light is present. In a quick test for light leaks, the apparent stray light will decrease rapidly as the slits are opened. The stray light error encountered most frequently in spectrophotometry produces a decrease in an absorption peak and leads to negative deviations from the Lambert-Beer law. If it is assumed that none of the

stray light is absorbed by the sample, the measured

absorbance is related to the true transmittance by the expression [9]^obs =iog [d-ns+n- (8) where S is the instrumental stray light expressed as a

fraction. The effects of several levels of unabsorbed

stray light on absorbance are given in table 2. Most

470-OM OL - 73 - 7

samples absorb sufficiently in other regions of the spec- trum to filter out a proportion of the stray liyht. Thus,

indirect estimates of instrumental stray light based on

deviations from lineanty are generally low. Table 2. Effect of unabsorbed straylight on absorbance

A A^bt S = 0.0001 S = 0.001 S = 0.01

0.1 0.1000 0.0999 0.0989

.5 .4999 .4990 .4907

1.0 .99% .9%1 .%26

1.5 1.499 1.487 1.384

2.0 1.9% 1.959 1.701 D. Nonparallel Incident Radiation A perfectly parallel beam of radiation must come

from a point source and can carry only an infini- tesimal amount of energy. The light beam in a spectro- photometer always has some finite angular size. As a

result, the average light path is greater than the per- pendicular distance between the cuvet faces. Meehan

[3] has considered the case in which the incident

radiation is convergent or divergent in one dimension such as corresponds, respectively, to radiation focused by a cylindrical lens or leaving a narrow slit. If 0 and R are the angles of incidence and refraction, re- spectively, the path length of the extreme ray (fig. 2)

is 6/cos R and the absorbance of this ray is A/cos R. Other rays enter at smaller angles and the observed absorbance is given by

h b H

Figure 2. Nonparallel incident radiation [3]. Reprinted by permission of John Wiley & Sons. Incorporated.

<4ob, =

j""""" 1/cos RdR. (9)

For this relation to hold, the incident beam must be

uniform over its cross section. Evaluation of this integral for Rm„ of 2-10° (Gm„ = 2.7

0-13.40

) gives the following percent errors in A (table 3). Thus, for angles of incidence up to 5 degrees, the absorbance

error is less than 0.1 percent.

E. Multiple Reflections On passage of light through a cuvet containing

solvent or sample solution, some radiation is reflected

69
Table 3. Dependence of absorbance error on nonparailelism of incident radiation Rmtx , degrees ©ra «x. degrees Percent error in A

2.7

5.3 8.0

10.7 13.4

at each of the two air-glass and the two glass-liquid surfaces. For perpendicular incidence, the fraction / reflected on passing from a medium of refractive index n.i to a second having retractive index n-i is given by the Fresnel expression

L/ir+TizJ

(10)

For an air-glass surface, / = 0.04 and for a glass- liquid surface, /= 0.0035. When more than one surface

is involved, the effects of multiple reflections must be

considered. The essential question when dealing with

liquid filters is whether the solvent completely compensates for such reflections. According to Goldring

et al. [11], it does not. They consider the case where the

reflections from all surfaces perpendicular to the

light beam on the two sides of the sample are grouped

together to form two effective reflection coefficients, n

and r2 , as shown in figure 3. Considering only first and

second order reflections, the observed absorbance is

WVT-Tr;

FIGURE 3. Internal multiple reflections (11). Reprinted by permission of the American Chemical Society.

^obi = log— =log

Po (l-r, + r,r») Poni-r. + r^r2

) (ID

which, after transformation and series expansion, can be reduced to the following expression:

^ob. = ^ + 0.4343(l-P)

1-r, (12)

Considering only the reflections from the two cuvet

faces and that ri = r2 = 0.05, the variation of A obs with A isA 0.1 0.2 0.5 1.0 2.0 A^ 0.1004 0.2007 0.5010 1.0011 2.0011

Thus, because of internal multiple reflections, the measured absorbance should always be larger than the

true absorbance with the percentage difference being

greatest at low absorbancea. Verification of these

deviations is difficult experimentally. Goldring et aL

[11], however, have suggested ways of minimizing

reflection effects. These include positioning the cuvet

at a small angle to the beam and stopping down the detector surface, immersing the cuvet in a fluid con- tained in a larger rectangular cuvet turned at an

angle to the beam, or by using a cuvet with prismatic windows. In each case, the effective path length must

be determined separately. Other alternatives consist

of using cuvets constructed of absorbing materials or immersion of a glass filter into the sample. For a limited wavelength range, reflection errors can also be reduced by coating the external window surfaces

with anti-reflection layers. In addition to reflections from the cuvets, reflections from other surfaces must also be considered. Gibson

[1] has pointed out that reflections may occur from the surfaces of the slit and detector and has suggested a means for checking this source of error. A thin glass

plate is placed in its normal position at right angles

to the beam and then at a small angle from this position. In the second position the reflected energy is directed out of the beam while scarcely affecting the

true absorbance.

F. Cuvets The majority of absorbance measurements are performed on solutions and, in such instances, the cuvet becomes an integral part of the measurement

system. Presently, the uncertainty in the length of the

light path can be a limiting factor in the determination

of molar absorptivity. Nonparailelism of the end windows can be of even greater consequence. However, the effect can be minimized by using the same

cuvet orientation for all measurements. The uncertainty in path length is nevertheless greater. Cuvets are available in a variety of shapes and

sizes and may be made of glass or silica. Construction remains largely an art and cuvets may vary considerably in their quality. Two methods of assembly

are commonly used. These are: (1) fusion using only heat and (2) the use of intermediate, low-melting

glasses. At present, each has its disadvantage. The

first technique is the more desirable, but our ex- perience, primarily with rectangular cuvets, indicates

that the edges are not always completely fused. These surfaces exhibit capillarity and may become

serious sources of contamination. Under such conditions, the cleaning of these cuvets is difficult, if not

impossible. The use of low-melting fluxes on the other hand can produce strains because of differences in coefficients of expansion and cuvets so constructed

are generally more fragile. Cuvets are frequently offered in matched sets. This may be considered more of a convenience than a necessity since this terminology is normally used only

to describe the transmission of the window material.

70
Path length and parallelism of end plates are of more

fundamental importance. Unfortunately, no generally accepted tolerances have been established in this country for the construction of cuvets. The British Standards Institution, however, has published a set of specifications [12] which merit our consideration and

possible adoption. They recommend that the path

length be specified to ±0.02 mm and the end windows

be flat over a defined beam area to four Newton fringes per centimeter in mercury green light. It should be

feasible for greater accuracy, especially for the 10 mm

cuvets, to calibrate a limited number by appropriate metrology techniques to ±0.01 mm.

This section is a report of our efforts to develop

well characterized liquid absorbance standards. Three

general types of liquid filters are discussed. These are

(1) individual solutions of high purity materials, (2) empirical mixtures and (3) aqueous solutions of organic

dyes. Much of the emphasis has been placed on deter- mining the optimum conditions for preparation of these

filters and, once established, what the absorbing

species or ionic compositions of these systems are. By combining this information with the instrumental

considerations developed in the first part of this paper,

it is hoped that the resulting data represent a step toward obtaining more meaningful absorptivity measurements.

A. Instrumentation

Absorbance measurements at a fixed wavelength were performed manually on a high precision double- beam spectrophotometer provided with a double monochromator. The accuracy of the photometric

scale of this instrument was established with the NBS high-accuracy spectrophotometer described by Mavrodineanu [13]. The wavelength scale was checked

with a mercury lamp. Potassium halide solutions [9, 10] were used to assess stray light in the ultraviolet. Room temperature was maintained between 24 to 26 °C. The cuvet compartment and jacketed holders were thermostated by a circulating water bath. Copperconstantan thermocouples were used to measure

temerature differences between the bath and the sample solution. An electronic feeler gauge [13] was used to measure

the path length of the 10 mm rectangular cuvets normally used. Solutions, assumed to obey Beer's

law, were used for calibrating the 1 mm cuvets. The accuracy of the microbalance was established

with NBS-calibrated weights. Class A, 1 -liter volu- metric flasks were checked gravimetically. All dilutions were subsequently made by volume. To minimize

errors from cell positioning, borosilicate. Pasteurtype pipets were used to transfer solutions to and from

the cuvets.

B. Individual Solutions of High Purity Compounds

L. Potassium Dichromate. Numerous attempts have been made to use chormium (VI) solutions

as ultraviolet absorbance standards. In the early studies [7, 14, 15, 16, 17] alkaline solutions of potassium chromate were preferred. More recently, potassium dichromate in slightly acidic media has been described

[18, 19, 20]. The absorption spectra of these two sys- tems are quite different with the latter giving the more

desirable arrangement of maxima and minima (fig. 4). The approximate composition of chromium (VI) solutions may be derived from the following equilibria:

220 250 300 350 400

WAVELENGTH, nm

Figure 4. Absorption spectra of 10-« M Cr (VI) in 0.05 N KOH

and 10-»N HCIO4. H2 CrO< = H+ + HCr04- (13) K2 HCrOj- = H+ + CrOf (14) 2HCr04- = Cr2 0? + H2 0. (15) Some values reported for the equilibrium constants Ku K2, and £3 at 25 °C are as follows: £, = 0.16 [21] £2 = 3.2 X 10-7 [22] and 3.0 X 10~7 [23] £3 = 43.7 [21], 35.5 [24] and 33.0 [25]. Thus, at a pH greater than 10. chromium (VI) exists almost entirely as CrOf ions while, in weakly acidic

solutions, the predominant species is HC1O4" which

partially dimerizes to Cr2Of. The amount of dimer

will increase with increasing chromium concentration. The formation of H2Cr04 or CrOf can be es- sentially eliminated by maintaining the acidity near pH3.

III. Evaluation of Selected Liquid Filters

71
Much of the uncertainty that has arisen from the use

of acidic potassium dichromate solutions as potential ahsorbance standards can be attributed to the dimerization reaction (eq (15)). It is primarily this equi- librium which leads to the observed deviations from

Beer's law. Because of the range of values reported

for Ka, one of the first objectives of the present study was to redetermine this constant. The spectrophoto- metric method was chosen and is described in some

detail below.

a. Spectrophotometric Determination of the Dimerixation Constant for: 2HCr04~ = Crj Of + H,0. If HCrOr and Cr20f are the only Cr(VI) species present with molar absorptivities € t and €2, respectively, the apparent molar absorptivity, € m, will be given by

em =(l-er)ei + l/2-a€, (16) where a is the fraction of total chromium in the dichromate form. From eq (15) the thermodynamic

equilibrium constant of the dimerization reaction is K3 =

[Cr2Of] 72 [HCrOr] 2 7?

or, in terms of aK3 =

a 72 2(l-a) 2Crr"r?

(17)

(18)

where 71 and 72 are the activity coefficients of HCrOi"

and Cr207", respectively, and Crr is the total chro- mium concentration. Because the ionic strength never exceeded 0.01, the activity coefficients may be treated by the Debye-Huckel expression [25]

log 7i (19)

where Z is the ionic charge, / is the ionic strength and A has a value of 0.509 1/mol at 25 °C. Equations (16) and (18) can be rewritten to give, respectively.

I. Experimental Procedure The chromium (VI) solutions used in the subsequent

studies were prepared from NBS SRM-136c, potas- sium dichromate, which had been dried at 110 °C for two hours. The distilled water was shown to be free of reducing impurities by titration with a dilute potassium permanganate solution. Perchloric acid was used to maintain the pH at 3.0 ±0.1. The optimum wavelengths for the determination of K3 were chosen as follows: Two solutions were pre- pared, one containing 391 mg K2Crj07/l and the other, 40.0 mg K2Cr207/l. The former was transferred to a

1.023 mm cuvet and the latter to a 10.00 mm cuvet. A

differential scan was then made on a recording spectro- photometer equipped with a 0-0.1 ahsorbance (full scale) slidewire, using the less concentrated solution as the reference. The resulting spectrum is shown in figure 5. Relatively large differences in ahsorbance are seen in the wavelength regions near 275 and 385 nm

even though the total number of chromium atoms in each light path is the same. Thus, it is in these two

regions that the largest deviations from Beer's law

should be observed. Three wavelengths were subse- quently selected in each region for the determination

of &.-390, 385 and 380 and 280, 275 and 270 nm,

respectively.

€m=0(l/2 £2 -€,) + €, (20)

and

log Kz = log 2(l-a)2Crr 1 + /

1/2 (21) Assuming a value of K3 , one can calculate from eq

(21) the a values at various total chromium concentrations. If the choice of K3 is correct, the measured

values of e m should lie on a straight line of slope (1/2

€2 — «i) and intercept eu as seen from eq (20). The

best value of K3 is then determined by the method of

least squares.

FIGURE 5. Difference spectrum of 391 mg KiChOt/I in 1.023 mm

cuvet versus 40.0 mg KjCrzOr/l in 10.00 mm cuvet: pH = 3. 2. Results The apparent molar absorptivities calculated from ahsorbance measurements on the four chromium

concentrations used for the determination of K3 are given in table 4. Using the method of least squares,

these «m values were plotted against the a fractions calculated from eq (21) for various assumed values of K3 . The molar absorptivities were weighted according

to the reproducibility of the ahsorbance measurements. A computer was used for all calculations.

72
A typical graph of the residual standard deviation 36. The minimum in this curve is the best estimate

of the experimental Cm values from the best straight of K3 and, for the case shown, is 32.6. The K3 values

line fit is shown in figure 6 for K3 values of 28 through at the six wavelengths are summarized in table 5. Table 4. Apparent molar absorptivities, €m, of K2 Cr2 07 solutions at 25 °C; pH = 2.9 (HCIO4) KjCr207ConC. tm 1/mol-' cm"'

mg/1 390 nm 385 nm 380 nm 280 nm 275 nm 270 nm

100.06 589.2 748.7 865.1 1579.3 1718.6 1881.0 400.48 714.1 878.6 997.8 1758.2 1914.4 2069.8

699.86 806.2 973.8 1094.0 1890.0 2053.5 2208.0

1000.27 875.9 1045.6 1166.5 1988.2 2161.5 2313.6

FIGURE 6. Least squares determination of K3

Table 5. K3 values at 25 °C as determined at several wavelengths

A, nm Ka 390 32.4 385 33.3 380 33.8 280 33.5 275 32.6 270 31.6

b. Absorption Spectra of HC1O7 and Cr2 0*. The absorbances of three solutions containing nominally 20, 40, and 60 mg K2 Cr2 07/l (pH = 3.0±0.1) were measured in a 10.00 mm cuvet at 5 nm incre- ments over the wavelength range of 220-400 nm. A

close approximation of €(HCr07) was obtained by

extrapolating the apparent molar absorptivities to zero chromium concentration. This extrapolation is simplified since the variation of a is nearly linear over

this concentration range. Similar absorbance measure- ments were performed on three solutions containing 1000 mg K2 Cr2 07/l using a 1.023 mm cuvet and the corresponding a value was calculated from eq (21), with K3 = 32.9. The first approximations of 1/2- ^(CrjjOf) values were obtained by substituting the extrapolated e(HCr07) results and the above a

value into eq (20). The e(HCrO,") and 1/2 • e(Cr20?)

values were then refined bv successive approximations. The calculated absorption spectra of these two ions are shown in figure 7.

Avg. 32.9

Std. Dev. 0.8

Rel. Std. Dev. 2.5%

250 300 350 WAVELENGTH, nm

FIGURE 7. Absorption spectra of HCrtV and CrjOf

.

c. Molar Absorptivities of HCrOr and CrzOf at 350, 313, 257, and 235 nm. As shown previously

in figure 4, the absorption spectrum of weakly acidic solutions of K2 Cr2 07 exhibit maxima at 350 and 257 nm

73
and minima at 313 and 235 nm. These maxima and minima do not shift significantly over the concentration range of 20-100 mg K2 Cr2 07/l. This is con- sidered a practical range for most applications since it yields absorbances of about 0.1-1.5 for a 10 mm cuvet. The molar absorptivities of HCrOr. and Cr2 Of at 350,

313. 257, and 235 nm were determined by the pro- cedure described above and are given in table 6. The

efHCrOrJ values are estimated to be accurate to ± 0.5 percent. Because of the imprecision of the determination, the uncertainty of the dichromate

molar absorptivity values may be as large as ±5

percent. Table 6. Molar absorptivities, c, of HCrO" and Cr2 07= at 25 °C

«, 1 mol -1 cm -1

350 nm 313 nm 257 nm 235 nm

HCrO,- 1576 711 2100 1804

1/2 Cr2 0" 1788 823 3156 2688

Table 7 shows the agreement between the experi- mental and calculated apparent molar absorptivities

for five concentrations of potassium dichromate in dilute perchloric acid (pH = 2.92 ±0.02). The experi- mental values are the averages obtained on five individual samples measured at each concentration. The 95 percent confidence limits for any given set of measurements was 0.1 percent. The a values used are

also given. These are seen to vary nearly linearly over

the range of chromium concentrations employed.

d. Optimum pH and Choice of Acid. The

chromium (VI) solutions used in this study were pre- pared in dilute perchloric acid media having a pH of about 3. This acidity was selected because the existing

equilibria data indicated this pH limited the formation

of either H2 CrO.» or CrOi to less than 0.1 percent of the total Cr (VI) concentration. Perchloric acid was

chosen instead of sulfuric to prevent the formation of chromate-sulfate complexes. Tong and King [24] and

Davies and Prue [25] noted slight differences in the absorbances of dilute perchloric and sulfuric acid

solutions after correcting for differences in ionic strength. They attributed these differences to mixed

complex formation. More recently, Haight et aL [26] have shown that the conversion of HCrOi" to CrSOf is quantitative in 1 M HSO* solutions and that absorption spectrum of this complex is significantly different from

the HCrO^ spectrum. Because most of the previous work reported in literature has been done in sulfuric acid, it was con- sidered of interest to intercompare these two systems. The results are shown in each acid between pH 2 and 3. At 350 nm the results obtained in perchloric acid are consistently 0.1-0.2 percent higher than in sulfuric;

at 235 nm, they are lower. The relatively large increase

in absorptivity at 235 nm at pH 1.90, together with the above considerations, suggest the formation of additional chromium (VI) species in systems employing

dilute sulfuric acid solutions. (See table 8.) e. Effect of Temperature. The absorbances of a

solution containing 60 mg K2Cr2 07/l in dilute perchloric

acid (pH = 3) were measured over the temperature

range of 17 to 37 °C. The percentage change in absorb- ance for wavelengths of 350, 313, 257, and 235 nm is shown in figure 8. The results are uncorrected for

Table 7. Experimental and calculated values (E, C) of the apparent molar absorptivity of F^C^O? at 25 °C; pH = 2.9(HC104 ) Cm, 1 mol _l cm"' K2 Cr^07 Cone.

ig/1 a 350 nm 313 nm 257 nr EC

Diff.

