United States
Environmental Protection
Agency
Environmental Monitoring
Systems Laboratory
Las Vegas NV 89193-3478
Research and Development
EPA/600/S4-89/021 Sept. 1989
Project Summary
A Single-Laboratory
Evaluation of Osmium
Analytical Methods
Clifton L. Jones, Daniel A. Darby, Gayle Marrs-SmithrVernon F. Hodge, and
Wendy G. Ellis
The results of a single-laboratory
study of osmium analytical methods
are described. The methods studied
include direct-aspiration atomic ab-
sorption spectroscopy (EPA Method
7550), furnace atomic absorption
spectroscopy and inductively coup-
led plasma atomic emission spec-
troscopy using (a) direct nebulizatron
(heated and unheated), (b) contin-
uous nebulization and (c) volatil-
ization (batch and heated continu-
ous). Also presented are the results
of several methods of sample
preparation. The stability of osmium
concentrations in digests over a
three-week period are also presented.
Method performance data including
detection limits, optimum concentra-
tion ranges (linearity), spike recov-
eries, interferences, precision, accu-
racy, and recommended operating
parameters are presented and
discussed.
This Project Summary was devel-
oped by EPA's Environmental Moni-
toring Systems Laboratory, Las Vegas,
NV, to announce key findings of the
research project that is fully docu-
mented in a separate report of the
same title (see Project Report
ordering information at back).
Introduction
Osmium is a metal, the tetroxide of
which is considered toxic to man and
other biota and is included on the
hazardous substances list for the
Resource Conservation and Recovery
Act. A single-laboratory study was
undertaken to determine the performance
of the atomic absorption, direct-aspiration
method for osmium (Method 7550. SW-
846) and, secondarily, to investigate the
use of graphite furnace atomic absorption
spectroscopy (GFAAS) and inductively
coupled plasma atomic emission spec-
troscopy (ICP-AES) for the determination
of osmium. Included in the investigation
was an evaluation of sample preparation
and introduction procedures and a study
of the stability of osmium concentrations
in the sample digests as these factors
might affect osmium method perform-
ance. The major components of the study
are shown in Table 1
Sample-introduction methods compar-
ed include direct nebulization, heated
direct nebulization, continuous nebuliza-
tion, heated continuous volatilization and
batch volatilization Method 7550 contains
an aqueous sample-preparation method
and references Method 3050 for prepa-
ration of solid samples. The other two
sample preparation methods investigated
were a sodium peroxide fusion method
and a pressure-bomb digestion method.
Details for all these methods are included
in the Project Report.
Overall method performance param-
eters reported include:
• Detection limits
• Optimum Concentration Ranges
(Linearity)
• Spike Recoveries
• Interferences
• Precision
• Accuracy
• Ruggedness
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Table 1. Study Components3
Sample-Preparation Sample-Introduction
Instrument Method Method
Flame AAS
Flame AAS
Flame AAS
Flame AAS
Furnace AASb
7550 (SW-846)
3050 (SW-846)
PB
Fusion
b
ON
ON
DN
ON
ICP-AES
ICP-AES
ICP-AES
ICP-AES
ICP-AES
PB
PB
PB
PB
PB
DN
HDN
CN
HCV
BV
aPB is pressure-bomb digestion, DN is direct nebulization, HDN is
heated direct nebulization, CN is continuous nebulization, HCV is
heated continuous volatilization and BV is batch volatilization.
bTestmg discontinued when use of radioactive 185osmium revealed
incomplete recovery.
Procedure
Performance characteristics of the
methods were determined using 9 types
of samples (4 liquid and 5 solid) that
included standard reference materials
from U.S. and Canadian agencies and a
hazardous waste site soil sample; all
were well-characterized matrices.
Instrument detection limits and optimum
concentration ranges were determined
with interference-free aqueous standards.
The stability of osmium concentrations in
digests of the spiked samples was moni-
tored twice a week for three weeks.
Seven elements, Al, Ca, Cr, Fe, Mg, Na
and V, were tested individually with
Method 7550 to determine if they inter-
fered (caused significant suppression or
enhancement) with the osmium signal.
Precision was determined from 5 con-
secutive 4-second readings at each
concentration.
The furnace AAS method was studied
with radioactive 18SQs to determine the
fate of osmium during the analysis.
These radiochemical studies were carried
out at the Scripps Institute of Ocean-
ography; gamma emissions at 646 KeV
and 875 KeV were used to monitor
yields.
Five different sample-introduction
methods (as listed above) were investi-
gated with ICP-AES to determine the
benefits for osmium determinations. The
first of these methods was conventional
direct nebulization. The other four
methods required some modification of
the sample-introduction hardware. The
plasma torch, spectrometer and com-
puter hardware were not altered for any
of the methods.
Results and Discussion
Method 7550
The average instrumental detection
limit obtained for osmium by AAS
Method 7550 was 0.3 mg/L. The average
method detection limit across all nine
sample matrices was also 0.3 mg/L. The
optimum concentration range in an
interference-free matrix extended from
0.9 mg/L to at least 100 mg/L. Precision
(%RSD) of the method ranged from 0.7
to 2.6 across the sample types with an
average of 1.5 percent. Method accuracy,
determined by assessing predigestion
spike recoveries for the nine matrices,
ranged from 84 percent to 98 percent
with an average of 91 percent recovery.
The seven elements listed above were
examined as possible interferents in the
determination of osmium by AAS Method
7550; of the seven elements tested only
three had any effects with calcium and
vanadium depressing the osmium signal
and chromium enhancing the osmium
signal. In general, the method was found
to be accurate, precise and relatively
insensitive to the presence of other
elements in the sample matrices.
