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Single-Laboratory Evaluation of Osmium Analytical Methods
Nevada Univ., Las Vegas
Prepared for:
Environmental Monitoring Systems Lab., Las Vegas, NV
Jun 89
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PB89-224893
EPA/600/4-89/021
June 1PS9
A SINGLE-LABORATORY EVALUATION OF OSMIUM ANALYTICAL METHODS
by
Clifton L. Jones, Daniel A. Darby, Gayle Marrs-Smith,
Vernon F. Hodge, and Vendy G. Ellis
Environmental Research Center
University of Nevada, Las Vegas
Las Vegas, Nevada 89119-9970
Contract Number 68--01-7159
Technical Monitor
Thomas A. Hinners
Quality Assurance and Methods Development Division
U.S. Environmental Protection Agency
Las Vegas, Nevada 89193-3478
ENVIRONMENTAL MONITORING SYSTEMS LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
LAS VEGAS, NEVADA 89193-3478
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TECHNICAL REPORT DATA
(Please read Instructions on the reverie before completing
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NOTICE
The information in this document has been funded wholly or in part by the
United States Environmental Protection Agency under Contract Number 68-01-7159
to the University of Nevada, Las Vegas, Nevada. It has been subjected to the
Agency's peer and administrative review, and it has been approved for publica-
tion as an EPA document. Mention of trade names or commercial products does
not constitute endorsement or recommendation for use.
ii
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ABSTRACT
The results of a single-laboratory study of osmium analytical methods are
described. The methods studied include direct aspiration atomic absorption
spectroscopy (EPA Method 7550), furnace atomic absorption spectroscopy and
inductively coupled plasma atomic emission spectroscopy using (a) direct
nebulization (heated and unheated), (b) continuous nebulization, and (c) vola-
tilization (batch and heated continuous). The results of several methods of
sample preparation are presented along with the results of osmium concentra-
tion stability over a three-week period. Method performance data including
detection limits, optimum concentration ranges (linearity), spike recoveries,
interferences, precision, accuracy, and recommended operating parameters are
presented and discussed.
This report was submitted in partial fulfillment of contract number
6R-01-7159 by the Environmental Research Center of the University of Nevada,
under the sponsorship of the U.S. Environmental Protection Agency. This
report covers work performed during the period of January 1988 to April 1989.
Hi
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CONTENTS
Abstract iii
Figures vi
Tables viii
Abbreviations and Symbols ix
Acknowledgment x
1. Introduction 1
2. Conclusions and Recommendations 2
3. Experimental Procedures 3
General Procedures 3
Method 7550 5
Furnace AAS Method 7
ICP-AES Methods 8
Sample Preparation Methods 15
Quality Assurance/Quality Control 17
4. Results and Discussion 18
Method 7550 18
Furnace AAS Method. 27
ICP-AES Methods 28
Sample Preparation Methods ..... 44
Concentration-Stability Study 48
References 50
Appendix
Recommended Changes to Method 7550 52
v
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FIGURES
Number Page
1. Apparatus for direct-nebulization ICP-AES ..... 10
2. Apparatus for heated direct-nebulization ICP-AES 10
3. Apparatus for continuous-nebulization ICP-AES 11
4. Apparatus for heated continuous-volatilization ICP-AES. ..... 11
5. Apparatus for batch-volatilization ICP-AES. ...... 12
6. Relationship between absorbance and distance of burner surface
from center-of-light beam for flame AAS determination
of 20 mg/L osmium 20
7. Relationship between absorbance and acetylene-to-nitrous oxide
ratio for flame AAS determination of 20 mg/L osmium 21
8. Relationship between absorbance and sample aspiration rate for
the flame AAS determination of 20 mg/L osmium ......... 22
9. Relationship between osmium signal intensity and temperature
for heated direct-nebulization ICP-AES 30
10. Comparison of time-integrated signals by continuous-nebulization
ICP-AES of a 1-mL injection of (A) 1-mg/L osmium standard and
(B) 1-mg/L copper 32
11. Relationship between emission intensity and temperature
for heated continuous-volatilization ICP-AES 33
12. Batch-volatilization ICP-AES purging cycle of 100 ng
of osmium injected into H2S04~£illed purging cell
with the cold trap in the heated condition and the flow
rate at 200 mL/min 36
13. Optimum concentration ranges (linearity) for direct-
nebulization ICP-AES of osmium at (A) the 225.5-nm and
(B) the 228.2-nm analytical wavelengths ..... 38
vi
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FIGURES (Continued)
Number Page
14. Direct-nebulization ICP-AES of unspiked Coal Fly Ash (CFA),
100 ug/L Os, and 0s blank samples at wavelengths adjacent
to, and including, the 225.5-nm analytical wavelength
for osmium 41
15. Spectral interference by iron at the osmium 225.5-nm analytical
wavelength during direct-nebulization ICP-AES 42
16. Spectral interference by chromium at the osmium 225.5-nm analyti-
cal wavelength during direct-nebulization ICP-AES 43
17. Suppression of 225.5-nm osmium signal in direct-nebulization
ICP-AES by HC1 45
IB. Suppression of 228.2-nm osmium signal in direct-nebulization
ICP-AES by HCl 46
vii
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TABLES
Number Page
1. Instrument Detection Limit for the Determination of Osmium
by Flame Atomic Absorption Spectroscopy Method 7550 23
2. Method Detection Limit for the Determination of Osmium by
Flame Atomic Absorption Spectroscopy Method 7550 24
3. Optimum Concentration Range (Linearity) for the Determination
of Osmium by Flame Atomic Absorption Spectroscopy Method 7550 . 24
4. Recovery and Precision of Osmium Determined by Flame
Atomic Absorption Spectroscopy Method 7550 25
5. Effect of. Al, Ca, Cr, Fe, Mg, Na, and V on the Observed
Concentration for 20-mg/L Solutions of 0s by Flame Atomic
Absorption Spectroscopy Method 7550 26
6- Precision for the Flame AAS Determination of Osmium in the Nine
Sample Digests 27
7. Retention of Radioactive 1850smium on the Graphite
Tube During Furnace AAS Analysis 28
8. Measurement Precision for 1-mg/L Osmium Signal Versus Temperature
for Heated Direct-Nebulization ICP-AES. ... 29
9. Measurement Precision for 1-mg/L Osmium Standard Versus
Temperature Using Heated Continuous-Volatilization ICP-AES. . . 31
10. Detection Limits and Relative Sensitivities for
ICP-AES Methods 37
11. Recovery Results For Direct-Nebulization and Batch-
Volatilization ICP-AES 39
12. Spectral Interference (Apparent ug/L of Osmium) Caused by
1000 mg/L of Tested Elements at 225.5 nm and at 228.2 nm. ... 40
13. Retention, of Radioactive l850smium During
Digestion of Solid Samples by Method 3050 47
14. Osmium Concentration (mg/L) by Flame AAS Over a 3-Week
Period in Digests of the 9 Matrices . . 49
viii
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LIST OF ABBREVIATIONS AND SYMBOLS
ABBREVIATIONS
AAS — atomic absorption spectroscopy
EPA — Environmental Protection Agency
g -- gram
GFAAS — graphite furnace atomic absorption spectroscopy
ICP -- inductively coupled plasma
ICP-AES — inductively coupled plasma-atomic, emission
spectroscopy
IDL — instrument detection limit
kg — kilogram
L — liter
m L --milliliter
MDL -- method detection limit
mg -- milligram
ng -- nanogram
nm — nanometers
NBS-SRM — U.S. National Bureau of Standards (now National Institute
of Standards and Technology)
Standard Reference Material
pg -- picogram
RSD — relative standard deviation
ug — microgram
v/v — volume/volume
v/v — weight/volume
SYMBOLS
A1 -- aluminum
Ca -- calcium
Ce — cerium
Co — cobalt
Cr — chromium
Cu — copper
Fe — iron
HCl — hydrochloric acid
HNO -- nitric acid
H,S0. -- sulfuric acid
2 4
Os — osmium
0s04 -- osmium tetroxide (or tetraoxide)
V — vanadium
ix
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ACKNOWLEDGMENT
We vish to acknowledge the Scripps Institute of J?,'™
for performing the furnace AAS and sample-preparation studies involving t
radiotracer. We express our appreciation to Thomas A. Hton-ra. the EPA Tasl-
Monitor (Environmental Monitoring Systems Laboratory - Las Vegas),
guidance and assistance in the execution of this project.
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SECTION 1
INTRODUCTION
Osmium tetroxide is listed among Hazardous Constituents in Appendix VIII
(Federal Register) and has the lovest Threshold Limit Value in air (at 0.0006
ppm) o£ any compound listed by the American Conference o£ Governmental
Industrial Hygienists (ACGIH). Methods of known precision and accuracy are
needed for the U.S. EPA to accomplish itt> mission. This study was conducted
to provide the needed information for atomic absorption osmium Method 7550 and
for other methods which seemed to offer improved detection limits. Since
there was uncertainty in the scientific community on the loss of osmium using
furnace atomic absorption, this issue as well as digestion losses were
resolved using radioactive ie5osmium. Based on evidence that ICP-AES offers a
lower detection limit than flame AAS, this technique was included in the study
to provide the performance data needed to add osmium to the list of elements
approved for determination by ICP-AES. Variations in sample introduction for
ICP-AES were investigated to evaluate improvements possible in the osmium
detection limit. Since osmium tetroxide produces a stronger measurement
signal than other forms of osmium as a result of its high volatility,
oxidative treatment of a digest could provide evidence that osmium was (or was
not) already present in the digest as the tetroxide.
A single-la^oratory study was undertaken to evaluate SV-846 Method 7550,
Osmium (Atomic Absorption, Direct Aspiration) and to investigate the use of
graphite furnace atomic absorption spectroscopy (GFAAS) and inductively
coupled plasma atomic emission spectroscopy (ICP-AES) for the determination of
osmium. The purpose of this study was to obtain method-performance data.
The study included a comparison of five methods of r'-.mple introduction for
ICP-AES: direct nebulization, heated direct nebulination, continuous
nebulization, heated continuous volatilization, and batch volatilization.
This report also describes a study of four sample-preparaiion methods and a
sample-digest stability study.