1577.6 713.5

1577.9 711.9 -0.3 +1.6

2113.6 1814.7 2108.7 1812.1 t4.9 +2.6

1579.9 713.3

1579.8 713.0 -0.1 +0.3

2120.8 1820.2 2118.8 1819.7 + 2.0 +0.5

60.12 0.0262 E 1581.4 713.8 2127.2 1827.1 C 1581.5 713.8 2126.7 1827.2

Diff. -0.1 0.0 + 0.5 -0.1 EC

Diff.

1584.0 715.0

1583.3 714.8

-r0.7 +0.2

2137.6 1835.8 2135.3 1834.4 + 2.3 +1.4

99.92 0.0423 E 1585 C 1584

Diff. +

1

5 715.6 2144.0 1841.5 0 715.0 2143.7 1841.4

.5 + 0.6 + 0.3 + 0.1

74
Table 8. Apparent absorptivities, a, of sulfuric and

perchloric acid solutions of K2Cr2 07 at 25°C; K2 Cr2 07 cone. =0.050 g/1

PH 350 nm 313 nm 257 nm 235 nm

H,S04 1.90 10.71, 4.837 14.44, 12.43, 1.98 10.71, 4.83, 14.44„ 12.4U

2.20 10.72* 4.83, 14.429 12.40, 3.00 10.73, 4.84, 14.42, 12.38, HCIO* 1.99 10.72, 4.84, 14.44, 12.39, 3.08 10.74, 4.84, 14.434 12.38,

Table 9. Apparent molar absorptivity of potassium chromate in 0.05 M Na2 HP04 and 0 05 M

KOH at 25 °C

cM, 1 mol" 1 cm-1 K,Cr04 Cone, M X 10* pH 373 nm 274 nm

7 9.2 482, 370, 14 9.1 482, 369,

21 9.1 481, 369,

0.05 M KOH

7 483, 370,

14 4824 369,

21 481 4 369,

l g

-i cm-i 0.05 M Na,HP04

1 1

1 1 0.4 1

0.20 -0.2

1

1 ^s. ' 3l3nm _

-0.4 1

1

V ^\350B257nm

-0.6

1 -0.8 1

1 1 235nm

15 20 23 30 35 40

TEMPERATURE, *C

FIGURE 8. Change in absorbance with temperature for acidic solutions of K,Cr,0,; pH = 3 (HC104 ).

thermal expansion of the solvent. The absorption minimum at 313 nm exhibits the smallest temperature dependence (—0.02 percent per °C) while the maxima

at 350 and 257 nm and the minimum at 235 nm have temperature coefficients of — 0.05 percent per °C over

the range 20 to 30 °C.

f. K2Cr2 07 in 0.5 M Na2 HP04 and 0.05 M KOH.

To overcome some of the difficulties in handling

strongly alkaline chromate solutions, Johnson [27] has recommended dissolving KzC^Ot in 0.05 M Na2 HP04 at pH 9. The apparent molar absorptivities obtained in this medium and in 0.05 M KOH are given in table 9. As seen from the data, both systems appear to exhibit deviations from Beer's law of about three parts per thousand over the concentration range studied. Al- though this deviation is the same magnitude and direction predicted from uncompensated internal multiple

reflections discussed previously, we have not been able

to demonstrate experimentally that such reflections are responsible for the observed results. Studies in which the envet windows are coated with various anti- reflection layers are continuing however.

From a consideration of the equilibrium data, the absorbances in the two systems should be nearly

identical. The ionic strength of the 0.05 M Na2 HP04 is 0.15 and, from Neuss and Rieman's work [22], the

ionization constant for the reaction HCr04 = H+ + CrO 4 is approximately 9xl0~7 mol/liter. Thus, at pH 9.1, only about 1 in 1000 chromium atoms is present as HCrO?. Since this species is less absorbing than CrOf at 373 and 274 nm. the apparent molar absorptivities in 0.05 M Na2 HP04 should agree with those obtained in 0.05 M KOH to within 0.1 percent. The absorbance of alkaline chromate solutions de- creases with increasing temperature at both absorption

peaks. Between 17 and 37 °C, the temperature coefficient at 373 nm is — 0.09 percent per °C; at 274 nm.

it is — 0.06 per °C. 2. Potassium Nitrate. Aqueous solutions of potas- sium nitrate exhibit a nearly symmetrical absorption peak with \max at 302 nm. This system has been

studied extensively by Vandenbelt [14] and by the

British Photoelectric Spectrometry Group in two

collaborative tests [28, 29]. Molar absorptivities re- ported vary from 7.06 to 7.20 liter mol-1 cm-1

. This

range of values undoubtedly results in part from the

fact that KNO3 solutions deviate significantly from

Beer's law. Careful reexamination of this system has yielded

the following apparent molar absorptivities (table 10). These are seen to decrease rather markedly with

increasing potassium nitrate concentration. Several

factors conceivably contribute to this behavior. In addition to possible multiple reflection effects already discussed some ionic interactions are undoubtedly

possible in this system because of the relatively high

concentrations of solute required coupled with the

fact that the N — 0 bond in the nitrate ion ha9 some

polar character. The potassium nitrate used in the above study was

analyzed for commonly encountered impurities. After drying at 110 °C. the water content was less than 0.02 percent. Flame emission analyses [31] indicated the following levels of alkali and alkaline earth impurities

75
(in parts-per-million): Li (< 0.2); Rb (13); Mg (0.2); Ca (1); Sr<4) and Ba (< 2). At 302 nm, the absorbance of KN03 solutions de- creases with increasing temperature. Over the range

17 to 37 °C, the temperature coefficient is —0.14 per- cent per °C. Solutions stored for up to six months showed no measurable change (=s 0.1 percent) in absorbance. Edisbury [29] has cautioned, however, that-some solu- tions may develop bacterial whiskers on standing.

Sterilization may be effected by prior boiling of the

distilled water. Table 10. Apparent molar absorptivity, e m , of aqueous

solutions of potassium nitrate at 25 °C

«„, 1 mol" 1 cm" 1 KN03 Conc..A/ 302 nm"

0.028 7.16o .056 7.14, .084 7.12, .112 7.10. .140 7.09,

Spectral bandpass = 1.0 nm

3. Potassium Hydrogen Phthalate. High purity potassium hydrogen phthalate (KHP) is readily available. It is presently issued by the National Bureau of Standards as a primary acidimetric standard. Its use

in the preparation of buffer solutions of known pH is well established. It has also been used as a spectral standard in the comparative evaluation of spectro- photometers [11, 32]. The absorbance of KHP solutions is dependent on

acidity, as shown in figure 9. In the present study,

dilute perchloric acid solutions were employed to minimize changes in ionic composition as a function

of pH. Acid concentrations much above 0.1 jV could

not be used because of the precipitation of potassium

perchlorate. Subsequently, all solutions were prepared

to contain 10 ml of perchloric acid per liter with a pH of 1.3. Ringbom [21] has given the first ionization constant

of phthalic acid as 1.6 X 10~3 at 25 °C at an ionic strength of 0.1. At the pH used, the absorbing system

consisted of 97 percent phthalic acid and 3 percent hydrogen phthalate ions. Table 11 summarizes the

absorptivity values obtained on these solutions. Absorbance decreases at the 275.5 nm maximum and

increases at the' 262 nm minimum with increasing temperature. Over the range 17 to 37 °C the temperature coefficients are —0.05 percent and +0.05 percent per °C, respectively. Perchloric acid solutions of KHP (NBS-SRM 84g) were examined for possible fluorescence, using a high

sensitivity spectrofluorometer. With 280 nm excitation, a faint fluorescence was detected with \max at approxi- mately 350 nm. The possible effects of this fluorescence on the absorbance data remain to be established.

250 300 350 WAVELENGTH , nm

FIGURE 9. Absorption spectra of KHP solutions at different

acidities.

Table 11. Apparent absorptivity, a, of potassium hydro- gen phthalate in dilute perchloric acid

at 25 °C; 10 ml HClO^liter

a, 1 g

_l cm"' NBS SRM 84p Cone, g/1 275.5 nm 262 nm

0.034 6.332 4.494 .142 6.31 9 4.48,

4. Cobalt and Nickel Sulfates and Perchlorates. Aquo cobalt (II) and nickel (II) ions have rela- tive broad absorption peaks with A. max at 512 and 394 nm, respectively (fig. 10). Solutions of either may serve

as useful absorbance standards. Most of the attention

to date, however, has been directed at cobalt sulfate which was first recommended by Gibson [1], In this study, the apparent molar absorptivities of Co(H2 0)£2 and Ni(H2 0)£2 were examined in sulfate and

perchlorate media. All solutions were prepared from

the high purity metals by dissolution in H2 S04-HN03 or HCIO4-HNO3 acids. Nitrate was removed by

repeated fuming until the acidity was reduced so that on dilution to volume, a pH of 1 could be obtained. The

results obtained in the two acid media are given in table 12. The values in sulfate solution are signifi- cantly higher than in perchlorate media for both

cobalt and nickel, suggesting that some complexation

of these ions by SO^ or HSO7 has occurred. A differ76
WAVELENGTH, nm WAVELENGTH, nm

FIGURE 10. Absorption spectra of Ni(HjO)*1 and Co(HiO)+l

.

Table 12. Apparent molar absorptivities, e m of acidic solutions of cobalt and nickel ions

at 25 °C; pH=1.0

fm (512 nm)

1 moH cm-1 Co Cone.

g/1 Sulfate Perchlorate

2.4 4.88, 4.80, 12.0 4.86, 4.79,

tm (394 nm)

1 mol-1 cm-1 Ni Cone.

g/1 Sulfate Perchlorate

2.3 5.I67 5.09,

11.5 5.154 5.07,

ential spectrum of 0.2000 molar cobalt solution in sulfate against the same cobalt concentration in perchlorate media is shown in figure 11. In sulfate solution, the formation of a second cobalt species with ^max = 528 nm is indicated. Similar behavior was also observed for nickel. The absorbances of cobalt and nickel solutions at their maxima are temperature dependent and increase

with increasing temperature. Between 17 and 37 °C

the temperature coefficients are 0.18 and 0.14 percent per °C at 512 and 394 nm, respectively. To explain

their nuclear magnetic resonance data on cobalt(II) solutions. Swift and Connick [33] have suggested that the octahedral Co(H2 OV"2 complex is in equilibrium

with the tetrahedral Co(H2 0)|2 complex and that for- mation of the latter is favored by increasing temperature. By analogy with halide complexes, the tetrahedral

Figure 11. Differential spectrum of cobalt sulfate versus cobalt

perchlorate; 10 mm cuvets; pH = 1. configuration would be expected to be approximately one hundred times more absorbing than the octahedral complex. A very small shift in this equilibrium could

result in a relatively large change in absorbance and

thereby account for the large temperature coefficient of this system. The same explanation may also be

valid for nickel solutions.

C. Liquid Filters of Improved Optical Neutrality Two approaches have been used in an attempt to improve the optical neutrality and extend the wavelength range of liquid filters. One is based on the use

of empirical mixtures, while the other employs watersoluble, organic "black" dyes.

In 1946, Thomson [34] described the preparation

of a grey inorganic solution consisting of chromic, cobal- tous and cupric sulfates and potassium dichromate. This solution, however, does not transmit below 300 nm. To extend the wavelength range farther into the

ultraviolet, a second exploratory solution (henceforth

referred to as the NBS composite) was prepared in which the cupric and potassium dichromate components of the Thomson mixture were replaced by p- nitrophenoL The absorption spectra of these solutions are compared in figure 12 to a commonly used glass

filter (Chance ON-10). The NBS composite was pre- pared in dilute sulfuric acid and had a pH of 1. No

fluorescence was observed under these conditions. Over the range 250-600 nm, the NBS composite

exhibits several broad maxima and minima whose

absorbances are less dependent on spectral bandpass

than the Chance ON-10 glass. Examples are given

in figure 13 at two wavelengths for each. The possible use of either Thomson solution or the NBS composite as an absorbance standard is limited by two factors, both of which arise from the presence of chromium (III) in these mixtures. First,

77
CHANCE ON -10 THOMSON SOLUTION NBS COMPOSITE

g 0.5 o

FIGURE 12. Absorption composite

400 WAVELENGTH

,

spectra of Thomas solution, an NBS

and a Chance ON-10 glass.

in which the observed increase in acidity is explained by the formation of —OH bridges. Recent studies [36] have shown that temperature, initial pH and the type

of anion present significantly affect the equilibrium composition. Although refluxing can reduce the aging

period from several months to several hours, this is of no practical advantage since the high temperature

products, on cooling, are no longer stable. In addition to the investigation of empirical mixtures,

several water-soluble organic dyes have also been

studied. Some typical absorption spectra are shown

in figure 14. All of these dyes have high absorptivities and the concentrations used were of the order of 50

milligrams per liter.

0.340 1 1 1 1 1 1

0.330

0.320 I0(445nm)

0.310 NBS(4l5nm) ^-^0N-I0l560nm)

0.300

0.290 yNBS (460 nm)

0.280 1 I I l 10 20 30 40 50 60 70 80 SPECTRAL BANDPASS, nm

FlCURE 13. Variation of absorbance with spectral bandpass for NBS composite and Chance ON-10 glass. an initial "aging" of 6-8 weeks period is needed in order to achieve adequate spectral stability at room

temperature. Secondly, and even more critical for our purpose, is that the absorbances of the aged

solutions exhibit a nonreversible temperature de- pendence. They must therefore always be stored at the temperature at which they were aged. Hall and Eyring [35] have studied the constitution of chromic salts in aqueous solution and have sug- gested that during the aging process Cr(H2 0)J 3 ions polymerize to yield species of the type H -, +5 (H2 0)5 Cr—O—Cr(H2 0)5 , H

[(H20)<Cr <^°^>Cr(H20)4] +4 H

and H H -, +7 (H2 0)5—€r—O—Cr(H2 0)<—O—€r<H2 0)5

200 300 ~ 400 ~ 500 600 WAVELENGTH , nm

Figure 14. Absorption spectra of aqueous solutions of selected dyes.

Cibalan Black [37], Neolan Black [37] and Alizarin Light Grey [38] were selected for further evaluation. Aqueous solutions of these dyes were found insensitive

to pH over the range 2-9, not affected by heat and

relatively light fast. Solutions continuously exposed

to fluorescent light exhibited an absorbance change of about 1 percent over a period of two months. These

initial results appeared sufficiently promising to warrant purification and more extensive testing. Members of the Organic Chemistry Section have

assisted in this effort. The purification of these dyes has proven extremely difficult, however, and yields of only a few hundred milligrams of each have been

obtained. Much larger quantities are needed if these dyes are to be used as Standard Reference- Materials. More efficient purification procedures are presently being sought.

D. Standard R«f»r*neo Material (SUM) 931 The first liquid absorbance standard to be issued by

the National Bureau of Standards is SRM 931. The

preparation and certification of this Standard Ref- erence Material are described below. The filters were prepared by dissolving high purity cobalt and nickel in a mixture of nitric and perchloric

acids. The weights of cobalt and nickel were chosen so

that the absorbances of their aquo complexes were

approximately equal at their absorption maxima. The absorbance of nitrate was adjusted to a comparable level by evaporation and subsequent addition

78
WAvaXNGTW.

Figure 15. Typical absorption spectrum of SRM 931.

of small amounts of nitric acid. The final pH of these mixtures was about 1. An absorption spectrum of a typical filter is shown

in figure 15. The peaks at 302 and 512 nm are due to absorption by NOj and Co(H2 0)^2

, respectively. The peak at 395 nm and the plateau at 650-700 nm

are due to Ni(HjO)+*. The filters are supplied in 10 ml ampoules which

have been prescored for easy opening. An SRM unit consists of three sets of filters, each set containing

a "Blank" (0.1N perchloric acid) and three absorbance

levels of the filter. Each set is packaged in an individual

tray for added convenience. The filters were calibrated by randomly selecting ampoules from each lot. The certified absorbance*

are as follows:

Table 13. Certification of NBS SRM 931

Net absorbance" Wavelength and (Bandpass), nm

Filter 302(1.0) 395(1.7) 512(2.0) 678(6.5) "A"— 'Blank' 0.307 ±0.003 0.3O4± 0.003 0.303 ±0.003 0.115±0.002 "B"- 'Blank' 0.608 ±0.005 0.605 ±0.005 0.606 ±0.005 0.229 ±0.003 "C"- 'Blank' 0.906 ±0.007 0.907 ±0.007 0.911 ±0.007 0.345 ±0.003

Net absorbances "A"— 'Blank,' mm cuvets at 25.0 °C.

lank,' and "C"— 'Blank' were determined using 10.00

The uncertainties are given at the 95 percent confi- dence level and include a possible systematic error of ±0.5 percent. Absorbances are certified at 25.0 °C. Absorbances

at other temperatures in the range 17 to 37 °C may be

calculated using the formula: A,=A2S[l + CA (t-2S))

where: At =Absorbance at temperature t (°C) An = Absorbance certified at 25.0 °C Ca = Fractional change in absorbance per "C The values of Ca at the four wavelengths are given

below.

