United State*
Environmental Protection
Agency
Environmental Sciences Research
Laboratory
Research Triangle Park NC 27711
EPA 600 2-80-025
January 1980
Research and Development
Preparation of
Standards for
Valence State
Measurements by
X-Ray Fluorescence
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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 nine series. These nine broad cate-
gories were established to facilitate further development and application of en-
vironmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The nine series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
6. Scientific and Technical Assessment Reports (STAR)
7. Interagency Energy-Environment Research and Development
8. "Special" Reports
9. Miscellaneous Reports
This report has been assigned to the ENVIRONMENTAL PROTECTION TECH-
NOLOGY series. This series describes research performed to develop and dem-
onstrate instrumentation, equipment, and methodology to repair or prevent en-
vironmental 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-600/2-80-025
January 1980
PREPARATION OF STANDARDS FOR VALENCE
STATE MEASUREMENT BY X-RAY FLUORESCENCE
by
Edward T. Peters and Kenneth T. Menzies
Arthur D. Little, Inc.
Cambridge, MA 02140
Contract No. 68-02-2750
Project Officer
Roy L. Bennett
Emission Measurement and Characterization Division
Environmental Sciences Research Laboratory
Research Triangle Park, NC 27711
ENVIRONMENTAL SCIENCES RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
RESEARCH TRIANGLE PARK, NC 27711
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DISCLAIMER
This report has been reviewed by the Environmental Sciences 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 does mention of trade
names or commercial products constitute endorsement or recommendation for use.
ii
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ABSTRACT
The preparation and characterization of standard samples representing
several valence states for sulfur, vanadium and chromium is described. These
standards will be used by the U.S. Environmental Protection Agency, which is
investigating the potential for determining valence state by high resolution
wavelength dispersive x-ray emission analysis. A total of forty (40) single
state and thirteen (13) multistate standards were prepared by dust generation
and collection on polycarbonate filters. The prepared samples and valence
states include sulfur (0, +4, +6, -2), vanadium (0, +4, +5) and chromium (+3,
+6). At least three standards were prepared for each valence state, with mass
concentration of the valence state element in the range 1 to 50 yg/cm2. The
prepared samples were coated with a thin layer of nitrocellulose by a wieking
procedure to provide a protective coating and to prevent loss of material.
Representative samples were analyzed for the uniformity of the deposit, mean
particle size and stability in air and x-ray irradiation. Duplicate samples
of each standard have been delivered to the Environmental Protection Agency.
This report was submitted in fulfillment of Contract No. 68-02-2750 by
Arthur D. Little, Inc. under the sponsorship of the U.S. Environmental Protec-
tion Agency. This report covers the period September 22, 1977 to September 21,
1978, and work was completed as of September 21, 1978.
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CONTENTS
Abstract iii
Figures. vi
Tables vii
Acknowledgement viii
1. Introduction 1
2. Conclusions 3
3. Recommendations 4
4. Compound Selection 5
5. Experimental Procedures and Results 7
Appendix
A. Sample Generation Apparatus (FRED) 32
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FIGURES
Number Page
1 Scanning electron micrographs of sulfur valence state
standards:
a) S° - Sulfur, 9-8, 9.5 pg/cm2, 3000X 22
b) S-2 - PbS, 37-5, 1.0 yg/cm2, 3000X 22
c) S+4 - La2(S03)3, 57-6A, 0.5 yg/cm2, 3000X 23
d) S+6 - BaS04, 30-2, 2.4 yg/cm2, Left - 3000X,
Right - 15,OOOX 23
2 Scanning electron micrographs of vanadium valence state
standards:
a) V° - Vanadium 25-1, 37.8 yg/cm2, 3000X 24
b) V+4 - V204, 45-2, 9.4 yg/cm2. 3000X 24
c) V+5 - V205, 13-5A, 0.9 yg/cm2, 3000X 25
3 Scanning electron micrographs of chromium valence state
standards:
) Cr+3 _ 0203, 33-1, 13.7 yg/cm2. 3000X 26
) Cr+6 _ PbCr04, 16-5A, 4.0 yg/cm2, 3000X 26
A-l FRED dust generation system 33
A-2 Cyclones & filter holders 34
vi
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TABLES
Number Page
1 Candidate Compounds for Valence-State Standards 6
2 Identification of Valence State Compounds 8
3 Characterization of As-Received Compounds 9
4 X-Ray Stability of Filter Substrates 11
5 Mass Loadings of Single Phase Valence State Standards 16
6 Mass Loadings of Multiphase Valency State Standards 18
7 Comparison of Generated Filter Standards to Starting Compounds . 20
8 Microscopic Evaluation of Filter Standards 21
9 Samples Used for X-Ray Stability Experiments 28
10 Sample Inventory 30
vii
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ACKNOWLEDGEMENT
The authors wish to express their gratitude to James Valentine for selec-
tion of candidate compounds and analytical methods, Stephen Spellenberg, Itamar
Bodek, Bruce Goodwin and Lawrence Damokosh for chemical analyses, Raymond
Cornish for scanning electron microscopy and William Wilson for x-ray analyses
and optical microscopy. Appreciation is expressed to Roy L. Bennett, EPA
Project Officer for his advice and support throughout the program.
viii
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SECTION 1
INTRODUCTION
The relative toxicity of various compounds depends largely on the valence
state of the elements. Most analytical methods used to characterize source
emission samples are quantitative for specific elements, but provide no infor-
mation about valence state or the form of chemical combination. X-ray diffrac-
tion can identify specific compounds present in a sample, thus identifying
valence state. However, the technique is limited to major, crystalline phases
present in amounts >50 to 100 yg per square centimeter of filter and thus is
generally not suitable for source emission samples.
The energy of outer-shell (valence) electrons of an element is altered
when the element combines with others to form a compound. Soft x-ray spectra
associated with valence electron transitions are likewise altered as a result
of chemical combination, losing the discrete energy characteristics of an
atomic orbital and assuming an energy band, the nature of which is dependent
upon the configuration of the molecular orbital. The "fine structure" of the
spectral band provides specific information about valence state of the elements.