There are reports in the literature of elements whose presence in
solution interfere with the determination of osmium by flame AAS (A, 5, 6) and
ICP-AES (7, 8); however, most of these elements are not likely to be present
at significant levels in environmental samples. Because! of the relative lack
of data regarding interference from the more common environmental elements,
several cf these elements were investigated in this study. For Method 7550,
the elements investigated are: Al, Ca, Cr, Fe, Mg, Na, and V. For the ICP-
AES methods, Al, Ca, Ce, Cr, Fe, Mg, and Na were tested. Hydrochloric acid
has been used in sample preparation methods (3) and in the preparation of
osmium standard solutions (8). The effect of various concentrations of
hydrochloric acid on the osmium signal during a determination was also
examined.
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SECTION 2
CONCLUSIONS AND RECOMMENDATIONS
The results of this single-laboratory study show that AAS Method 7550
(excluding the sample preparation procedure) is precise and accurate for
determining osmium in extracts and digests of a variety of liquid and solid
sample types. Conventional 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 ug/L for 1 mL) was obtained by batch volatilization of osmium
(as the tetroxide) into the ICP-AES instrument. Method 7550 is recommended
for osmium concentrations above 1 mg/L, conventional ICP-AES for osmium
concentrations above 1 yg/L, and batch-volatilization ICP-AES for osmium
concentrations below 1 yg/L.
Recovery studies, including the use of radioactive 145osmium, revealed
that furnace atomic absorption spectroscopy and several digestion procedures
cannot be recommended for osmium determinations. 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. The Appendix to this report contains the detailed
changes recommended for AAS Method 7550.
The 228.2-nm wavelength is recommended for direct nebulization ICP-AES
determinations to avoid the chromium and iron spectral interferences observed
a>- 225.5 nm. The batch-volatili2ation technique is recommended as a means to
avoid even minor spectral interferents because these non-volatile components
do not reach the plasma with this technique.
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SECTION 3
EXPERIMENTAL PROCEDURES
GENERAL PROCEDURES
The primary objective of this single-laboratory study was to determine
the method-performance parameters for the determination of osmium in solution
by acetylene-nitrous oxide flame AAS for inclusion into SW-846. Secondary
objectives included investigation of alternate methods of instrumental
determination, sample preparation, and a study of sample-digest stability.
The two alternate instrumental methods of osmium determination examined vere
furnace AAS and ICP-AES. The method-performance parameters studied include:
1.
Detection Limits,
2.
Optimum Concentration Ranges (Linearity)
3.
Spike Recoveries,
4.
Interferences,
5.
Precision,
6.
Accuracy, and
7.
Ruggedness Testing
All data were generated according to the procedures described in
"Guidelines for Selection and Validation of U.S. EPA Measurement Methods" (1)
and "Test Methods for Evaluating Solid Wastes," SV-846 (2).
The instrumental methods of analysis addressed in this report all require
sample digestion. Method 7550 contains an aqueous sample preparation method
and cites Method 3050 for \ he preparation of solid samples. The two other
sample preparation methods investigated were a sodium peroxide fusion method
[an adaptation of (3)1, and a pressure bomb digestion method (PARR Instrument
Manufacturing Company). The procedure used for the pressure-bomb digestion was
that recommended by the manufacturer.
The instrument detection limits and the optimum concentration ranges were
determined in aqueous standards that were free of interferences. For the
Method 7550 evaluation, the method detection limit, spike recoveries,
precision, and accuracy vere determined in digests of each of the nine
samples. The stability of osmium in the sample digests vas monitored twice a
week for a period of 3 weeks.
A thorough literature search revealed no mention of osmium reference
materials. Nine samples vere chosen for this study. Of the nine materials,
two were U.S. National Bureau of Standards - Standard Reference Materials
(NBS-SRM); two were certified materials from the U.S. EPA; one material was
from a U.S. EPA hazardous waste site; one vas a certified reference material
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from the National Research Council of Canada. These materials are
well-characterized reference matrices, but not reference materials with
certified values for osmium.
All aqueous and solid vaste samples were digested prior to analysis by
the methods under study. Four liquid and five solid samples were chosen to
evaluate the performance of Method 7550. These samples were also used to
investigate several sample preparation methods. A study of the stability of
osmium concentrations in digests of these sample matrices was also conducted.
To ensure consistency, standards and dissolved-osmium spikes were prepared
from one lot of 1000-mg/L osmium (as the tetrachloride) stock, standard
(Spectrum Chemical Manufacturing Corp.). All acids used in the study were
Ultrex Spectrograde (J.T. Baker brand) unless otherwise stated.
Liquids
All aqueous samples were digested by the procedure described in Method
7550 using HN03 and H2S04. The four aqueous samples used in this study were:
1. A standard solution of osmium (as the tetrachloride) prepared from
the osmium stock standard by serial dilution with a solution
containing 1 percent each (v/v) of HN03 and H2S04;
2. NBS-SRM No. 1643b, Trace Elements in Water}
3. A "synthetic interference solution" containing osmium (as the
tetrachloride) and 500 mg/L concentrations of Al, Ca, Fe, and Mg in
a solution containing 1 percent each (v/v) of HN0s and H?S04;
4. A TCLP extract of soil from a U.S. EPA hazardous waste site.
Solids
The solid samples were digested using several different sample
preparation methods, which are described in the Sample Preparation Methods
portion of this section. The five solid samples used in this study were:
1. NBS-SRM No. 1633a, Coal Fly Ash;
2. U.S. EPA Municipal Digested Sludge #0319}
3. UNLV-QAL/EPA - CLP Reference Materials - Solid Laboratory Control
Sample (0387);
4. Certified Reference Material No. MBSS-1, Marine Sediment (National
Research Council, Canada); and
5. Soil from a U.S. EPA hazardous waste site,
A sixth solid was used as a solid spiking material to evaluate sample
preparation methods* Spectromel No. 2 Standard Powder Mixture, containing
12•5 percent osmium metal (Johnson Matthey Chemicals Limited, England).
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METHOD 7550
This method is intended to provide the information needed to determine
osmium by acetylenc-nitrous oxide flame AAS.
Instrumentation
A Perkin-Elmer Model 5000 Atomic Absorption Spectrophotometer, fitted
with an acetylene-nitrous oxide burner head was used to obtain data for Method
7550. Deuterium background correction was used for all determinations.
Optimization of Instrument Variables
Prior to the collection of data for the method parameters, several
instrument variables were adjusted in order to optimize osmium absorbance in
the acetylene-nitrous oxide flame. These variables include the following:
Burner Height--
Burner height was measured in millimeters from the burner surface to the
center of the light beam from the hollow-cathode lamp. Changing the burner
height dramatically affects absorbance measurements as the zone containing the
highest concentration of osmium atoms is moved. Prior to this exercise, the
burner slot was aligned with the light beam by means of the burner controls
that adjust the in/out and rotational orientation of the burner assembly.
Fuel-to-Oxidant Ratio—
The recommended acetylene-nitrous oxide flame is a rich, red reducing
flame. Once the burner position was optimized, the fuel-to-oxidant ratio was
varied in order to achieve the conditions that yielded the highest absorbance
from a 20-mg/L solution of osmium in an aqueous matrix containing 1 percent
each (v/v) HN03 and H2S04. Under the proper reducing conditions, the
population of atomic osmium is maximized.
Sample Flow Rate—
Changing the sample flow rate (aspiration rate) changes the absorbance by
increasing or decreasing the number of atoms in the light path.
Results from this optimization of burner height, fuel-to-oxidant ratio,
and sample flow rate for Method 7550 are reported in Section A of this report.
Detection Limits
Instrument Detection Limit (IOL)—
The IDL of a given analyte is the concentration that corresponds to three
times the standard deviation of seven replicate measurements of a blank
solution, according to the definition on page "THREE-2" in SW-8A6 (2). The
JDL for osmium was determined while aspirating an aqueous solution of 1
percent each (v/v) HN03 and H2S04 into the flame.
Method Detection Limit (MDL)—
The MDL takes into account that some sample-digest matrices can enhance
the absorbance of osmium and thereby increase the apparent sensitivity.
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Likewise, the matrix can also depress the absorbance and thereby decrease the
apparent sensitivity. Therefore, the digests of the 12 liquid and solid
samples were subjected to standard additions of osmium in order to produce a
calibration line for each matrix, according to the procedure on page "ONE-15"
in SW-846 (2). IDL and MDL values for Method 7550 are reported in Section 4.
Optimum Concentration Range
The absorbance values of a set of osmium standards prepared in an aqueous
matrix containing 1 percent each (v/v) HNOj and H2S04 (free of interferences),
ranging from a blank solution to 100 mg/L, were measured.
Spike Recoveries
Three portions of each of the liquid and solid samples vere spiked to
bring the concentration of osmium to 10 mg/L above the endogenous levels.
Interferences
Three different matrices were tested to determine the effect they would
have on the determination of osmium at the analytical wavelength of 290.9 nm.
These matrices were:
1. An aqueous solution containing 1 percent each (v/v) HN03 and HjS04
(as described in Method 7550),
2. An aqueous solution containing A percent each (v/v) HN03 and H2S04
(the acid composition after pressure-bomb digestion), and
3. 6-molar HCl.
Each matrix was tested alone and with 20 mg/L osmium.
Seven elements, Al, Ca, Cr, Fe, Mg, Na, and V, vere tested individually
to determine whether they interfere with the determination of osmium at the
290.9-nm analytical wavelength. First, a 20-mg/L solution of osmium
containing 1000 mg/I, of the element in question was measured, and the
absorbance was compared to that from a 20-mg/L, interference-free osmium
standard. When an interference was observed for a particular element (5
percent suppression or enhancement of the osmium signal), its concentration
was varied from 100 mg/L to 5000 mg/L to determine the extent of the
interference.
A 1000-mg/L solution of each element containing no osmium was also
analyzed at 290.9 nm to evaluate spectral interferences. When a spectral
interference was observed, the concentration of the interfering element was
varied from 100 mg/L to 5000 mg/L, and the measured absorbance was compared to
that from a blank solution. All solutions used for interference studies were
prepared in an aqueous matrix containing 1 percent each (v/v) HNOj and H?S04.
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Precision
Each concentration or absorbance measurement reported in this study was
the average of five consecutive four-second readings. These five readings
were used to calculate the RSD. Precision data are presented in Section 4.
Stability Studies
All solutions were reanalyzed twice a week over a three-week period to
determine the stability of osmium in the sample digests.
FURNACE AAS METHOD
The furnace AAS method for the determination of osmium was studied alone
and in combination with radiochemical techniques in order to determine the
fate of osmium during analysis.