X, nm

302

395

512 678

Ca -0.0014 + 0.0014 + 0.0018 + 0.0014

The absorbances of these niters will also depend on

the spectral bandpass used. To insure that the measured absorbances do not differ by more than ±0.1 per- cent from the true values, 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.

IV. Conclusion The availability of well characterized materials from which the analyst prepares his own solutions is preferable to the issuance of ampouled liquids. Although the former are not as convenient to use, they

offer potentially greater accuracy because of their better stability. The potassium dichromate and potassium hydrogen

phthalate used in this study are issued by NBS as primary oxidimetric and acidimetric standards (SRM

136c and 84g, respectively). The potassium nitrate is presently issued as a thermal analysis standard (SRM 756). A product of comparable purity may be

79
obtained by repeated recrystallization of reagent-grade

material from distilled water. The cobalt and nickel

solutions are best prepared from the high purity metals which are commercially available. To certify absorptivities for these materials, one must resolve the question as to how well these systems obey Beer's law. For acidic potassium dichromate, the increased dimerization of HCrOi" to CriOf with

increasing chromium concentration primarily accounts

for the relatively large deviations observed. With

potassium nitrate, one can only speculate that ionic interactions are enhanced at the higher concentrations. In neither instance are these deviations suffi- ciently large to limit the usefulness of these materials

as liquid absorbance standards. Such deviations do

require that the absorptivity data be used with proper

precautions. Additional studies are needed to explain the 0.2-

0.3 percent deviations from Beer's law obtained for the various concentrations of alkaline chromate, po- tassium hydrogen phthalate, cobalt perchlorate and

sulfate, and nickel perchlorate and sulfate. Although

internal multiple reflection effects appear to be of the proper magnitude to account for these differences,

initial attempts to measure such effects with mag- nesium fluoride-coated cuvets have not been success- ful While extremely durable, magnesium fluoride reduces reflections by about 50 percent and more

efficient coatings will probably be needed to resolve

this problem. If internal reflections are responsible

for the observed deviations, the absorptivity data

given should not be corrected when using liquid absorbance standards for checking the accuracy of the photometric scale since these reflections are included in every measurement. However, in the determination of molar absorptivity, which is con- sidered a fundamental property of the material, ap- propriate corrections should be applied. Until this question can be resolved, it is recommended that these materials be used at concentrations which yield an absorbance of at least 0.5 when measured in a 10 mm cuvet. With this restriction, it is believed that the uncertainty of the absorptivity data does not exceed ± 0.5 percent at the 95 percent confidence leveL

The authors wish to express their appreciation to Radu Mavrodineanu for providing the calibrated Schott filters and to David S. Bright for performing

the computer calculations on the HCrOr/CrzOf

equilibrium.

V. References

[1] Gibson, K. S., Nat. Bur. Stand. Circular 484 (1949).

[2] Harrison, G. R., M.I.T. Wavelength Tables (John Wiley & Sons, Inc. New York. N.Y., 1969).

[3] Meehan. E. J.. Treatise on Analytical Chemistry', Kolthoff.

I. M., and Elving, P. J., Editors. Part 1, Volume 5, Chapter 54

(John Wiley & Sons, Inc. New York, N.Y., 1964).

[4] Hardy, A. C, and Young, F. M.. J. Opt. Soc. Am. 39. 365

(1949). [5] Eberhardt, W. H.. J. Opt Soc. Am. 40, 172 (1950).

[6] Broderson. S.. J. Opt. Soc. Am. 44. 22 (1954).

[7] Hogness. T. R.. Zscheile. F. P.. and Sidweil, A. E., J. Phys. Chem. 41, 379 (1937).

[8] Perry, J. W., Photoelectric Spectrometry Group BulL 3. 40

(1950). [9] Slavin, W., Anal Chem. 35, 561 (1963).

[10] Poulson,R,E..AppLOpt3,99(1964).

[11] Goldring, L. S., Hawes, R. C, Hare. G. H.. Beckman. A. 0.. and Stickney, M. E., Anal. Chem. 25, 869 (1953).

[12] British Standard 3875. British Standards Institution (19651

[13] Mavrodineanu, R., Nat. Bur. Stand. (U.S.), Tech. Note 584, 178 pages (1971). O. Menis and J. I. Shultz, Eds.

[14] Vandenbeit, J. M., Forsyth, J., and Garrett, A.. AnaL Chem. 17,

235(1945).

[15] Haupt. C. W..J. Opt. Soc. Am. 42. 441 (1952).

[16] Haupt. G. W., J. Res. Nat. Bur. Stand. 48,414(1952).

[17] Vandenbeit. J. M.. and Spurlock, C. H.. J. Opt. Soc. Am. 45,

967(1955).

[18] Gridgeman, N. T., Photoelectric Spectrometry Group Bull. 4, 67(1951).

[19] Ketelaar, J. A. A.. Fahrenfort. J., Haas. C. Brinkman, G. A., Photoelectric Spectrometry Group BulL 8, 176 (1955).

[201 Vandenbeit. J. M.. J. Opt. Soc. Am. 50.24(1960).

[21] Ringbom. A., Compiexation in Analytical Chemistry, (John Wiley & Sons, Inc., New York. N.Y.. 1963), p. 294.

[22] Neuss, J. D., and Rieman, W., J. Am. Chem. Soc. 56, 2238

(1934).

[23] Howard. J. R., Nair, V. S. K.. and Nancollas. G. H.. Trans. Faraday Soc. 54, 1034 (1958).

[24] Tong, J., and King. E. L.. J. Am. Chem. Soc. 75, 6180 (1953).

[25] Davies, W. G., and Prue. J. E., Trans. Faraday Soc. 51. 1045

(1955).

[26] Haight. G. P., Richardson. D. C. and Coburn. N. H., J. Inorg. Chem. 3, 1777 (1964).

[27] Johnson, E. A., Photoelectric Spectrometry Group BulL 17,

505(1967).

[28] Edisburv, J. R., Photoelectric Spectrometry Group BulL 1, 10(1949).

[29] Edisburv, J. R., Photoelectric Spectrometry Group BulL 16.

441(1965).

[30] Malitson. I., private communication.

[31] Rains. T. C. private communication.

[32] Ewing, G. W., and Parsons. T., AnaL Chem. 20, 423 (1948).

[33] Swift, T. J., and Connick. R. E., J. Chem. Phys. 37. 307 (1962).

[34] Thomson. L. C, Trans. Faraday Soc. 42, 663 (1946).

[35] Hall. H. T., and Eyring, H., J. Am. Chem. Soc. 72 , 782 (1950).

[36] Burke, R. W., and Deardorff. E. R., Nat. Bur. Stand. (U.S.), Tech. Note 544. 153 pages (1970), O. Menis and J. I. Shulu,

Eds.

[37] Ciba Chemical and Dye Company, Fair Lawn. New Jersey.

[38] Industrial Chemicals Division. Allied Chemical Corporation,

Buffalo, New York. (Paper 76A5-733)

80
JOURNAL OF RESEARCH of the National Bureau of Standards—A. Physics and Chemistry

Vol. 80A. No. 4. July-August 1976 Considerations for the Use of Semi-Transparent Metallic Thin Films as Potential Transmittance Standards in Spectrophotometry*

R. Mavrodineanu

Institute for Materials Research, National Bureau of Standards, Washington, D.C. 20234

(May 20, 1976) Various characteristics of evaporated metal-on-fused silica filters are discussed in relation to their optical transmission properties. Special metal holders provided with shutters were designed to be used with these filters, and are described in detail. Transmittance measurements, performed in various conditions, are reported and indicate that the evaporated metal-on-fused silica filters might present an acceptable material as transfer standards

in spectrophotometry. Key words: Evaporated metal-on-quartz; filters, transmittance; neutral filters; standard

reference materials; transmittance characteristics; ultraviolet-visible filters. The use of solid materials, in the form of glass

filters and of solutions of inorganic and sometime

organic compounds, to test the correct functioning

of spectrophotometers is a common practice [l].

1 A

variety of materials are available from the National Bureau of Standards (NBS) which can be used as transfer standards to verify the accuracy of the transmittance scale and the short and long term

stabilities of conventional spectrophotometers. Two

Standard Reference Materials (SRM's) have been

issued by NBS for the verification of the accuracy

of the transmittance scale; these are SRM 930 and SRM 931. SRM 930 is a solid material which consists of a

set of three glass filters having nominal transmit- tance of 10; 20; and 30 percent. They are certified for transmittance in the visible spectral range from

400 nm to 635 nm. A detailed description of this SRM is given in NBS Special Publication 260-51

[2]. SRM 931 is a liquid standard which consists

of a solution of cobalt and nickel in dilute perchloric acid contained in glass ampoules [3]. The trans- mittance of these solutions is certified from 302 nm

to 678 nm and should be used in conjunction with

curvettes having a known light path; such curvettes

are available from NBS as SRM 932 [4].

. Both SRM's 930 (the glass filters) and 931 (the

liquids) are limited in their spectral transmittance range and require the use of spectral bandpasses, from 2.2 nm to 6.5 nm for the glass filters and from

1.0 nm to 6.5 nm for the liquid filters depending

of wavelength, when accurate transmittance values

are sought.

•This work was supported in part bv the National Institute of Oeneral Medical Sciences.

1 Figures in brackets indicate the literature references at the end of this paper.

The need to provide similar transfer standards, but

with expanded spectral range to the ultraviolet and

with less stringent spectral bandpass requirements, has resulted in an investigation to find adequate

materials for this purpose. Such materials should

fulfill the following conditions: (a) be transparent

in the spectral range of interest, usually between 200 nm and 800 nm; (b) have a transmittance inde- pendent of wavelength (optically neutral) ; (c) have

a spectral transmittance independent of temperature

;

(d) have low reflectance and be free of interferences;

(e) be nonfluorescent; (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. Thus, various solid materials were examined and the final choice was

the evaporated metal-on-(non-fluorescent) fused

silica type filter. The transmission chare cteristics of such filters are illustrated in figure 1 and are compared with that

of three glass filters and Ronchi ruling on glass. The major limitation of the evaporated metal-on- quartz filters results from their intrinsic property of attenuating the incident radiation by reflecting rather than absorbing part of it. As a consequence

of this property, this type of filter could generate

stray radiations in the sample compartment of con- ventional spectrophotometers and is susceptible to produce interreflections when used with instruments equipped with lenses. To determine the practical value of such filters as a

transfer transmittance standard, comparative meas81
Table I. Comparison between the -percent transmittances ( % T) measured on three incouel-on-silica filters at NPL and NBS

EVAPORATED METAL ON QUARTZ

[iNCONELj

RONCHI RULING ON GLASS

100 LINES/in.

200 300 400 500 600

X, NANOMETERS

Figure 1. Spectral characteristics for five transparent

materials from 200 nm to 700 nm. uremonts were performed with the National Physical Laboratory in England (NPL), and the results are given in table I. In this case the inconel-on-fused

silica filter was protected by a clean fused silica plate held in place with an organic cement. Except

for a filter which showed some mechanical Haws in its structure, the reproducibility of transmittance meas- urements was as good as that obtained for the absorbing glass filters (table II). The error which would result from positioning the inconel-on-fused

silica was measured by rotating from 0° angle to

3° —6°—and 9°. From' the results obtained (table

III) it can be concluded that in the case of the high- accuracy spectrophotometer used at NBS a position- ing error within 3° can be tolerated. The interreflection error affecting the measure- ments when a filter is inserted in the radiation path between the two lenses of a spectrophotometer was examined in detail [5]. The measured error for a glass filter established for the high-accuracy spec- trophotometer was one or two 10~ 4 transmittance

units; a value which is about four times larger was

found for an evaporated metal-on-fused silica filter. It should be mentioned here that both the positioning and interreflection errors are, for a given material, instrument dependent. As a result of these considerations a decision was made to produce a limited number of sets of evaporated inconel-on-fused silica and to test these filters in actual measurements performed on the con- ventional spectrophotometers manufactured in the United States. One of the filters from a set is shown in figure 2. From left to right one can see the main body of the

filter holder made from an aluminum alloy anodized

black. This body is provided with dove-tail grooves which can accept a front and rear shutter made

NBS, %T

Wavelength NBS, %T ' NPL, <r0 T nm

1 2 average

450. 0 24. 87 24. 88 24. 87, i 24. 93

550. 0 23. 7S 23. 82 23. 80 23. 86

650. 0 23. 3S 23. 39 23. 38$ 23. 46

450. 0 49. 35 49. 33 49. 34 • 49. 56

550. 0 47. 60 47. 60 47. 60 47. 81

650. 0 46. S5 46. 85 46. 85 47. 14

450. 0 72. 17 72. 20 72. 18s 72. 30

550. 0 72. 05 72. 11 72. 08 72. 20

650. 0 72. 20 72. 34 72. 27 72. 33 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 differ- ences found in the measurements of this filter may be due to

this flaw. Table II. Comparison between the percent transmittances Cy0 T) measured on three Schott NG-4 glass filters at NPL

and NBS

Wavelength nm

NBS. %T

March 12. 1971 NBS, %T

Mav 18, 1971 NBS, rcT- NPL, To T

average i February

440. 0 ' 12. 92 12. 91 12. 91 s 12. 93

465. 0 | 14. 965 j 14. 98 14. 973 1 15. 01

590. 0 : 11. 70 11. 64 11.67 11. 67

635. 0 ! 12. 72 12. 68 12. 70 . 12. 72

440. 0 19. 625 19. 58 19. 603 . 19. 62

465. 0 i 22. 3S, 22. 35 22.367 22. 43

590. 0 19. 06 IS. 95 19. 00s 19. 01

635. 0 20. 455 20. 37 20. 41 3 ; 20. 47

440. 0 32. S9 32. 86 32.87, i 32. 98

465. 0 i 35. 52 35. 54 35. 53 35. 66

590. 0 i 31. 16s 31. 10 31. 133 i 31. 21

635. 0 ! 32. 56$ 32. 52 32. 543 ; 32. 62 Average difference between NBS and NPL percent T

values =—0.19 percent Table III. Percent transmittance (^cT) measured on a Schott neutral glass filter 2 mm thick, and an inconel-on-fused silica

filter 2 mm thick at 500 nm, for normal incidence and for the angle of 3°; 6°; and 9° %T

\. Filter

j Angle i

Schott NG-4

Glass

Inconel-on-fused silica

l Front j Back

i 0° 2S. 13 29. 91 29. 87

3°

| 28. 10 29. 98 29. 87

6° 2S. 03 29. 84 29. 82

9°

i 27. 98 29. 92 29. 90

82
Figure 2. Filter holder with 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; 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. from a black plastic (Delrin 2). The last three units

illustrate the holder with these shutters on. A more

detailed dimensional drawing of the filter holder is shown in figure 3. The shutters provide two functions

:

one is to protect the filter from contamination, the other to detect if stray radiant energy (SRE) is produced in the spectrophotometer compartment

as a result of reflections generated by the incident beam on the semi-transparent metal layer. The

determination of stray radiant energy generated

in the sample compartment of the spectrophotometer, due to the reflecting propreties of these filters, may

be assessed as follows: A background signal may be measurable when the instrument shutter at the photomultiplier tube is closed. Another slightly higher background signal may be detectable with

the instrument shutter open and with the filter placed in the beam, in the sample compartment, with both sliding shutters closed. Both of these signals should be very small. A third signal may be detected when the front sliding shutter at the filter holder is removed and the rear sliding shutter is closed. If under these circumstances a signal is detected, it is very likely due to SRE produced by reflections re- sulting from the semi-transparent mirror which

scatter from the walls of the cell compartment. This

should also be low in magnitude. The transmittance

of the filter is measured where both sliding shutters

at the filter holder are removed.

1 In order to describe materials and experimental procedures adequately, it was

occasionally necessary to identify commercial products by manufacturer's name

or label. In no instance does such identification imply endorsement by the National Bureau of Standards, nor does it imply that the particular product or equipment is necessarily the best available for that purpose.

Under these circumstances a source of systematic

error can be from multiple reflections between the

lenses in the instrument and the filter surfaces [5]. An indication of the existence and magnitude of such

interreflection phenomena can be obtained by comparing the differences between the transmittance measurements and the certified values for SRM 930 and 931 to the corresponding differences for the

inconel-on-fused silica filters. Each set is made from three filters and one blank

placed in individual metal holders which are pro- vided with the front and rear sliding shutters. The

selection of inconel as the semi-transparent metal

layer was made on the basis of its relatively good

optical neutrality in the spectral range from 250 nm

to 700 nm. The" fused silica substrate is of optical quality and non-fluorescent. All the silica filters, including the blank, were ground and polished at the same time and together to a parallelism of 0.02 mm

and a flatness of less than 2 fringes (mercury 546.1 nm). The nominal dimensions are 30.5 mm by 10.4 mm by 2 mm thick, and the nominal transmittances

of these filters are 1 percent; 20 percent; 30 percent; and 90 percent.3 The transmittances of each set were measured at 250 nm, 300 nm, 340 nm, 400 nm, 440 nm, 465 nm, 500 nm, 546.1 nm, 590 nm, and 635 nm.