The measurement of x-ray emission lines, as applied to chemical bonding, is
applicable only to transitions in the outer orbitals and hence, involves long
wavelengths. Special x-ray analysis apparatus is needed to measure these
wavelengths.
The Environmental Protection Agency requires the preparation of valence
state standards to evaluate and calibrate methods which employ high resolution
wavelength dispersive x-ray fluorescence spectrometry to determine various
chemical oxidation states of elements of environmental concern in pollution
samples. The present program was undertaken to prepare standard samples of
sulfur, vanadium and chromium in various valence states. The scope of work
for this study is as follows:
The contractor shall prepare standards meeting the following criteria:
1. Single and multistate standards for three elements shall be prepared:
sulfur, vanadium and chromium.
a. The sulfur standards shall include the following species:
elemental, sulfite, sulfate, and sulfide.
b. The vanadium standards shall include oxidation states represented
by: V, V02, and V205.
c. The chromium standards shall include oxidation states represented
by: Cr203 and Cr03>
1
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d. Compounds other than oxides may be used.
2. Single (containing only one of the valence species) standards shall
be prepared at three concentrations ranging from 1 micrograms/cm2
to 50 micrograms/cm2 for each oxidation state.
3. Multi-state standards shall be prepared consisting of combinations of
oxidation states of a given element on the same specimen. Chemical
reactivity may make this impossible with some combinations.
4. Total deposition (loading) on each standard shall be limited to 500
micrograms or less per square centimeter.
5. Samples shall be uniformly dispersed over a circular area of at least
30 mm diameter at the center of a 47 mm substrate.
6. Due to high instability of some species, a method for stabilizing the
reactive oxidation states shall be developed.
7. Stability of the samples shall be demonstrated. This includes expo-
sure to x-ray tube primary radiation.
8. Duplicate sets of all standards shall be delivered to EPA, Research
Triangle Park, NC.
The end product of this program is the delivery of replicate sets of single
and multistate standards of sulfur, vanadium and chromium compounds to the
Environmental Protection Agency.
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SECTION 2
CONCLUSIONS
The present program was conducted to generate standard samples of various
valence states for sulfur, vanadium and chromium for use by the Environmental
Protection Agency in determining the valence state of these elements when found
in source emission samples. A total of forty (40) single state and thirteen
(13) multistate standards have been prepared, consistent with the program re-
quirements. Based upon the evaluation of proposed standard samples, the fol-
lowing conclusions were reached:
• It is possible to prepare valence state standards of sulfur, vanadium
and chromium compounds (representing those that may be encountered in
source emission samples) by collection on filter substrates in a dust
generation apparatus using selected compounds as the source material.
2
t Elemental valence state mass concentrations of ^1 to 50 yg/cm can
be prepared.
• Long-term stability of prepared standards to atmospheric exposure
and x-ray irradiation has been demonstrated for samples protected by
coating with a layer of nitrocellulose.
t Replicate standards prepared on 47 mm diameter polycarbonate filters
have been prepared at concentrations ranging from 0.1 to >50
for the following elements and valence states:
S: 0, -2, +4, +6
V: 0, +4, +5
Cr: +3, +6
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SECTION 3
RECOMMENDATIONS
A total of fifty-three (53) standards representing various valence states
of sulfur, vanadium and chromium, have been delivered to the Environmental
Protection Agency (EPA). The following additional studies are recommended:
i "7
• Carry out measurements of the elemental mass concentration for the
various standard samples, by x-ray emission analysis, including
measurements of S, V and Cr (and other elements that are present,
such as La, Pb and Ba), using appropriate calibration samples.
Determine the agreement between prepared and measured concentrations.
It is expected that these analyses will be made by the EPA.
• Using a high resolution x-ray spectrometer, measure the x-ray spectra
(sensitive to valence state) for the standards and determine the
suitability of these standards for estimating the valence (oxidation)
states of sulfur, vanadium and chromium present in source emission
samples.
t Based upon the above, prepare additional standards for other combina-
tions of valence state and mass concentration.
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SECTION 4
COMPOUND SELECTION
The program scope of work specified the preparation of standards repre-
senting several oxidation states for sulfur, vanadium and chromium, including
some multistate standards. It is expected that these standards will be useful
for determining the suitability of research-type spectrometers to detect a
difference in spectra for the various states at the concentration levels of
interest. Once such an instrument is identified or developed, additional
valence state standards and various combinations of multistate standards can
be prepared.
It was agreed to prepare standards for the four oxidation states of sulfur
and for fewer oxidation states for vanadium and chromium. After consultation
with the Project Officer, it was agreed to prepare three oxygen states for
vanadium, including the pure element, and two for chromium. In the case of
chromium compounds, the +6 state is recognized as being especially toxic.
CrOs is not stable at elevated temperatures, transforming to Cr20s. However,
the +6 state should be retained by formation of a salt, such as PbCr04_ A
list of the oxidation states for which standards were to be prepared, together
with a selection of candidate compounds, is given in Table 1.
The specific compounds to be used in the standards for all three elements
were selected, as far as possible, with emphasis on the likelihood of occur-
rence in stack emission samples and on their relative stability. All compounds
except for the mercury sulfite complexes and the barium and lanthanum sulfite
salts, are available in powder form from commercial sources.
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TABLE 1. CANDIDATE COMPOUNDS FOR VALENCE-STATE STANDARDS
Element
Valence
(Oxidation)
State
Compounds/Comments
Vanadium
Chromi urn
Sulfur
0
+4
+5
+3
+6
0
_o
+4
V metal, powder (or as wire for vapor
deposition)
V204, powder
VoOg, most stable oxide
Cr20o, powder, most stable oxide
PbCKL, powder
Sublimed sulfur ("flowers")
Most likely as sulfide of cadmium (CdS),
mercury (HgS) or lead (PbS); there may be
some stability problem due to air oxidation,
and all three compounds may have to be
tested.
This oxidation state is the most difficult
to stabilize; complex salts such as
Hg(S03)~£ are known to be very stable to
oxidation in solution, and are potentially
useful if they can be prepared as solid
salts; sulfite salts of barium (BaSOs) or
lanthanum (La2(SOs)3) may be stable although
recent studies at Arthur D. Little, Inc.
show that the corresponding calcium salts
are oxidized rather easily in air.
