United States
Environmental Protection
Agency
Research and Development
Environmental Monitoring
and Support Laboratory
P.O. Box 15027
Las Vegas NV89114
EPA-600/7-79-081
March 1979
£EPA
Acid Dissolution Method
or ®
sJ
In Sol
Evaluation of an Interlaboratory
T
rison with Results of a
Interagency
Energy-Environment
Research
and Development
Program Report
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental
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3. Ecological Research
4. Environmental Monitoring
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EPA-600/7-79-081
March 1979
ACID DISSOLUTION METHOD FOR THE ANALYSIS OF PLUTONIUM IN SOIL
Evaluation of an interlaboratory collaborative test
and comparison with results of a fusion method test
by
E. L. Whittaker and G. E. Grothaus
Quality Assurance Branch
Monitoring Systems Research and Development Division
Environmental Monitoring and Support Laboratory
Las Vegas, Nevada 89114
ENVIRONMENTAL MONITORING AND SUPPORT LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
LAS VEGAS, NEVADA 89114
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DISCLAIMER
This report has been reviewed by the Environmental Monitoring and Support
Laboratory, U.S. Environmental Protection Agency, and approved for publication.
Mention of trade names or commercial products does not constitute endorsement
or recommendation for use.
ii
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FOREWORD
Protection of the environment requires effective regulatory actions which
are based on sound technical and scientific information. This information must
include the quantitative description and linking of pollutant sources, trans-
port mechanisms, interactions, and resulting effects on man and his environment.
Because of the complexities involved, assessment of specific pollutants in the
environment requires a total systems approach which transcends the media of
air, water, and land. The Environmental Monitoring and Support Laboratory-Las
Vegas contributes to the formation and enhancement of a sound monitoring data
base for exposure assessment through programs designed to:
• develop and optimize systems and strategies for moni-
toring pollutants and their impact on the environment
• demonstrate new monitoring systems and technologies by
applying them to fulfill special monitoring needs of
the Agency's operating programs -
This report presents the results of an Interlaboratory Collaborative test
of an acid-dissolution method for measuring the plutonium concentrations in
soil type samples when the plutonium on the soil is in a very insoluble refrac-
tory form. Those results are compared with the analytical results of a clas-
sical fusion method. The purpose of the comparison is to show equivalency of
two methods, one being a shorter and less expensive method. Since the chemical
form of plutonium in the environment is not always predictable, this report
should be of interest to all analysts and others responsible for monitoring
soil for plutonium contamination. For further information contact the Quality
Assurance Branch of the Monitoring Systems Research and Development Division of
the EMSL-Las Vegas.
GeorgerB. Morgan
Director
Environmental Monitoring and Support Laboratory
Las Vegas
iii
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ABSTRACT
The data from an interlaboratory collaborative test are presented. A
statistical analysis of the data is also presented. From that analysis,
statements are made of the combined within-laboratory precision, the
systematic error between laboratories, the total error between laboratories
based on a single analysis, and the method bias.
Soil samples used for the test contained plutonium in a highly
refractory form, a very insoluble form, and therefore, difficult to measure
the true concentration. Plutonium concentrations in those samples ranged
from 0.1 to 10 dpm/g of soil.
A comparison is made between the acid dissolution method and a
fluoride-pyrosulfate fusion method which was tested in a similar study
using the same test samples.
±V
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CONTENTS
Page
FOREWORD iii
ABSTRACT iv
LIST OF TABLES vi
INTRODUCTION 1
SUMMARY 1
CONCLUSIONS AND RECOMMENDATIONS 2
COLLABORATIVE TEST - CALCULATIONS AND DATA 3
REFERENCES 6
APPENDIX 23
Tentative Method For The Analysis of Plutonium-239 and Plutonium-238
In Soil (Acid Dissolution Technique)
-------
LIST OF TABLES
Page
Table 1. Acid Dissolution Method, Test Sample H 7
Table 2. Acid Dissolution Method, Test Sample F 8
Table 3. Acid Dissolution Method, Test Sample E 9
Table 4. Acid Dissolution Method, Test Sample G (G-9') 10
Table 5. Acid Dissolution Method, Test Sample G (G-8) 11
Table 6. Summary of Results - Acid Dissolution Method 12
Table 7. Fusion Method, Test Sample A 13
Table 8. Fusion Method, Test Sample B 14
Table 9. Fusion Method, Test Sample C 15
Table 10. Fusion Method, Test Sample D (D-9) 16
Table 11. Fusion Method, Test Sample D (D-8) 17
Table 12. Summary of Results - Fluoride - Pyrosulfate Fusion Method 18
Table 13. Summary of Precision Data, Acid Dissolution Method 19
Table 14. Summary of Precision Data, Fluoride - Pyrosulfate 19
Fusion Method
Table 15. t-Test to Detect Method Bias, Acid Dissolution Method 20
Table 16. t-Test to Detect Method Bias, Fluoride - Pyrosulfate 20
Fusion Method
Table 17. Comparison of Results for Two Laboratories that 21
Participated in Both Methods
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INTRODUCTION
In response to the need for the U.S. Environmental Protection Agency
(EPA) to promulgate a guideline for the protection of people against
harmful health effects resulting from soil contaminated with plutonium,
a method of analysis has been tested and published (Hahn et al, 1977).
