EPA-600/2-76-126

May 1976 Environmental Protection Technology Series

DEVELOPMENT OF

THIN CALIBRATION STANDARDS FOR

X-RAY FLUORESCENCE ANALYSIS

Environmental Sciences Research Laboratory

Office of Research and Development

U.S. Environmental Protection Agency

Research Triangle Park, North Carolina 27711

it,
RESEARCH REPORTING SERIES

Research reports of the Office of Research and Development, U.S. Environmental

Protection Agency, have been grouped into five series. These five broad

categories were established to facilitate further development and application of environmental technology. Elimination of traditional grouping was consciously

planned to foster technology transfer and a maximum interface in related fields. The five series are:

1. Environmental Health Effects Research

2. ~ Environmental Protection Technology

3. Ecological Research

4. Environmental Monitoring

5. Socioeconomic Environmental Studies

This report has been assigned to the ENVIRONMENTAL PROTECTION

TECHNOLOGY series. This series describes research performed to develop and

demonstrate instrumentation, equipment, and methodology to repair or prevent

environmental degradation from point and non-point sources of pollution. This

work provides the new or improved technology required for the control and

treatment of pollution sources to meet environmental quality standards.

Polls, P. A., Kuehner, E. C, CcsGstt, V/. A.,

Development of a pert icul ate reference sample

on membrane filters for the standardization of

x-ray fluorescence spectrometers, (Proc. 24th

Annual Conf. on Applications of X-Ray Analysis,

Denver, CO, Aug. 6-8, 1975), Chapter in

Advances in X-Ray Analysis, R. VJ. Gould,

C. S. Barrett, J. E. Kev.'kirk ana C. 0. Ruud,

Eds., 19, 463-472 (Kenaall/Hunt Publ . Co.,

Dubuque, Iowa, 1976)

.

Pella, P. A., Kuehner, E. C, Cassatt, W. A.,

Development of thin calibration standards for

| x-ray fluorescence analysis, Environm.ental | Protection Technology Series, E?A~600/2-76-126,

j

11 pages (available as PE253252 from the | National Technical Information Service, jformaSpringfield, Va . , 22151, May 1976).

! NESIR 75 -970 (R)

310
EPA-600/2-76-126

May 1976

DEVELOPMEUT OF THIN CALIBRATION STANDARDS

FOR X-RAY FLUORESCENCE ANALYSIS

p. A. Pella, E. C. Kuehner, and W. A. Cassatt

National Bureau of Standards

Washington, DC 2023^

Contract No. EPA-IAG-lOO (D)

DPA-L4G-Di4-F8il9

Project Officer

T. G. Dzubay

Atmospheric Chemistry and Physics Division

Environmental Sciences Research Laboratory

Research Triangle Park, NC 27711

U.S. EWIRONlvIENTAL PROTECTION AGENCY

OFFICE OF RESEARCH AND DEVELOPMEIJT

ENVIROMENTAL SCIEI^CES RESEARCH LABORATORY

RESEARCH TRIANGLE PARK, NC 27711
DISCMIMER

This report has been reviewed by the Environmental Science Research

Laboratory, U.S. Environmental Protection Agency, and approved for publication. Approval does not signify that the contents necessarily reflect

the views and policies of the U.S. Environmental Protection Agency, nor endorsement or recommendation for use.
I

INTRODUCTION

Within the past several years, an increasing niimber of x-ray analyses of

enviroriciental and "biological samples have "been reported in which the

National Bureau of Standards Standard Reference Material (SRM) 1571

orchard leaves -was used as a trace element calibration standard [l-3].

This botanical SRI4 is well suited to such applications because it contains

a number of trace elements such as S, 01, K, Ca, Or, Mn, Fe, Co, Ni , Cu,

Zn, As and ?b which are important in the analysis of environmental samples.

In the analysis of ambient air particulates, the sample analyzed is often

in the form of thin deposits on either membrane or Whatman type filters.

Therefore, it is desirable to have a calibration sample in the same form.

The preparation of orchard leaf deposits on membrane filters to be used

for this purpose is described in this report.

The NBS SRi-1 as certified requires that at least 250 mg samples be taken

to insure that the trace element distribution is representative of the

bulk material. To permit smaller samples to be used and to minimize the

attenuation of x-rays in the individual particles, the material was reground.

A number of different coatings were investigated to provide good sample

protection, and at the same time obtain low attenuation of the x-rays from

elements suc'h as phosphorus and sulfur. Different coating materials that

were investigated consisted of polyisobutylene , heat-sealed overlays of

polypropylene and Parafilm, and vapor-deposited Parylene films.