+6
BaSO», powder
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SECTION 5
EXPERIMENTAL PROCEDURES AND RESULTS
STARTING MATERIALS
Valence State Compounds
Approximately lOOg (or greater) samples were ordered for each of the
candidate valence state compounds, except for S+4. The powders utilized for
the preparation of standards are identified in Table 2. Each of the compounds
was initially evaluated by optical microscopy to estimate the particle size
distribution and by x-ray methods to determine the presence of secondary ele-
ments and phases. The results of these evaluations are summarized in Table 3.
Second procurements of PbS and BaS04 from other sources contained similar
impurities as the first procurement; these compounds were accepted for use
despite the minor phase and element interference. A second phase was observed
in the Vg04 sample. Although it could not be identified, the absence of trace
elements suggests that it is a higher oxidation state of vanadium (not ^^05),
estimated to be about 5 percent. Based on phase stability after several weeks
exposure to laboratory air, this compound was accepted for standard preparation.
In many cases, the compounds contained fairly large particles or agglom-
merates. Although the latter would tend to break down by air jet milling
during sample generation, it was decided to use an impactor stage having
approximately a 4 pm - 50% cutoff at the entrance to the sample generation
chamber, (Appendix A), to minimize the presence of large particles on the
filter standards.
With respect to the S standard, initial attempts to prepare a mercuri-
sulfite complex standard by reacting HgCl2 and NaCl with Na2$03 were unsuccess-
ful due to oxidation of the product to sulfate. Therefore, attempts were
initiated to prepare lanthanum sulfite by reaction of LaCls-S^O and NaSOq.
The La2(S03)s) precipitate was carefully vacuum filtered and washed and then
placed in a dessicator under argon to minimize oxidation. An aliquot of the
wet solid was analyzed for total oxidizable sulfur by 12 titration, and another
aliquot of dry solid was similarly analyzed. No differences were observed.
An analysis of the La/SOs ratio indicated no difference between the wet and dry
solids. Therefore, it appeared that there is no oxidation of the sulfite
during drying. A second 50g sample was prepared, and x-ray diffraction analy-
sis indicated that the material was fully reacted (there was no LaCl3). On the
basis of a consistent x-ray diffraction pattern after a two-week exposure to
laboratory air and a lack of detectable sulfate by chemical analysis, the pre-
pared La2(SOs)3 powder was determined to be stable and was accepted as the
standard.
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TABLE 2. IDENTIFICATION OF VALENCE STATE COMPOUNDS
Element/
Valence State
S°
S"2
S+4
s+6
v°
v+4
v+5
Cr+3
Cr+6
Compound
S
PbS
La2(S03)3
BaS04
V
V2°4
V2°5
Cr203
PbCrO,
Supplier
Fisher
Fisher
ADL
Fisher
Alfa
Alfa
Fisher
Alfa
Fisher
Lot
Grade Particle Size Number
Laboratory
Science
Experimental
Reagent
_
-
Certified
_
Certified
Powder
Powder
Powder
Powder
-325
Powder
Powder
-325
Powder
704110
L173
-
012177
040777
730793
042077
5363
8
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TABLE 3. CHARACTERIZATION OF AS-RECEIVED COMPOUNDS
Compound
S
PbS
La2(S03)3
BaSO.
V2°4
V2°5
Cr203
PbCrO,
Secondary Phases
(X-Ray Diffraction)
2% PbSO,
Minor Elements
Particle Size*
5% second phase
(X-Ray Fluorescence) (Optical Microscopy)
I (trace)
Sr
8-10 pm
1, 10 (B)
2-5 (A)
4, 20 (B)
1-3
1-3
2-5 (A)
3-5 (A)
*A - Agglomerates up to 50 ym also present
B - Biomodal size distribution
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Filter Media
Candidate filter media were evaluated for stability to x-ray radiation.
For this purpose, membrane filters of polycarbonate, mixed esters of cellulose
and a copolymer of acrylonitrite and vinylchloride were exposed to radiation
from a chromium target x-ray tube operated at 45 KV and 20 ma for a period of
ten hours. The area of x-ray illumination was well defined by the development
of a light brown color in the filters. Flexure tests after exposure showed
severe embrittlement to the mixed esters and copolymer filters and slight
embrittlement to the polycarbonate filter. Subsequently, a set of Teflon-on-
Teflon filters were obtained and similarly evaluated. These filters were the
least stable to x-ray irradiation, splitting after 3 hours exposure time. A
summary of the filter evaluation studies is given in Table 4.
Although quite fragile and subject to electrostatic charging effects, the
polycarbonate filters appear to be the best substrate material for the valence
state standards and were used throughout the program.
SAMPLE PREPARATION
All standards were prepared in a dust generation system (FRED) described
in Appendix A. After preliminary runs, FRED was fitted with two stages of
a cascade impactor to provide a particle size distribution with an upper size
cutoff of approximately 4 urn (50 percent cutoff, aerodynamic diameter). The
12 available sample ports were modified to hold six 47 mm diameter filter
cassettes alternately with six 25 mm diameter filter cassettes. The smaller
filters were generally used for chemical analysis (usually four filters) and
for x-ray analysis and microscopic characterization. The 47 mm filters were
preserved as standards, although they were occasionally analyzed to check
agreement with the 25 mm diameter filters.
At the beginning of the program a series of six 47 mm and six 25 mm filters
from the same run (with elemental sulfur) were analyzed to determine the equiv-
alence of collected masses on the two size filters. The results were as
follows:
Filter Mass S° ( g) Filter Mass S° ( g)
Large 1 313 Small 1 346
(47 mm) 2 289 (25 mm) 2 250
3 271 3 338
4 280 4 258
5 274 5 275
6 283 6 (Sample Lost)
Mean 285 Mean 293
CV 0.053 CV 0.155
10
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TABLE 4. X-RAY STABILITY OF FILTER SUBSTRATES*
Filter
Material
Polycarbonate
Mixed Esters
Cellulose
Acrylonitrile/
Supplier
Nuclepore
Mi 11 i pore
Gel man
Designation
-
AA
DM 450
Pore Size (ym)
0.4
0.8
0.45
Copolymer
Teflon-on-Teflon Ghia P147-PL02
*A11 filters were 47mm in diameter
11
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The results are statistically equivalent, although a somewhat poorer
coefficient of variation is noted for the 25 mm diameter set. This may be the
result of anisokinetic sampling due to the relatively small diameter of the
filter holder opening.