To ensure the dissolution of very insoluble refractory forms, that
method (a sequential fluoride pyrosulfate fusion method) is necessarily
a tedious and rigorous exercise in analytical chemistry which, for large
numbers of samples, would involve considerable time and expense. It was
economically expedient then that another method be selected and tested
which required less time and cost of materials. The method selected (an
acid dissolution method) had been developed by the U.S. Atomic Energy
Commission (now The Department of Energy) as its regulatory guide for
the measurement of plutonium in soil (AEC Regulatory Guide 4.5, May
1974). This method is described in the Appendix to this report.
The results of an interlaboratory collaborative test of the acid
dissolution method and a comparison with results of the fusion method
are reported here. A statement is made of the equivalency of the two
methods.
SUMMARY
The method in detail form, as tested, and as given in this report
(Appendix), was documented by the Methods Development and Analytical
Support Branch of the Monitoring Systems Research and Development Division
of the Environmental Monitoring and Support Laboratory (EMSL-Las Vegas).
The interlaboratory collaborative test was conducted by the Quality
Assurance Branch of the same Division.
The AEC 4.5 method is an acid dissolution method, considerably
simpler in detail than the sequential fluoride-pyrosulfate fusion method,
and is widely used. A concern in the selection of the acid dissolution
method, and the reason for a comparative study of the two methods, was
whether or not the acid dissolution method would dissolve the highly
refractory forms of plutonium and thereby account for such forms in the
analysis. It is required that an EPA recommended method account for the
insoluble forms of plutonium since the chemical form of environmental
plutonium cannot always be predicted. It is believed by a number of the
users of the acid dissolution method that it will account for the highly
-------
refractory forms of plutonium.
The reference soil materials that were prepared (by AEC's Health Services
Laboratory at Idaho Falls, Idaho) for the interlaboratory collaborative test of
the sequential fluoride-pyrosulfate fusion method were also used to test the
acid dissolution method. See Hahn et al, 1977, Appendix page A-24 for the
procedure for standard soils preparation. The identification codes of the
reference soil samples for the test of the acid dissolution method were changed
from what they were for the test of the fusion method.
Of 37 laboratories invited to participate in the collaborative test of
the method, 15 indicated they would participate. Reference samples and stand-
ards were sent to the 15 laboratories. Analytical data were returned by 6
laboratories.
Summary data are also given for the test of the fusion method for a
comparison of the results of the two methods.
CONCLUSIONS AND RECOMMENDATIONS
The question of the ability of the acid dissolution method to account for
the highly refractory forms of plutonium has been answered in this study. Had
the method failed to dissolve those refractory forms known to exist in the
samples, the method would have shown a definite low bias. A comparison of the
grand average values with the respective known values in Table 6 indicates that
there is a positive but not serious bias in the results of the method. Also,
in Table 15 the calculated values of t are all lower than the critical values
of t, indicating no serious bias in the method for the range of plutonium
activity of 0.1 - 10 disintegrations per minute per gram (dpm/g) of soil.
Analytical results of labs 4 and 5 were not used in the statistical
analysis of the data in Tables 6, 13, and 15. Lab 5 data were rejected because
3 of the 4 mean values of the replicate data reported by Lab 5 failed the
"extreme mean" test (Steiner, 1975). Lab 4 data were rejected because the data
failed the "Ranked Results" test (Youden, 1975). It is obvious from observing
the data in Table 6 that Labs 4 and 5 had a definite systematic error in their
analytical effort in this study.
No elaborate comparison can be made here between the acid dissolution
method and the fluoride-pyrosulfate fusion method because of the limited
number of collaborators in the two studies. However, there is sufficient
evidence to conclude that both methods are adequate methods for monitoring
soil for plutonium. The data for the coefficients of variation (%) in Tables
13 and 14 indicate that the fusion method gave better precision than the acid
dissolution method. A comparison of the known values to the grand average
values in Tables 6 and 12 shows that the fusion method gave a little better
accuracy than the acid dissolution method.
-------
It should be noted that two of the laboratories that participated in the
fusion method also participated in the acid dissolution method. Lab 1 of the
acid dissolution method is lab 4 of the fusion method and lab 6 of the acid
dissolution method is lab 5 of the fusion method. Table 17 gives a comparison
of the results of those labs with the two methods. Lab 1 got notably better
results with the acid dissolution method than it did (lab 4) with the fusion
method. Lab 5 got slightly better results with the fusion method than it did
(lab 6) with the acid dissolution method.
The acid dissolution method is a somewhat shorter method than the fusion
method. One analyst made the observation that the acid dissolution method
required only about one half of the time that was required for fusion method.
For the sake of economics then, the acid dissolution method would be the one of
choice, although other factors favor the fusion method.
Since neither method shows a significantly low bias, as might be antici-
pated by the knowledge of the refractory form of the plutonium activity in the
samples, both methods are adequate for monitoring environmental soil for
plutonium. Insufficient data in this study tend to magnify those indicated
differences as shown by the data reported. A larger set of data from more
participating laboratories would likely diminish those differences somewhat.
The two methods, in the opinion of the authors, are essentially equivalent
methods.