Sets of these filters were sent to the manufacturers

of spectrophotometers for evaluation and tests on

the individual instruments. The results of these

i The filter holders and the evaporated inoonel-on-fosed silica filter, were made

In the NBS Instrument Shops.

83
\

Cut CO«>»^W CWvt-**""^^

Figure 3. Details of the filler holder with shutters. Nominal dimensions are in millimeters.

Table IV. Percent transmittance (%T) measured on four inconel-on-fused silica filters at 10 wavelengths and on the NBS-IMR

high-accuracy spectrophotometer and on two conventional instruments A and B %T

^\Wavelengthnm

Instrumenv\^

250 300 340 400 440 465 500 546 590 635

NBS-IMR

High-accuracy Spectrophotometer

1. 45

21. 39

28. 68

91. 42

1. 81

21. 27

29. 92

92. 34

1. 86

20. 33

30. 20

92. 66

1. 94

18. 93

29. 96

92. 90

2. 04

18. 33

29. 70

93. 01

2. 13

18. 09

29. 55

93. 05

2. 27

17. 90

29. 44

93. 11

2. 49

17. 83

29. 44

93. 16

2. 70

17. 86

29. 49

93. 19

2. 90

17. 95

29. 57

93. 23 Spectrophotometer A

1. 53

21. 37

28. 67

91. 59

1. 84

21. 24

29. 87

92. 60

1. 93

20. 31

30. 19

93. 32

2. 00

18. 91

29. 95

93. 20

2. 12

18. 29

29. 70

93. 30

2. 20

18. 07

29. 58

93. 30

2. 34

17. 89

29. 49

93. 42

2. 55

17. 82 29.46

93. 46

2. 75

17. 84

29. 52

93. 50

2. 95

17. 92

29. 60

93. 58 Spectrophotometer B

1. 49

21. 6

28. 8

91. 4

1. 85

21. 5

30. 0

92. 6

1. 90

20. 5

30. 2

92. 6

1. 97

19. 1 30. 0

93. 1

2. 07

18. 5

29. 9

93. 3

2. 16

18. 3

29. 7

93. 3

2. 31

18. 1 29. 5

93. 3

2. 53

18. 0

29. 5 93.3

2. 83

18. 3

29. 9

93. 9

3. 06

18. 5

30. 0

93. 9

tests will permit to establish the usefulness of these

filters as SRM's in spectrophotometry. In the mean time comparative measurements were performed on two conventional spectrophotometers

available at NBS, and the"results are assembled in table IV. These results are preliminary measurements

and should be considered with caution. The data seem to indicate, however, that the evaporated

metal-on-fused silica niters may be an acceptable material for use as transfer standards in spectrophotometry.

Note.—The semi-transparent metallic surface of the niters described in this paper is not protected from environmental

exposure and careless handling. A solution to this problem was found recently. It consists in placing on ton of the fused

silica plate carrying the semi-transparent evaporated metal

layer a clear fused silica plate, about 1.5 mm thick, adequately ground and polished, to achieve an optical contact with the evaporated semi-transparent metal layer surface. The two

fused silica plates are slightly beveled, and an optical cement

is applied to protect the edges. Under these conditions the two fused silica plate assembly, with the semi-transparent evaporated metal layer in between, behaves optically as if it were a single semi-transparent plate, and provides full protection of the metal layer. Several sets of these new filters were made for evaluation in actual working conditions.

84
References

[11 Hand, R. N., Practical spectrophotometric standards,

Clin. Chem, 15, 839-863 (1969).

[2] Mavrodineanu, R., and Baldwin, J. R., Glass filters as a standard reference material for spectrophotometry

selection, preparation, certification, use, SRM 930, Nat. Bur. Stand. (U.S.), Spec. Publ. 260-51, 118 pages (Nov. 1975).

[3J Burke, R. W., Deardorff, E. R., and Menis, O., Liquid absorbance standards, J. Res. Nat. Bur. Stand.

(U.S.), 76A. (Phvs. and Chem.), No. 5 469-482

(Sept.-Oct. 1972).

[4] Mavrodineanu, R., and Lazar, J. W., Standard quartz

cuvettes for high accuracy spectrophotometry, Nat.

Bur. Stand. (U.S.), Spec. Publ. 260-32, 26 pages (Dec.

1973) and Clin. Chem. 19, (9) 1053-1057 (1973).

[5] Mielenz, K. D., and Mavrodineanu, R., Reflection

correction for high-accuracy transmittance measure- ments on filter glasses, J. Res. Nat. Bur. Stand. (U.S.), 77A (Phys. and Chem.), No. 6, 699-703 (Nov.-Dec.

1973). (Paper 80A4-909)

85
JOURNAL OF RESEA RCH of the National Bureau of Standards—A. Physics and Chemistry

Vol. 80A, No. 4, July-August 1976 Acidic Potassium Dichromate Solutions as

Ultraviolet Absorbance Standards*

R. W. Burke and R. Mavrodineanu

Institute for Materials Research, National Bureau of Standards, Washington, D.C. 20234

(May 18, 1976) The absorbances of five concentrations of potassium dichromate in 0.001 M perchloric

acid have been determined at eight wavelengths in the ultraviolet on the National Bureau

of Standards Institute for Materials Research high-accuracy spectrophotometer. Four of the wavelengths—235, 257, 313, and 350 nm—correspond to absorbance maxima or minima in the HCr04 ~ spectrum and are useful wavelengths for checking the accuracy of the absorbance

scale of narrow bandpass spectrophotometers. Although partial dimerization of HCrO«~

to Cr2 C>7" produces small positive deviations from Beer's law at these wavelengths, the ap- parent absorptivities calculated for each concentration are reproducible to one part in a thousand. The estimated uncertainties in the absorptivity values are ±0.7 percent at 0.1 absorbance (A) and ±0.2 percent near ,4=1. These uncertainties include all known sources

of possible systematic error and the 95 percent confidence level for the mean. The remaining

four wavelengths used for measurement are near two predicted isosbestic points in the HCr04 ~/Cr2 07" spectra. The absorptivities at 345 nm are sufficiently independent of concentration that this wavelength can be used for checking absorbance linearity to one part

in a thousand over the range A = 0.2-1. Key words: Absorbance linearity; accuracy; acidic potassium dichromate solutions; calibra- tion of ultraviolet spectrophotometers; liquid filters; transfer standards; ultraviolet absorb- ance standards.

I. Introduction At present, there are no certified standards

available from the National Bureau of Standards (NBS) for checking the accuracy of the absorbance

scale of spectrophotometers throughout the ultraviolet. The number of analytical" applications in

this important region of the spectrum, however,

continues to increase relatively sharply and the need for such standards is becoming increasingly

acute. The area with perhaps the most serious need for ultraviolet absorbance standards at the moment is clinical chemistry. Workers in this field, for example,

frequently use molar absorptivity as an index of purity of their biological standards. Only recently, Burnett [1]

1 has discussed the importance of ac- curate molar absorptivity measurements in the

clinical laboratory. He especially emphasizes the need for spectrophotometry accuracy in clinical enzymology. Not only must the molar absorptivity

of the substrate or enzyme-catalyzed reaction product be known accurately but also the individual absorbance measurements on the test sample must

be accurate. This dual requirement for accuracy

,*!?is

, ~'ork "'as supported i n part bv , hc xation:li i nstltme o( Gencial

ledlcal sciences.

i Figures in brackets indicate the literature references at the end of this paper.

in this instance arises because high-purity and well- characterized enzyme preparations are not yet

routinely available for use as standards. A program has been under way at NBS in the

Institute for Materials Research (IMR) since 1969

for the development and issuance of visible and

ultraviolet transfer standards for use as Standard

Reference Materials (SRM's). Two such standards

are currently available: (1) SRM 930, consisting of a

set of three neutral Schott NG glass filters and (2) SRM 931, an empirical inorganic solution available

at three concentrations in 10-ml ampoules. Neither of these SRM's meets the present needs for absorbance

standards in the ultraviolet. The glass filters do not

transmit below about 350 nm while the inorganic

solution is certified only at one wavelength in the

ultraviolet (302 nm) and with an uncertainty of ± 1 percent. Transfer standards capable of being certified throughout the range of 200-350 nm and with a smaller uncertainty are being investigated, however. The most promising solid filter at the moment for

this purpose is the metal-on-quartz type. Some

recent experiences with this filter are discussed in the paper immediately following [2]. Of the chemical or liquid-type filters that have been proposed, the weakly acidic potassium dichromate system is considered the best. A critical evaluation of this system, as well as several other candidate materials, was presented in a previous paper [3].

2H-427 O 76 - 6

86
In this paper, we present absorptivity values for

five concentrations of potassium dichromate in 0.001 M perchloric acid. These concentrations effec- tively span the absorbance range of .1=0.1-1.5 when measured in 10 mm cuvettes. The absorptivities were calculated ftom transmittance measure- ments at eight wavelengths in the ultraviolet on the IMR high-accuracy spectrophotometer. Four of the wavelengths—235. 257, 313 and 350 nm—correspond

to absorbance maxima and minima in the HCr04 ~ spectrum and arc useful wavelengths for checking

trie accuracy of the absorbance scale of narrow

bandpass >pectrophotometers. Although partial di- merization of HCr04 ~ to Cr207 ~ produces small positive deviations from Beer's law at these wavelengths,

the apparent absorptivitics calculated for each concentration are reproducible to one part in a thousand. The estimated uncertainties in the absorptivity values are ±0.7 percent at 0.1 absorbance (A) and ±0.2 percent near A=\. These uncertainties include

all known sources of possible systematic error and

the 95 percent confidence level for the mean. The

remaining four wavelengths used for measurement

are near two predicted isosbestic points in the HCrO«~/Cr2 07 " spectra. The absorptivitics at 345 nm

are sufficiently independent of concentration that

this wavelength can be used for checking absorbance

linearitv to one part in a thousand over the range 4=0.2-1.

II. Experimental Procedure The potassium dichromate (K2 Cr2 07 ) used in this study was a special lot of analytical reagent grade

material obtained from the J. T. Baker Chemical Company.2 Drying studies performed at 105 °C

indicated that the surface moisture of this material was less than 0.01 percent. Xo measurement of possible occluded water was made. However, a recent paper [4] by Voslnmori and Sakaguchi has shown that K2 Cr2 07 typically contains 0.01 to 0.02 percent occluded water which can be removed only by crushing and drying at 350 °C. Coulometric assay of the undried potassium dichromate gave a purity, expressed as an oxidant,

of S9.972 ±0.005 percent at the 95 percent confidence level. Emission spectroscopy indicated that the only significant elemental impurities present were sodium and rubidium. Their concentrations were estimated to be 0.02 and 0.03 percent,

respectively. The concentrations of the K: Cr207 solutions used throughout this study are expressed on a weight/

weight basis. Milligram samples of K: Cr2 07 were weighed to the nearest microgram on a microbalance.

After dissolution in distilled water shown to be free of reducing impurities (see Discussion), 1 ml of 1 M

perchloric acid was added and the solutions were

• In order to describe materials and experimental procedures adertuatelv. it is occasionally necessary to identify the sources o( commercial produris hr the manufacturer's name. In no instance does such identification implv endorsement by the National Bureau of Standards, nor docs it imply that the particular Product Is necessarily the best available for that purpose.

diluted approximately to volume in 1-liter volu- metric flasks. Each flask was fitted with a double- nbbed Teflon stopper (Ivontes Glass Company,

Vineland, New Jersey) to prevent evaporation. The

weight of each solution was determined on a single- pan top-loadins balance having a sensitivity of 0.01 e. XBS-calibrated weights were used to establish the accuracy of the balances. The concentrations of the solutions were then calculated after correcting all weights to vacuum. A solvent blank was prepared by

diluting 1 ml of 1 M perchloric acid to 1 liter with

distilled water. The Institute for Materials Research high- accuracy spectrophotometer was used for performing

the transmittance measurements which, in turn, were

converted to absorbance. The design and construction of this instrument have been described in detail by one of us (RM) in reference (5) and will not be repeated here. Similarly, the quartz cuvettes used

are also of XBS design and construction and have

been described previously [G]. These cuvettes are currently available through the XBS Office of Standard Reference Materials as SRM 932. Each

cuvette is certified for path length and parallelism to ±0.0005 mm.

Prior to use the cuvettes were cleaned by soaking

in concentrated (18 M) sulfuric acid for several hours. In order to minimize the heat of mixing, they were

then transferred consecutively to 12, 6 and 3 M

sulfuric acid before rinsing with distilled water. After

rinsing thoroughly, the cuvettes were air-dried under

an inverted Petri dish that served as a dust-protective

cover. For sample measurements, five calibrated cuvettes were placed in separate holders in the rotating sample compartment (5] of the IMR high-accuracy

spectrophotometer and a reference filter (inconel- on-quartz) was placed in a sixth position. The

cuvettes were left in their respective holders for the duration of the experiment. All transmittance/' absorbance measurements were made relative to

air in a temperature-controlled room at 23.5 ±0.3

°C. The transfer of solvent blank and sample solu- tions to and from these cuvettes was made by means

of borosilicatc, Pasteur-type, disposable pipettes. After being rinsed with "the test solution, a final transfer for measurement could be made in 10—15

seconds, after which time the cuvette was immediately stoppered with a snugly-fitting Teflon

stopper. Six sets of solutions having nominal concentrations of 20, 40, 60, SO and" 100 nig K2Cr:0T/kg were prepared. Each concentration within a set was measured a minimum of six times at the eight wavelengths of interest. The absorbance. .-1=— log

( ^sample/ i»nk) i was computed for each wavelength and concentration from the average of the six transmittance measurements (T). Absorptivities were then calculated after correcting the absorbances

for systematic errors due to cuvette path length, beam geometrv and internal multiple reflections. No correction was applied for the reflections dis- cussed by Mielenz and Mavrodineanu [7] from

87
internal components such as the lenses and slit jaws because these are adequately compensated

for by the blank. A detailed account of these corrections and calculations will be given in an XBS 260

Special Publication which is now in preparation.

III. Discussion and Results The ultraviolet absorbance spectrum of a 40 mg

kg-1 solution of potassium dichromate in 0.001 M

perchloric acid is shown in figure 1. The four wavelengths selected for certification of absorptivity of

this absorbing system, namely 235, 257, 313 and

350 nm, are also indicated. The maxima and minima

are sufficiently broad that serious restrictions are not placed upon instrumental spectral bandwidth

requirements. The half bandwidths of the 257 and

350 nm peaks, for example, are approximately 60 nm so that an instrumental spectral bandwidth of

3 nm or less is sufficient for obtaining at least 0.999

of the maximum peak intensities.

1 1

K

2 Cr

2 07 = 40 mg/kq

\

257

350 \ 313 / \

1

The first experiment performed on the IMR high- accuracy spectrophotometer was the determination

of the rinse behavior and reproducibility of the transmittance/absorbance measurements of the sol- vent blank. In the initial studies, measurements

were made at 235 nm only, since any problems

associated with the transfer of solution were expected

to be the greatest at the shortest wavelength used. The results obtained for the five cuvettes employed

are shown in figure 2. All exhibit a similar rinse pattern, attaining a minimum and constant absorb- ance value after 5-6 rinses (the term rinse as used

here and throughout the remainder of this paper

refers to the exchange of one cuvette volume for a second one). Once the blank measurements were in control, similar absorbance measurements were

then made on the first series of K;Cr;07 solutions. Five concentrations having nominal concentrations

of 20, 40, 60, SO and 100 mg K,Cr: 07 kg were

measured. Table I summarizes the results of these measurements together with the final solvent blank

values for 235 nm. The cycle of measurements shown

here was then extended to the other seven wave350 WAVELENGTH, nm Figure 1. Absorbance spectrum of KjCrjOr in 0.001 M per- Figure 2. Apparent absorbances of the blank solvent in the five chloric acid. curvettes used—Nos. 14, 16, IS, 25, and 26 ref. [6\. Table I. Reproducibility of absorbance measurements at t35 nm for a cycle of solvent and sample rum

Cuvette No.