For each standard, the first run was targeted for the heavy concentration
level (50 yg of the valence state element per square centimeter) with mass
loadings determined by gravimetric analysis. Adjustments in the Wright dust
feed gear settings and collection time was then used for setting conditions
for the other desired concentration levels (1 and 10 pg element/cm2).
SAMPLE ANALYSIS
Valence/element and total mass concentration was determined for each
sample set by the analysis of at least four filters (two filters were used
for some of the multistate standards). The mean mass (ug) of the measured
element was used to calculate the mass concentrations, with the effective
area of the 47 mm diameter filter taken as 13.85 cm2.
Analysis Procedure
S° - Sulfur--
Elemental sulfur was analyzed by the thiocyanate/acetone colorimetric
technique*. For this purpose the filters are leached with a solution of ace-
tone and water (95% acetone by volume), then reacted with cyanide to give
thiocyanate. This is measured colorimetrically after addition of an acetone
solution of ferric chloride. The procedure is as follows:
The filters are placed in screw cap jars, an appropriate amount of 95%
acetone (usually 25 ml) is added, and the jar sealed using a sheet of teflon
to prevent contact with the cap. The filters are leached for at least one
hour, then an aliquot containing between 2 and 200 micrograms of sulfur is
removed and reacted with 5 ml of 0.1% NaCN (in 95% acetone). The volume is
made up to 25 ml and is allowed to stand 5 minutes.
A 5 ml aliquot is then taken and mixed with 5 ml of 0.2% Fed3 solution.
The absorbance of the resulting ferrithiocyanate (corrected for the reagent
blank) is measured at 465 ym, using 1cm cells in a Coleman Model 55 UV-Visible
Spectrophotometer.
Filter blanks and spiked filters are run initially to ensure that inter-
ferences are not present and that recovery of S° from the filter is 100%. A
calibration curve is generated during each set of analyses.
*Skoog and Bartlett, Analytical Chemistry 26, 1954, 1008-1011
12
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s-2 _
Lead content was measured by atomic absorption spectrophotometry. The
lead sulfide samples were digested in hot concentrated hydrochloric acid.
They were then diluted as required for analysis, and aspirated into a Perkin-
Elmer Model 503 spectrophotometer. Appropriate standards were prepared and
analyzed for a calibration curve. Recovery of filter spikes was checked and
found to be 97.6%.
S+4 - La2(S03)3~
Lanthanum content was measured by plasma emission spectrophotometry (Spec-
trophotometrics DC-Arc Plasma Spectrophotometer). The lanthanum sulfite samples
were digested in hot 10% HC1 for 2 hours and diluted as required for analysis.
Appropriate standards were prepared and analyzed for a calibration curve.
Recovery was found to be 100%.
S+6 - BaS04«
Barium content was analyzed by plasma emission spectrophotometry. The
barium sulfate samples were desorbed in 20 ml of a mixture of 10 parts concen-
trated ammonium hydroxide and 90 parts 0.1 N EDTA. The samples were warmed
gently to dissolve the barium sulfate and then diluted as required for analy-
sis. The samples were aspirated into a Spectrophotometrics DC-arc plasma
spectrophotometer. Appropriate standards were prepared and analyzed for a
calibration curve. Recovery was checked and found to be 97.9%.
V° - Vanadium-
Vanadium content was analyzed by atomic absorption spectrophotometry.
Some difficulty was encountered in desorbing the vanadium metal from the
Nuclepore filter. Recoveries of only 44% were observed with 0.1 N nitric
acid. Therefore, hot concentrated nitric acid was used to desorb the material.
Great care had to be taken to prevent the filters from curling during the
digestion process. The samples were diluted as necessary and aspirated into
a Perkin-Elmer Model 503 Spectrophotometer. Recovery was found to be approxi-
mately 100%.
v+4 - V2o4-
Vanadium content was determined by atomic absorption spectrophotometry.
The vanadium oxide samples were digested in hot concentrated nitric acid,
diluted as required for analysis and aspirated into a Perkin-Elmer Model 503
Spectrophotometer. Appropriate standards were prepared and analyzed for a
calibration curve. Recovery was checked.
V - v 0 —
v V2u5
Atomic absorption analysis of vanadium was carried out by dissolving the
from the filter in 0.1 N HC1 at about 80°C. Samples were diluted as
appropriate and aspirated or injected into a nitrous oxide/acetylene flame
13
-------
or graphite tube, respectively, depending on the concentration. Appropriate
standards and blanks were run along with the unknowns.
Cr+3 - Cr203-
The chromium content was analyzed by atomic absorption spectrophotometry.
The chromium oxide samples were digested in a mixture of hot concentrated
nitric acid and concentrated hydrochloric acid. They were then diluted as
required for analysis and aspirated into a Perkin-Elmer Model 503 spectro-
photometer. Appropriate standards were prepared and analyzed for a calibration
curve. Recovery was checked and found to be 95.4 percent.
Cr+6 - PbCr04—
Chromium content was analyzed by atomic absorption spectrophotometry. The
samples were desorbed in 0.1 N HC1 at about 80°C and after appropriate dilution,
were aspirated into a nitrous oxide/acetylene flame for analysis. Appropriate
standards and blanks were run along with the unknowns.
S"2, S+6 - PbS, BaS04-
Duplicate filters were analyzed for lead content by digestion in concen-
trated nitric acid and measurement by DC arc plasma emission spectroscopy.
A second set of duplicate samples were analyzed for barium content by the
procedure given above. Sulfur content for the two valence states were calcu-
lated based on the measured metal content and stoichiometry.
