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

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                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.
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.

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                                   EPA-600/2-76-126
                                   May 1976
  DEVELOPMENT 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 20231*
         Contract Ho.  EPA-LAG-IOO(D)
               Project Officer

                T. G.  Dzubay
 Atmospheric Chemistry and Physics Division
 Environmental Sciences Research Laboratory
      Research Triangle Park, NC 27711
    U.S. ENVIRONMENTAL PROTECTION AGENCY
     OFFICE OF RESEARCH AND DEVELOPMENT
 ENVIRONMENTAL SCIENCES RESEARCH LABORATORY
      RESEARCH TRIANGLE PARK, NC 27711

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                              DISCLAIMER
     This report has been reviewed by the Environmental Science Research
Laboratory., U.S. Environmental Protection Agency, and approved for pub-
lication.  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.
                                   11

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                                  I

                             INTRODUCTION

Within the past several years, an increasing number of x-ray analyses of
environmental 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 [1-3].
This botanical SRM is well suited to such applications because it contains
a number of trace elements such as S, Cl, K, Ca, Cr, Mn, Fe, Co, Ni, Cu,
Zn, As and Pb 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 SRM 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 such 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 milligrams
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 trans-
ferred to a 50 ml beaker containing about Uo ml of spectrograde cyclo-
hexane.  The beaker containing the orchard leaves was placed in an

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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 pm pore size).
The glass filtering funnel was constructed so .that deposits on the 37 mm
membrane filters were 29 mm in diameter.  The net weight of the air-dried
deposit was used to calculate the area! density of the deposit in mg/cm .
All weighings were performed in a room in which the relative humidity was
maintained at 5Q±k 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*0.  The procedure consists of first vaporizing the
di-para-xylylene dimer by heating it at 175°C at 760 mm Hg (l.OlxlO5 N/m2).
This vaporized material passes through a deposition chamber maintained at
room temperature, .and 0.1 mm Hg (13-3 N/m2) 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/m2).  The film thickness is monitored in the depo-
sition 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 heat-
sealed 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 [U].  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 cyclo-
hexane 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.

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                        RESULTS AND DISCUSSION

Neutron activation analysis (NAA) of the reground orchard leaves showed
that the concentrations of iron, nickel, cobalt and chromium 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 inhomo-
geneities (not greater than five percent).  These results indicated that
the reground SRM 1571 was satisfactory for use provided that recertifi-
cation 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 ym  (3.1 ym equivalent diameter).  Less than 0.5 percent of
the particles were within 82.5 and 105 pm » with no particles observed
above 105 ym .

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.  Of the several samples examined
having areal densities of 1.0 to 2.0 mg/cm , variations in uniformity did
not exceed 0.02 mg/cm2.

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
background corrected peak intensities of coated and uncoated samples having
the same areal densities using EL wavelength-dispersive x-ray spectrometer.

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The values obtained for various thicknesses of Parylene and of thin poly-
propylene and Parafilm coatings are summarized in tables 1 and 2, respec-
tively.  For Parylene coatings 75 to 193 nm thick, little change in
attenuation was noted.  Also, the x-ray background 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.
No. of
Samples

   5
   3
   2
       Average Attenuation Values of Parylene Coatings'
                                          2
          sample areal density =2.0 mg/cm

                               Percent Attenuation
                    Parylene
                  Thickness, nm
                       75
                       75
Element

   P
   S
   K
Average

  3
  2
  1.1*
                                                  1-7
                                                  1.3
5
5

5
2
         7
         5
         7

         5
         3
                117.5
                117-5

                158.0
                158.0
                158.0

                166.5
                166.5
                166.5

                193.0
                193.0
   P
   K

   P
   S
   K

   P
   S
   K

   P
   K
                                                  3.1
                                                  1.7
  5.2
  2.9

  5.0

  2!6

  7.3
  3.7
0.50
0.1*1*

0.90

0.35

0.81
0.92
0.69

0.88
0.20
         3
         3
         2
            2500
            2500
            2500
                                 P
                                 S
                                 K
             33
             28.9
             11. U
           1.5
           0.71
 Calculated from the expression I -I /I  x 100; where I  = peak
                       ^         u  c  u               u   ^
 intensity of uncoated sample, and I  = peak intensity of coated sample.
 S = standard deviation of a single measurement.