Solvent Sample Sample Solvent

After 5 rinses After 5 rinses After 7 rinses After 5 rinses

14 a 05198 a 30128 a 30141 a 05213

16 . 05099 . 55052 . 55057 . 05093

18 . 05310 . 80066 . 80074 . 05321

25 . 05002 L 04863 1. 04855 .05022

26 . 05064 1. 30351 1. 30333 .05065

Ref. filter . 55913 . 55925 . 55936 .55922

Inconel-on-quartz

88
lengths of interest. In all, six sets of K2 Cr207 solu- tions were measured. Typically, two volleys consist- ing of three transmittance/absorbance measurements

were made on each of the 30 solutions. The first volley was made after rinsing the cuvettes, initially containing solvent, with five rinses of sample and was repeated again after two additional rinses. Since no systematic increase in absorbance was ever observed when the results of the first volley of measurements were compared to the second, the six measurements were invariably averaged. The ab- sorbances were obtained by subtracting the solvent blank values from the sample readings. After applying appropriate corrections for beam geometry and

internal multiple reflections, the corrected absorb- ances (Aton) were used to calculate the desired

absorptivities, using the relationship Absorptivity=^4corr/6c, where 6=internal light path in cm and c=concentration of K2 Cr207 solution in g kg-1

. The absorptivities computed for the five concentrations of K2 Cr2 07 solutions used are summarized in table II. The values tabulated were determined from least-squares plots

of the experimental values and were subsequently normalized to the concentrations shown. The

uncertainties given include all known sources of possible systematic error and the 95 percent (2a) confidence interval for the mean. The random

component of these uncertainties, based on standard

deviations computed from residuals resulting from

fitting the data to the various concentration levels for each wavelength, is 0.07 percent at the 2a level. It is observed that all absorptivities in table II increase with increasing K2 Cr207 concentration. These deviations from Beer's law are produced by the

fact that, in weakly acidic media, chromium (vi) ions

exist as two distinct absorbing species—HCrOr and

its dimerization product, Cr2 07 ~. The equilibrium be- tween these two species may be represented as

and the corresponding dimerization constant, KD , is given bv [HCrOr] Kn 2 (2) =

Although eq. (2) predicts that the formation of Cr2 07 ~

is strictly a quadratic function of K2 Cr207 concentration, the value of KD = ?>2.% (mol kg -1

) -1 obtained

previously [3] is of such magnitude that the percent- age of total chromium present as the Cr2 07 " ion is very nearly a linear function of the K2 Cr2 07 con- centration for the range of solutions studied. Calculated values of HCrOr/Cr207 " speciation in this system are given in table III. Table III. HCrOr/Cr2 07- speciation in 0.001 M perchloric acid solutions of K2 Cr2 07 . K,Cr2 07 Percent Cr as Percent Cr as Cone, g kg-1 HCrOr Cr2 07-

0. 020 99. 10 0. 90

. 040 98. 22 1. 78

. 060 97. 38 2. 62

. 080 96. 56 3. 44

. 100 95. 77 4. 23

2HCrOr =Cr20: =+H2 0 (1)

The direction and magnitude of the expected

deviations from Beer's law for the acidic potassium dichromate system can be ascertained when the

speciation data in table III are combined with the

spectral characteristics of the two chromium (vi) species. Figure 3 shows the relation of the absorbance

spectra of the HCrOr and Cr2 07 " ions. At wavelengths of 235, 257, 313 and 350 nm, the Cr2 07 " ion

is shown always to have a larger absorptivity than

the HCrOr ion. Hence, the measured absorptivities at these wavelengths should increase with increasing K2 Cr2 07 concentration. From the same argument, the data in figure 3 also suggest that there are two wavelengths near 320 and

345 nm where Beer's law is obeyed. In an attempt to determine these isosbestic points experimentally, absorbance measurements were also made on the

Table II. Absorptivities of K2 C2 07 in 0.001 M perchloric acid at 23.5 °C

Absorptivity, kg g~ l cm-1 K2 Cr2 07 Cone, g kg" 1 235(1. 2)>nm 257(0.8)nm 313(0.8)nm 350(0.8) nm Uncertainty '

"0. 020

. 040

. 060

. 080

. 100

12. 243

12. 291

12. 340

12. 3S8

12. 436

14. 248

14. 308

14. 869

14. 430

14. 491

4. 797

4. 804

4. 811

4. 818

4. 825

10. 661

10. 674

10. 687

10. 701

10. 714

0. 034

*. 022

<*. 020 020

019

" Nominal concentration ; all weights corrected to vacuum.

6 Wavelcneth and, ( ), spectral bandwidth.

c Includes estimated systematic errors and the 95 percent confidence interval for the mean.

d For wavelength of 313 nm, the uncertainty is reduced to half of these values for K2 Cr2 0; concentrations of U.U4U, u.uou, 0.080 and 0.100 g kg"'.

89
Figure 3. Absorbance spectra of the HCr04 ~ ion and its dimerization product, Crj07~ same solutions used above at 322, 323, 345 and 346 nm. The corresponding absorptivity values are given

in table IV. Although small systematic deviations from Beer's law are still present, the absorptivities calculated for 345 nm are sufficiently constant that

the acidic K2 Cr207 system can be used at this wavelength, over the concentration range shown, to check

the absorbance linearity of narrow bandpass spectro- photometers to one part in a thousand. The variation of absorptivity of the acid K2 Cr2 07 system with temperature is the smallest that we

have observed for any liquid filter that we have

studied to date. For the four wavelengths recom- mended for checking the accuracy of the absorbance

scale—235, 257, 313, and 350 nm—the absorptivities decrease with increasing temperature. Over the range 20-30 °C, the corrections are, respectively, —0.05, -0.05, —0.02, and -0.05 percent per degree Celsius. The -0.02 percent correction found at 313 nm cor- responds closely to the correction predicted from the thermal expansion of the solvent. Until further eval- uation can be made of the temperature dependence

of 345 nm wavelength recommended for checking absorbance linearitv, measurements should be re- stricted to 23.5 ±1 °C. Several considerations were involved in the selec- tion of perchloric acid for acidifying the K2 Cr2 07 solutions. First, perchloric acid was preferred over

the sulfuric acid which has been used traditionally because perchlorate ion, unlike sulfate, has no tend- ency to form mixed complexes with chromium (vi) species. Secondly, the choice of 0.001 M acid rather

than 0.01 Mwas based on two factors: (1) the 0.001 M

acid represented a practical compromise for mini- mizing conversion of HCr04 " to either H2 Cr04 or CrO«" and (2) the lower acid concentration substantially lowered the oxidation potential (~13 percent)

of the chromium (vi) ions and hence improved the

solution stability of the proposed K2 Cr2 07 standards. A final point that needs to be emphasized concerns

the quality of the distilled water used for preparing

the standard solutions. Acidic potassium dichromate

is a potential oxidant and consequently the distilled water used must be shown to be free of reducing

impurities in order that the absorptivity data given

in this paper be valid. A simple but yet definitive

test is outlined in figure 4. In this study a 500-fold

dilution of a 100 mg kg-1 standard solution of K2 Cr2 07 was made with the distilled water in question and its absorbance measured at 350 nm. Not

only did the measured absorbance agree with the predicted value, thus indicating no reduction of chromium (vi), but also subsequent measurements showed that this K2 Cr207 solution standard should

be stable for at least two months provided it is ade- quately protected against evaporation. In instances where this test shows the distilled water to be suspect, it is recommended that the water be redistilled from alkaline potassium permanganate in order to preoxidize the impurities.

Finally, the general use of solid and liquid transfer standards for calibrating the absorbance scale of a

precision commercial spectrophotometer is illus- trated in figure 5. The measurements on the glass

0.002S Eist 0.0020

E| 0.0015 500-fOLDOILUTIONOF100mg K Cr 0 /kg

(Lj , 2 4 6 8 10 12 14 > ' 30 60 DAYS AFTER PREPARATION Figure 4. Test of the distilled water for reducing impurities. Table IV. Absorptivities of K2 Cr2 07 in 0.001 M perchloric acid near two predicted isosbestic wavelengths; temperature 23.5 °C

Absorptivity, kg g~' cm-1 K2 Cr2 0 7 Cone, g kg-1 322 (0.8) b nm 323 (0.8) nm 345 (0.8) nm 346 (0.8) nm

• 0. 020 5. 845 6. 065 10. 593 10. 615

. 040 5. 842 6. 062 10. 595 10. 621

. 060 5. 838 6. 059 10. 598 10. 627

. 080 5. 835 6. 056 10. 600 10. 632

. 100 5. 831 6. 053 10. 602 10. 638 •Nominal concentration: all weights corrected to vacuum.

b \Vavelength and, ( ), spectral bandwidth.

90
8 r

ABSORBANCE Figure 5. Correction AA for the absorbance scale of a precision commercial spectrophotometer.

filters were performed at 440 nm while the K2Cr2 07 measurements were made at 350 nm. At both wavelengths the absorbances measured on the precision spectrophotometer are high and should be corrected by subtracting the appropriate A^4 values.

IV. Conclusion

Following widespread use in the collaborative

testing of spectrophotometers for more than 25 years, the absorptivitics of the acidic potassium dichromate system have now been determined with a known

accuracy from measurements performed directly on

the XBS Institute tor Materials Research high- accuracy spectrophotometer. Later this year, crystalline K?Cr20T , together with detailed instructions on

preparing solutions from it, will be available from NBS through its Office of Standard Reference

Materials as an SRM. In conjunction with the cali- brated quartz cuvettes previously issued (SRM 932),

it is believed that this material*will provide a valid check of the accuracy of the absorbance scale of narrow bandpass spectrophotometers in the ultra- violet from 235-350 nm.

The authors gratefully acknowledge the assistance

of the following persons at XBS during various phases of this work: K. D. Mielenz, for helpful discussions of fundamental spectrophotometry pa- rameters; G. Marinenko, for performing the coulometric assay; J. A. Norris, for the the emission spectrographs analyses; H. H. Ku, for the statistical analysis of the data; and R. W. Seward, for supplying through the Office of Standard Reference Materials the samples of potassium dichromate.

V. References

[1] Burnett, R. W.f J. Res. Nat. Bur. Stand. (U.S.), 76

A

(Phys. and Chcm.), No. 5, 483-4S9 (Sept.-Oct. 1972).

[2] Mavrodineanu, R., J. Res. Nat. Bur. Stand. (1976), in press.

[3] Burke, R. W., Deardorff. E. R., and Menis, O., J. Res. Nat. Bur. Stand. (U.S.), 76 A (Phvs. and Chem.). No. 5, 4G9-4S2 (Sept.-Oct. 1972).

[4] Yoshimori, T. and Sakaguchi, N., Talanta 22, 233-238

(1975).

[5] Mavrodineanu, R., J. Res. Nat. Bur. Stand. (U.S.), 76A

(Phys. and Chcm.), No. 5, 405-425 (Sept.-Oct. 1972).

[6] Mavrodineanu, R., and Lazar, J. W., Nat. Bur. Stand.

(U.S.), Spec. Publ. 200-32, 26 paces (Dec. 1973), and

in Clin. Chem. 19, 1053-1057 (1973).

[7] Mielenz, K. D., and Mavrodineanu. R., J. Res. Nat.

Bur. Stand. (U.S.), 77A (Phvs. and Chem.), No. 6, 699-703 (Nov.-Dec. 1973). (Paper 80A4-908)
U S. Department of Commero

Juaniu M Krepe

Secretary

Nalinnal Buflraxi nt Standard* F.m«l Ambler Aclinic Ihnpcmr Rational ^uram of jiianriartfe

(KErttftcate

Standard Reference Material 930D

Glass Filters for Spectrophotometry

R. Mavrodineanu and J. R. Baldwin

This Standard Reference Material is intended as a reference source for the verification of the transmittance and absorbance scales of spectrophotometers. It consists of three individual glass filters in their metal holders and one empty filter holder. The holders are provided with shutters to protect the glass filters when not in use. These shutters must be removed at the time of measurements and be replaced after the measurements have been completed. Each filter bears an identification number, and the upper left corner has been removed to indicate correct placing in the metal holder.

Filter & Set

Ident. Number

TRANSMITTANCE (T) TRANSMISSION DENSITY (-log 10T)

Wavelength and (Bandpass) nmWavelength and (Bandpass) nm

440.0

-(2.2) 465.0(2.7) 546.1

(6.5) 590.0(5.4) 635.0

(6.0) 440.00

(2.2) 465.0(2.7) 546.1(6.5) 590.0

(5.4) 635.0(6.0)

Date of Certification: The transmittance values (T) can be converted to percent transmittance (%T) by multiplying by 100. The

transmission densities are calculated from the measured transmittance (T). These values should be indicated by

the absorbance scale of the spectrophotometer if the filters are measured against air. The transmittance values given were measured against air at an ambient temperature of 23.5 °C. The transmittance values are estimated to be accurate to within ±0.5 percent at the time of certification. This

uncertainty includes the effects of the random and systematic errors of the calibration procedure, as well as possible transmittance changes of the filters during the period of calibration. Aging of the glass may cause some filters to change transmittance by about ±1 percent over a period of approxi- mately one year from the date of calibration. Improper storage or handling of the filters may also cause changes

[5]. 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. In cases where verification is desirable, the filters should be returned to the National Bureau of Standards for cleaning and recalibration. This Standard Reference Material was issued with the assistance and advice of K. D. Mielenzand J. R. DeVoe. 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 T. W. Mears and

R. W. Seward. Washington, D.C. 20234 August 1, 1977 92

J. Paul Cali. Chief

Office of Standard Reference Materials
The transmitlance measurements were made using the high-accuracy spectrophotometer designed and built at the NBS Institute for Materials Research [1]. This instrument represents the primary transmittance standard;

its transmittance accuracy was established using the double-aperture method of linearity testing [1,3,5,6]. The

estimated standard deviation obtained from a set of 20 measurements of the high-accuracy spectrometer used

for transmittance measurements on glass filters of the SRM 930 type is 5 x 10

5 transmittance units for transmit- tance between 0. 1 and 0.3. The bounds of the systematic error of the instrument are estimated to be + 10 trans- mittance units. The neutral glasses for the filters were provided by Schott of Mainz, Germany, and are designated as "Jena Color and Filter Glass." The glass material was prepared by the manufacturer especially for SRM 930. It was

fine-annealed and was selected for best homogeneity and minimum of inclusions and striae. The glass filters were aged at NBS for about one year prior to certification. They are of the type NG-4 and NG-5, and their nominal transmittances are 10, 20, and 30 percent [2,5]. The glasses were selected to provide a means to verify the transmittance scale of conventional spectrophotometers at three levels. 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). The

empty filter holder is provided to be used in the reference beam of the spectrophotometer so that approximately

equivalent conditions of stray radiations are achieved for both beams. The transmittance of the filters depends upon the intrinsic properties of the material, wavelength, spectral bandpass, geometry of the optical beam, temperature, and positioning of the filter. A change of ambient temperature

of 1 or 2 °C from 23.5 °C will not significantly affect the calibration [5]. Changes in the transmittance may be caused by surface conditions, aging of the glass, exposure to a harmful atmosphere, or careless handling as indicated on the face of this certificate [2,3,4,5]. The effective spectral bandpasses used to determine the certified values are given on the face of the certificate and the transmittance measurements are made by producting 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 glass filter. The filters were measured in the spectrophotometer in a

position perpendicular to the incident light beam as shown in the attached figure. SRM 930D 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 glass filter surface with particulate matter due to static charges is minimized through the metallic nature of the 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. Prior to certification measurements, each filter was examined for surface defects and thorough/ cleaned [5]. 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 degraded the accuracy of the certified values. As the Standard Reference Material is a transfer standard, the only means

available to verify its integrity is remeasuring its transmittance with a primary standard instrument similar to

that used in this certification [1,4,5]. In most cases where verification, or recertification, of the transmittance

values is desirable, it will be most expeditious to return the filters to the National Bureau of Standards for measurement. Prior to shipment, the NBS Office of Standard Reference Materials should be contacted regarding the conditions and cost of verification and /or recertification. We wish to acknowledge the cooperation of 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 National Institutes of Health, Bethesda, Maryland.
REFERENCES

1. R. Mavrodineanu, An Accurate Spectrophotometer for Measuring the Transmittance of Solid and Liquid

Materials, NBS Journal of Research 76A, No. 5, 405-425 (1972).

2. R. Mavrodineanu, Solid Materials to Check the Photometric Scale of Spectrophotometers, NBS Tech. Note

544, O. Mcnis and J. I. Shultz, Eds.,pp. 6-17, U.S. Government Printing Office, Washington, D.C. 20402

(Sept. 1970); ibid, NBS Tech. Note 584, pp. 2-21 (December 1971).

3. K. S. Gibson, Spectrophotometry, NBS Circ. 484 (Sept. 1949).

4. Collected Papers from NBS Conference on Accuracy in Spectrophotometry and Luminescence Measurements, NBS Journal of Research 76A, No. 5, 375-510 (1972).

5. R. Mavrodineanu and J. R. Baldwin, Glass Filters as a Standard Reference Material for SpectrophotometrySelection, Preparation, Certification, Use, SRM 930, NBS Special Publication 260-51, U.S. Government

Printing Office, Washington, D.C. 20402, 1975.

6. K. D. Mielenz and K. L. Eckerle, Spectrophotometer Linearity Testing Using the Double-Aperture Method,

Appl. Optics 11, 2294-2303 (1972). METAL HOLDER FOR THE COLORED GLASS FILTERS*

FRONT TOP METAL HOLDER

57 X 13 X 13

EXPOSED FILTER SURFACE

29 X 8

COLORED GLASS FILTER

1.5

30.5 x || x 2.0 (Of 1.5)

DIMENSIONS IN mm

•Shutters not illustrated.

94
^aitnnal ^Bureau of j§tatuiarrta

(Heritftcaie

Standard Reference Material 931c

Liquid Absorbance Standards for

Ultraviolet and Visible Spectrophotometry

This Standard Reference Material is certified as solutions of known net absorbances at specific spectral wavelengths. It is intended primarily for use in the calibration and checking of accuracy of the photometric scale of narrow bandpass

spectrophotometers and for routine critical evaluation of daily working standards used in spectrophotometry. This Standard Reference Material is applicable for calibrating those instruments that provide an effective spectral bandpass

of I.S nm or less at 320 nm, 2.0 nm or less at 395 nm, 3.3 nm or less at 512 nm,and 8.5 nm or less at 678 nm [1].

Net Absorbance

3 Wavelength and (Bandpass), nm

Filter 302(0.8) 395 (0.8) 512(0.8) 678(0.8) "1" -"Blank" 0.305 1 0.001 0.309 ± 0.001 0.307 ±0.001 0.116 ± 0.001

-11" -"Blank" .608 ± .002 612 ± .002 .609 ± .002 .232 1 .001 "HIVBtank" .938 ± .002 .912 ± .002 .904 1 .002 .346 1 .001

'Net absorbances ("I" - "Blank". "II" - "Blank", and "III" - "Blank") were determined using 10.00-mm cuvettes (SRM 932) at 25 °C. See Instructions

for Use. The transmittance measurements leading to the certification of this SRM were performed on the high accuracy spectrophotometer located in the NBS Center for Analytical Chemistry. The design and construction of this instrument have been described previously [2]. This instrument is the primary transmittance standard; its accuracy has been verified using the double aperature radiation-addition principle. The bounds of systematic error of this instrument are estimated

to be 1 10

4 transmittance units. These liquid absorbance filters were calibrated at the wavelengths and conditions indicated by measuring the transmittance. T, of the "Blank" and solutions "I", "11", and "III" against air as a reference. The values of T were used to calculate the corresponding apparent absorbances. A., using the relationship A,=-logioT. The certified net absorbances were obtained by subtracting the apparent absorbances of the "Blank" solution from the apparent absorbances calculated for solutions "I". "II". and "111". The uncertainties of the certified values include all known

sources of possible systematic error and the 95 percent confidence level for the mean.