COLLABORATIVE TEST - CALCULATIONS AND DATA
In the interlaboratory collaborative test of the acid dissolution method,
reference samples H, F, E and G are the same sample materials as samples A, B,
C and D respectively in the fluoride-pyrosulfate fusion test. The plutonium-
239 activity levels are 0.1 dpm/g (A/H), 1.0 dpm/g (B/F; C/E), and 10 dpm/g
(D/G) in the reference samples. The plutonium-238 activity levels are only
1/65 of the plutonium-239 amounts and therefore only samples D/G are high
enough to be measured (0.1 dpm/g level). In the tables, the activities given
for samples H, F, E/A, B, C are plutonium-239 activities and the plutonium-238
and plutonium-239 activities are indicated separately for samples D/G by the
designations D-8, D-9/G-8, G-9.
I
Standard solutions of plutonium-239, -238 and plutonium-236 were sent to
each participating laboratory along with 35g of each reference soil material so
that all labs could be calibrated to the same standard. The detailed acid
dissolution method (Appendix) contains a section on calibration and standard-
ization (as did the fusion method). Therefore no subsequent effort was made to
ensure that all labs were in fact calibrated to the same standard.
Individual analytical results for replicate samples for the six labs are
tabulated by sample designation in Tables 1-5. Also_included in the tables for
each lab are the averages of the replicate results, X the experimental
-------
(within lab) standard deviation, S.; and the ratio of the average value to the
known value. )(• is simply the arithmetic mean of the replicate results of each
lab. S- is a measure of the precision for each lab for a given sample and is
calculated from Equation 1.
TT \ 2
(1)
where X- = the individual results reported by Lab i
"X. = the mean of the individual results for Lab i
n. = the number of replicates reported by Lab i
Table 6 gives a summary of the data from Tables 1-5. It lists the mean values
according to laboratory and sample. It gives a ranking of the mean values
according to laboratory and sample and a sum score for each laboratory (Youden
1975) . For six labs and five samples the lower and upper limits for the
ranking score are 7 and 28 respectively. Such a ranking is used to identify
laboratories which show a pronounced systematic error or bias. Table 6 pre-
sents the grand average of the mean values from four laboratories for each
sample. Equation 2 (Steiner, 1975) was used to determine any extreme means in
the data.
Xn - X(n-l)
where X = the highest (suspect) value
X, _, » = the next highest value
X-, = the lowest value
For low values the numerator in Equation 2 would be the lowest (suspect) value
subtracted from the next lowest value.
Table 6 also lists three specific standard deviations for each sample which are
needed to evaluate the method as to the limits of error that can be expected
when a typical group of analysts use the method. Those standard deviations are
S / the combined within laboratory standard deviation; S, , the standard devi-
ation of the 4-lab mean value data for each sample; and S, , the standard
deviation of the systematic errors. Sr is calculated from Equation 3.
m n.
(Ini-m) Sr2 = Z I1 (X-^Xi)2 (3)
where n. = the number of replicate analysis by Lab i
m = the number of labs
X- = the individual results for Lab i
"X. = the mean value for the individual results of Lab i
S is calculated from Equation 4.
-------
sd = tz(xi ~ x)2/ to-1* I*
where )( = the grand average of the lab mean values
"X. = the mean value for the individual results of Lab i
m = the number of labs
S^ is calculated from Equation 5. (Youden 1975) .
S = (S2 - S
where K = the number of replicate analyses on a given sample by the
participating labs.
g
b values in Table 6 were calculated using K = 3. However, in labs where
routine soil analyses are performed, a single analysis is apt to be the usual
extent of plutonium determination. Therefore the precision for single analysis
needs to be determined. For that determination S, needs to be recalculated for
single analysis in place of triplicate analyses. This was done using Equation
5 using K = 1 and rearranging the equation to
Sd ' Sb2+ Sr2
Sd =
-------
Comparing the grand average values to the known values for each sample in
Table 13 shows that there is a positive bias but not a serious bias to the data
of the acid dissolution method. However, the data were subjected to the t-test
(test for systematic error in a method) for all of the samples (representing
three levels of plutonium activity) by using Equation 7 (Youden 1975).
t = (X - R) m /sd' m~l degrees of freedom (7)
where x = tne 9rand average value for each sample
R = the known value
m = the number of collaborators
S = the standard deviation of the data (taken from Table 6)
For comparison, results of the t-test are presented in Table 15 for the acid
dissolution method and in Table 16 for the fusion method.
All of the data for the tables for the fusion method was treated by the
same calculations as for the acid dissolution method. The mean value of
sample B for Lab 6 was rejected and not used in the statistical analysis for
sample B because it failed the extreme mean test. That was the only mean value
of Lab 6 that failed the test, therefore the other mean values for Lab 6 are
included in the statistical analysis for the respective samples.
REFERENCES
1. Hahn, P. B., E. W. Bretthauer, P. B. Altringer, and N. F. Mathews.
"Fusion Method for the Measurement of Plutonium in Soil: Single-
Laboratory Evaluation and Interlaboratory Collaborative Test," EPA-600/7-
77-078, July 1977.
2. U.S. Atomic Energy Commission Regulatory Guide 4.5, May 1974. "Measure-
ments of Radionuclides in the Environment - Sampling and Analysis of
Plutonium in Soil."