° +5
V°, V - Vanadium, V205«
Duplicate filters were analyzed for V?0,- content by polarography. A 5 ml
aliquot of W NH.C1 adjusted to pH 9.9 witn cone. NH4OH was added to ex-
tract the V20s- The material dissolved readily (within one minute) and in no
case was any suspended material noticed. An aliquot of the extract was added
to 5 ml of the buffer (pH 9.9) and the differential pulse polarogram was taken
The height of the peak at -1.27V was taken as a measure of the V205 concentra-
tion. A standard curve was prepared covering the range of 1-12 yg of V (added
as V205) in 5 ml buffer. The standard stock solution was stable for at least
5 hours. The sensitivity of the procedure is 0.32 yA per 1 yg V(V205) in 5 ml
solution (3.9 x 10-% Vanadium).
Some preliminary experiments were made to determine if the presence of
V, V20s and V£04 would interfere in the procedure.
For V metal: 9.3 mg V was added to 5 ml of the buffer and
extracted for 20 minutes. The resulting suspension was
allowed to settle and the supernatant analyzed. A total of
112± 10 yg V (as V20s) was found. This is equivalent to
only 1.2% of the V present as
For V204: 10 mg was added to 5 ml of buffer. Very little
of the material dissolved in 10 minutes. The resulting
solution yielded 309 yg V present as V20s (5.0% of V).
14
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For V20s: 5 mg was dissolved in 5 ml of buffer. The solution
gave a weak wave at -1.175V and showed no V205. The response
was only ^5% that of a similar concentration of
These experiments indicate that the presence of these other species on
the filter would contribute little to the V20s signal if they were present
at comparable levels.
Total vanadium was determined by digestion in concentrated nitric acid
and analysis by plasma emission spectroscopy.
Cr+3, Cr+6 - Cr203, PbCr04~
Mixed phase standards for Cr+3 (Cr203)/Cr+6 (PbCHty) were analyzed by
atomic absorption spectrophotometry using the procedures described above for
the individual valence states.
Experimental Results
The experimental results, including valence state/element total mass con-
centrations are given for the single and multistate standards in Tables 5 and 6,
respectively. The forty (40) single state standards range in concentration
from 0.094 to 75 pg/cm2 (valence state/element) and 1 to 512 pg/cm2 (total).
The thirteen (13) multistate standards range in valence state/element concen-
tration from 1 to 74 yg/cm2, with total mass concentration ranging up to
200 yg/cm2.
SAMPLE PROCESSING AND EVALUATION
Mounting and Coating
Sample holders designed to contain 47 mm diameter filters and to be used
in the EPA's x-ray apparatus were supplied by the EPA. Before mounting, the
samples were coated with a thin layer of nitrocellulose (Parlodian) to provide
protection from the atmosphere (to minimize the opportunity for oxidation) and
to prevent the loss of material. For this purpose, a 0.6 cm thick pad of poly-
urethane is placed in a 10 cm Petri dish and filled with a 0.5 percent solution
of nitrocellulose in amyl acetate to the top of the polyurethane. Pieces of
10 cm diameter filter paper (such as Whatman 41) are placed on the polyurethane
pad. Three to five seconds are required to wet the top piece with the nitro-
cellulose solution. The filter to be treated is placed particle side up on a
clean 5 cm diameter filter paper, which is placed on the stack. After coating
(requiring about 5 seconds), the 5 cm diameter filter is removed and placed
aside. After some drying (about 30 seconds), the filter is picked up w"th
tweezers, held until almost dry (20-30 seconds) and placed in the holder. The
slightly wet periphery of the filter will adhere to the mounting ring when dry.
At the slow wi eking rates employed, there was no evidence for particle
migration to the clean edge of the filter. After drying, heavily loaded
samples could be treated quite aggressively with no loss of material. To
determine the increase in mass due to treatment, three clean Nuclepore filters
were weighed before and after parlodian treatment; the change in weight,
15
-------
TABLE 5. MASS LOADINGS OF SINGLE PHASE VALENCE STATE STANDARDS
Element/
Set No. Valence State
9a S°
9b
9c
10 V+5
11
12
13
4.C
15 Cr+6
16
17
18
20 V°
21
22
24
25
26
27
28 S+6
29
30
31
32 Cr+3
33
34
35 S"2
Date
Compound Generated
S 12-20-77
V205 12-30-77
1-04-78
PbCr04 1-17-78
1-19-78
V 1-25-78
1-26-78
2-03-78
BaS04 2-06-78
2-16-78
Cr203 3-02-78
3-07-78
PbS 4-06-78
Mass Concentration*
Ji
4
4
4
4
2
4
4
4
4
4
4
2
3
2
3
3
4
5
6
4
4
3
5
3
4
4
MZ
54.0
9.5
0.69
75.8
42.2
6.56
0.92
27.6
4.01
0.46
59.6
25.6
17.5
1.13
32.3
37.5
7.87
0.62
75.2
8.66
2.38
0.28
43.7
13.7
1.41
6.21
MJ_
54.0
10
1
136
75
11
2
163
24
3
353
26
18
1
32
37
8
1
512
59
16
2
64
20
2
46
CV
14
19
46
10
16
17
16
5
4
4
1
4
9
17
2
10
13
25
1
5
3
8
1
2
3
2
(continued)
16
-------
TABLE 5 (CONTINUED)
Element/
Set No. Valence State
36
37
38
39
40
42 V4"4
43
44
45
46
54 S+4
55
56
57
Date M<
Compound Generated M
I!
4
4
4
4-10-78 4
4
V~0, 4-28-78 4
c 4
5-03-78 4
4
5-08-78 4
4
La2(S03)3 4
4
4
4
iss Concentration*
MZ
5.30
1.01
0.094
14.8
65.7
3.65
36.2
0.10
9.39
1.75
16.6
13.2
3.1
0.49
MT
38
8
1
110
490
6
59
<1
15
3
72
57
13
2
CV
2
3
4
2
2
10
4
60
10
2
1
1
6
1
N = Number of samples analyzed
p
MZ = Mass concentration (mean) of valence state element (yg/cm )
2
MT = Total mass concentration (mean)(pg/cm )
CV = Coefficient of variation (percent)
17
-------
TABLE 6. MASS LOADINGS OF MULTIPHASE VALENCY STATE STANDARDS
CO
Set No.