Instrumentation
A Perkin-Elmer Model 5000 Atomic Absorption Spectrophotometer, fitted
with a Model HGA-500 Graphite Furnace/Programmer, an AS-40 Auto Sampler, and a
Data System 10, was utilized to obtain data for osmium determination using
furnace AAS alone. Standard graphite tubes and pyrolytically coated platforms
were used.
For the radiochemical studies of furnace AAS, a Varian Model 1275 AAS
with a GTA 95 Temperature Program Controller, pyrolytically coated graphite
tubes (without platforms) was used. The radioactivity counts were obtained
using a 3-inch sodium iodide scintillation crystal with a 1-inch well (gamma
detector) coupled to a Nuclear Data Model 62 pulse-height analyzer. The
radiochemical studies were performed at the Scripps Institute of Oceanography.
185 0s Tracer (15)
A carrier-free 1850s (half-life of 93.6 days) was produced using a
cyclotron at McGill University, Montreal, Canada. An ultra-pure rhenium foil
target (0.025 mm thick, Alfa Products #00279) was bombarded with protons fov
20 hours (20 MeV tandem, ca. 4 microcuries/ua hr.) which produced about
76 microcuries as of April 6, 1987.
After delivery to the Scripps Institute of Oceanography, the irradiated
part of the rhenium foil was cut into multiple small pieces (ca. 1.5 mm) and
used as required. Each rhenium piece was placed into a 50-mL distillation
flask and dissolved with 250 uL of concentrated HNOj, followed by distillation
of the ia50s as 0s04. Ceric sulfate was used as the oxidant in a solution of
H2S0fl (ca. 15 percent H20). The 1,50s was collected into a mixture of
ethanol:9 M HC1 (1:2) and transferred into a teflon digestion vessel upon
verification of the la50s activity.
The 0s04 was heated in a drying oven at 120° C for 2 hours to reduce the
0s+# to 0s+4, followed by evaporation of the solution to dryness at low heat.
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The 1850s was dissolved in 6 M HC1, diluted to the desired concentration, and
transferred into a glass liquid scintillation vial. The gamma peaks at 646
KeV and 875 KeV vere used to determine the yields for all samples.
Optimization of Instrument Variables
The following parameters were varied in an attempt to optimize the osmium
signal:
Drying Temperature—
The drying temperature was varied from 80° C to 2008 C. The time for the
drying step vas varied from 10 seconds to 60 seconds.
Ashing Temperature—
The ashing temperature was varied from 200° C to 1500° C. The time for
the ashing step was varied from 1 second to 30 seconds.
Atomization Temperature—
The atomization temperature was varied from 2700° C to 3000° C. The time
for the atomization step was varied from 4 seconds to 10 seconds.
At successive stages of the furnace temperature program cycle, the
program was halted and the radioactivity was determined for both the furnace
and the pyrolytically coated graphite tube. The results of this study are
presented in Section 4.
ICP-AES METHODS
Five different methods of sample introduction were investigated in order
to determine the feasibility of using ICP for osmium determination. The first
method uses unmodified direct nebulization. The other four methods involve
some modification of the sample introduction hardware. The torch,
spectrometer, and computer hardware were not altered for any of the methods.
However, different analytical wavelengths and integration times have been
used. The five sample introduction methods are:
1. Direct nebulization,
2. Heated direct nebulization,
3. Continuous nebulization,
4. Heated continuous volatilization, and
5. Batch volatilization.
Each of these methods was studied in detail to determine relative
sensitivity, detection limits, and measurement precision for osmium
determination. The continuous direct-nebulization sample introduction method
was included to show osmium standard volatility during nebulization relative
to a standard composed of non-volatile copper ions. Heat was applied in
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three of the methods to exploit the volatile nature of 0s04 (8,16). Each of
these sample introduction methods vill be explained separately.
Ins trumenta t ion
An Applied Research Laboratories Model 35000 sequential ICP-AES
spectrometer with a spectral bandpass of 0.27 nm, equipped with a Tylan mass-
flow controller and a Digital POP 11/23 computer was used to obtain data for
the evaluation of the ICP-AES methods. The ICP-AES was operated with a
standard extended torch at an applied power of 1200 watts.
Direct Nebulization—
The ICP-AES sampling train was fitted with a Gilman peristaltic pump and
a Meinhard concentric nebulizer for the direct nebulization method of sample
introduction (Figure 1).
Heated Direct Nebulization—
The sampling train is the same as that for the direct nebulization method
above, except that a cylindrical heating rod (0.5-inch diameter, maximum 70
watts power) is placed in front of the nebulizer. The heater is surrounded by
a 10-turn glass coil (1.2 mm i.d., Technicon part no. 157-0226-01) through
vhich the sample flows (Figure 2).
Continuous Nebulization—
The sampling train is the same as that for the direct nebulization
method, except that the liquid drained from the nebulization chamber is
recycled to the peristaltic pump so that the sample stream is continuously fed
to the nebulizer in a closed loop (Figure 3).
Heated Continuous Volatilization--
The sample stream is fed through a 10-turn glass coil (1.2 mm i.d.,
Technicon part no. 157-0226-01) surrounding a cylindrical heating rod (0.5-
inch diameter, maximum 70 watts power). The heated sample stream flows into
a glass phase separator (Varian VGA-76, part no. 20-100559) at which point the
vapor enters the ICP torch through a tygon tube 10 cm long and 5 mm i.d. The
liquid phase is drained into a waste container (Figure A). This apparatus is
a simplified version of that derigned by Bazan (16), and it is based on the
same principles.
Batch Volatilization—
This sample introduction apparatus utilizes an 80-mL purging cell
containing 22 mL H2S04 and 0.3 g eerie ammonium nitrate (ACS grade), vhich is
placed in a 1-L beaker filled with boiling water (Figure 5). Also
investigated was a 500-mL purging cell containing 100-mL of 10 percent H2S0^
with 20 g/L potassium dichromate (ACS grade). A liquid sample is injected *
through a septum into the purging cell. Argon is used as the carrier gas.
Vapor from the purging cell, containing volatile 0s04, enters a glass
condenser tube 20 cm long and 15 mm i.d. vhich is cooled with water flowing
through close-vound tygon tubing. The sample vapor then continues through a
U-shaped glass drying tube (Ace 14/35) 40 cm long and 20 mm i.d. This tube
was used with and without magnesium perchlorate (ACS grade, screened to remove
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Direct Nebulization
Figure 1. Apparatus for direct-nebulization ICP-AES.
Figure 2. Apparatus for heated direct-nebulization ICP-AES.
10
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Continuous Nebulization
Figure 3. Apparatus for continuous-nebulization ICP-AES
11
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Batch Volatilization
Sample
Injection Port
Boiling Water
Nichrome Wire
Variable
Transformer
Dry Ice Ethanol
Hot Plate
Figure 5. Apparatus for batch-volatilization ICP-AES.
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particles smaller than 1 mm). This apparatus is loosely based on one
described by Tao et al. (8).
The vapor proceeds through a U-shaped glass tube 70 cm long and 5 mm
i.d., which is filled with crushed borosilicate glass screened to ca. 1-mm
size. The tube rests in a vacuum Devar flask, containing a dry ice/ethanol
slurry, to trap the 0s04< The cold bath is raised and lowered about the
stationary apparatus.
Nichrome wire is wound around the cold trap tube and connected to a
variable transformer (Variac 0-120 volt) to facilitate heating. Once the
OsO has been collected in the trap, the cold bath is lowered, and voltage is
applied to the Nichrome wire heater. The 0s04 is thereby re-volatilized and
introduced directly into the torch.
Optimization of Instrument Variables
For the majority of data collected, the sequential spectrometer stepper
motor was commanded to the peak position of osmium emission, derived by direct
nebulization of a 1-mg/L osmium standard. The position of maximum signal
intensity was located by integrating at the stepper motor positions around,
and including, the expected position for maximum intensity.
Instrument variables were optimized prior to the collection of data for
the method parameters. Several adjustments were made in order to maximize the
osmium signal. These instrument variables are addressed separately for each
type of sample introduction.
Direct Nebulization--
Optimization consisted of routine adjustment of torch height and
peristaltic pump flow rate to achieve minimum detection limit for the
instrumental tuning element manganese, at the analytical wavelenth 257.6 nm,
with 1200 watts of applied RF power to the ICP-AES load coil. Argon carrier
gas flow rate was 1.0 L/minute. The peristaltic pump was adjusted to a sample
flow rate of 1 mL/minute.
Heated Direct Nebulization—
In addition to the routine optimization procedure above, the temperature
of the sample stream was varied using a variable transformer (Variac, 1-120
volts) to control the voltage to the heating rod.
Continuous Nebulization—
The optimization procedure was the same as that for direct nebulization.
Heated Continuous Volatilization—
Parameter optimization consisted of varying the temperature of the sample
stream using a variable transformer to control the voltage to the heating rod.
In order to maximize the osmium signal, the argon carrier gas flow rate was
varied using a mass-flow controller. The results of this study are presented
in Section 3.
13
-------
Batch Volatilization--
Parameter optimization involved, in addition to that for direct
nebulization, the investigation of two purging cell solutions}
1. 10-percent (v/v) H2S04 with 20 g/L potassium dichromate, and
2. concentrated H S04 with 15 percent added water (to reduce surface
tension) and 12 mg/mL eerie ammonium nitrate.
The drying tube vas used with and without magnesium perchlorate. Three
different packing materials were used in the cold traps borosilicate glass
wool, 3-mm borosilicate glass beads, and crushed borosilicate glass (screened
to ca. 1 mm).
The length of time for cold trapping was also optimized by monitoring the
osmium vapor signal with the cold trap in the heated condition and noting the
amount of time required for complete volatilization of osmium tetroxide from
the purging cell. The signal integration time was also varied. The argon
carrier gas flow rate (and therefore the purging rate during the cold trapping
cycle) was varied using a mass flow controller. The injection volume was held
constant at 1 mL for each determination.
Detection Limits
Detection limits were calculated using the method outlined in SW-846.
The detection limit is defined as three times the standard deviation of seven
integrated signals from a blank solution at the wavelength of interest. The
detection limit was determined for each of the sample introduction methods.
Using the direct-nebulization sample introduction method, the detec.ion limits
were determined using the two most sensitive analytical wavelengths for
osmium, 225.5 nm and 228.2 nm (17).