Furnace AAS Method
Method performance parameters could
not be obtained using furnace AAS
because the osmium absorption sigi
were not reproducible. The results f
the use of the radioactive tra
suggested that the osmium salt
converted to the metal or a refracl
carbide in the graphite furnace during
charring and atomization cycles.
maximum furnace temperature (3000
is not high enough to volatilize ;
recover all of the osmium within a use
time frame.
ICP-AES Methods
The instrumental detection limits
tained for ICP-AES were 0.3 gg/L \
both the direct-nebulization and hea
direct-nebulization sample introducl
techniques, 1.4 ng/L with heated conti
ous volatilization, and (for 1-mL inject
0.03 pg/L for batch volatilization.
optimum concentration range with dii
nebulization extended from 0.001 mg/l
at least 20 mg/L at the measurem
wavelengths of 225.5 nm and 228.2 i
With heated direct nebulization <
heated continuous volatilization the rai
extended from 0.001 and 0.004 m<
respectively, to an upper limit of m
than 10 mg/L for both. Batch volatilizal
yielded (based on a 1-mL injection)
optimum range from 0.09 yg/L to
ng/L. Precision (as %RSD) for ICP-A
methods was less than 0.5 with dir
nebulization, heated direct nebulizat
and heated continuous volatilization <
was 6 percent with batch volatilization.
Method accuracy for ICP-AES dire
nebulization and batch-volatilizat
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methods were determined by assessing
pre-digestion spike recoveries for two
different sample matrices. Note that this
estimate of the method accuracy
(recovery) includes any loss of osmium
during the sample preparation using the
pressure-bomb digestion method. With
direct nebulization the recovery data
were biased high by approximately 100
percent. One possible explanation is
spectral interference from an iron
emission line on the 225.5-nm
wavelength used to measure osmium.
Both iron and chromium were found to
contribute significant spectral interference
at the 225.5-nm wavelength. When the
228.2-nm wavelength was used, more
realistic values of 80 percent recovery
were obtained. With batch volatilization
the recovery obtained was 77 percent.
Cerium, used as an oxidant in sample
preparation, contributes to the osmium
signal at 228.2 nm. However, this latter
emission line is free of other major
interferences, and it is only 30 percent
less sensitive than the 225.2-nm
wavelength.
Sample Preparation
Osmium spike recovery obtained using
the sample-preparation procedure in
Method 7550 was 71 percent on average.
Digestion Method 3050 for soil sample
preparation was also unsatisfactory; less
than 50 percent of the radiotracer was
recovered. The sodium peroxide fusion
method yielded spike recoveries typically
around 80 percent and was considered
adequate for solid samples if the spikes
were made with osmium metal. When
spikes of dissolved osmium (as the
tetrachloride) were allowed to air dry prior
to fusion, recoveries fell to 20 percent.
The pressure-bomb digestion method
was found to give recoveries ranging
from 84 to 98 percent (average 91
percent) across the sample matrices.
Stability of Osmium
Concentrations
Digests, of the nine matrices spiked
with 10 mg/L osmium (as the tetra-
chloride) were found to be stable in
osmium concentration for at least a 3
week period.
Conclusions and
Recommendations
Results of this single-laboratory study
show that flame atomic absorption
spectroscopy (AAS) Method 7550
(excluding the sample preparation pro-
cedure) is precise and accurate for
determining osmium in extracts and
digests of a variety of liquid and solid
sample types. AAS Method 7550 should
be revised to show that the analytical
wavelength is 290.9 nm and not 290.0
nm. Conventional inductively coupled
plasma atomic emission spectroscopy
(ICP-AES) with direct nebulization offers
an instrumental detection limit that is
1000-fold lower than the 0.3-mg/L value
obtained for AAS Method 7550. The
minimum instrumental detection limit
achieved (0.03 ng/L for 1 ml) was
obtained by batch volatilization of
osmium (as the tetroxide) into the ICP-
AES instrument. Method 7550 is recom-
mended for osmium concentrations
above 1 mg/L, conventional ICP-AES for
osmium concentrations above 1 iig/L, and
batch-volatilization ICP-AES for osmium
concentrations below 1 ng/L.
Recovery studies, including the use of
radioactive 185 osmium, revealed that
furnace atomic absorption spectroscopy
and several digestion procedures cannot
be recommended 'or osmium deter-
minations, the digestion procedures in
Method 7550 and in Method 3050 should
be avoided for osmium. The pressure-
bomb digestion procedure, with osmium
recoveries in the range of 84 to 98
percent, is recommended and is
described in detail in the Project Report.
The Appendix to the Proiect Report
contains the detailed changes recom-
mended for AAS Method 7550.
The 228.2-nm wavelength is recom-
mended for direct nebulization ICP-AES
determinations to avoid the chromium.
and iron spectral interferences observed
at 225.5 nm. The batch-volatilization
technique is recommended as a means
to avoid even minor spectral interferents
because these non-volatilize components
do not reach the plasma with this
technique.
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Clifton L Jones, Daniel A. Darby, Gayle Marrs-Smith, Vernon F. Hodge, and Wendy
G. Ellis are with the University of Nevada, Las Vegas, NV 89119-7159.
Thomas A. Hinners is the EPA Project Officer (see below).
The complete report, entitled "A Single-Laboratory Evaluation of Osmium Analytical
Methods," (Order No. PB 89-224 893/AS; Cosf: $75.95, subject to change) will be
available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Monitoring Systems Laboratory
U.S. Environmental Protection Agency
Las Vegas, NV 89193-3478
United States
Environmental Protection
Agency
Center for Environmental JJgseach
Information
Cincinnati OH 45268 —
0 2 5 .:
Official Business
Penalty for Private Use $300
EPA/600/S4-89/021
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