3. Youden, W. J. 1975. "Statistical Techniques for Collaborative Tests,"
Statistical Manual of the AOAC, Association of Official Analytical Chem-
ists, Washington, D. C.
4. Steiner, E. H. 1975. "Planning and Analysis of Results of Collaborative
Tests." Statistical Manual of the AOAC, Association of Official Analytical
Chemists, Washington, D. C.
5. Annual Book of ASTM Standards, Part 31, 1977.
-------
Table 1
Acid Dissolution Method
Test Sample H, PU-239
(Known Value: 0.113 +_ 0.001 dpm/g)
,ab
1
2
3
4
5
6
Lab Values
(dpm/g)
0,103
0.122
0.117
0.146
0.150
0.148
0.146
0.120
0,135
0.114
0.130
0.008*
8.70
1.9
7.09
0.173
0.182
0.163
Xi
(dpm/g)
0.114
0.148
0.134
0.122
5.90
0,173
Si
(dpm/i
0.010
0.002
0.013
0.011
3.55
0.010
Ratio
1.01
1.31
1.19
1.08
52.2
1.53
*Rejected by T-test (ASTM D-2777) for 3 observations and 5% significance.
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Table 2
Acid Dissolution Method
Test Sample F, PU-239
(Known Value: 0,901 + 0,003 dpm/g)
Lab Lab Values x- s- _ Ratio
(dpm/g) (dpm/g) (dpm/g) x/Known
1 0,929
0.944 0.894 0.073 0.99
0.810
2 1.30
1.11 1.23 0.104 1.37
1.28
3 1.04
0.965 0.985 0.044 1.09
0.954
4 0.503
0.303 0.422 0.105 0.47
0.460
5 4.85
4.07 4.48 0.391 4.97
4.51
6 1.05
0.999 0.980 0.078 1.09
0.894
8
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Table 3
Acid Dissolution Method
Test Sample E, PU-239
(Known Value: 1.03 + 0.01 dpm/g)
Lab Lab Values Xi s- _ Ratio
(dpm/g) (dpm/g) (dpm/g) x/Known
1 0.977
1.02 1.02 0.047 0.99
1.07
2 1 .72
1.37 1.53 0.18 1.49
1 .51
3 1.19
1.23 1.20 0.023 1.16
1.19
4 0.656
0.202 0.457 0.23 0.444
0.512
5 5.07
5.46 5.22 0.21 5.07
5.14
6 1.19
1.17 1.15 0.048 1.12
1 .10
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Table 4
Acid Dissolution Method
Test Sample G, PU-239 (G-9)
(Known Value: 8.99 + 0.03 dpm/g)
Lab Lab Values x- s- _Ratio
(dpm/g) (dpm/g) (dpm/g) x/Known
1 8.72
8.84 8.78 0.061 0.98
8.80
2 13.0
13.0 12.9 0,23 1 .43
12.6
3 10.5
9.73 9.95 0.49 1.11
9.60
4 2.61
3.01 2.95 0.31 0.33
3.23
5 4.74
4.52 4.61 0.11 0.51
4.58
6 10.0
10.1 9.77 0.54 1.09
9.15
10
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Table 5
Acid Dissolution Method
Test Sample G, PU-238 (G-8)
(Known Value: 0.1381 0.001 dpm/g)
Lab Lab Values x- s- _ Ratio
(dpm/g) (dpm/g) (dpm/g) x/Known
1 0.159
0.161 0.159 0.002 1.15
0.157
2 0.179
0.206 0.188 0.016 1.36
0.179
3 0.155
0.152 0.151 0.005 1.09
0. 145
4 0.155
0.036 0.076 0.069 0.55
0.036
5 ND
ND
ND
6 0.126
0.148 0.128 0,019 0.93
0.110
11
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Lab
Table 6
Summary of Results - Acid Dissolution Method
Ranked Results
G-8
H
G-9
G-8
SCORE
1
2
3
4a
5a
6
0.114
0.148
0.134
0.122
5.90
0.173
0.894
1.23
0.985
0.422
4.48
0.980
1.02
1.53
1.20
0.457
5.22
1.15
8.78
3,2.9
9.95
2.95
4.6?,
9.77
0.3,59
0.3,88
0.153.
0.076
0.128
1
4
3
2
6
5
2
5
4
1
6
3
2
5
4
1
6
3
3
6
5
1
2
4
4
5
3
1
(3)
2
12
25
19
6
23
17
Known
X
Sd
Sr
Sb
0.3,38
0.156
0.0327
0.0126
0.0319
The limits for acceptance by the score of
the ranked results are 7-28. Lab 4 re-
sults were therefore rejected in the
statistical analysis.
Labs 4 and 5 data were not used in the statistical analysis.