Optimum Concentration Range
The linear range was determined for the direct nebulization sample
introduction method at two analytical wave'engths, 225.5 and 228.2 nm. The
linearity of signal response versus osmium concentration was investigated
using a set of interference-free osmium standards ranging from a ulank
solution to 20 mg/L osmium. These standards were prepared in an aqueous
matrix containing 1 percent each (v/v) of HNOj and HjSO^.
Spike Recoveries
Reference materials (determined to be free of osmium) were spiked with
10 ng/mL osmium. All osmium spiking solutions were prepared from one lot of
1000-mg/L osmium tetrachloride stock standard (Spectrum Chemical Manufacturing
Corporation). Spike recoveries were evaluated using the direct-nebulization
and batch-volatilization sample introduction methods.
14
-------
Interferences
Osmium-free solutions containing known concentrations of Al, Ca, Ce, Cr,
Fe, Mg, and Na were analyzed using direct nebulization to check for possible
spectral interference at two osmium analytical wavelengths, 225.5 nm and 228.2
nm. Solutions of the individual elements were prepared at concentrations of
either 500 mg/L or 1000 .mg/L by serial dilution from stock, standards (Spex
Industries, Inc.).
Since osmium is sometimes determined in solutions of hydrochloric acid
(3), an experiment was conducted to study the effects of varying
concentrations of hydrochloric acid in osmium-free and osmium standard
solutions that had been prepared in 1 percent each (v/v) HN03 and HjS04. The
Hydrochloric acid concentrations examined were 0 percent (v/v), 1 percent
(v/v), 3 molar, and 6 molar.
Precision
Since samples and standards were all measured in triplicate, precision
data are available for each of the sample introduction methods, and are
presented in Section 3.
Accuracy
Spike recoveries were used to assess the method accuracy (sample
preparation in combination with instrumental determination).
SAMPLE PREPARATION METHODS
The samples were prepared using three different methods. Preparation of
liquid samples was performed according to SV-846 Method 7550 for osmium and
according to a pressure-bomb procedure (Parr Instrument Company). The
preparation of solids was performed according to SW-846 Method 3050 (as
Prescribed by Method 7550), a sodium peroxide fusion method (3), and the
pressure-bomb procedure.
Method 7550 Aqueous Sample Preparation
Liquid samples were spiked at appropriate levels and digested according
to the procedure given in Method 7550, using HN03 and H2S04« Borosilicate
glassware and Spectro grade acids were used.
A test was performed to determine whether the Method 7550 digestion
Procedure would release osmium from a water sample containing ascorbic acid
(ACS grade, Fisher Scientific). It was thought that ascorbic acid would serve
to reduce osmium to a non-volatile oxide (14). An aqueous solution containing
0-2 gram/L ascorbic acid was spiked with 1 mg/L osmium and was digested using
the procedure outlined in Method 7550. Thirty percent hydrogen peroxide (ACS
grade), eerie ammonium nitrate (8), and ammonium peroxydisulfate (ACS grade)
vere used to reoxidize the reduced osmium. The results of this experiment are
presented in Section 3.
15
-------
Method 3050 Solid Sample Preparation
As specified in Method 7550, the Method 3050 digestion procedure was used
to prepare solid samples. Borosilicate glassware, Spectrograde HN03 and HC1,
and ACS grade H202 were used.
In order to quantitate osmium losses occurring during the Method 3050
digestion, and to determine at which point during the procedure the losses
were occurring, samples were spiked with radioactive 0s. At progressive
stages of the digestion procedure, the total radioactivity vas counted;
percent loss of 0s corresponds to the percentage loss of total osmium.
Sodium Peroxide Fusion Method for Solid Samples
Solid samples were prepared by sodium peroxide fusion in 30-mL zirconium
crucibles. A 0.5-gram solid sample was mixed with 5 grams of sodium peroxide
(ACS grade). The mixture was brought to red heat (about 550° C) over a Bunsen
burner for 10 minutes with constant swirling. After cooling, 15 mL of water
were slowly added to the reaction mixture. After further cooling, the
crucible contents were transferred to a 50-mL beaker. Then slowly, with
stirring, 13 mL conc. HCl and 3 mL conc. HN03 were added. (All osmium spikes
were added prior to fusion.) Several solid samples were prepared in this
manner and results from the analyses are tabulated in Section 3.
Pressure-Bomb Method
Samples were prepared using 23 mL teflon-lined acid digestion bombs (Parr
Instrument Company No. 4745). The quantities of samples and HN03 were those
recommended by the manufacturer. Several solid and liquid samples were
prepared >:sing this method. Results from the analyses of the digests are
presented in Section 5.
Liquid Samples—
Ten mL sample and 2 mL concentrated HN03 were placed in the pressure
bomb and heated at 150° C for 2 hours. After cooling, 2 mL concentrated
H S04 were added and the digest was then diluted to 50 mL with Type II water.
Tne resultant acid composition was 4 percent each of HN03 and H2S04.
Solid Samples—
A 0.1-gram sample and 2 mL concentrated HN03 were placed in the pressure
bomb and heated at 150° C for 4 hours. After cooling, 2 mL concentrated
H2S0 were added and the digest vas diluted to 50 mL with Type II water. The
resultant acid composition was 4 percent each of HN03 and H2S04.
Osmium Concentration Stability
In order to determine whether osmium is stable in sample digests for
extended periods of time, each of the target samples was digested using the
pressure-bomb digestion method and analyzed twice weekly over a period of 3
weeks by Method 7550. The four liquid and five solid samples (listed under
General Procedures) were digested in triplicate; they had been spiked such
that they contained 10 mg/L osmium in the final volume of 50 mL.
16
-------
QUALITY ASSURANCE/QUALITY CONTROL
Accuracy
Since no reference materials are available with certified values for
osmium, homogeneous materials were the next acceptable alternative. To ensure
consistency, all osmium standards and osmium spiking solutions were prepared
from one lot of 1000 mg/L osmium tetrachloride stock standard (Spectrum
Chemical Manufacturing Corporation). Spectromel No. 2 Standard Powder
Mixture, containing 12.5 percent osmium metal (Johnson Matthey Chemicals
Limited, England) was used as a solid spiking material in experiments
involving the sodium peroxide fusion method.
Preparation Blank Analysis
For each set of sample digests, at least one preparation vessel served as
a blank and received all of the reagents used in the sample preparation
procedure. Preparation blanks were analyzed along with the rest of the
samples in the set.
Precision
Samples to be analyzed by flame AAS vere prepared in triplicate; those to
be analyzed by ICP-AES were prepared in duplicate.
Continuing Calibration Verification
A solution of known osmium concentration was used to check for drift
after every set of not more than 10 samples. When more thar a 5-percent
drift was observed, the instrument was recalibrated, and the samples were
analyzed again.
17
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SECTION A
RESULTS AND DISCUSSION
This section will first present results for the determination of osmium
in the nine matrices by flame AAS Method 7550. The method-performance and
experimental results for furnace AAS and ICP-AES methods are then examined,
followed by examination of the results of the sample preparation and the
concentration-stability study.
METHOD 7550
The method-performance parameters and experimental results include:
1. Optimum Conditions and Ruggedness Testing,
2. 1DL,
3. MDL,
4. Optimum Concentration Range (Linearity),
5. Spike Recovery,
6. Interferences, and
7. Stability Studies.
All analytical results are reported in units of mg/L or ug/L for the
solutions. The standard deviation values are reported to two significant
figures following the guidance of the Association of Official Analytical
Chemists (17). All of the data reported (except burner height) were obtained
by employing automatic spectral-background correction (deuterium lamp) and
with the optimum measurement conditions discussed below.
Optimum Conditions
In the initial instrument set-up, a solution of known concentration is
aspirated into the flame, and the analyst attempts to adjust the instrument in
order to maxirai2e the absorbance signal from the test solution. In the
optimization of flame AAS, the analyst normally adjusts the burner position,
gas flow rates, and sample flow rates. The extent to which these parameters
can be varied without significantly decreasing the absorbance is a measure of
the ruggedness of the method.
In the current study, before data were collected fot any of the
method-performance parameters, the absorbance signal vas optimized by varying
the burner height, acetylene-nitrous oxide flow rate, and sample aspiration
rate.
Burner Height--
The results for the adjustment of burner height are shown in Figure 6.
The observed absorbance from a 20-mg/L solution of osmium in 1 percent (v/v)
18
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each of HN03 and H2S04 varied from 0.006 to 0.134 absorbance units as the
burner surface to center-of-beam distance vas increased from 2 mm to 4 mm. A
distance of 3.6 mm was chosen, because it gave the highest absorbance
reading.
Fuel-to-Oxidant Ratio—
It is known that an acetylene-rich flame is necessary for the best osmium
sensitivity. As shown in Figure 7, a ratio of acetylene-to-nitrous oxide of
1.4 yields the highest absorbance for a 20-mg/L solution of osmium.
Sample Flow Rate--
It can be seen from Figure 8 that the absorbance from a 20-mg/L solution
of osmium can vary by a factor of five in response to changes in the
aspiration rate. The maximum absorbance value was observed at 4.8 mL/min.
However, at this point, any small change in sample flow rate, possibly due to
a change in sample viscosity, will result in a large decrease in absorbance
and thus an apparent decrease in osmium concentration.
Summary of Optimization Procedure
The follcving initial set-up procedure by which analysts should seek the
optimum absorbance conditions for their particular instruments is
recommended:
1. Place the burner surface approximately 3.6 mm below the
center of the hollow-cathode light beam.
2. Set the acetylene-to-nitrous oxide gas flow ratio at
approximately 1.4.
3. Set the sample flow rate to approximately 4.8 mL/min,
using an aqueous solution.
4. Adjust all three variables while aspirating a 20 mg/L
solution of osmium until the optimum absorbance signal
is obtained.
Instrument Detection Limit (IDL)
The IDL is defined in SW-846 as three times the standard deviation of
seven consecutive measurements of a reagent blank's signal, or,
IDL = KS„
where; K * 3
SD - the standard deviation of the average
instrument noise.
For this study, a solution containing 1 percent (v/v) HN03 and 1 percent
(v/v) H2S04, free of interferents, vas used as the reagent blank..
19
-------
ro
o
0)
a
c
o
JD
L_
o
in
JD
<
2.0
2.5 3.0 3.5
Burner Height (mm)
Figure 6. Relationship betveen absorbance and distance of burner surface froa center-of-light
beam for flame AAS determination of 20 mg/L osmium.