EXPERIMENTAL

Sample deposits were prepared by filtration of a suspension of the reground

material in cyclohexane. Attempts were made to prepare deposits from an aerosol suspension, but were not successful.

A. Grinding of the Orchard Leaves SRM 1571

A high velocity air impact grinder was used to reduce the particle sizes.

The impactor produces two size fractions having relatively broad size

distributions. The "fines" fraction was collected after each pass through

the grinder and was reground four times. The particle size distribution

of the collected material was characterized by automated image analysis.

Data were taken by examining nine regions of four different sample

preparations. These samples were prepared by dispersing a few milligrajns

of material in cyclohexane with ultrasonic stirring. About three drops

of this suspension were placed on a microscope slide, covered with a cover glass and allowed to dry.

B. Preparation of Deposits on Membrane Filters

From 2 to 15 mg of dried reground orchard leaves were weighed and transferred to a 50 ml beaker containing about ^^0 ml of spectrograde cyclohexane. The beaker containing the orc'nard leaves was placed in an

1
ultrasonic bath for about one minute to disperse the particles. The

suspension was transferred to a vacuum filtering unit and the sides of

the beaker were washed down with cyclohexane . The material was filtered

rapidly through a 37 mm preweighed membrane filter (0.8 ym pore size).

The glass filtering funnel was constructed so that deposits on the 37 nmi membrane filters were 29 rm in diameter. The net weight of the air-dried

deposit was used to calculate the areal density of the deposit in mg/cm^.

All weighings were performed in a room in which the relative humidity was maintained at 50±^ percent.

C. Preparation of Parylene-Coated Samples

The samples were coated at the Union Carbide Corporation with a thin film

of Parylene N — a polymeric material developed by Union Carbide Corporation

(U.S. Patent No. 33^275^) • The procedure consists of first vaporizing the

di-para-xylylene dimer by heating it at 175°C at 760 mm Hg (l.OlxlO^ N/m^).

This vaporized material passes through a deposition chamber maintained at

room temperature, and 0.1 mm Hg (13.3 N/m^) which contains the samples to

be coated. The samples are rotated in the chamber to insure a uniform

coating of Parylene. The exit of the deposition chamber is connected to

a cold trap (-70°C) and pumped with a mechanical pump to a pressure of

0.001 mm Hg (0.133 N/m^). The film thickness is monitored in the deposition chamber using a piezoelectric frequency sensor. More accurate

thickness measurements were made at Union Carbide after removal of the

samples using a UV spectrophotometer. The thickness is calculated from

a calibration curve by comparing the absorbance of the film with that

of a film of known thickness measured interferometrically

.

D. Preparation of Thin-Film Overlays of Parafilm and Polypropylene

A sheet of Parafilm (American Can Co. ) was stretched by hand as thinly as possible without causing the sheet to tear, and was placed directly on the

sample which was held by suction to a filtering funnel. A polypropylene

sheet (0.08 mm thick) was next placed on top of the Parafilm and was heatsealed to the sample using a hot air gun.

E. Mounting the Parylene-Coated Filter Deposit

Filter holders for mounting the deposits were supplied by EPA, and are described in an EPA report [h]. These holders, designed to accommodate

37 mm diameter membrane filters, were modified by increasing the platform

diameter by 2 mm. This permitted the filter deposit to be centered in

the holder more easily and also allowed for some expansion of the membrane

filter before cementing in the holder. The filter deposit was held on a filtering funnel by applying suction. A thinned mixture of cement prepared

by mixing equal parts of Barge all-purpose cement, ethyl acetate and cyclohexane was brushed on the platform of the holder. The filter was cemented

to the holder platform by pressing the holder flat against the outer edge of

the filter deposit and then allowed to dry.

2
RESULTS ML DISCUSSION

Neutron activation analysis (NAA) of the reground orchard leaves showed

that the concentrations of iron, nickel, cobalt and chromiiim were much

higher than in the certified SRM 1571- Therefore, it was important to

determine if these introduced elements were homogeneously distributed

throughout the material. For this reason, sixteen samples of reground

material (200 mg each) were taken and six elements, including iron and

chromium, were analyzed by NAA. An analysis of variance of these con- centrations in the material indicated that there were no gross inhomogeneities (not greater than five percent). These results indicated that

the reground SRM 1571 was satisfactory for use provided that recertification of the introduced elements could be accomplished. An analysis

of a number of these elements using NAA, atomic absorption spectrophotometry

(AAS) , and photon activation analysis (PAA) are included at the end of

this report. Further work on the analysis of this material is still in

progress

.