47
48
49
50
51
52
53
58
59
60
61
62
63
4-iciiicnu/ uaie
Valence State Compounds Generated N
Cr+3, Cr+6 Cr203, PbCr04 5-19-78 4
4
4
4
4
4
4
V°, V+5 V, V205 2,2
2,2
2,2
S"2, S+6 PbS, BaS04 2,2
2,2
2,2
MZ1
73.7
47.5
7.93
1.05
12.8
1.50
0.35
,0
2.2
0.68
15.2
2.8
0.5
MZ2
15.8
9.92
2.04
0.37
18.9
3.77
0.57
39.3
13.0
1.0
8.8
1.6
0.2
CV1
2.6
4.5
8.3
16.4
20.0
13.5
7.9
1.7
1.3
12.3
0.4
0
0.5
CV2
3.9
1.1
1.5
1.6
2.4
1.7
2.8
6.8
4.3
5.0
0
4.3
0
MT
201
128
34
4
131
25
4
70
25
3
173
32
5
* N = Number of samples analyzed
MZ1, MZ2 = Mass concentration (mean) of valence state elements (yg/cm )
CV1, CV2 = Coefficient of variation for elements 1 and 2 (percent)
MT = Total mass concentration (mean)
-------
corresponding to about 300 yg, is about two percent. This is insignificant
with respect to x-ray scattering (background). Several additional filters
were similarly processed, but were fastened to mylar washers, and were subse-
quently used for characterization of the deposit and for stability measurements,
Deposit Characterization
Moderate to high concentration samples of each single phase valence state
were analyzed by x-ray diffraction and fluorescence within a period of two to
four months after preparation for comparison to the original powders. The
major purpose of this analysis was to identify the presence of phase trans-
formation. The results are presented in Table 7. The x-ray diffraction traces
were identical to those of the starting compound, with no evidence for a phase
change in any of the standards. The low levels of contaminant elements
observed in some of the samples is attributed to some carryover from prior
runs in the dust generation apparatus. The levels are low, however, and should
have no adverse influence on the standards.
Portions of more lightly loaded filters (1-10 iag/cm2 valence/element) were
prepared for examination by scanning electron microscopy by coating with a thin
layer of gold. Preliminary studies were carried out to evaluate the uniformity
of the deposit on the filter and reproducibility among filters for the same
run. Samples for the S~2 and V+4 valence states were examined to evaluate par-
ticle distribution as a function of:
• central vs. peripheral location on filters
• replicate filters
P P
• very light (M3.1 pg/cm ) and light (^1 yg/cm ) loadings
Photographs for each condition were taken at about 2000 and 5000 magnifi-
cations, and also at about 500 magnifications for lightly loaded samples. A
comparison of photographs indicated samples from various locations on the
filter and from replicate filters were equivalent.
Subsequently, light and moderately heavy filter loadings of all generated
compound standards were examined by the SEM, with photographs taken at about
2000, 10,000 and sometimes 500 magnifications. Particle size analysis was
carried out on selected SEM photographs using a Ladd digitizer. Particle dis-
tributions were observed to be uniform in all cases, with particle sizes
ranging from about 0.2 to 3 or 4 pm. The data is summarized in Table 8. It
should be noted that the mass concentration calculated from the number and
average size of particles agrees quite well with the measured results, further
verifying the uniformity of the deposit. Representative scanning electron
micrographs of the various single standards are given in Figures 1 through 3.
Sample Stability
Preliminary experiments were conducted early in the program in which
filter samples of six standards having heavy mass loadings (^50 pg/cm^) were
subjected to x-ray irradiation. The samples represented the following
19
-------
TABLE 7. COMPARISON OF GENERATED FILTER STANDARDS TO STARTING COMPOUNDS
Compound
S
PbS
La2(S03)3
BaSO*
Additional Phases*
(X-Ray Diffraction)
nc
nc
nc
nc
Additional Elements*
(X-Ray Fluorescence)
nc
Ba, S
nc
nc
V
V2°4
V2°5
Cr203
PbCr04
nc
nc
nc
nc
nc
nc
Pb, Ba, S
nc
Ba, S
nc
*nc - no change from starting compound
20
-------
TABLE 8. MICROSCOPIC EVALUATION OF FILTER STANDARDS*
Compound Sample No. Magnification n d
s°
s-2
s+4
s+6
V
v+4
y+5
Cr+3
Cr+4
S
PbS
La2(S03)3
BaS04
V
V2°4
V2°5
Cr203
PbCr04
9-8
37-5
57-6A
30-2
25-1
45-2
13-5A
33-1
16-5A
2,200
11,000
2,200
11,000
2,200
2,200
2,200
2,200
11,000
MZ
36 1.28 4.4 9.5
143 0.2a 2.2 1.0
63 1.92 3.2 0.5
104 0.23 0.6 2.4
31 1.58 20.5 37.8
161 0.79 6.0 9.4
69 0.98 1.0 0.9
34 1.98 26.5 13.7
46 0.45 1.0 4.0
N = number of particles counted
d = average Martins diameter for particles counted
2
MZ = mass concentration of valence element (yg/cm ) estimated
by electron microscopy
MZ = actual mass concentration of valence element (pg/cm )
21
-------
3389-4
(a)
3350-5
(b)
Figure 1 Scanning Electron Micrographs of Sulfur Valence
State Standards:
a) Sr - Sulfur, 9-8, 9.5yg/cm2, 3000X
b) S~2 - PbS, 37-5, l.Oyg/cm2, 3000X
22
-------
"fM"' ^ ^Ifc,
\
^
.