-------
0.00
1-1
1.2 1.3 1.4 1.5
Acetylene —to —Nitrous Oxide RGtio
1.6
Figure 7. Relationship betveen absorbance and acetylene-to-nitrous oxide ratio for
flame AAS determination of 20 mg/L osmium.
-------
ro
to
0.200
0.150 -
QJ
U
c
D
-p 0.100 -
o
cn
JD
<
0.050
0.000
1.0
2.0 3.0 4.0
Aspiration rate (mL/min)
5.0
Figure 8. Relationship between absorbance and sample aspiration rate for the
flame AAS determination of 20 mg/L osmium.
-------
Results for the determination of the IDL by absorbance readings on non-
consecutive days are listed in Table 1. The IDL determined from the
absorbance readings is 0.28-mg/L. This agrees with the value given in Method
7550, 0.30 mg/L.
TABLE 1. INSTRUMENT DETECTION LIMIT FOR THE DETERMINATION
OF OSMIUM BY FLAME ATOMIC ABSORPTION
SPECTROSCOPY METHOD 7550
Date IDL (mg/L)
09-21-88 0.36
09-23-88 0.27
09-28-88 0.21
Average 0.28
Method Detection Limit (MDL)
The MDL is defined in SW-846 as the minimum concentration of an analyte
that can be measured in a sample which has been processed through the
preparative procedure. The MDL for this study was determined for the nine
sample digests by the following relationship:
MDL = KS0/m
where: KS D = the IDL
m = the slope of the calibraton line in the
sample digest.
Portions of each digest were spiked to contain osmium at three
concentrations. The results from this procedure produced a calibration line
for each of the nine samples. The MDLs of the nine matrices were used to
calculate an average MDL of 0.30 mg/L (Table 2).
Optimum Concentration Range
The optimum concentration range is the range over which there is a linear
relationship between absorbance and concentration. This method parameter was
determined by preparing several osmium standard solutions ranging from blank
to 100 mg/L. The solutions were prepared in 1 percent (v/v) each HN03 and
H^S04, which was free from interfering elements. The test for deviation from
linearity was defined as that point on the curve where the predicted
absorbance differed by more than 5 percent from the calculated absorbance
value. This value was obtained from a linear regression of the absorbance
versus concentration at the blank level, 2.0 mg/L, and 5.0 mg/L. The results
of this test are presented in Table 3. Although the predicted absorbance did
not differ by more than 5 percent from the calculated absorbance value, it was
23
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TABLE 2. METHOD DETECTION LIMIT FOR THE DETERMINATION OF OSMIUM
BY FLAME ATOMIC ABSORPTION SPECTROSCOPY METHOD 7550
Matrix
Slope
MDL (mg/L)*
Trace Elements in Water
0.83
0.34
Osmium Spectrometric Solution
0.85
0.33
Synthetic Interference Solution
0.99
0.28
TCLP Extract of Hazardous Waste
0.83
0.34
Coal Fly Ash
1.11
0.25
Municipal Digested Sludge
0.98
0.29
Marine Sediment
0.96
0.29
Laboratory Control Sample
0.96
0.29
Hazardous Waste Site Soil
0.99
0.28
Average
0.94
0.30
sThe method detection limit is defined by MDL = 3 Sp/m where Sp is the
standard deviation of the instrument background signal and m is the slope
of the calibration line in the sample matrix.
TABLE 3. OPTIMUM CONCENTRATION RANGE (LINEARITY) FOR THE DETERMINATION OF
OSMIUM BY FLAME ATOMIC ABSORPTION SPECTROSCOPY, METHOD 7550
Os Added
(mg/L)
Absorbance
Observed Calculated
Percent'
Error
0.00
0.001
0.001
2.00
0.009
0.011
-19
5.00
0.027
0.027
0.5
10.0
0.051
0.053
-4.1
20.0
0.103
0.106
-2.7
30.0
0.153
0.158
-3.4
40.0
0.202
0.211
-4.3
50.0
0.253
0.264
-4.1
60.0
0.301
0.316
-4.8
70.0
0.355
0.369
-3.8
80.0
0.407
0.422
-3.5
90.0
0.468
0.474
-1.3
100.
0.508
0.527
-3.6
'Percent Error = ((observed absorbance - calculated absorbance)/calculated
absorbance] x 100.
24
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decided not to increase the osmium concentration above 100 mg/L; it is
unlikely that environmental samples would contain osmium at such high levels.
Spike Recoveries
Each of the nine samples was digested by the pressure-bomb method
(Section 3). No osmium could be detected in the samples. So, three portions
of each of the nine samples were spiked prior to digestion with 10 mg/L of
osmium as the tetrachloride. Table 4 shows the spike recovery data. Greater
than 94 percent recovery was obtained from the liquid samples. Spike
recoveries for the soils were all in the 80-percent range, jxcept for Coal Fly
Ash which is at 94 percent. An average of 91 percent recovery was obtained
for all of the samples digested. The lower-than-100-percent recovery is
likely due to physical losses of osmium during the digestion procedure, or due
to interfering elements.
TABLE 4. RECOVERY AND PRECISION OF OSMIUM DETERMINED BY
FLAME ATOMIC ABSORPTION SPECTROSCOPY METHOD 7550
0s Added
Percent*
Mat rix
(mg/L)
Recovery
Trace Element;": in Water
10.0
96
Osmium Standard Solution
10.0
94
Synthetic Interference Solution
10.0
98
TCLP Extract of Hazardous Waste
10.0
96
Coal Fly Ash
10.0
94
Municipal Digested Sludge
10.0
84
Marine Sediment
10.0
88
Laboratory Control Sample
10.0
85
Hazardous Waste Site Soil
10.0
86
Average
91
'Average of three digestions.
Interferences
Three different matrices were evaluated to determine whether they
interfere with the detection of osmium. The 1 percent (v/v) each HNO and
HjSO. solution and the 4 percent (v/v) each HN03 and H2S0^ solution (t^ie
resultant matrix of the digestion procedures) gave the same absorbance reading
for a 20-mg/L solution of osmium. The 6-M HCl matrix gave a significantly
lover absorbance reading, and thus was not used for any of the analyses.
Seven elements, Al, Ca, Cr, Fe, Mg, Na, and V, were tested to determine
whether or not they interfere with the quantification of osmium. Interference
data are presented in Table 5. Al, Fe, Mg, and Na did not interfere with the
25
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TABLE 5. EFFECT OF Al, Ca, Cr, Fe, Mg, Na, AND V ON THE OBSERVED
CONCENTRATION FOR 20-mg/L SOLUTIONS OF Os BY FLAME
ATOMIC ABSORPTION SPECTROSCOPY METHOD 7550
Interfering Interfering Element Concentration (mg/L)
Element
100
500
1000
5000
Al
a
a
20.0
a
Ca
19.4
19.3
19.0
17.9
Cr
20.7
21.3
22.9
34.3
Fe
a
a
19.9
a
Mg
a
a
20.0
a
Na
a
a
19.9
a
V
19.4
19.2
19.0
18.3
"Not tested at these concentrations because no interference vas detectable
when the interfering element was at a concentration of 1,000 mg/L.
detection of osmium when their concentrations were 1000 mg/L or lower and
were not investigated further. Ca and V both suppressed the osmium signal
slightly; this is probably due to viscosity and surface tension effects rather
than a chemical interference, but this was not experimentally verified.
Chromium caused a large increase in the apparent osmium signal. This is
because chromium has an absorption line very close to that of osmium; a
solution containing 1000 mg/L of chromium alone gave an apparent osmium
concentration reading of 3 mg/L, while 5000 mg/L chromium gave a signal
corresponding to 15 mg/L osmium. These effects were reproducible in the
presence of 20 mg/L osmium, as shown in Table 5, which suggests a spectral,
rather than a physical interference.
Precision
Measurement precision was determined from five repetitive concentration
readings for each sample. The relative standard deviation (RSD) was used as
an estimate of precision. Table 6 shows the RSD for each of the nine sample
digests. The RSDs for osmium at about the 10-mg/L level are excellent. All
RSDs are better than 2.6 percent with an average of 1.5 percent.
Accuracy
The accuracy of the osmium results obtained with Method 7550, in
combination with the pressure-bomb method of sample preparation was
determined using spike recovery results for the nine matrices, The samples
were spiked with 10 mg/L 0s (as the tetrachloride) prior to digestion. The
spike recoveries ranged from 84 to 98 percent, averaging 91 percent (Table 4).
26
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TABLE 6. PRECISION FOR THE FLAME AAS DETERMINATION OF OSMIUM
IN THE NINE SAMPLE DIGESTS
0s Added 0s
Measured
Percent
Matrix
(mg/L)
(mg/L)
RSD
Trace Elements in Water
10.0
9.83
0.9
Osmium Standard Solution
10.0
9.26
1.5
Synthetic Interference Solution
10.0
9.80
0.9
TCLP Extract of Hazardous Waste
10.0
9.59
0.7
Coal Fly Ash
10.0
9.29
1.5
Municipal Digested Sludge
10.0
8.61
1.5
Marine Sediment
10.0
8. 77
2.6
Laboratory Control Sample
10.0
8.70
1.8
Hazardous Waste Site Soil
10.0
8.88
2.0
Average
1.5
FURNACE AAS METHOD
Optimum Conditions
No reproducible osmium signal could be obtained using furnace AAS,
although various combinations of drying, ashing and atomization conditions
were used. Since osmium tetroxide boils at 130° C (11), it is possible that
the osmium is lost during the drying step of the furnace temperature cycle.
Alternatively, osmium may be reduced to the refractory metal which has a
boiling point of greater than 5300° C (11). Because graphite sublimes at
about 3650° C (11), the maximum temperature for the atomization step was
limited to 3000° C. It is possible that osmium metal is produced during the
charring cycle and subsequently is not volatilized during atomization.
In an effort to determine which of these scenarios was occurring,
radioactive 185osmium was injected into the furnace and the program
temperature cycle was initiated. The program was then terminated at various
stages and the total radioactivity was determined. Table 7 displays the total
percent osmium remaining after different temperature steps.
It is apparent that most of the osmium (81 percent) remains in the
graphite tube after being held at 500° C for 30 seconds. Therefore, it is
unlikely that the majority of the osmium is in the form of 0s0.. After the
atomization steps at 2000° C and 30008 C for 2 seconds, much of the osmium (52
percent and 27 percent, respectively) remains on the furnace walls. Clearly,
osmium is probably in the elemental form and is slowly being volatilized from
the furnace walls. Furnace AAS analysis of osmium might be improved by the
use of matrix modifiers that would release oxygen to osmium during the
charring and atomization steps.