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Table 7
Fusion Method
Test Sample A; PU-239
(Known Value: 0.113 +_ 0.001 dpm/g)
Lab Lab Values x- si _Ratio
(dpm/g) (dpm/g) (dpm/g) X/Known
1 0.178
0.089 0.111 0.059 0.98
0,067
2 0.12
0.09 0.103 0.015 0.91
0.10
3 0.223
0.157 0.180 0.037 1.59
0.160
4 0.136
0.14(0 0.138 0.003 1.22
5 0.13
0.13 0.123 0.012 1.09
0.11
6 0.12
0.09 0.103 0.015 0,91
0.10
13
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Table 8
Fusion Method
Test Sample B, PU-239
(Known Value: 0.901 + 0.003 dpm/g)
Lab Lab Values x- S. _Ratio
(dpm/g) (dpm/g) (dpm/g) x/Known
0.844
1.04 0.932 0.101 1.03
0.910
0.75
0.88 0.813 0.065 0.90
0.81
0.887
0.857 0.881 0.022 0.98
0.899
0.94
0.98 0.947 0.031 1.05
0.92
0.82
0.83 0.840 0.026 0.93
0.87
0.40
0.58 0.490 0.127 0.54
14
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Table 9
Fusion Method
Test Sample C, PU-239
(Known Value: 1.03 +_ 0.01 dpm/g)
Lab Lab Values x- s- _Ratio
(dpm/g) (dpm/g) (dpm/g) x/Known
1 1 .11
1.09 1.11 0.022 1.08
1. 13
2 0.89
0.91 0.92 0.042 0.89
0.97
3 1.00
1.04 1.02 0.018 0.99
1.02
4 1 .10
1.05 1.05 0.050 1.02
1 .00
5 0.90
0.94 0.92 0.028 0.89
6 0.78
0.71 0.72 0.060 0.70
0.66
J.5
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Table 10
Fusion Method
Test Sample D, PU-239 (D-9)
(Known Value: 8.99 +_ 0.03 dpm/g)
Lab Lab Values X- s- _Ratio
(dpm/g) (dpm/g) (dpm/g) x/Known
1 11.3
9.32 9.98 1.14 1.11
9.32
2 8.4
8.4 8.23 0.29 0.92
7.9
3 8.21
8.22 8.18 0.058 0.91
8.12
4 9.71
9.17 9.39 0.28 1.04
9.29
5 7.79
8.26 8.02 0.33 0.89
6 7.03
6.65 6.66 0.36 0.74
6.30
16
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Table 11
Fusion Method
Test Sample D, PU-238 (D-8)
(Known Value: 0.138 +_ 0.001 dpm/g)
Lab Lab Values x- S. Ratio
(dpm/g) (dpm/g) (dpm/g) x/Known
0.244
0.133 0.185 0.056 1.34
0. 178
0.12
0.12 0.113 0.012 0.82
0.10
ND
0.493
ND
0.166
0.187 0.173 0.012 1.25
0.167
0.15
0.13 0.140 0.014 1.01
0.16
0.13 0.130 0.030 0.94
0.10
17
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Lab
Summary of Results -
Average Values (Xj» dPm/g)
B
Table 12
Fluoride - Pyrosulfate Fusion Method
D-9
D-8
Ranked Results
B C D-9
D-8
SCORE
1
2
3
4
5
6
0.111
0.103
0.180
0.138
0.123
0.103
0.932
0.813
0.881
0.947
0.840
0.490a
1.11
0.92
1.02
1.05
0.92
0.72
9.98
8.23
8.3,8
9.39
8.02
6.66
0,185
0.113
0.173
0.140
0.130
3
1.5
6
5
4
1.5
5
2
4
6
3
1
6
2.5
4
5
2.5
1
6
4
3
5
2
1
5
1
(3)
4
3
2
25
11
20
25
14.5
6.5
00
Known
"x
Sd
\JL
s
r
Sv,
0.113
0.126
0.0295
0.0315
0.0232
0.901
0.883
0.0576
0.0571
0.0472
1.03
0.957
0.138
0.0407
0.136
8.99
8.41
1.160
0.550
1.120
0.3.38
0.148
0.0300
0.0418
0.0178
The limits for acceptance by the score
of the ranked results are 7-28. Lab 6
score is close enough to 7, therefore
was not rejected.
aThis mean value was not used in the statistical analyses because it failed the extreme mean test.
-------
Table 13
Summary of Precision Data
Acid Dissolution Method
Coefficients of Variation (%)
Sample
H
G-8
F
E
G-9
Known
(dpm/g)
0.113
0.138
0.901
1.03
8.99
X
(dpm/g)
0.142
0.156
1.02
1.22
10.35
within
lab
6.7
8.1
7.7
7.8
3.6
between
labs
17.4
20.4
19.2
25.2
22.8
Total
(single analysis)
18.7
22.0
20.7
26.4
23.1
Table 14
Summary of Precision Data
Fluoride - Pyrosulfate Fusion Method
Coefficients of Variation (%)
Sample
A
D-8
B
C
D-9
Known
(dpm/g)
0.113
0.138
0.901
1.03
8.99
X
(dpm/g)
0.126
0.148
0.883
0.957
8.41
within
lab
25.0
28.2
6.5
4.3
6.5
between
labs
18.4
12.0
5.3
14.2
13.3
Total
(single ana!
31.0
30.7
8.4
14.8
14.8
19
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Table 15
t-Test to Detect Method Bias
Acid Dissolution Method
Sample
H
G-8
F
E
G-9
m
4
4
4
4
4
t-calc.
t-crit.