The particle size distribution of the reground material is shown in figure

1. The area of about 80 percent of the 3000 particles examined was between

zero and 7-5 (3.1 pm equivalent diameter). Less than 0.5 percent of

the particles were within 82.5 and 105 ym^, with no particles observed

above 105 Mm^.

To determine if the cyclohexane treatment altered the trace element

composition of the orchard leaves, NAA was performed on cyclohexane

extracts and on the material after treating with cyclohexane. A number

of elements, including Br, Mn, Fe , K and Na, were analyzed. The results

showed that only sodium was affected by this treatment, and about 50

percent was extracted.

A simple technique was developed for measuring the uniformity of the

deposits. This consisted of directing a laser beam about 2 mm in diameter

normal to the sample, and measuring the intensity of the light transmitted

[5]- Using a Beer's law calibration of absorbance vs areal density, the

uniformity of the deposit was measured. 0^ the several samples examined

having areal densities of 1.0 to 2.0 mg/ cm , variations in uniformity did

not exceed 0.02 mg/cm^.

A number of coating materials were investigated and compared with regard

to their ability to adhere to the sample and to provide protection from

abrasion and humidity. These consisted of vapor-deposited Parylene films

and heat-sealed overlays of polypropylene using Parafilm as a binder.

Also, orchard leaf deposits were prepared from suspensions in cyclohexane

containing from 0.1 to 10 percent by weignt of polyisobutylene . Another

important factor considered was the x-ray attenuation of these coatings,

especially from elements of low atomic number such as phosphorus and sulfur.

The attenuation of P, S, K, and CaKa lines was measured by comparing the

backgroiind corrected peak intensities of coated and uncoated samples having

zhe same areal densities using a wavelength-dispersive x-ray spectrometer.

3
The values obtained for various thicknesses of Parylene and of thin polypropylene and Parafilm coatings are summarized in tables 1 and 2, respectively. For Parylene coatings 75 to 193 nm thick, little change in

attenuation was noted. Also, the x-ray backgroiind intensities from these

samples were essentially the same as the uncoated samples except that

the background of the 2500 nm thick sample was increased significantly.

The attenuation values of the polypropylene-Parafilm coatings could not

be reduced any further because thinner films were not available. In

order to produce adherent deposits using polyisobutylene, as much as one percent or greater by weight of the polymer in cyclohexane was used.

However, the amount of polyisobutylene retained on the samples varied

considerably making it difficult to obtain reproducible attenuation data.

For this reason, this coating material was considered unsatisfactory.

Table 1. Average Attenuation Values of Parylene Coatings'

sample areal density =2.0 / 2 mg/cm

No. of Parylene Percent Attenuation

Samples Thickness, nm Element Average

5 75 P 3 1.7

3 75 S 2 1.3

2 75 K l.k

5 117.5 P 3.1 0.50

5 117.5 K 1.7 O.hh

^ 3 158.0 P 5-h 0.90

2 158.0 5.2

k 158.0 K 2.9 0.35

7 166.5 P 5.0 0.81

5 166.5 S 0.92

7 166.5 K 2.6 0.69

5 . 193.0 P 7.3 0.88

3 193.0 K 3.7 0.20

3 2500 P 33 1.5

3 2500 S 28.9 0.71

2 2500 K 11. U

'Calculated from the expression I -I /I x 100; where I = Deak

u c u u

* intensity of uncoated sample, and = peak intensity of coated sample.

S = standard deviation of a single measurement.
Table 2. Attenuation Values of Polypropylene

and Parafilm Coatings

Sample #

Total Coating

^ Thickness, mg/ cm P

Percent

S

. Attenuation

CI K Ca

1 1.2 28,.1 23.2 Ik.l 10.9 8.5

2 1.1 2i|, 23.6 16.2 ll.h 7.9

3 1.2 26 .3 2h.k 13.6 10.1 8.3

Average 26 .3 23.7 14.6 10.8 8.2

The ruggedness of samples coated with Parylene films (75 to 200 nm thick) were tested by an air jet which impinged on the sample surface. Effects

of humidity were observed when the samples were placed in a chamber maintained at 100 percent relative humidity.

The trace element composition of the starting material (di-para-xylylene

dimer) was analyzed at Union Carbide Corporation. Of the 27 elements

analyzed, sodium, magnesium, iron and aluminum were present in concentrations from 2 to 9 ppm. Silicon was 31 ppm, and the remaining ' elements

were either below 1 ppm or were not detected. These resiilts indicate

that the Parylene material should be of sufficient purity for application

in this work. Since the Parylene films appeared to furnish good sample

protection, including high resistance to humidity, high purity, good

adherence, and acceptable x-ray attenuation for the low atomic number

elements, 60 samples were prepared and delivered to EPA containg from

0.1 to 5-0 mg/ cm of orchard leaves coated with 117-5 nm of Parylene.