27
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TABLE 7. RETENTION OF RADIOACTIVE 185"OSMIUM ON THE GRAPHITE
TUBE DURING FURNACE AAS ANALYSIS
Temperature
Time
Gas Flow
Percent 0sa
Step No.
(°C)
(sec.)
(L/min.)
Remai ni ng
1 ramp
amb.-110
30
3
2 hold
110
45
3
96±2
3 ramp
110-500
70
3
4 hold
500
30
3
81 ± 3
5A ramp
500-2000
1
0
6A!> hold
2000
2
0
52±5
5B ramp
500-2000
1
0
6Bb hold
3000
2
0
27±4
3T8r,0smium with 5 ng stable osmium.
incomplete atomization occurs at both 2000° C and 3000" C. Temperatures
above 3000° C were not explored, due to instrument limitations.
A set of ideal instrument conditions for the furnace AAS determination of
osmium was not obtained in this study. Similar difficulties ha"s been
experienced by others (9,10).
ICP-AES METHODS
In this section, the results obtained from the evaluation of the four
ICP-AES sample introduction methods investigated are discussed.
Optimum Conditions
All optimization procedures were performed using the analytical
wavelength of 225.5 nm tjnless otherwise indicated.
For the majority of. the data collected, the sequential spectrometer
stepper motor was commanded to the peak position of osmium emission. This was
achieved by first nebulizing a 1-mg/L osmium standard and integrating the
signal at the stepper mfltor positions around and including the expected
position of maximum intensity. Once the position of maximum intensity was
found, the stepper motottvas commanded to this position. All ICP-AES data
were collected in this rfanner, with the exception of the Fe and Cr scans,
which are discussed in the section on interferences.
Direct Nebulization—
General optimization of the plasma consisted of adjustment of the plasma
torch height, such that l/ith manganese (as the instrumental tuning element) a
detection limit of 0.6 Jfg/L at the 257.6-nm analytical wavelength was achieved
with 1200 watts of applied power, using a two-second integration time. The
28
-------
sample flow rate was adjusted to 1 mL/minute. Th*» plasma support gas was
delivered at 0.8 L/minute. The sample carrier gas flow rate was 1.0 L/minute.
Measurement precision for a 1-mg/L osmium standard typically had an RSD of
less than 0.5 percent.
Heated Direct Nebulization--
In genetal, optimum conditions used were the same as those used for
direct nebulization. To find the optimum temperature for heating the sample
stream, a variable transformer was used to increase the voltage step wise to
the sample stream heating rod. A 1-mg/L osmium tetrachloride standard was
aspirated, and signal integrations were taken every two seconds. The osmium
signal versus temperature is plotted in Figure 9. A linear relationship was
assumed between zero voltage applied at ambient temperature, and the voltage
applied for the sample stream to boil at 100° C.
As expected, increasing the teiverature of the sample stream caused an
increase in the volatility of osmium from aerosol droplets formed during
nebulization. The net signal increase at a point just prior to boiling was
about twice that for unheated direct nebulization. The precision of the
signal measurements remained constant up to approximately 100° C. As the
sample stream began to boil, measurement precision decreased dramatically.
Precision data (RSD) relative to temperature are presented :!n Table 8.
TABLE 8. MEASUREMENT PRECISION FOR 1-mg/L OSMIUM SIGNAL VERSUS
TEMPERATURE FOR HEATED DIRECT-NEBULIZATION ICP-AES
Tempera ture
(°C)
RSD
25
0.27
50
0,22
75
0. 49
ca. 100 not
boiling)
0.29
The detection limit at ambient temperature was 0.3 ug/L. At just less
than boiling, the detection limit fas also 0.3 ug/L, even though the
sensitivity had doubled.
It was observed that bubbles from the boiling sample stream caused
sporadic 25-percent increases in the osmium signal. This phenomenon may be
the result of a more turbid flow through the 1.2-mm i.d. glass coil
surrounding the heater. No further investigations on this subject were
performed.
Continuous Nebulization—
In order to further investigate the volatile nature of OsO , a
continuous nebulization apparatus was employed (Figure 3). A 1-mL aliquot of
a 1-mg/L osmium tetrachloride standard wus injected, and the resulting osmium
signal was integrated every 2 seconds. For comparison, a 1-mL aliquot
29
-------
20,000
1 5,000
_>>
' cn
c
CD
i 10,000
o
c
cn
CO
5,000
0
25 50 75 100
Temperature (°C)
Figure 9. Relationship between osmium signal intensity and
temperature for heated direct-nebulization ICP-AES.
30
-------
of a 1 mg/L copper standard was also injected in the same fashion. Plots of
signal response over a 10-minute period for these two elements are shown in
Figure 10.
Heated Continuous Volatilization—
This method of sample introduction was employed to take advantage of the
volatility of 0s04, especially at elevated temperature. Additionally, if
elements causing spectral interferences were present in a sample, separation
of the gas and liquid phases would remove most interferences. These
interfering elements would be present in solution as aquo-ligands and would
remain in the liquid phase, while the gas phase would contain the OsO .
Determining the optimum conditions for this sample introduction method
consisted of varying power to the sample stream heating rod. A plot of ap-
proximate temperature versus signal for a 1-mg/L osmium standard is presented
in Figure 11. Corresponding measurement precision data (RSD) are shown in
Table 9.
TABLE 9. MEASUREMENT PRECISION FOR 1-mg/L OSMIUM STANDARD VERSUS
TEMPERATURE USING HEATED CONTINUOUS-VOLATILIZATION ICP-AES
Temperature
(° C)
RSD
25
4.7
50
0.7
75
0.3
ca. 100 (not
boiling)
0.4
When the sample stream temperature is increased to a point just below
boiling, the osmium signal increases to a level of about eight times that of
signal at ambient temperature. Note, however, that the sensitivity at ambient
temperature is about eight times less than that for direct nebulization.
Measurement precision was poor at ambient temperature (RSD of 4.7 percent),
but improved with heating to less than 1 percent RSD. Precision decreased
dramatically as the sample stream began to boil. The detection limits at
ambient temperature and at a temperature just below boiling were determined
to be 2.1 pg/L and 1.4 Pg/L, respectively.
Batch Volatilization—
The optimization of this method of sample introduction involved designing
the purging cell, choosing the ideal material for the cold trap, and optimiz-
ing flow conditions.
Two types of purging cells were investigated. The first was a 500-mL
cell containing 100 mL 10-percent H2S04 with 2 percent (w/v) potassium
31
-------
Time (sec.)
Figure 10, Comparison of time-integrated signals by continuous-nebulization
ICP-AES of a 1-mL injection of (A) 1-mg/L osmium standard and (B) 1-mg/L copper.
32
-------
8000
c 6000
0J
c 4000
cr>
'if)
2000
0
0 25 50 75 100
Temperature (°C)
Figure 11. Relationship between emission intensity and temperature
for heated continuous-volatilization ICP-AES.
33
-------
dichromate as the oxidant. The second cell was 100-mL cell containing 22 mL
concentrated H;S0 and 3 mL of water (to reduce surface tension). To this
solution, 0.3 g of eerie ammonium nitrate was added as the oxidant. The
sample volume injected into the purging cell was 1 mL for all standards and
samples.
The first arrangement proved unsatisfactory because a large amount of
water vapor was generated along with the 0s04 during the purging cycle. The
water vapor caused the the magnesium perchlorate in the drying tube to become
saturated after only five determinations. In addition, dry magnesium per-
chlorate seemed to display some adsorptive properties towards osmium.
The second cell proved more useful in that very little water vapor
escaped the sulfuric acid solution; so little that the drying tube could be
used without the magnesium perchlorate. Approximately 15 determinations could
be performed before it was necessary to change the purging cell solution.
Ceric ammonium nitrate was used as the oxidant instead of potassium dichro-
mate. The 100-mL purging cell with H S04 and ceric ammonium nitrate was used
for all remaining investigations of tne proper cold trapping material and
argon flow conditions.
Cold-trap efficiency was investigated using three materials: borosili-
cate glass wool, 3-mm glass beads, and crushed borosilicate glass screened to
approximately 1 mm. The glass wool was found to bind 0s04 irreversibly. The
3-mm diameter glass beads trapped 0s04 during about the first 4 minutes of the
purging cycle, but then the osmium signal began to increase, indicating possi-
ble leakage, or perhaps saturation of surface area. Best results were ob-
tained using the 1-mm crushed glass, which was used during the remainder of
the investigation.
Optimum argon flow rates were determined for both the purging and cold
trap heating cycles. For the purging cycle, 200 mL/minute was found to be
sufficient. The optimum carrier flow rate for delivering the 0s04 to the
torch from the heated cold trap was 0.8 L/minute. This was determined by
varying the argon carrier gas flow rate while purging a 1000-ng injection of
osmium with the cold trap in heated condition. Although this flow rate would
seem to be the same as that of the plasma (0.8 L/minute), a darker central
channel could be seen. The actual carrier flow rate was, therefore, more than
0.8 L/minute. This is probably due to expansion of the argon gas when the
cold trap was heated.
To determine the length of time required for integration during the
trap-heating cycle, the signal from 100 ng of volatilized osmium from the
heated cold trap was monitored, using repetitive two-second integrations.
This showed that all of the osmium was released from the trap in less than
12 seconds. Therefore an integration time of 15 seconds was chosen.
The length of time required for the purging cycle was determined by
injecting 100 ng of osmium into the purging cell with the cold trap in the
heated condition. The osmium signal was then monitored using 15-second
integrations for several minutes. The relation of signal intensity versus the
time in minutes is presented in Figure 12. A purging time of 10 minutes was
34
-------
sufficient to remove all of the 0s04 from the reaction cell, and vas used for
the remainder of the investigation. Measurement precision, using 1-mL
aliquots of a 10-ug/L osmium standard, had a 6-percent RSD.
In summary, the optimum conditions and apparatus arrangement for the
batch volatilization are as follows:
1. Utilize a purging cell with sulfuric acid and enough vater to reduce
surface tension, with eerie ammonium nitrate added as a strong oxidant.