2.29
1.10
1.18
1.21
1.15
3.18
3.18
3.18
3.18
3.18
Table 16
t-Test to Detect Method Bias
Fluoride - Pyrosulfate Fusion Method
Sample
A
D-8
B
C
D-9
m
6
5
5
6
6
t-calc.
t-crit.
1.08
0.75
0.70
1.30
1.23
2.57
2.78
2.78
2.57
2.57
20
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Table 17
Comparison of Results for Two Laboratories
that Participated in Both Methods
% Deviation from the Known Value
Sample Lab 1 Lab 4 Lab 6 Lab 5
(dpm/g) Acid Dissolution Fusion Acid Dissolution Fusion
.113 0.88 (+) 22.1 (+) 53.1 (+) 8.8 (+)
.901 0.78 (-) 5.1 (+) 8.8 (+) 6.8 (-)
1.03 0.97 (-) 1.9 (+) 11.6 (+) 10.7 (-)
8.99 2.3 (-) 4.4 (+) 8.7 (+) 10.8 (-)
.138 15.2 (+) 25.4 (+) 7.2 (-) 1.4 (+)
U.S EPA Headquarters Library
Mgi! cooe 3404T
snnsyivar.ia Ave
shiagtor,. DC 2C
202-566-0556
_, i«w '-i-ive o«fyf '<
1200 Pennsylvania Avenue NW
Washington, DC 20460
-------
APPENDIX
TENTATIVE METHOD FOR THE ANALYSIS
OF PLUTONIUM-239 AND PLUTONIUM-238 IN SOIL
(Acid Dissolution Technique)
Edited by
Paul B. Hahn, Neil F. Mathews,
Erich W. Bretthauer
Monitoring Systems Research
and Development Division
Environmental Monitoring
and Support Laboratory
Las Vegas, Nevada 89114
The method described in this Appendix was
distributed to the participating laboratories
for the interlaboratory collaborative test.
U.S. ENVIRONMENTAL PROTECTION AGENCY
OFFICE OF RESEARCH AND DEVELOPMENT
ENVIRONMENTAL MONITORING AND SUPPORT LABORATORY
LAS VEGAS, NEVADA 89114
23
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PREFACE
The analytical method described in this document was
developed by the U.S. Atomic Energy Commission as their
Regulatory Guide Method for the measurement of plutonium in soil
The method was selected for collaborative testing on the
basis of both theoretical considerations and its widespread
use to determine its suitability as an Environmental Protection
Agency Reference Method. Data from the collaborative tests
will be used to determine, on a statistical basis, the limits
of error which can be expected when the method is used by a
typical group of analysts.
24
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CONTENTS
Preface
Sections
1 Scope and Application
2 Summary
3 Interferences
4 Apparatus
4.1 Instrumentation and Accessories
4.2 Laboratory Equipment
4.3 Labware
5 Standards, Acids, Reagents
5.1 Standards
5.2 Acids
5.3 Reagent s
6 Calibration and Standardization
6.1 Calibration of the 2?r Alpha Counter
and the Alpha Spectrometer
6.2 Purification of the Plutonium-236 Tracer
6.3 Standardization of the Plutonium-236 Tracer
7 Step by Step Procedure for Analysis
i
7.1 Sample Decomposition
7.2 Sodium Hydroxide Precipitation
7.3 Ammonium Hydroxide Precipitation
7.4 Ion Exchange Separations
7.5 Electrodeposition
7.6 Quality Assurance Program
25
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8 Calculation of Results
8.1 Calibration of the 2ir Alpha Counter
8.2 Standardization of the Plutonium-236 Tracer
8.3 Calculation of Plutonium Concentrations
in the Aliquot of Soil Taken for Analysis
8.4 Calibration of the Alpha Spectrometer and
Calculation of the Plutonium Recovery
8.5 Propagation of Uncertainties
Sample Calculations
References
Figure 1 Disposable Electrodeposition Cell and Support
26
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SOIL ANALYSIS PROCEDURE
1. SCOPE AND APPLICATION
1.1 This method covers the analysis of soils for plutonium
at levels greater than 0.01 dpm/g in a variety of chemical and
physical forms known to exist in soils encountered in the United
States. The method is expected to adequately handle soils
containing plutonium in a non-refractory form and there is
documented evidence that the method should also be applicable
to soils containing certain refractory forms of plutonium
(U.S. Atomic Energy Commission Regulatory Guide 4.5, May 1974),
(Sill 1975b).
1.2 The minimum detection level (MDL) of the method will
depend on both the background counting rate of the alpha
spectrometer and the amount of plutonium-238 and plutonium-239
contamination in the plutonium-236 tracer. Plutonium-236
having only a few hundredths percent of plutonium-238 and
plutonium-239 contamination is now commercially available and
is recommended for this procedure. For an analysis of 10 g of
soil, employing 10 dpm of plutonium-236 tracer, a 1000-minute
counting time on a spectrometer having a 17% counting efficiency
and a 0.010-cpm background over each energy region of interest,
and realizing an 80% plutonium recovery, the MDL is estimated
to be 0.008 dpm/g.
27
-------
1.3 The precision and accuracy of the method have not, as
yet, been extensively documented. The method is proposed for
interlaboratory collaborative test to determine the limits of
precision and error which can be expected when it is used by
a typical group of analysts.