3413-5
(O
3391-1
(d)
Figure 1 (Continued) Scanning Electron Micrographs of Sulfur
Valence State Standards:
c) S+4 - La2(S03)3, 57-6A, 0.5yg/cm2, 3000X
d) S+6 - BaSO,, 30-2, 2.4yg/cm2, Left - 3000X,
Right - 15.000X
23
-------
•V'
• .vr-i-<*5ir-r*=
. , "• «J,,, . *
• »> •>
3395-1
(b)
Figure 2 Scanning Electron Micrographs of Vanadium Valence
State Standards:
a) V° - Vanadium, 25-1, 37.8ug/cm2, 3000X
b)
- V0, 45-2, 9.4yg/cm, 3000X
24
-------
• „ '
•- •. ,
•,; .
L.
3351-1
.^
*-":f
(c)
Figure 2 (Continued) Scanning Electron Micrographs of
Vanadium Valence State Standards;
c)
, 13-5A, 0.9yg/cm, 3000X
25
-------
I
3393-2
-r
_ t*r .. ,* »
.•
3356-3
(b)
Figure 3 Scanning Electron Micrographs of Chromium Valence
State Standards:
a) Cr+-} - Cr203, 33-1, 13.7ug/cm2, 3000X
b) Cr+b - PbCr04, 16-5A, 4.0ug/cm2, 3000X
26
-------
oxidation states: S°, S+6, V°, V+5, Cr+3, Cr+6. Initially, uncoated samples
were subjected to x-ray exposures represented by a chromium target tube oper-
ated at 45 KV and 25 ma, with the sample in vacuum. After 23 hours, the S
standard (9-8A) burned through at the center. The BaS04 and a duplicate S
sample were carbon coated and re-exposed. The BaS04-carbon coated sample sur-
vived a 100 hour exposure, but with an apparent reduction in S intensity by
about 40 percent. The replicate S standard showed a 3-fold reduction in S
intensity after carbon coating. This sample also failed after approximately
20 hours x-ray exposure.
The failure of these samples was attributed to embrittlement of the sub-
strate filter and eventual fracture due to stresses during sample loading and
unloading. The loss in intensity after exposure was felt to be the result of
sample loss during handling, as these initial evaluation samples were not
coated with nitrocellulose.
After all the single phase standards were generated, samples representing
moderate to heavy concentrations for each of the single state standards were
treated with parlodian, attached to mylar washers and trimmed to 25 mm diam-
eters. These samples were treated according to the following schedule for
10 days:
16 hours/day - ultraviolet irradiation in closed containers
8 hours/day - sunlight (window) exposure in laboratory air
0.5 hours/day - held in vacuum
0.5 hours/day - irradiated by 45 KV - 25 ma x-rays (chromium
target tube) in vacuum
These evaluation samples were examined each day for evidence of degrada-
tion. Also, x-ray intensities of the elements of interest were measured at
the start and end of the exposure schedule. After several days, the samples
assumed a saddle-shape due to annealing of the polycarbonate substrate and
insufficient resistance by the mylar washer. Therefore, final x-ray inten-
sities were more variable than would generally be expected. The standard
samples mounted in the EPA holders (described in the next section) will not
suffer this warping problem. The only noted degradation was an embrittlement
of the polycarbonate substrates, resulting in very fragile samples. In two
cases, attempts to flatten the samples prior to x-ray intensity measurements
resulted in the samples breaking. An identification of the samples and results
are given in Table 9.
No deterioration (during the 10-day test period) was noted for exposure
to laboratory air, ultraviolet radiation or vacuum exposure. The relative
changes in x-ray intensity after a cumulative 5-hour x-ray irradiation are
probably due to sample distortion. There was no evidence for sample loss or
transformation (phase change) as a result of the stability evaluation.
As the elemental sulfur standard was broken during these tests, a dupli-
cate (coated) standard was subjected to an 8-hour x-ray exposure. X-ray inten-
sity of the sulfur K« line (PET analyzing crystal) agreed to initial intensity
27
-------
TABLE 9. SAMPLES USED FOR X-RAY STABILITY EXPERIMENTS
Valence
ro
oo
Component
Sample No.
Mass Concentration
X-Ray Intensity - (cps)
s°
s-2
s+6
v°
v+4
v+5
Cr+3
Cr+6
S
PbS
La2(S03)3
BaS04
V
V2°4
V2°5
Cr203
PbCrO,
9-6
40-2A
54-1A
28-1
25-1 A
43-1A
10-1
32-4
18-5
(yg/cm )
54.0
229
57.8
75.2
132
126
75.8
43.7
59.6
Initial
4.3
8.6
22.6
15.1
1.8
6.4
4.6
3.1
13.6
Final*
•*
7.5
17.5
15.9
2.0
3.6
7.5
18.4
Change (%)
.
-13
-22
+5
+11
-44
mm
+143
+35
* Blank values represent samples that were broken during attempt to flatten
to original geometry.
-------
within 4 percent. It is concluded that there are no evaporative losses of
elemental sulfur from coated filters in the level of vacuum employed in x-ray
spectrometers.
IDENTIFICATION OF STANDARDS
The identification of standards by valence state and mass concentration
of the valence/state element is given in Table 10. Total mass concentrations
and an inventory of replicate samples available for use as standards is also
given.