2. Set the water trap in place without a desiccant.
3. Set up the cold trap filled with ca. 1 mm crushed borosilicate glass.
4. Set the argon carrier flow rate at 0.2 L/minute for the purging cycle.
5. Purge for 10 minutes.
6. Set the argon flow rate at 0.8 L/minute for heating the cold trap.
7. Employ a signal integration time of 15 seconds.
Detection Limits
The detection limits were determined for each of the ICP-AES methods,
except fot continuous nebulization. The detection limits and relative
sensitivities for each of the methods are presented in Table 10. Detection
limits using heated nebulization and heated continuous volatilization were
determined at the temperature just prior to boiling. Sensitivities were
determined relative to the direct-nebulization method.
The detection limits obtained using direct nebulization and heated direct
nebulization were both 0.3 Ug/L at the 225.5-nm wavelength. Heating the
sample stream increased the sensitivity by a factor of two; however, the blank
noise increased proportionally.
The heated continuous-volatilization method of sample introduction
provided the worst detection limit; although, as stated earlier, this method
would afford freedom from spectral interferences.
The best detection limit vas obtained using the batch volatilization
method of sample introduction. Data were gathered using a 1-mL sample volume
produced a detection limit of 30 pg, about a ten-fold improvement over that by
direct nebulization. Larger sample volumes (within reason) would afford even
lower detection limits.
Optimum Concentration Range
The linear range for osmium was determined using only the direct
nebulization sample introduction method. Presented here are the recommended
35
-------
u>
CT\
to
c
Ln
800 -
600 -
400 -
200 -
6 8
Time (min.)
Figure 12. Batch-volatilization ICP-AES purging cycle of 100 ng of osmium injected into
H2S04-filled purging cell vith the cold trap in the heated
condition and the flow rate at 200 mL/min.
-------
TABLE 10. DETECTION LIMITS AND RELATIVE SENSITIVITIES FOR ICP-AES METHODS
Method
Wavelength
Detection Limit
(Mg/L)
Relative
Sensitivity
Di rect
Nebulization
225.5 nm
228.2 nm
0.3
0.5
o >-»
Heated Direct
Nebulization
225.5 nm
0.3
2
Heated Continuous
Volatilization
225.5 nm
1.4
2
Ba tch
Volatilization
225.5 nm
0.03*
320
"For 1-mL sample volume.
concentration ranges for the heated direct-nebulizatiou, continuous-heated
volatilization, and batch-volatilization sample introduction methods.
Direct Nebulization—
The linear range for this sample introduction method was determined by
aspirating a series of standards, and noting any deviation from theoretical
concentrations. The standards used ranged from a blank solution to a 20-mg/L
osmium standard. The linear range was investigated at both the 225.5-nm and
228.2-nm analytical wavelengths. The results are presented in Figure 13.
Data points represent the average result of triplicate measurements of these
solutions. As shown in Figure 13A, the increase in the osmium signal at the
225.5-nm line is linear to approximately 20 mg/L. At the 20-mg/L level, the
deviation from the theoretical value is 3.8 percent. Similarly, in Figure
13B, one car. see that at the 228.2-nm line, the increase in the osmium signal
is linear (within 5 percent of the theoretical value) up to 20 mg/L. The
deviation at this point was only 2.0 percent. The linear range for osmium at
this wavelength probably extends up to 30 mg/L. However, this upper range was
not investigated.
Heated Direct Nebulization—
The linear range for osmium was not investigated using this method of
sample introduction. However, since the sensitivity of this method is twice
that of direct nebulization, a linear range of 10 mg/L might be assumed.
Heated Continuous Volatilization—
The linear range for osmium was not investigated using this method of
sample introduction. However, given the sensitivity of the method, a linear
range of 10 mg/L might be assumed.
37
-------
20.00
cn
E 16.00
o
L.
c
0)
u
c
o
o
0.00
oi
E
o
L.
c
aj
a
c
o
CJ
0.00
A (225.5 rim)
¦ calculated
~ observed
20.00 -
B (228.2 nm)
• calculated
~ observed
0.00 4.00 8.00 12.00 16.00
Concentration 0s (mg/L)
20.00
Figure 13. Optimum concentration ranges (linearity) for direct-nebulization
ICP-AES of osmium at (A) the 225.5-nm and
(B) the 228.2-nm analytical wavelengths.
38
-------
Batch Volatilization—
The useful range for this method was determined to be from 30 ng/L to
ion |jg/L osmium. This is assuming a 1-mL injection volume. Larger sample
volumes would allow quantitation at lower concentrations. Samples containing
more than 100 |Jg/L osmium should be analyzed using direct nebulization, which
is simpler, faster and possesses a greater linear range.
Spike Recoveries
Two of the target samples (NBS 1633a, Coal Fly Ash, and MEoS-l, Marine
Sediment) were spiked at the 10-yg/L level and analyzed using the direct-
nebulization and batch-volatilization sample introduction methods. A total of
three aliquots of each sample were prepared for each matrix using pressure-
bomb decomposition. Two of the aliquots were spiked with osmium such that a
concentration of 10 ug/L would result in the final volume of 50 mL. The third
aliquot was not spiked.
Samples were analyzed using direct nebulization at the 225.5-nm and
228.2-nm wavelengths, and by batch-volatilization at the 225.5-nm wavelength.
Analytical results are presented in Table 11.
TABLE 11. RECOVERY RESULTS FOR DIRECT-NEBULIZATION AND
BATCH-VOLATILIZATION ICP-AES
DIRECT NEBULIZATION
225.5 nm 228.2 nm
(Wg/L) (Ug/L)
BATCH VOLATILIZATION
225.5 nm
(Pg/L)
1633A (unspiked)
12.5
<0.5
<0.03
1633A Spike 1
20.4
8.1
7.5
1633A Spike 2
20.4
7.5
8.0
MESS (unspiked)
n/a
0.85
<0.03
MESS Spike 1
n/a
8.2
7.7
MESS Spike 2
r./a
8.0
7.7
The results presented in Table 11 using direct nebulization at 228.8 nm
and batch volatilization at 225.5 nm are consistently biased low by about 20
percent relative to the theoretical value of 10 pg/L. This low bias seems to
be due to osmium losses during sample preparation through volatilization,
rather than due to the analytical method. (See Sample Preparation Methods
section.)
Results for the Coal Fly Ash using direct nebulization at 225.5 nm are
biased high by approximately 100 percent. A scan of the wavelengths adjacent
to, and including, the 225.5-nm wavelength was performed while aspirating the
39
-------
unspiked Coal Fly Ash digest. The results of this scan are shown in
Figure 14. Apparently, there is a spectral interference at the 225.5-nm
wavelength, probably caused by the presence of iron (see Interferences, this
section). For this reason, the Marine Sediment digest emissions were
monitored only at the 228.8-nm wavelength. Analysis of the unspiked Marine
Sediment digest by direct nebulization did result in a small positive signal.
This is probably the result of minor spectral interferences.
Interferences
Seven elements were investigated to determine i£ their presence caused
spectral interferences at the 225.5-nm and 228.2-nm wavelengths. The direct-
nebulization method was used for this interference study. The elements
investigated were Al, Ca, Ce, Cr, Fe, Mg, and Na. Cerium was included because
it may be used as an oxidant in some sample preparation methods (8). The
results of the interference study for the elements listed above are shown in
Table 12.
TABLE 12. SPECTRAL INTERFERENCE (APPARENT ug/L OSMIUM) CAUSED BY
1000 mg/L OF TESTED ELEMENTS AT 225.5 nm AND 228.2 nm
Apparent ug/L of Interference
Element 225.5 nm 228.2 nm
Al
2
2
Ca
ft
Ce
_ a
150
Cr
102 (380)b
5
Fe
104 (145)b
•
Mg
2
Na
*No interference was detected.
bPeak of interfering element just slightly off osmium analytical
wavelength.
As is apparent in Table 12 and Figures 15 and 16, the interfering effects of
iron and chromium are significant. The magnitude of these interferences
prohibits the use of the 225.5-nm wavelength for the analysis of soil samples
or high-level aqueous samples usirig the direct-nebulization method. At the
228.2-nm line, only cerium gave a significant positive interference. There-
fore, the use of this element as the oxidant in sample preparation is not
recommended for direct-nebulization ICP-AES analysis at the 228.2-nm
wavelength.
40
-------
i I > * ti k ¦+¦
N
n
cn
Q
U3
H
N
GO *
01
in
H
to m r>.
n cn
-------
* 1000 pg/L Fe
-• 0s Blank
+ 100 pg/L 0s scd.
>1
¦P
•rH
Ifi
G
a)
-p
c
H
iH
j
M
N
N
r
-------
>1
¦p
•H
W
C
Q)
C
H
CO
G
•H
C/D
* 1000 ug/L Cr
— 0s Blank
+ 100 Mg/L 0a standard
Nr^ntn«ro(a«-NCD'rcnir>'-iU3Mr>«»ntntoiD-^r«r-jm
• •••••••••••• • • •••••••«•«»•
ininininininininininininin ininintotDtDtouitototocouto
intnininininininininininin tnintnintninininininintninin
NNNNNNNMMNNN M N N N M N N N N N M M M N N
M r-I M M N N N M N M M f J CI ri t-l M N f-l Cv» C-l N N N t-J M CM N
Wavelength, nanometers x 10
Figure 16. Spectral interference by chromium at the osmium 225.5-nm
analytical vavelength during direct-nebulization ICP-AES.
A3
-------
The effects of increasing concentrations of HC1 (1 percent (v/v), 3
molar, and 6 molar) on a 1-mg/L osmium standard in a matrix of 1 percent (v/v)
each HN03 and H2S04 vere studied. Figures 17 and 18 show an increasing
negative interference with increasing hydrochloric acid concentration relative
to a 1-mg/L osmium standard in 1 percent (v/v) each HN03 and H2S04 without
HC1. This negative interference was the same at both analytical wavelengths.
It is possible that hydrochloric acid caused changes in the viscosity or
surface tension of the solution such that osmium transport to the plasma was
reduced (physical interference). The formation of larger aerosol droplets
during nebulization might also be responsible for this result.
SAMPLE PREPARATION METHODS
The results for the four methods of sample preparation investigated are
discussed individually below.
Method 7550 Aqueous Sample Preparation
Two aqueous matrices (TCLP extract of soil from a hazardous waste site
and Type II water) were spiked with 10 mg/L osmium and digested in duplicate
by the procedure given in Method 7550. The average recovery was 71 percent.
Flame AAS Method 7550 was used for the determination.