1.4 This method is recommended for use by experienced
technicians under the supervision of a radiochemist or other
qualified person who fully understands the concepts involved
in the analysis and instrument calibrations. Furthermore, the
method should be utilized only after satisfactory results are
obtained by the analyst when replicate standard soil samples are
analyzed.
2. SUMMARY
2.1 The principle of the analytical procedure follows
(U.S. ^uomic Energy Commission Regulatory Guide 4.5, May 1974):
A known quantity of plutonium-236 tracer is added to the sample
which is .decomposed by sequential nitric acid-hydrofluoric acid
and nitric acid-hydrofluoric acid-hydrochloric acid digestions.
Boric acid is added to complex the fluoride ion and to aid in
the extraction of plutonium from any remaining insoluble
residue. Sequential iron hydroxide precipitations are performed
with sodium hydroxide and ammonium hydroxide to respectively
remove amphoteric elements and calcium and to eliminate soluble
fluorides. The hydroxide precipitate is dissolved in nitric
acid saturated with boric acid and plutonium is isolated and
purified by two successive anion exchange separations. The
28
-------
Plutonium is electroplated on stainless steel disks and is
determined by alpha spectrometry. The chemical yield, counting
efficiency, counting time, etc., are the same for all plutonium
isotopes and simplify calculations. In addition to the activity
of plutonium-236 added and the weight of the sample, only the
total number of counts of plutonium-236, plutonium-239, and/or
plutonium-238 recovered is necessary to calculate the concentra-
tion of plutonium-239 and/or plutonium-238 in the sample.
3. INTERFERENCES
3.1 Reagents, glassware, and other sample processing
hardware may cause contamination. All of these materials must
be demonstrated free from contamination under the conditions
of the analysis. Specific selection of reagents and sample
processing hardware is detailed in the procedure.
3.2 Possible procedural interferences are noted when apt
to be encountered.
4. APPARATUS
4.1 Instrumentation and Accessories (as described or
functionally equivalent)
4.1.1 A windowless 2ir gas flow proportional counter.
4.1.2 An alpha spectrometer capable of 40- to 50-keV
resolution of actual samples electrodeposited on flat, mirror-
finished stainless-steel planchets with a counting efficiency
greater than 17% and a background less than 0.010 cpm over
each designated energy region. Resolution is defined as the
full width half maximum (FWHM) in keV, the distance between
29
-------
those points on either side of the alpha peak where the count
is equal to one-half the maximum count (Heath, 1964).
4.1.3 Disposable electrodeposition cells are constructed
from 20-ml, linear-polyethylene, liquid scintillation vials.
See Figure 1. A 1.59-cm (5/8-inch) hole is cut in the bottom
for introduction of the anode. The foil-lined caps are re-
placed by 22-mm Polyseal caps having a GCMI 400 thread design.
The tubular portion of the polyethylene liner is removed and
the conical portion retained as a cover for the assembled cell.
A 0.36-cm (9/64-inch) hole having a beveled inside edge is
bored through the center of the cap. A 1.91-cm (3/4-inch)
diameter washer with a 0.32-cm (1/8-inch) hole is cut from
0.08-cm (1/32-inch) neoprene and placed in the cap. The shank
of a hollow brass rivet (Dot Speedy Rivets, #BS4830, Carr
Fastener Co., Cambridge, Massachusetts) is passed through the
washer and cap to serve as an electrical contact for the planchet
cathode, (Talvitie, 1974). The cell support and cathode socket
consist of a non-insulating banana jack attached to a Lucite
base.
4.1.3.1 The cathodes are 1.91-cm (3/4-inch) diameter,
0.38-mm (15-mil) thick, type 304 stainless steel planchets
pre-polished to a mirror finish. The exposed cathode area is
2.3 cm^. Prior to use the planchets are degreased with detergent
and/or acetone, immersed in hot concentrated nitric acid for
10 minutes, rinsed, and stored under distilled water until
needed.
30
-------
ANODE
SCINTILLATION
VIAL
44«««PLANCHET
CAP
ASSEMBLY
BASE
Figure 1. Disposable electrodeposition cell and support
31
-------
4.1.3.2 The anode is a 1.27-cm (1/2-inch) diameter,
0.08-cm (1/32-inch) platinum or platinum-iridium disk having
six 0.32-cm (1/8-inch) perforations and attached at the center
to a 10-cm (4-inch) length of 0.16-cm (1/16-inch) platinum or
platinum-iridium rod.
4.1.3.3 To assemble the cell, the planchet is centered
on the threaded end of the cell and held in place by vacuum
applied through one of the holes of a two-hole rubber stopper
butted against the other end. The cap assembly is screwed on
and leakage checked by adding water to the cell and observing
the rise of air bubbles when the vacuum is reapplied. Flexing
the cell by alternately applying and releasing the vacuum
improves the seal of leaky cells. The combined resilience of
planchet and washer maintains the liquid seal and electrical
contact during electrolysis.
4.1.3.4 Electrolysis is conducted without stirring using
an electroplating unit such as a 10-volt, 5-ampere Sargent-
Slomin Electrolytic Analyzer. The platinum anode is lowered
into the solution until the bottom edge of the anode is about
2 mm above the shoulder of the cell. (If set too deeply, gas
bubbles will be trapped and cause fluctuations of the current.)