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             Table 2.  Attenuation Values of Polypropylene
                         and Parafilm Coatings
              Total Coating ?           Percent Attenuation
Sample #    Thickness, mg/cm      P      S      Cl     K      Ca

  1               1.2            28.1   23.2   Ik.I   10.9   8.5

  2               1.1            2k.k   23.6   16.2   11.4   7-9

  3               1.2            26.3   2k.k   13.6   10.1   8.3

Average                          26.3   23.T   lk.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 main-
tained 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 concen-
trations from 2 to 9 ppm.  Silicon was 31 ppm, and the remaining elements
were either below 1 ppm or were not detected.  These results 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.

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             Table 3.  Preliminary Results of the Analysis
                  of Reground SRM 1571 Orchard Leaves
          No. of
a = AAS

b = PAA


° = NAA

Values in parenthesis are not certified.
Certified Values
Element
Fea
,, .a
,T.b
ivi
Crc
Mn°
KC
BrC
Zn°
i4eas. Average, yg/g Rel. Std. Dev.
3 .
3
1
15
16
16
15
15
791
22.5
21.5
41.97
102.1
1.49 (wt. %}
10.48
24.99
0.2
0.5
	
3-70
1.84
1.53
2.58
3.22
% of SRM 1571, yg/g
300120
1.310.2

(2.3)
91+4
1.4710.03 (wt. %}
(10)
25+3
                                  6 '

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                                  II

                              CONCLUSIONS

Reference samples consisting of uniform layers of reground orchard leaves
of known composition, i.e., NBS SRM 1571? on membrane filters were pre-
pared for the standardization of x-ray fluorescence spectrometers.  These
samples have areal densities of 0.1 to 5.0 mg/cm2, and are coated with a
thin polymer film for protection against abrasion and moisture.

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                              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  (19T3).

3.  F. W. Reuter, III, Anal. Chem. Vj_, 1763  (1975)-

k.  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 Deter-
      mining Uniformity of Particulate Deposits on Membrane Filters,  in
      press.
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.

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AREA
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                                                          4
Figure 2.  Reference sample mounted in holder (actual size 5x5 cm),
                              10

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TECHNICAL REPORT DATA
(Please read /nurucHons on the reverse before completing)
1. REPORT fvO.
EPA-600/2-76-126
2.
J. TITLE A\D SUBTITLE
DEVELOPMENT OF THIN CALIBRATION STANDARDS F(
X-RAY FLUORESCENCE ANALYSIS
7. AUTHORiSI
P. A. Pella, E. C. Kuehner,
3. RECIPIENT'S ACCESSIOr+NO.
5. REPORT DATE
DP May 1976

6. PERFORMING ORGANIZATION CODE
8. PERFORMING ORGANIZATION REPORT NO.
•>
and W. A. Cassatt
9. PERFORMING ORG VNMZATION NAME AND ADDRESS
Analytical Chemistry Division
National Bureau of Standards
Washington, DC 20234
12. SPONSORING AGENCY NAME AND ADDRESS
Environmental Science Research Laboratory
Office of Research and Development
U. S. Environmental Protection Agency
Research Triangle Park, NC 27711
10. PROGRAM ELEMENT NO.
' ' 1A1010
11. CONTRACT/GRANT NO.
EPA-IAG-100 (D)
DPA-IAG-D4-F849
•13. TYPE OF REPORT AND PERIOD COVERED
Pin^l fi/l/7/74 - 6/1/75
14. SPONSORING AGENCY CODE' '
EPA-ORD
15. SUPPLEMENTARY NOTES
16. ABSTRACT •
Particulate reference samples have been prepared for the standardization of
X-ray fluorescence spectrometers used in the analysis of air particulate matter.
Uniform layers of reground orchard leaves of known 'composition, i.e., National
Bureau of Standards Standard Reference Material 1571, were deposited on membrane
filters and coated with a thin polymer film for projection against abrasion and
moisture. These samples have been prepared with mass per unit area of 0.1 to 5.0
mg/cm • V
/
* • , <»
* f
17.
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* X-ray fluorescence
* Chemical analysis
* Standards
Calibrating
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