A photograph of a typical reference sample mounted in a holder is shown

in figure 2.

5
Table 3. Preliminary Results of the Analysis

of Regroiand SRM 1571 Orchard Leaves

Element

^ Ho . of

I'leas

.

Average, pg/g Rel. Std. Dev. %

Certified Values

of SRM 1571, Mg/g

Fe^ 3 . 791 0.2 300±20

3 22. 5 0.5 1.3±0.2

Ni^' 1. > 21.5

Cr^ 15 . hi. 91 3.70 (2.3)

Mn" 16 102.1 l.dh 9l±h

16 1.1+9 (wt. ^) 1.53 l.U7±0.03 (vt. %)

Br" 15 10.1i8 2.58 (10)

Zn" 15 2I1.99 3.22 25±3

^ = PAA .

: ^

Values in parenthesis are not certified.

6
II

CONCLUSIONS

Reference samples consisting of uniform layers of reground orchard leaves

of known composition, i.e., NBS SRI^l I5TI5 on membrane filters were prepared for the standardization of x-ray fluorescence spectrometers. These

samples have areal densities of 0.1 to 5-0 mg/cm^, and are coated with a thin polymer film for protection against abrasion and moisture.

7
REFERENCES

1. J. A. Cooper, Nucl. Instr. and Meth. 106, 525 (1973).

2. R. D. Giauque, F. S. Goulding, J. M. Jaklevic and R. H. Pehl , Anal.

Chem. k^, 671 (1973).

3. F. W. Reuter, III, Anal. Chem. kj_, I763 (1975).

h. F. S. Goulding and J. M. Jaklevic, EPA Report No. EPA-R2-73-182

,

April 1973.

5. E. C. Kuehner and W. A. Cassatt, A Laser Microphotometer for Determining Uniformity of Particulate Deposits on Memtrane Filters, in

pre s s . . ^ ^

Identification of any commercial product does not imply recommendation or endorsement by the National Bureau of Standards, not does it imply that

the material or equipment identified is necessarily the best available

for the purpose.

8
6

V

lVAd31NI 3ZIS NIHIIM S3n0llMVd dO lN30d3d

Figiire 1. Particle size distribution of orchard leaves

9
10
TECHNICAL REPORT DATA

(Fkcsc read /iiilriK iujus on ih.c rcvcrxc hcjore complclint:)

1. REFCRT \0, 2. EPA-600/2-76-126

3. RECIPIENT'S ACCESS 1 OI^ N O.

TITl^ A'*~''~^5LJ5TiTL£ DEVEILOPMENT OF THU^" CALIBRATION STANDARDS FOR

X-RAY FLUORESCENCE ANALYSIS

5. REPORT DATE

May 1976

6. PERFORMING ORGANIZATION CODE

P. A. Pella, E. C. Kuehner, and W. A. Cassatt

8. PERFORMING ORGANIZATION REPORT NO.

9. PERFORr.iir.G ORG %\lZATIOrvJ NAME AND ADDRESS

Analytical CheniLStiry Division

National Bureau of Standards

Washington, DC 20234

10. PROGRAM ELEMENT NO. lAlUlU

11. CONTRACT/GRANT NO. EPA-IAG-100 (D)

DPA-IAG-D4-F849

12. SP0\S0R:\G AGENCY r^AME AND ADDRESS

iinvironrnenuax ocience Kesearcn ijaDoratory

Office of Research and Development

U. S. Environmental Protection Agency

Research Triangle Park, NC 27711

>13. TYPE OF REPORT AND PERIOD COVERED

r 1 nH 1 0/ X/ 1 / 1 Ht—=—O/ X/ / D

14. SPONSORING AGEfslCY CODE

EPA-ORD

13, O w r r l_ C ' » 1 C 1 > ' r"^ 111 IS W 1 ^

16. ABSTRACT

Particulate reference sairples have been prepared for the standardization of

X-ray fluorescence spectraneters used in the analysis of air particulate matter.

Uniform layers of reground orchard leaves of Jaicwn ccirposition, i.e., National

Bureau of Standards Standard Reference Material 1571, were deposited on membrane

filters and coated with a thin polymer film for proJ:ection against abrasion and

moist^e. These samples have been prepared with mass per unit area of 0.1 to 5.0

mg/cm .

17. KEY WORDS AND DOCUMENT ANALYSIS

a. DESCRIPTORS ^ b.lDENTIFIERS/OPEN ENDED TERMS c. COSATI Field/Group

* X-ray fluorescence

* Chemical analysis

* Standards

Calibrating

20F

07D

14B

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