It was not known whether osmium, which might be reduced in solution,
would be oxidized by the Method 7550 procedure (1 percent nitric acid, 15
minutes heating). Ascorbic acid (200 mg/L) was used to reduce 1 mg/L osmium
in several test solutions. These solutions were digested by the procedure in
Method 7550 and analyzed by direct nebulization ICP-AES at the 225.2-nm
analytical wavelength. The results showed that osmium remained reduced; no
observable signal was detected when the digestion was repeated with 1 mL of
H SO added prior to, instead of after, heating.
2 4
Three strong oxidants, Ce+4, ammonium peroxydisulfate, and 30-percent
hydrogen peroxide, were tested to see if they could oxidize the reduced osmium
(as well as oxidize the excess ascorbic acid). These reagents were added
separately to 100-mL aqueous samples before digestion, along with the nitric
acid, at the following levels: 50 mg/L eerie ammonium nitrate; 100 mg/L
ammonium peroxidisulfate; and 1 mL 30-percent hydrogen peroxide.
The results of the analysis of these digests showed that both Ce+4 and
hydrogen peroxide oxidized the reduced osmium at this low level of heating;
osmium recovery was 82 percent. AmmonJ , peroxydisulfate, however, did not
fully oxidize the osmium at these temperatures, and the recovery was only
12 percent. It should be noted again that cerium gives a spectral interfe-
rence in the direct nebulize ion ICP-AES analysis of osmium at the 228.2-nm
wavelength.
44
-------
CP
3
¦X
c
a
-10
-20
1000
c \
o cn
33
i/i
C o
Oi
4.
O
o
Q
900 -
800 -
700
600 •
500
Figure 17. Suppression of 225.5-nm osmium signal in
direct-nebulization ICP-AES by HCl.
45
-------
500
135 HCI
3M HCI
6M HCI
Figure 18. Suppression of 228.2-nm osmium signal in
direct-nebulization ICP-AES by HCI.
46
-------
Method 3050 Solid Sample Preparation
Initial investigation of the Method 3050 digestion procedure for solid
samples shoved very low spike recoveries (less than 15 percent). A radio-
active tracer was used to determine at which steps in the digestion procedure
osmium losses were occurring. Radiotracer studies were performed at Scripps
Institute of Oceanography. The results of this work are presented in Table
13.
TABLE 13. RETENTION OF RADIOACTIVE 1#50SMIUM DURING DIGESTION
OF SOLID SAMPLES BY METHOD 3050
Percent 1850s Remaining After Treatment With:
Sample
HN03* HjOjh
HClc
Taken to Drynessd
LCS (0287)'
66±11
53 + 13
41+6
7±2
1633Af
29±5
28±6
27±5
4+2
Municipal'
Digested Sludge
76+9
60±9
57±10
48+10
MESS-lf
14 + 3
66+10
62+10
55+11
Cu-Ni-Cof
s
58+15
57±15
35±5
Reference Ore
'After step 7.27
bAfter step 7.4.
cAfter step 7.5.
dNot in procedure. Taken to dryness to determine loss of osmium.
'Average of 4 digestions.
fAverage of 2 runs.
From these results, it is apparent that most of the osmium (>30 percent)
is lost, probably through volatilization, during the first step of nitric acid
addition. Comparatively little osmium (>13 percent on average) is lost during
the subsequent addition of H202. After the addition of HC1, the amount of
osmium lost was very small for some samples, and as much as 22 percent for the
Cu-Ni-Co Reference Ore. All samples, with the exception of the Municipal
Digested Sludge, lost significant amounts of osmium when taken to dryness.
The anomalous results for the Municipal Digested Sludge may perhaps be
explained by this sample's very high organic content. Osmium in this
digestion might be, in part, reduced by organic matter that has not been
47
-------
destroyed by the acids and HO,. The consistently low recoveries of osmium
for the Coal Fly Ash, NBS-SRH l633a, might be explained by its relative lack
of reduced species. Further investigation is needed to determine vhether the
above speculations are indeed true.
Sodium Peroxide Fusion Method for Solid Samples
Fusion of 0.5 grams of solid sample with 5 grams sodium peroxide gave
good results, with osmium recoveries averaging 80 percent when a solid spiking
material was used (Spectromel No. 2 Standard Powder Mixture containing 12.5
percent osmium metal). The recovery was only 20 percent; however, when a
dissolved osmium tetrachloride standard was added to the solid sample and then
allowed to dry before the fusion.
Pressure-Bomb Method
Initial investigations showed that sample digestion using a pressure bomb
yielded good (>85 percent) osmium spike recovery for both liquid and solid
samples. Two sets of samples were digested using this method. One set,
composed of the nine samples, was spiked with 10 mg/L dissolved osmium
chloride, pressure-bomb digested, and analyzed using flame AAS Method 7550. A
second set of two sample matrices (Coal Fly Ash, NBS-SRM 1633a and Marine
Sediment, MESS-1) was spiked with 10 ug/L osmium, pressure-bomb digested, and
analyzed by ICP-AES, using direct nebulization and batch volatilization.
The first set, composed of the nine samples, was prepared in triplicate,
spiked prior to digestion with 10 mg/L osmium, and pressure-bomb digested.
The results of analysis obtained by Method 7550 are presented in Table 4. As
stated previously in Section 2, spike recoveries were good. They ranged from
84 percent to 98 percent, with an average recovery of 91 percent. These
sample digests were stored in glass containers and subsequently used in the
Concentration-Stability Study, which is discussed below.
The results from the analysis of the second set of samples by ICP-AES
methods have been stated previously in the spike-recovery portion of the ICP-
AES Methods section heading above.
CONCENTRATION-STABILITY STUDY
The stability of the osmium concentration in pressure-bomb digests of the
nine samples was evaluated over a 3-week period. The osmium concentration in
each of the solutions was measured twice weekly by using flame AAS Method
7550. As shown in Table 14, osmium remains stable in the sample digests, when
stored in glass containers under normal laboratory conditions, for at least 3
weeks; all concentration values stayed within 10 percent of the initial
values.
48
-------
TABLE 14. OSMIUM CONCENTRATION (mg/L) BY FLAME AAS OVER
A 3-WEEK PERIOD IN DIGESTS OF THE 9 MATRICES
Day
Matrix 0 4 7 11 14 18 21
(measured as mg/L)
Trace Elements in Water
9.61
9.80
9.79
9.41
9.56
9.71
9.
.95
Osmium Standard Solution
9.35
9.31
9.37
8.90
9.19
9.34
9.
.42
Synthetic Interference Solution
9.83
9.17
9.66
9.52
a
9.80
9.
80
TCLP Extract of Hazardous Waste
9.56
8.85
9.28
9.22
«
9.38
9.
62
Coal Fly Ash
9.35
8.87
8.51
8.65
8.97
8.81
8.
.76
Municipal Digested Sludffe
8.35
8.63
8.35
8.45
d.40
8.84
8.
.32
Marine Sediment
8.81
8.99
8.93
8.97
8.63
8.78
8.
.65
Laboratory Control SampLe
8.49
9.33
9.08
8.87
8.36
8.69
8.
.97
Hazardous Waste Site So111
8.62
8.92
8.21
8.42
8.10
8.57
8,
.46
"Not determined.
-------
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13. Summerhays, K.D., P.J. Lamonthe, and T.L. Fries. Volatile Species in
Inductively Coupled Plasma Atomic Emission Spectroscopy: Implications
for Enhanced Sensitivity. Appl. Spectrosc., 37(1)125—28f 1983.
14. Cotton, F.A., and G. Wilkinson. Advanced Inorganic Chemistry, A
Comprehensive Text. Interscience Publishers, New York, New York. 1972.
866 pp.
15. Bazan, J,M. Enhancement of Osmium Detection in Inductively Coupled
Plasma Atomic Emission Spectrometry. Anal. Chem., 59:1066-1069, 1987.
16. Winge, R.K., V.J. Peterson, and V.A. Fassel. Inductively Coupled Plasma
Emission Spectroscopy: Prominent Lines. Appl. Spectrosc., 33(3):
206-219, 1979.
17. Horwitz, W., ed. Statistical Manual of the AOAC. The Association of
Official Analytical Chemists, Arlington, Virginia, 1975. 126 pp.
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APPENDIX
RECOMMENDED CHANGES TO METHOD 7550
METHOD 7550
It is recommended that the existing Method 7550 be revised as follows:
2.0 SUMMARY OF METHOD
2.1 Replace this section with:
Prior to analysis by Method 7550, samples must be prepared for
direct aspiration. All sample types (aqueous, sludge, oils, greases
and waxes) should be digested by the pressure-bomb digestion
procedure discussed in this method.
3.0 INTERFERENCES
A new section, 3.5, should be created. It should state the following:
3.5 Samples that contain high concentrations of chromium will exhibit a
positive interference.
5.0 REAGENTS
5.4 The matrix of the standard solutions should be changed from "IX
(v/v) HNO and IX (v/v) H SO " to "AX (v/v) HNO- and AX (v/v)
H SO ."
2 4
6.0 SAMPLE COLLECTION, PRESERVATION, AND HANDLING
6.2 This section states that plastic and glass containers are suitable.
This should be changed to:
Teflon bottles should be avoided because of absorption of osmium.
7.0 PROCEDURE
7.1 and 7.2 These sections should be completely revritten as follows;
7.1 Sample preparation: All samples should be digested according to the
bomb digestion method described herein. The bomb vessel size for
the following procedure is 23 mL. As the bomb vessel size
increases, the sample and reagent portions may also be increased
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accordingly. Follow all manufacturer's directions on the proper use
and assembly of the bomb.
7.2 Pressure-Bomb Digestion of Samples
7.2.1 For aqueous samples, transfer a 10-mL aliquot of the well-mixed
sample to the bomb vessel. For sludges, oils, gteases, or
waxes, weigh out 0.100 g of the sample and transfer to the bomb
vessel.
7.2.2 For aqueous samples, add 2 mL HN0s to the bomb vessel and close
properly. For solid samples, add 2 mL HN03 to the bomb vessel
and close properly.
7.2.3 Place bombs.,with aqueous samples in a muffle furnace at 150° C
for 2 hours Place bombs with solid samples in a muffle
furnace at 50° C for A hours.
7.2.4 After the bombs have been allowed to cool, carefully transfer
the entire contents of each into a 50-mL volumetric flask. Do
not filter the solutions. Add 2 mL H2S04 and bring to volume
with deiomeed water.
7.3 This section stdftes that a wavelength of "290.0-nm" should be used.
This wavelength is incorrect. The wavelength should be changed to
"290.9-nm".
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