While no long-term stability studies have been made on this lot (931c), studies on previous lots (93 1,93 la. and 93 lb) 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 purchase. The preparation of the filters was performed by E.R. Deardorff, Inorganic Analytical Research Division, and the transmittance measurements were performed by M .V. Smith. Office of Standard Reference Materials. Technical leader- ship for the preparation and measurements leading to certification was provided by R.W Burke, Inorganic Analytical Research Division. The overall direction and coordination of technical measurements leading to certification were performed under the chairmanship of E.L. Garner. The technical and support aspects involved in the preparation, certification, and issuance of this Standard Reference Matenal were coordinated through the Office of Standard Reference Materials by R.W. Seward.

Washington. D.C. 20234

February 12. 1982 George A. Unano. Chief

Office of Standard Reference Materials
Temperature Dependence

Absorbances at various temperatures (17 to 35 °C) may be calculated using the equation

A, = A25[l+CA<t-25)]

where: A( = Absorbance at temperature t (°C) A25 = Absorbance certified at 25.0 °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

Preparation of Filters The filters were prepared by dissolving high-purity cobalt and nickel in a mixture of nitric and perchloric acids. The

absorbance spectrum of the resulting solution is shown in the following figure. The maxima at 302 and 5 1 2 nm are due to absorbance by NO" and Co(H20)6*,*respectively. The maximum at 395 nm and the plateau at 650-700 nm are due to Ni(H20)6

.** The pH of these solutions is about 1.

Instructions for Use

This material is for use as a spectrophotometry absorbance standard.

1. Select two clean 10.00 mm cuvettes free of scratches. At least one should be fitted with a ground glass or Teflon

stopper to minimize evaporation. Reserve it for all sample measurements.

2. Mark each of the cuvettes to assure the same orientation in the spectrophotometer.

3. Place the cuvettes in their respective holders 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 rinses, first with isopropyl alcohol and then with distilled water, are generally adequate.

4. Obtain the optical mismatch of the cuvettes at 302, 395, 5 1 2, and 678 nm, usingthe spectral bandpass limitations given on the face of the certificate.

5. Empty the cuvettes by suction without removing them from their holders, refill with distilled water and measure the absorbances again at each of the above wavelengths.

96
6. Repeat the emptying and refilling operation until constant absorbance readings are obtained.

7. Using the liquid filters provided, measure, in turn, the absorbance of the "Blank", T, "11", and "111" against distilled water. 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.)

8. Subtract the appropriate "Blank" reading from the absorbances obtained for "I", "II", and "III". These net absorbances should agree with the certified values within the uncertainties specified. Consult the manufacturer of the instrument if they do not. The above instructions are for use with the standard 10-mm rectangular cuvette. For calibration of the several spectrophotometry systems used in various automated instruments, the user is referred to the instruction manual for the

particular instrument. 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. 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 6S6.3 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 following

restrictions are placed on the size of the spectral bandbass selected: To obtain ±0. 1 percent of the true value, 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. For ±0.2 percent, the respective bandpasses should not exceed 2.2, 2.9, 4.8, and 113 nm. Additional information on the

effect of spectral bandpass on the absorbances of these filters is given in the figure below. These curves are not to be used, however, to correct the measured absorbances. +2.00 ^ +1.00 0 0.00 z1 -i-oo

Id^ -2.00 <g -3.00

to5-4.00

-5.00 0 2 4 6 8 10 12 14 16 18 EFFECTIVE SPECTRAL BANDPASS, nm

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. In addition, it is recommended that this Standard

Reference Material not be used after three years from the purchase date.

References

[I] R. W. Burke, E. R. Deardorff, and O. Menis, J. Research, Nat. Bur. Stand. 76A, 469-482 ( 1972).

[2] R. Mavrodineanu, J. Research, Nat. Bur. Stand. 76A, 405-425 (1972).

Note: The above papers 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. Government Printing Office. Washington, D.C. 20402. 1973.

97
U. S. DepuUMOl of Corai

Philip M.«luun>ct

Ctaraty >-•»•'

ukmml BamtoTtuadanh

Emot ABMr. CHraaor jNattmtal J&mtm of ji>tanttartte

(Efcritfttate

Standard Reference Material 932

Quartz Cuvette for Spectrophotometry

R. Mavrodineanu and J. W. Lazar

This Standard Reference Material consists of a single, accurately calibrated cuvette that is issued for use in the production of accurate spectrophotometric data on liquids. The design and nominal dimensions of the all-quartz cuvette

are shown in Figure 1. The pathlength of the cuvette is defined by the distances between the two optically transparent windows measured at several heights within the cuvette. Cuvettes issued as Standard Reference Material 932 have a nominal pathlength of 10 mm. The pathlength and parallelism are certified, at the time of measurements, with an

uncertainty of ±0.0005 mm as determined by measurements (at 20 °Q taken at the positions indicated below. Cuvette number is issued with this certificate. For this cuvette the following measurements were obtained:

Date of Certification: The overall direction and coordination of the technical measurements leading to certification were performed under the chairmanship of O. Menis and J. A. Simpson. 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 T. W. Mears.

Height Pathlength mm mm

Washington, D.C. 20234 December 2, 1980

(Revision of certificates dated 11-5-73, 6-16-77, and 8-2-78)

George A. Uriano, Chief

Office of Standard Reference Materials

98
The original cuvettes were designed and produced at the National Bureau of Standards (NBS) using special techniques and non-fluorescent optical-quality fused quartz as described in NBS SP 260-32(1]. The transparent windows are attached to the body of the cuvette by direct fusion, and the unit was stress-relived by annealing. The radiation pathlength measurements of the cuvette were performed using electronic feeler-gauge type instruments capable of a

resolution of 5 parts in 10

6

. To preserve the integrity of the certified values, the cuvette must be handled with great care and should be held only by the frosted-quartz side windows. When not in use, it should be stored in the container provided for this purpose. Extended exposure to laboratory atmosphere and dusty surroundings is to be avoided. Improper handling of the cuvette and the use of solutions that can corrode or en-ode the quartz could degrade the

certified pathlength values. In cases where verification of the pathlength is desirable, the cuvette should be returned to NBS for examination and, if needed, recalibration. Prior to shipment the NBS Office of Standard Reference Materials should be contacted regarding the conditions and cost of the verification and/or recertiflcation. The development and production of SRM 932 is a result of the combined efforts of the Center for Analytical Chemistry,

the Center for Mechanical Engineering and Process Technology, and the Instrument Shops Division. The radiation pathlength measurements of all of these cuvettes have been performed, at NBS, by E. G. Erber of the Mechanical

Processes Division. Reference:

[ 1] Mavrodineanu, R., and Lazar, J. W., Standard Reference Materials; Standard Quartz Cuvettes for High Accuracy

Spectrophotometry, Nat. Bur. Stand. (U.S.), Spec. Publ. 260-32 (December 1973). Superintendent of Documents,

U.S. Government Printing Office, Washington, D.C. 20402, 55 cents. QLI

radiation pathlength"

6

(FROSTED)

(TRANSPARENT) 1 ; ^

2 U— io —J i T

_1_

6 MEASUREMENT

1 LEVEL

, —BOTTOM

2 MEASURMENT

LEVEL

All dimensions in millimeters. T12.5 _1.

INCIDENT I RADIATION BEAM

99

r-ebruary 12. 1982

Office of Standard Reference Materials
U.S. D*p*rtm»rtf of Commerce Juuiu'M. Krep*

Secretary EmM Ambler, Acriixt DiretuwRational ^nrtmx af Standards

(Herttftcais

Standard Reference Material 935

Crystalline Potassium Dichromate

for Use as an Ultraviolet Absorbance Standard

R. W. Burke and R. Mavrodincanu

This Standard Reference Material 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**, 6», at 23.5 °C.

This SRM is intended to be used as a reference standard for the verification of the accuracy and linearity of the absorbance scale of absorption spectrometers that can provide an effective spectral bandpass of 1.2 nm or less at 235 nm, and 0.8 nm or less at 257, 313, 345, and 350 nm. Such verification is accomplished by comparing the measured apparent absorbanccs, A», to the A, calculated from the certified €« values as described under

"Instructions for Use." Table 1 gives the certified values of €, in kg-g^-cm"

1 for five concentrations of the SRM 935 potassium dichromate in 0.001 N perchloric acid at 23.5 °C and the indicated wavelengths and spectral bandpasses for a

I -cm internal pathlength.Table 1. C«, Apparent Specific Absorbance,

3 kg g

' cm

1 Nominal Wavelength and 'Bandpass) nm

Concentration gkg"' 235.0(1.2) 257.0(0.8) 313.0(0.8) 345.0(0.8)

b 350.0(0.8) Uncertainty' 0.020 12.260 14.262 4.805 10.604 10.672 ±0.034

.040 12.304 14.318 4.811 10.603 10.682 ± .020

d .060 12.347 14.374 4.816 10.602 10.692 ± .020

d .080 12.390 14.430 4.821 10.601 10.701 ± .020

d .100 12.434 14.486 4.827 10.600 10.711 ± .020

d

*6a 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. Wavelength 345.0 nm is near one of the two isosbestic points in HCrOI/CrjOf spectra. Because it is on the slope of the composite

spectrum, reproduction of the €, values is dependent on wavelength accuracy. Measurements at this wavelength should be made only for

verification of the linearity of the absorbance scale. '£« 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, the uncertainties given include all known systematic errors and the 95 percent confidence

interval of the mean.

At wavelength 313.0 nm, the uncertainty is reduced to ±0.010. '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 absorbanccs are given in reference I. ••The nomenclature used in this certificate is that recommended by K. D. Mielenz, Anal. Chem. 48, 1093-1094(1976), which is reproduced

in toe Appendix of NBS Special Publication 260-54.

Washington, D.C. 20234 J. Paul Cali, Chief June 1

, 1 977 Office of Standard Reference Materials

100
The overall direction and coordination of the technical measurements leading to this certificate were per- formed under the joint chairmanship of I. L. Barnes and J. R. DeVoe. 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 R. W. Seward. We wish to acknowledge: The contributions of 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. PREPARATION AND CERTIFICATION

The details of the preparation and certification of SRM 935 are provided 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 935 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 6, values were calculated for each wavelength using the relation: D^D, A^

b x c bxc 1 ' where:

6« = apparent specific absorbance A* = apparent absorbance

D, = transmittance density of the sample solution, -logioT, Db= transmittance density of the blank solution, -logioTb b = internal cuvette pathlength, cm

c The crystalline potassium dichromate used for SRM 935 is a special lot of analytical reagent grade material obtained from the J. T. Baker Chemical Co., Phillipsburg, N.J. Assay: A coulometric assay of the purity of the undried material was performed by G. Marinenko of the NBS

Analytical Chemistry Division. The purity of SRM 935, expressed as an oxidant, was found to be 99.972 ± 0.005 percent where the uncertainty figure represents the 95 percent confidence interval for the mean based on 1

1

degrees of freedom. In addition, the material was examind by optical emission spectrometry for trace elemental

impurities by J. A. Norris of the NBS Analytical Chemistry Division. The only significant impurities detected were sodium and rubidium. Their concentrations were estimated to be in the range of 0.02 and 0.03 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 935 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 935 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 trans- ferred 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.

101
The accuracy of the absorbance scale of the spectrometer being tested is ascertained by comparing the measured

apparent absorbances, A„ of a series of 0.001 N perchloric acid solutions containing 0.020 to 0.100 gram K2 Cr2 Ch/ kg to the A, values calculated from the certified €, values. Although the C. data in table 1 are given for nominal concentrations of 0.020, 0.040, 0.060, 0.080, and 0. 100 g K2Cr2Cb/kg, the C. values for concentrations between these nominal concentrations can be determined by linear interpolation. Using the appropriate C

values, the calculated A, values at 23.5 °C are obtained from the expression: A, = €. x b x c (2)

Calculations An example of the calculation of A. for one concentration of K2Cr207 under a specified set of conditions is shown below. Calculations of A, 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 C. for concentrations of 0.040 and 0.060 g-kg"

1 are 10.682 and 10.692, respectively. The corresponding €. for c = 0.04375 g-kg is:

e. = 10.682 + ^

7f~^° (10.692- 10.682) €.= 10.682 + 0.0019

€. = 10.684 The calculated apparent absorbance. A,, from equation 2, is: A, = 10.684 x 0.9982 x 0.04375 A, = 0.4666 The uncertainty, AA«, in the calculated A, is determined from the combined uncertainties in b, and c in equation 2, provided no other systematic errors are present. Thus: AA« = bc|Ae.|+e.c|Ab|+€.b|Ac| (3) To evaluate AA,, A€. is taken from column 7 of table 1 and the Ab and Ac values must be determined experi- mentally.

In the experiments performed to obtain the €, values in table 1 , the uncertainties for b and c did not exceed 1 part

in 10 and 2 parts in 10 , respectively. The solution of equation 3 gives: AA«= 1(0.044) (0.020) + 10.7(0.044) (0.0001) + 10.7(1) (0.0000088) = 0.0010

" Thus, the uncertainty of A,, 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 A. measured and A, 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.

102
• IFigure 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,

6t, cuvette pathlength, b, and concentration, c.

o I I i

' '

i

'

i

'

i • 0.2 0.4 0.1 0.1 1.0 Absarhaaca Temperature Correction: Although 6* values in table 1 are certified at 23.5 °C, SRM 935 can be used as an absorbance standard at other temperatures in the range 20 to 30 °C provided corrections are made to the €» values. Over this range the apparent specific absorbances decrease linearly with increasing temperature for all the wavelengths given in table I. The corresponding temperature coefficients, k, for these wavelengths are given in table 2.

Table 2. Variation of 6, with Temperature Over the Range 20 to 30 °C.

A, am Temperature Coefficient, k

The value of €. at any temperature in the range 20 to 30 °C can be calculated from the certified value and the appropriate temperature coefficient using the relation:

ej

3' 5 = apparent specific absorbance certified at 23.5 °C. k = temperature coefficient, percent per °C. REFERENCES

1. Burke, R. W., and Mavrodineanu, R., Acidic Potassium Dichromate Solutions as Ultraviolet Absorbance

Standards, J. Res. Nat. Bur. Stand. (U.S.), MA (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, Washington, D.C 20234.

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).

Percent per degree Celsius 235 257

313 345 350

-0.05

-0.05 -0.02

-0.08

-0.05

where: ti = apparent specific absorbance at temperature t (°Q

103
U.S. Department if ("nmmcn-f

.luaniti M. Kirps

t>*ctrUry

N»i»«v«l 8uraaoe< Standard FjM Ambkw. Llirm.w ^fattnnal ^Bureau of jitanrtarrfs

Ol^rtificat^

Standard Reference Material 2009

Didymium Glass Filter for Checking the

Wavelength Scale of Spectrophotometers

Kenneth L. Eckerie and William H. Venable. Jr.

Serial Number.

This SRM 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, anywhere from 14 to 24 wavelength corrections can be determined from 400 to 760 nm.

Detailed instructions on the use of this SRM and examples of its use are given in NBS Special Publication

260-66. Each didymium-glass filter is identified by the SRM number and a serial number. The wavelengths of the transmittance minima as obtained from measurements on two filters representative of the melt are given in Table I . These values are given for seven equally spaced values of the half-height width of triangular passbands. The minima number is identified in the figure that illustrates the spectral transmittance as a function of wavelength. The wavelength values of nine points of inflection on the spectral transmittance curve

as obtained on two filters are given in Table 2. These inflection points are representative of the melt and are also

identified in the figure. These inflection points should only be used with the transmittance minima as described

in Sections 2.2 and 2.3 in SP 260-66. The measurements on which these tables are based were made at 25 °C with a high-precision reference spectro- photometer that has a wavelength accuracy of 0.04 nm. Table 3 indicates the estimated random (as obtained from 4 sets of measurements on a single filter) and systematic errors of the transmittance minima given in Table

I. Table 2 also indicates the range of the measured wavelengths of the inflection points. 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. The technical and support aspects involved in the preparation, certification, and issuance of this SRM were

coordinated through the Office of Standard Reference Materials by R. K. Kirby. The spectral transmittance as a function of wavelength for a filter representative of the melt is given in Table 4. These values are not certified but are provided for use as specified in SP 260-66. They shouldnot be used to check

the photometric scale of a spectrophotometer.

It is recommended that the filter be handled only by its edges and 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 isoprophyl alcohol, and nnse again with distilled water. Dry after each rinsing by wiping lightly with optical lens tissue. Washington, D C. 20234 George A. Uriano, Chief January g, 1980 Office of Standard Reference Materials 104
400 500 600 700 Wavelength (nm)

Spectral transmi ttance of a typical didymium glass filter. Numbers indicate the principal points of minimum transmittance and letters indicate the principal points of inflection. SRM 2009 10c
Table I Certified Wavelengths (nm) of the Transmitunce

Bandwidth Minimum No.