29
-------
Valence State
TABLE 10. SAMPLE INVENTORY
2
Compound Set Mass Concentratlon-ug/cm Samples*
Valence Element Total
S° S 9c
9b
9a
S"2 PbS 38
37
36
35
39
40
S+4 La2(S03)3 57
56
55
54
S+6 BaS04 31
30
29
28
V° V 27
22
26
21
20
24
25
V+4 V204 44
46
42
0.69
9.5
54.0
0.09
1.01
5.30
6.21
14.8
65.7
0.49
3.1
13.2
16.6
0.28
2.38
8.66
75.2
0.62
1.13
7.87
17.5
25.6
32.3
37.8
0.10
1.75
3.65
0.69
9.5
54.0
0.67
7.5
39.5
46.3
110
490
2.6
16.7
71
89
2.0
17.3
63
547
0.62
1.1
7.9
17.5
25.6
32.3
37.8
0.16
2.9
• 5.9
3
3
0
4
2
4
4
3
4
4
4
4
4
4
4
4
4
4
4
4
4
2
3
1
2
4
4
(continued)
30
-------
TABLE 10. (continued)
45
43
V+5 V205 13
12
11
10
_L*3
Cr * Cr203 34
33
32
Cr+6 PbCr04 17
16
15
18
S"2, S*6 PbS,BaS04 63
62
61
V°, V*5 V, V205 60
59
58
Cr+3,Cr46 Cr203,PbCr04 53
52
51
50
49
48
47
1 IVL*).3 WVllWdlWl Hi* 1
Valence Element
9.39
36.2
0.92
6.56
42.2
75.8
1.41
13.7
43.7
0.46
4.01
27.6
59.6
0.2, 0.5
1.6, 2.8
8.8, 15.2
0.68, 1.0
2.2, 13.0
-vO, 39.3
0.35, 0.57
1.50, 3.77
12.8, 18.9
1.05, 0.37
7.39, 2.04
47.5, 9.92
73.7, 15.8
1 Vdl-UH/ UN
Total
15.1
59
1.6
11.7
75
135
2.1
20.0
64
2.9
24.9
172
370
4.9
30.9
168.7
2.5
25.4
70.2
3.9
24.5
130.5
3.7
22.9
128.1
201.2
ooi'ip ica
3
4
3
3
2
3
4
3
2
4
3
2
3
4
4
4
4
4
4
4
4
3
4
4
4
4
Number o* 47mm diameter filters (not treated with parlodian) in
addition to the duplicate set of standards.
31
-------
APPENDIX A
SAMPLE GENERATION APPARATUS (FRED)
"FRED" is an apparatus constructed at ADL to generate test atmospheres
containing dust for occupational hygiene studies. The device, which is shown
in Figure^.!, consists of a Wright Dust Feeder, an aerosol charge neutralizer,
and an 80-liter stainless steel dust chamber.
Dust samples to be dispersed in the apparatus are compressed into a
special holder which is attached to the Wright Dust Feeder. In operation, the
holder (and the compacted dust) is slowly rotated and driven against a sta-
tionary scraper blade which continuously removes dust from the compacted sur-
face at a uniform rate. Compressed air entering the container entrains and
removes the dust particles. Both the compressed air flow and the dust holder
advance rate may be varied to provide a wide range of dust concentration.
There is also an auxiliary port for introducing additional compressed air for
further dilution. Dust output may be estimated from knowledge of the compact
advance rate, compact density, and air flow rate.
Dust particles may accumulate electrical charge in the dispersal process;
consequently, the dispersed dust is passed through a charge neutralizer
(Thermo-Systems Model 3054) before it is introduced into the dust chamber.
The neutralizer is a 10-mi11icure krypton-85 beta source. Beta rays interact
with air molecules to generate both positive and negative ions. The ions be-
come attached to the dust particles of opposite charge.
The dust chamber was constructed from two stainless steel stock pots.
The pots are held together by three latches with a soft rubber gasket seal
between the pots. The system is operated at a positive pressure with respect
to the atmosphere. Effluent aerosol from the dust chamber is vented to a hood.
There are 12 ports on the dust chamber for extracting samples. Twelve
filter holders may be mounted inside the chamber, as is shown in Figure A-2.
Flow rate through the samplers is controlled by 12 critical orifices. The
orifices are mounted on the U-shaped vacuum line shown in Figure A-l.
32
-------
Figure A-1. FRED dust generation system.
33
-------
Figure A-2. Cyclones & filter holders.
34
-------
TECHNICAL REPORT DATA
(rlease read Instructions on the reverse before completing
EPA-600/2-80-025
TITLE AND SUBTITLE
3. RECIPIENT'S ACCESSION MO.
REPARATION OF STANDARDS FOR VALENCE STATE
MEASUREMENT BY X-RAY FLUORESCENCE
5. REPORT DATE
January 1980
6. PERFORMING ORGANIZATION CODE
MITHOR(S) ~~~ ~~~ "
Edward T. Peters and Kenneth T. Menzies
8. PERFORMING ORGANIZATION REPGHT NO.
. PERFORMING ORGANIZATION NAME AND ADDRESS
Arthur D. Little, Inc.
Acorn Park
Cambridge, Massachusetts 02140
10. PROGRAM ELEMENT NO.
1AD712B BC025 (FY-79)
11. CONTRACT/GRANT NO.
68-02-2750
12. SPONSORING AGENCY NAME AND ADDRESS
Environmental Sciences Research Laboratory - RTP, NC
Office of Research and Development
U.S. Environmental Protection Agency
Research Triangle Park. N.C. 27711
13. TYPE OF REPORT AND PERIOD COVERED
Final
14. SPONSORING AGENCY CODE
EPA/600/09
15. SUPPLEMENTARY NOTES
16. ABSTRACT
The preparation and characterization of standard samples representing several valence
states for sulfur, vanadium, and chromium are described. The standards will be used
by the U.S. Environmental Protection Agency to investigate the potential for deter-
mining valence state by high resolution wavelength dispersive x-ray emission analysis.
A total of 40 single state and 13 multistate standards were prepared by dust genera-
tion and collection on polycarbonate filters. The prepared samples and valence
states include sulfur (0, +4, +6, -2), vanadium (0, +4, +5), and chromium (+3, +6).
At least three standards were prepared for each valence state, with mass concentration
of the valence state element in the range 1 to 50 ug/crrr. The prepared samples were
coated with a thin layer of nitrocellulose by a wieking procedure to provide a pro-
tective coating and to prevent loss of material. Representative samples were
analyzed for the uniformity of the deposit, mean particle size and stability in air,
and x-ray irradiation.
17 KEY WORDS AND DOCUMENT ANALYSIS
a. DESCRIPTORS
* Standards
* Sulfur
* Vanadium
* Chromium
* Valence
X-ray fluorescence
18. DISTRIBUTION STATEMENT
RELEASE TO PUBLIC
b.lDENTIFIERS/OPEN ENDED TERMS
19. SECURITY CLASS (This Report j
UNCLASSIFIED
20. SECURITY CLASS (This page)
"UNCLASSIFIED
. COS ATI Field/Group I
07B
06E
20F
21 NO OF PAGES
43 I
22. PRICE 1
EPA Form 2220-1 (Rev. 4-77) PREVIOUS EDITION is OBSOLETE
35
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