12

34

56

1

9

10

Minima for the Indicated Bandwidth*

1.5 nm 3.0 nm 4.5 nm 6.0 nm 7J nm 9.0 nm 10.5 nm

402.42 401.81 401.69 401 66 401.42 400.95 431.50 432.48 440.27 440 52 441 84 442 52 442.37 442.08 441.33 445.59 445.14 472.72 472.58 472.88

478.89 479.34 479.28 478.31 477.36 476.50 475.65

513.45 513.61 513.89 514.31 515.38

529.58 530.02 529.90 529.47 529.27 529.12 528.88

572.69 573.27 574.21 575.11 576.59

585.34 585.54 585.77 586.02 585.99 585.35 584.42 623.62 624.02

629.53 629.41 628.56 627.03 627.02

684.66 684.68 684.71 684.72 684.71 684.66 684.58

739.86 739.96 740.24 740.91 742.01 742.97 743.65

748.28 748.10 Table 2 Wavelengths and Transmittances at Nine Selected Points of Inflection

SRM 2009

Point Wavelength Range* Traremittance 1 Identification (nm) (nm) A 406.44 +.08 0.7760 -.06 B 429.43 +.05 .7359 -.05 C 449.49 +.06 .6516 -.06 D 484.84 +.10 .6758 -.15 E 536.50 +.06 .5805 -.09 F 568.15 +.08 .4023 -.04 G 599.05 +.05 .3348 -.07 H 733.39 +.06 .4719 -.03

I 756.45 +.01 .4177 -.02 •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. 106
Table 3 Estimated Random and Systematic Errors of the Transmittance Minima

Band Nominal Standard Deviation for Indicated Bandwidth Number Wavelength

of Minimum 1.5 nm 3.0 nm 4.5 nm 6.0 am 7.3 run 9.0 run 10.5 nm

Transmittance

1 402 nm

(O O^r""

1 0 018 nra 0.020 nm 0.021 nm 0.025 nm 0.029 nm

2 431 00,3

(0.06) 0010 UIU _____ 3 440 *JJJ

0.013 0.050 0.028 0.009 0.005 SS 0029 ----- 5 473

(J;JJj 0.0.2 0.032 -

6 479

(0?5) 0013 0 009 0 009 0 009 0 0M

(025)

7 573

(°°^ 0.016 0.014 0.013 0.010

8 530

( °°Jj

0.010 0.010 0.011 0.010 0.011

9 573 (005) 0010 0 010 0 012 0 014

10 585 °°°

7 0.004 0.007 0.008 0.008 0.007 °??7

(0.06) (0._5)

0.06, -

12 630 °TM 0.120 0.171 0.133 0.091

13 685

( °°j9

0.029 0.024 0.017 0.014 0.014

14 740 0.013 0.01 1 0.010 0.009 0.009 «« -

-

T Value* in parentheses are estimates of the systematic error. The estimates for the 1 .5 nm bandwidth were obtained from the data taken ai

1.5 nm intervals as compared to data taken at 0. 15 nm intervals. The method of estimating the systematic errors lortne 10.5 nm bandwidth

is described in Section 3.4 in SP 260-66. SRM 2009

107
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.

Wave uAT1 AT Wave AT AT Length OtalluaLU Sy s tern*TM Length Sys tem~

(nm) 1 Error mm; T

380. 00 . 61832 000054 . 00010 "JOT J o JL

.

jU .63896 • OOOOR U U \J U O 5J . 00010

383. 00 . 66210 oooofi4 .00010 "} Q A J 0 H . 3 u .68291 00007Q .00010

386. 00 . 70112 000047 . 00010 *3 Q T Jo/. 3 U . 71885 nnnn7

i

.00010

389. 00 . 73590 .00010 ion K ft jU .74954 000040 .00010

392. 00 . 7 6 201 oooor ~\ .00010 J3J. 3 u .77104 0000 5 5 .00010

395. 00 . 77620 oooofi4 .00010 J Q C j y d .

<^ ft 3 U .77123 on n n 7 a .00010

398. 00 . 75013 oooorq .00010 399

.

C ft 3U .72419 * 00007Q U U U U / 7 .00010

401. 00 . 70966 000070 . 00010 A ft "J c ft .69505 00007

i

• U U U U / X .00010

404. 00 . / J D 0 J • UUUUJ

/

. 00010 /( ft c 4 U 3 . C ft 3 U .75886 • 000047 U U U U *t / .00010

407. 00 .77756 . ooooar U U *J u o o .00010 408. 3 U . 80673 • 00007? u u u u / «• .00010

410. 00 .82277 oooo^fl .00010 411. ft 3 U .82813 • oooo UUUUJO^ a .00010

413. 00 . 83115 nooofio . 00010 414. 50 .83287 • U U U U / J .00010

416. 00 . 83222 oooofio .00010 417. 50 .83200 • 000079 \J \J \J \J 4m J .00010

419. 00 . 83277 000059 .00010 420. 50 .83844 • OOOOfifi U U U U U U .00010

422. 00 .83711 oooofi4 .00010 423. 50 .83328 .000052 .00010

425. 00 .83511 • 000049 u u u u *i .00010 426. 50 .83414 • 00004 U U U U *t J5 .00010

428. 00 .81620 oooo .00010 429. 50 . 72877 • 000021 U U U U & J .00010

431. 00 . 62212 • onU U noU U Qfi J u .00010 432. 50 .63478 • 0000 UUUUJO?Q .00010

434. 00 . 64183 . 00010 435. 50 .63392 .000059 .00010

437. 00 .61858 • 000049 U U U U *t 3 .00010 438. 50 .60090 • 000018 UUUUJO .00010 440.00 .59163 .000064 .00010 441. 50 . 59496 .000053 .00010

443. 00 .60082 .000063 . 00010 444. 50 .59576 .000068 .00010

446. 00 .59315 .000055 .00010 447. 50 .60769 .000044 .00010

449. 00 .63551 .000083 .00010 450. 50 .66313 .000060 .00010

452. 00 .68002 .000069 .00010 453. 50 .69110 .000057 .00010

455. 00 .70206 .000027 .00010 456. 50 .70341 .000033 .00010

458. 00 .69051 .000025 .00010 459. 50 .66807 .000049 .00010

461. 00 .64890 .000057 .00010 462. 50 .63966 .000060 .00010

464. 00 .63837 .000070 .00010 465. 50 .63353 .000040 .00010

467. 00 . 62094 .000072 .00010 468. 50 .60572 .000053 .00010

470. 00 .59042 .000008 .00010 471. 50 .57213 .000072 .00010

473. 00 . 56160 .000033 .00010 474. 50 .58414 .000031 .00010

476. 00 .59528 .000052 .00010 477.50 .56003 .000052 .00010

479. 00 . 54953 .000070 .00010 480. 50 .55659 .000036 .00010

482. 00 .57326 .000025 .00010 483. 50 .62009 .000040 .00010

485 00 .67764 .000052 .00010 486. 50 .72126 .000072 .00010

488 00 .75412 .000045 .00010 489 50 .78321 .000082 .00010

491 00 .80556 .000035 .00010 492 50 .81759 .000060 .00010

494 00 . 82117 .000063 .00010 495 50 .81916 .000038 .00010

497 00 .81233 .000096 .00010 498 50 .80115 .000044 .00010 SRM 2009

108
TABLE 4 Wave AT AT Length Standard System-

(nm) T Error atic

500. 00 .78689 . UOOObO . 00010

503. 00 .75433 n a a a a i

. 000047 . 00010

506. 00 I'll o? . / Zlo 3 . 0000 / u . 00010

509. 00 . 65070 a a a a c a

. OOOObO .00010

512. 00 .53799 . 000044 . 00010

515. 00 C 1 C 1 T

. 00003b . 00010

518. 00 CQCA1 . UUUU1Z . 0 0 0x0

521. 00 . b / y 3 3 . oooo4y . 00010

524. 00 . H J D / 0

C\C\C\C\ A A . UUU 1U 527.00 . UUUU j / n n m n . UU01U

530. 00 nnnni n . UUUU LZ . 00010

533. 00 . jo? Jo . 00004b . 00010

536. 00 C A A C "7

. oooob/ nnm n . 0 0 U 10 539. 00 . D50D

. OOOObb . UUU 1U 542. 00 1 Q "> A ~l n n n n e o . UUUUoz nnni n . UUU1U

545. 00 Q A C\C\C

. o 4 UUo . 00004 j nnm n . U0010 548.00 . obi J / . 0000 / j . 00010

551. 00 QCCQ7 . ob jo / A A n A C Q

. OOOObo nnni n . 00010

554. 00 . 864 82 . 0000b/ . 00010

557. 00 . UUUU J

nnm n . 00010

560. 00 . o 4 Z j / . 00004b . 00010

563. 00 . / y / z o

t\f\f\f\ A Q

. 00004o . UUU1U

566. 00 . 66681 • UUUUji . 00010

569. 00 '.25719 .000033 .00010

572. 00 .02604 .000010 .00010

575.,00 .03443 .000011 .00010

578.,00 .05117 .000012 .00010 581.00 .06863 .000007 .00010 584.00 .01549 .000007 .00010

587.,00 .01577 .000016 .00010

590.,00 .04702 .000023 . 00010

593,.00 . 07588 .000013 .00010

596..00 .19036 .000034 .00010

599..00 .32378 .000038 .00010

602,.00 .44667 .000041 .00010

605 .00 .56569 .000048 .00010

608 .00 .62507 .000047 .00010 611 .00 .68076 .000072 .00010

614 .00 .79774 .000073 .00010

617 .00 .87427 .000086 .00010 SRM 2009

(cont

Wave AT AT Length Standard System-

(nm) T Error atic

501. 50 .77050 n n n n c o. 0000b 2 . 00010

50450 . 73899 a n a a a c

. 00004b .00010

50750 .69622 n a a a c a

. UOOUb 4 . 00010

51050 . 58889 . 000023 .00010

513. 50 .51737 a a n a c a .000050 .00010

516

50 .56547 A A A A 1 O. 000033 . 00010

51950 .59603 A A A A ^ O

. 00003 8 .00010

522

50 . 514 64 A A A A C O. OOOObo .00010

525. 50 . 4 0684 . uuuuz y a a m n . 0001U

528. 50 . 36734 A A A A 1 Q . uoooiy .00010

531. 50 . 37185 A A A A A C. 00004b . 00010

534. 50 .45072 A A A A "7 .000037 .00010

537. 50 . 63283 . 00003b A A A 1 A. U0010

540. 50 . 75111 A A A A *} O

. UOOOZo .00010

543. 50 . o z u y j

A A A A C A . OOOObO nnn i n . 00010

546. 50 . 85304 . OOOObo . 00010

549. 50 . oojj/ nnnn^io . 0U0043 nnm n . 00010

552. 50 . 86526 A A A A *3 C. 00003b .00010

555.,50 . ob3b3 . nnnmo UUUU J o a n n i n . 00010

558.,50 . 85316 A A A A O O

. UUOOoo . 00010

561..50 . 82504 A A A A C A

. OOOObO . 00010

564..50 . / bl 34 A A A A "3 1. 000031 n n n 1 A . 00010

567..50 AAAA-3 A . UUUU ju .00010

570.,50 .07751 .000011 .00010

573..50 .02536 .000013 .00010

576,.50 .03857 .000013 .00010

579,.50 .06944 .000014 .00010 582.50 .03836 .000014 .00010

585 .50 .01027 .000006 .00010

588 .50 .02979 .000022 .00010

591 .50 .05748 .000009 .00010

594 .50 .11929 .000028 .00010

597 .50 .26661 .000020 .00010 600.50 .38146 .000043 .00010

603 .50 .51047 .000043 .00010

606 .50 .60327 .000074 .00010

609 .50 .64527 .000041 .00010

612 .50 .73522 .000054 .00010

615 .50 . 84664 .000067 .00010

618 .50 .88467 .000056 .00010

109
TABLE 4 (cont)

Wave AT AT Length Standard System-

(nm) T Error atic

620. 00 .88413 .000042 .00010

623. 00 .87047 .000037 .00010

626. 00 .87359 .000066 .00010

629. 00 .87008 .000069 .00010

632. 00 .87569 .000048 .00010

635. 00 .88828 .000124 .00010

638. 00 .89322 .000035 .00010

641. 00 .89755 .000085 .00010

644. 00 . 90014 .000052 .00010

647. 00 .90124 .000041 .00010

650. 00 .90106 .000034 .00010

653. 00 .90218 .000054 .00010

656. 00 .90301 .000082 .00010 659.00 .90198 .000049 .00010

662. 00 .89951 . 000104 .00010

665. 00 .89155 .000045 .00010

668. 00 . 87844 .000079 .00010

671. 00 .86691 .000077 .00010

674. 00 .85883 .000035 .00010

677. 00 . 84419 .000077 . 00010

680. 00 .81611 .000076 .00010

683. 00 .77773 .000111 .00010

686. 00 77287 .000111 .00010

689. 00 . 80954 .000080 .00010

692. 00 .84399 .000066 .00010

695 00 .87138 .000058 .00010

698 00 .88764 .000054 .00010

701 00 . 89577 .000056 .00010

704 00 .89957 .000049 . 00010 707.00 .90166 .000056 .00010

710 00 . 90225 .000066 . 00010

713 00 . 90041 .000056 .00010

716 .00 . 89593 .000042 .00010

719 .00 .88755 .000075 .00010

722 .00 .87336 .000061 .00010

725 .00 .84758 .000054 .00010 728.00 .79437 .000071 .00010

731 .00 .66643 .000039 . 00010

734 .00 .40413 .000053 .00010

737 .00 .17772 .000064 . 00010 SRM 2009

Wave AT AT Length Standard System-

(nm) T Error atic

621. 50 .87711 .000071 .00010

624. 50 . 87083 .000060 .00010

627. 50 .87310 .000069 .00010

630. 50 .87085 .000064 .00010

633. 50 .88221 .000043 .00010

636. 50 .89117 .000089 .00010

639. 50 .89543 . 000062 .00010

642. 50 .89905 .000051 .00010

645. 50 .90080 .000043 .00010

648. 50 .90112 .000060 .00010

651. 50 .90149 .000079 .00010

654. 50 .90300 .000066 . 00010

657. 50 .90256 .000044 .00010

660. 50 .90115 .000091 . 00010

663. 50 .89656 .000071 . 00010

666. 50 .88540 .000079 .00010

669. 50 .87177 .000085 .00010

672. 50 .86284 .000056 . 00010

675. 50 .85369 .000076 .00010

678. 50 .83115 .000053 .00010

681. 50 .79779 .000063 . 00010

684 50 .76323 .000051 .00010

687 50 .79255 .000057 .00010

690 50 .82685 .000045 .00010

693 50 .85936 .000052 .00010

696 50 .88070 .000036 .00010

699 50 .89254 .000029 .00010

702 50 .89798 .000040 .00010 705.50 .90039 .000060 .00010

708 .50 .90231 .000071 .00010

711 .50 .90168 .000071 .00010

714 .50 .89850 .000060 .00010

717 .50 .89232 .000040 .00010

720 .50 .88137 .000049 .00010

723 .50 .86256 .000086 .00010

726 .50 .82620 .000070 .00010

729 .50 .74399 .000039 .00010

732 .50 .55165 .000033 .00010

735 .50 .26498 .000040 .00010

738 .50 .14784 .000028 .00010

no
Wave AT Length Standard

(nm) T Error

740. 00 . 14400 .000017

743. 00 . 16928 .000022

746. 00 . 19302 .000027

749. 00 . 17678 .000040

752. 00 . 24335 .000050

755. 00 . 34682 .000095

758. 00 . 47017 .000099

761. 00 . 57256 .000071

764. 00 . 64917 .000089

767. 00 . 71596 .000088

770. 00 . 76342 .000040

773. 00 . 78603 .000082

776. 00 . 78665 .000098

779. 00 . 75800 .000083 SRM 2009

(cont

)

AT WaveSystem- Length

atic (nm) .00010 741.50

. 00010 744.50

.00010 747.50

.00010 750.50

.00010 753.50

.00010 756.50

.00010 759.50

.00010 762.50

.00010 765.50

.00010 768.50

.00010 771.50

.00010 774.50

.00010 777.50

.00010 780.50

AT AT Standard SystemT Error atic

.14841 .000015 .00010 .19331 .000020 .00010 .17683 .000014 .00010 .20220 .000026 .00010 .29149 .000049 .00010 .40869 .000063 .00010 .52523 .000164 .00010

.61237 .000077 .00010 .68412 .000093 .00010

.74294 .000080 .00010

.77754 .000144 .00010

.78934 .000082 .00010 .77668 .000050 .00010 .73040 .000120 .00010

m
US. Department of Commerce

Elliot L. Richardson. SenrUn

National Bimi of Standards Enwi Aabter. Acum Uiitcuo Rational ^urau af ^tnndnvds

(HtvtiixtciU

Standard Reference Material 2030

Glass Filter tor Transmittance Measurement

R. Mavrodmcanu and J. R. Baldwin

This Standard Reference Material is intended as a reference source for one-point verification of the transmit- tance and absorbance scales of spectrophotometers at tne given wavelength and measured transmittance. It consists of one glass filter in its holder and one empty filter holder. The filter bears an identification number. For protection, the metal holder is provided with two shutters that should be removed during measurements.

responding transmission density, —logioT, is: Date of Certification: . The transmittance value (T) can be converted to percent transmittance (%T) by multiplying by 100. The trans- mission density is calculated from the measured transmittance (T). This value should be indicated by the absorbance scale of the spectrophotometer if the finer is measured against air. The transmittance value given was measured against air at an ambient temperature of 23.5 °C, and is estimated

to be accurate to within ±0.5 percent at the time of certification. This uncertainty includes the random and

systematic errors of the calibration proceoure. as well as transxniitance changes of the filter during the period of

calibration.

It is possible that aging of the glass may cause some filters to chunpt- transmittance by about ±1 percent over a

period of approximately 1 year from the date of calibration. ImproDer storage or handling of the filter may also cause changes [5]. In cases where verification is desirable, the filter snouid be returned to the National Bureau of Standards for cleaning and recalibration.

It is recommended that the f»»*-r ; r the holder be handled only by the edees and with soft plastic (polyethylene) gloves and optical lens tissue When not in use. it should be stored in us holder, with the shutters on, and in the box provided for this purpose. Extended exposure to laboratory atmosphere and dusty surroundings should be avoided. This Standard Reference Material was issued with the assistance and advice of K.. D. Mielenz and J. R. DeVoe. The technical and sunoort aspect involved in the preparation, certification, and issuance of this Standard

Reference Material were coordinated through the Office of Standard Reference Materials by R. W. Seward,

The transmittance. T, at A 465.0 nm and for a spectra! bandpass of 2.7 nm is: . ThecorWashmgton. D.C. 20234 September 23, 1976

J. Paul Cali, Chief

Office of Stanoara Reference Materials

112
The transmittance measurements were made using the high-accuracy spectrophotometer designed and built at the Institute for Materials Research, National Bureau of Standards [ 1 ]. This instrument represents the primary

transmittance standard; its transmittance accuracy was established using the double-aperture method of

linearity testing [ 1 ,3,5,6]. The reproducibility (relative standard deviation obtained from a set of 20 measurements) of this high-accuracy spectrometer for transmittance measurements on glass filters similar to the one

used to produce this SRM, is approximately 0.02 percent. The combined uncorrected systematic error of the instrument is estimated to be not more than ±10~4 transmit- tance units. As indicated on the face of this certificate, SRM 2030 is intended to be used as a one-point verification of the transmittance (absorbance) scales of spectrophotometers. If these scales require verification at other wavelengths and transmittanccs, "Glass Filters for Spectrophotometry," SRM 930c or subsequent issues, should be

used. The neutral glass for the filter was provided by Schott of Mainz, Germany, and is designated as "Jena Color and

Filter Glass "

[2,5]. 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). The empty filter holder is provided to be used in the reference beam of the spectro- photometer so that approximately equivalent conditions of stray radiations are achieved for both beams. The transmittance of the filter depends upon the intrinsic properties of the material, spectral bandpass, wavelength, geometry of the optical beam, temperature, and positioning of the filter. A change of ambient temperature of 1 or 2 ° C from 23.5 ° C will not significantly affect the calibration [5].Changes in the transmittance maybe caused by surface conditions, aging of the glass, exposure to a harmful atmosphere, or careless handling as indicated on the face of this certificate [2,3,4,5]. The effective spectral bandpass used to determine the certified value is given on the face of the certificate and the transmittance measurements are 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 glass filter. The filter was measured in the spectrophotometer in a position perpendicular to the incident light beam as shown in the attached figure. Prior to certification measurements, the filter was examined for surface defects and thoroughly cleaned [5]. 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 degraded the accuracy of the certified value. Because the Standard Reference Material is a transfer standard, the only means

available to verify its integrity is remeasuring its transmittance with a primary standard instrument similar to that used in this certification [ 1 ,4,5]. In most cases where verification, or recertification, of the transmittance value is desirable, it will be most expeditious to return the filter to the National Bureau of Standards for measurement. Prior to shipment the NBS Office of Standard Reference Materials should be contacted regard- ing the conditions and cost of verification and /or recertification. REFERENCES

[ 1] R. Mavrodineanu, An Accurate Spectrophotometer for Measuring the Transmittance of Solid and Liquid

Materials. NBS Journal of Research 76A, No. 5, 405^25 (1972).

[2] R. Mavrodineanu, Solid Materials to Check the Photometric Scale of Spectrophotometers, NBS Tech. Note 544, O. Menis and J. I. Shultz, ed., pp 6-17, U.S. Government Printing Office, Washington, D.C. 20402 (Sept. 1970), ibid NBS Tech. Note 584, pp 2-21 (December 1971).

[3] K. S. Gibson, Spectrophotometry, NBS Circ. 484 (Sept. 1949).

[4] Collected Papers from NBS Conference on Accuracy in Spectrophotometry and Luminescence Measurements, NBS Journal of Research 76A, No. 5, 375-510 (1972).

[5] R. Mavrodineanu and J. R. Baldwin, Glass Filters as a Standard Reference Material for Spectrophotometry-Selection, Preparation, Certification, Use SRM 930, NBS Special Publication 260-51, U.S. Govern- ment Printing Office, Washington, D.C. 20402, (1975).

[6] K. D. Mielenz and K. L. Eckerle, Spectrophotometnc Linearity Testing Using Double-Bcam-Aperture Method, Appl. Optics, 11, 2294-2303 (1972).

113
METAL HOLDER FOR THE COLORED GLASS FILTERS

FRONT

! W1.5 BASE

.METAL HOLDER

57 X 13 X 13

EXPOSED FILTER SURFACE

29 X 8

COLORED GLASS FILTER

30.5 X II x 2.0 (Of 1.5)

DIMENSIONS IN

114
U. S. Department i)( Commerce

Mi>colm Hjldngt

Secret ;irv

Ernen Amhk-t Ifcrecioi Rational ^mtnn of jzttanttarrfe

Certificate of (Ealibraiifltt

Standard Reference Material 2031

Metal-on-Quartz Filters for Spectrophotometry

This Standard Reference Material is intended for use in the verification of the transmittance and absorbance scales of conventional spectrophotometers in the ultraviolet and visible regions of the electromagnetic spectrum. It consists of three individual filters in their metal holders and one empty filter holder. Two filters, having nominal transmittances of 10 and 30

percent, were produced by evaporating different thicknesses of chromium metal on 1.5-mm thick fused silica plates that had

been precision ground and polished. These metal films are protected by 1.5-mm 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 90

percent. The metal holders for these filters are provided with shutters to protect the filters when not in use. The shutters must

be removed at the time of measurement and be replaced after the measurements have been completed. Each filter holder

bears the set and filter identification number.

Set and

Filter ldent. Number

TRANSMITTANCE (T)

Wavelength, nm

250.0 280.0 340.0 360.0 400.0 465.0 500.0 546.1 590.0 635.0

Set and

Filter ldent. Number

TRANSMITTANCE DENSITY (-logioT)

Wavelength, nm

250.0 280.0 340.0 360.0 400.0 465.0 500.0 546.1 590.0 635.0

Date of Certification: Washington, D.C. 20234 George A. Uriano, Chief September 20, 1982 Office of Standard Reference Materials (Revision of Certificate dated 6-1-79) . c
The transmittance values (T) can be converted to percent transmittance (%T) by multiplying by 100. The transmittance

densities were calculated from the measured transmittance (T). These transmittance densities should be indicated by the absorbance scale of the spectrophotometer when the filters are measured against air. All of the certified transmittance values were obtained by measuring against air at an ambient temperature of 23. 5 °C. The uncertainties of the certified transmittance values of these filters do not exceed 1.0 percent. These uncertainties include

0.5 percent for random errors of the calibration procedure, as well as 0.5 percent for possible changes in the transmittance

with time. The long-term stability of the filters with the chromium film has not been rigorously established. Measurements

to date, however, suggest that the transmittance of the chromium-coated filters could change by as much as 0.5 percent in the

first year after calibration. Therefore, this SRM should be returned to the National Bureau of Standards for free verification of the transmittance values on the first anniversary of the certification date. Information regarding subsequent verifications or recalibrations, including costs for such service, will be included with this first verification report. Before returning this SRM to NBS, information regarding such shipment should be obtained from the Office of Standard Reference Materials, Room B311, Chemistry Building, National Bureau of Standards, Washington, DC 20234. Telephone: (301) 921-2641. 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. The transmittance measurements were made using the high-accuracy spectrophotometer designed and built in the NBS

Center for Analytical Chemistry [/ ]. This instrument is the primary transmittance standard; its transmittance accuracy was

established using the double-aperture method of linearity testing [1,3,5,6]. Transmittance measurements for SRM 2031 are made by producing the vertical image of the slit (about 8 mm by 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 filters were measured in the spectrophotometer in a position perpendicular to the incident light beam. A spectral bandpass of 1 .6 nm was

used for measurements at all wavelengths. Because the transmittances of these filters exhibit an appreciable optical

neutrality, the dependence of transmittance on bandpass is not critical and wider bandpasses may be used in routine measurements. For a quantitative discussion of this subject, the user should consult reference 5, pp. 32 and 33. The transmittance, T, was measured against air in the reference beam; hence it includes the reflection losses that occur at the

air-filter interface. Under these circumstances the measured transmittance, T, has a corresponding transmittance density, defined as -logio T. The internal transmittance, Ti, of a material is defined as the transmittance of the material corrected for

reflection losses. This is obtained experimentally when the measurements are made against a blank sample in the reference beam. The absorbance, A, of a material is related to this internal transmittance, Ti, by the expression, A = logioT. 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). 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 transmittance of the filters depends upon the intrinsic properties of the material, wavelength, spectral bandpass, geometry of the optical beam, temperature, and

positioning of the filter. While changes in ambient temperature of 1 or 2 °C from 23.5 °C have not significantly affected the

calibration, the effect of temperature variations exceeding 2 °C have not been investigated. Changes in the transmittance may be caused by surface conditions, aging of the matenal, exposure to a harmful atmosphere, or careless handling [2,3,4,5]. SRM 2031 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 particulate matter due to static charges is minimized through the metallic nature of

the container. A fiat 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.

116
The filter is shown in the assembled unit with its front surface facing up. The filter, in its filter holder, should be placed in the cuvette compartment of the spectrophotometer with its front surface facing the incident light beam

and the rear surface facing the photodetector.
Prior to certification measurements, each filter was examined for surface defects and the condition of the optical contact [5]. S hould 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

203 1 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 [1,5]. In most cases, where verification or recertification of the transmittance values is desirable, it will be most expeditious to return the filters to the National Bureau of Standards for measurement.

Further information concerning the selection, preparation, and properties of SRM 2031 will be found in reference 5. The research, development, and initial production of this SRM were conducted by R. Mavrodineanu and J.R. Baldwin, NBS

Inorganic Analytical Research Division. The transmittance measurements were performed by R. W. Burke, Inorganic Analytical Research Division, and M.V. Smith,

Office of Standard Reference Materials. Technical leadership for the preparation and measurements leading to certification was provided by R.W. Burke. The overall direction and coordination of technical measurements leading to certification were performed under the chair- manship of E.L. Garner. 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 R.W. Seward. We wish to acknowledge the cooperation of 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. The contributions of-H.E. Bennett and J.M. Bennett of the Michelson Laboratory, Physical Optics Branch, Naval Weapons

Center. China Lake, California, who helped in the initial production of SRM 2031 are also gratefully acknowledged.

References

1. R. Mavrodineanu, An Accurate Spectrophotometer for Measuring the Transmittance of Solid and Liquid Materials, NBS Journal of Research 76a. No. 5, 405-425 (1972).

2. R. Mavrodineanu, Solid Materials to Check the Photometric Scale of Spectrophotometers, NBS Tech. Note 544, O. Menis, and J.I. Shultz, Eds., pp. 6-17, U.S. Government Printing Office, Washington, D.C. 20402 (Sept. 1970); ibid, NBS Tech. Note 584, pp. 2-21 (December 1971).

3. K..S. Gibson, Spectrophotometry, NBS Circ. 484 (Sept. 1949).

4. R. Mavrodineanu, Considerations for the Use of Semi-transparent Metallic Thin Films as Potential Transmittance Standards in Spectrophotometry. NBS Journal of Research 80A, No. 4, 637-641 (1976).

5. R. Mavrodineanu and J.R. Baldwin. Metal-on-Quartz Filters as a Standard Reference Material for Spectrophotometry, SRM 2031. NBS Special Publication 260-68, U.S. Government Printing Office. Washington, D.C 20402, 1979.

6. K..D. Mielenzand K.L. Eckerle, Spectrophotometer Linearity Testing Using the Double-Aperture Method, AppL Optics

11. 2294-2303 (1972).

118
S. Department of Cummerr*

luiniu M. Krep*

Secretary

Naltniul Iuim M Sl.nd.rtl. Kmtu AmMrr. Uirrrlnc ^aitmtal £5uram of ^twxAzxiiz

Certificate

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 SRM 2032 of known pathlengths and concentrations, and comparing the result with the certified values of the

specific absorbance, t. Specific Absorbance*

t (L g"' cm" ), vs Wavelength, \ (nm), at 23.5 °C

A 240 245 250 255 260 265 270 275

t 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" ha* been avoided

since it is ambiguously defined. See K. D. Mielenz. Anal. Chem. & 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, 1 979 Office of Standard Reference Materials

119
Supplementary Information

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.

Certification Procedure: The certified specific absorbances were measured in the NBS Center for Analytical Chemistry high-accuracy spectrophotometer [I, 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~' were prepared and measured against distilled water, using standard fused-silica cuvettes with pathlengths between 10.001 and 10.009 mm1

. 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.

was used to calculate the specific absorbances, €, 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 ± 14% 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

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.

t = A/cC (1)

(2)

"iisued by NBS is SRM 932. Quara Cuvettes for Spectrophotometry.

120
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'. 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 = 1 0 g L~ ') with a 1 cm pathlength exhibits a sharp cutoff in transmittalce 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) T(X) + x(X)

(3)

1 + x(X) X(X)=

T'fX)-T(X)

x(X) - }

j _ 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 2032, the true transmittance, T(X), of KI

solutions may be calculated as: T(X)=.0-

t(X)ce where t(kj 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. Tint with isopropyl alcohol and then with distilled water, are generally adequate.

121
The apparent transmittanccs 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-to- noise ratios should be used. Many instruments do not permit the direct measurement of transmittanccs 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, 405-425 (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).

122
U S Department of Commcrct

Philip M. Klutznick

Secretary Enm Ambto. Orccior ^aitanal ^nrtZLK of jltandartte

Certificate

Standard Reference Material 2033

Crystalline Potassium Iodide with Attenuator

Heterochromatic and Isochromatic 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 I %. 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 ol known pathlengths and concentrations, and comparing the results with the certified values of the specific absorbance, t. Specific Absorbance

2

t (Lg cm" 1 ), vs Wavelength, X (nm), at 23.5 °C

X 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 unil concentration. The term "absorptivity" has been avoided since it is ambiguously defined. See K. D. Mielenz. Anal. Chem. 4S. 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 X255 nm is the transmittance of both filters at X255 is SRM 2033 was issued with the technical assistance of J. R. Baldwin, R. W. Burke, A. L. Cummmgs, 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, 1 980 Office of Standard Reference Materials 123
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 k W 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 spectro- photometer [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~' 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/ci (I) was used to calculate the specific absorbances, t, from the measured absorbances. A, and the known values of concentration, c, and pathlength, S, . 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 r

- If -•»•<?. a

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.

h lssued by NBS as SRM 932. Quartz Cuvette for Spectrophotometry. SRM 2033

124
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 0.0 1 % 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:

1 + x(X) JHzJIi) (4)

1 - 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"'(Mc£

where t(X) is the certified specific absorbance given on the face of this certificate.

Soft glass containers and pipettes contain residual amounts of U V absorbing material, but can be used after proper cleaning. Several n rises, first with

isopropyl alcohol and then with distilled water, are generally adequate. SRM 2033

125
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-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 1 00, 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 trans- mittances lower than I 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 caoable 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 wails 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 f 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

126
HBSQ14A ikiv. 2-»ct U.S. DEPT. O* COIW. 1. PUBLICATION OR 2. Performint Organ. Rap »ri No. 1. Publication Data BIBLIOGRAPHIC DATA REPORT NO. April 1983 SHEET (See instructions) NBS SP 260-81

4. TITLE AND SUBTITLE Standard Reference Materials: Accuracy in Analytical Spectrophotometry

5. AUTHOR(S)

R. W. Burke and R. Mavrodineanu

S. PERFORMING ORGANIZATION (If joint or other thon NBS. see instructions) NATIONAL BUREAU OF STANDARDS DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

1. Type of Report & Period Covarad Final

S. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City, State, ZIP)

same as above

10. SUPPLEMENTARY NOTES " _ ~°

Library of Congress Catalog Card Number: 83-600512

I | Document describes a computer program; SF-185. FlPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less foctuol summary of most significant information, If document includes a significant bibliography or literature survey, mention it here) This publication describes the activities undertaken since 1969 within the Center for Analytical Chemistry at the National Bureau of Standards (NBS) in the field of high-accuracy spectrophotometry. The first part of this work presents

a summary of the Standard Reference Materials (SRM's) that have been developed for checking the proper functioning of ultraviolet and visible spectrophotometers

and includes a description of the high-accuracy spectrophotometer specially constructed in the Center for Analytical Chemistry and subsequently used for performing all of the transmittance measurements. The second part of this publication is devoted to a critical discussion of the analytical factors that

can affect the accuracy of selected spectrophotometric procedures that have been widely used at NBS in the characterization of various SRM's.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons) absorbance; accuracy; antimony; arsenic; cobalt; comparative measurements; copper; high-accuracy spectrophotometry; nickel; polarization; scatter; spectral bandpass;

spectrophotometric determinations; Standard Reference Materials; stray light; transmittance; transmittance stability; wavelength accuracy.

13. AVAILABILITY

1 xl Unlimited

I | For Official Distribution. Do Not Release to NTIS

fv] Order From Superintendent of Documents, U.S. Government Printing. Office, Washington, D.C.

20402.

I | Order From National Technical Information Service (NTIS), Springfield, VA. 22161

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