4.2 Laboratory Equipment (as described or equivalent)
4.2.1 Pulverizer — Arthur H. Thomas 3367-D05 pulverizer,
pulverizes one pound quartz ore 0.64 cm in diameter to 0.15 mm
(100 mesh) in one minute, requires 1-hp motor.
4.2.2 Balance -- Mettler top-loading balance, capacity
32
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1200 g, precision ± 0.05 g.
4.2.3 Drying oven — maximum temperature 110°C, including
trays to fit.
4.2.4 Infrared drying lamp.
4.2.5 Centrifuge — for 220-ml centrifuge bottles.
4.2.6 Hot plate -- capable of providing a temperature
range of 10° above ambient to 370°C.
4.2.7 Burner — Meker type.
4.2.8 vHot water bath -- to accommodate 220-ml centrifuge
bottles.
4.2.9 Anion exchange resin columns -- Dowex 1x4
(100-200 mesh nitrate form). Remove the fines from an appro-
priate amount of resin by repeated suspension in distilled
water and decantation. Decant the water and add a volume of
concentrated (16 M) nitric acid approximately equal to 50% of
the volume of the resin slurry. Using 8 M nitric acid from a
wash bottle, transfer sufficient resin to a 1.3 cm ID ion
exchange column to give a 10-cm bed of settled resin. Convert
the resin to the nitrate form by passing 100 ml of 8 M nitric
acid through the column at maximum flow rate.
4.3 Labware (as described or functionally equivalent)
4.3.1 Pipets.
4.3.1.1 Automatic pipets with disposable tips — optional
sizes.
4.3.1.2 Disposable pipets — 2-ml glass eye-dropper type,
with 2-ml bulb.
33
-------
4.3.1.3 Volumetric -- 1 ml, treated with silicone water
repellent to eliminate drainage and calibrated "to contain" by
blowing residual liquid from the tip.
4.3.2 Dropping bottles.
4.3.3 Beakers -- glass, 150 ml, 250 ml and other assorted
sizes.
4.3.4 Beakers — PTFE "Teflon," 250 ml.
4.3.5 Graduated cylinders -- assorted sizes.
4.3.6 Ion exchange columns -- 1.3 cm ID x 15 cm with
reservoir to hold 100 ml of solution.
4.3.7 Centrifuge bottles — 220-ml capacity.
4.3.8 Millipore filter holder -- Pyrex, to accommodate
47-mm filter.
4.3.9 Membrane filters — 47-mm diameter, 0.45 ym pore
size DM-450.
4.3.10 Polyethylene wash bottles -- optional sizes.
4.3.11 Teflon FEP bottles -- optional sizes.
4.3.12 Safety glasses.
4.3.13 Timer — minute intervals.
4.3.14 Scissors.
4.3.15 Spatulas -- optional sizes.
4.3.16 Rubber policemen.
4.3.17 Stirring rods — Teflon and glass,
4.3.18 Stainless steel planchets -- 5.0 cm and 1.91 cm
disks pre-polished to a mirror finish on one side.
34
-------
4.3.19 pH paper — pH range 1-12.
5. STANDARDS, ACIDS, REAGENTS
5 .1 Standards
5.1.1 National Bureau of Standards (NBS) artericium-241
point source -- approximately 3 x 105 dpm, deposited on
platinum and certified to ± 1% of its stated activity-
5.1.2 Plutonium-236 solution -- 2.5 x 10^ dpm of
plutonium-236 in 2 M nitric acid in minimal solution (available
from Oak Ridge National Laboratory).
5.1.3 Americium-241 solution -- 1 x 10^ dpm of americium-
241 in 2 M nitric acid in minimal solution.
5.2 Acids
All solutions are made with distilled water. All
acids are reagent grade and meet American Chemical Society (ACS)
specifications.
5.2.1 Boric acid -- crystalline.
5.2.2 Hydrochloric acid — concentrated (12 M), 4 M and
2 M.
5.2.3 Hydrofluoric acid -- concentrated (48% solution).
5.2.4 Nitric acid — concentrated (16 M), 8 M, 8 M
saturated with boric acid and 2 M.
5.3 Reagents
All solutions are made with distilled water. All
reagents listed are reagent grade and meet ACS specifications
unless otherwise defined.
5.3.1 Ammonium hydroxide -- concentrated (15 M).
35
-------
5.3.2 Ammonium iodide-hydrochloric acid solution -- 1 part
1 M ammonium iodide to 20 parts 12 M hydrochloric acid.
5.3.3 Anion exchange resin -- Dowex 1x4 (100-200 mesh,
nitrate or chloride form).
5.3.4 Iron carrier — 10 mg Fe(III)/ml in 2 M hydrochloric
acid.
5.3.5 Octyl alcohol.
5.3.6 Silicone water-repellent solution.
5.3.7 Sodium bisulfite — anhydrous.
5.3.8 Sodium nitrite -- crystalline.
5.3.9 Sodium hydroxide -- 50% solution.
5.3.10 Thymol blue indicator, sodium salt -- 0.02%
solution.
6. CALIBRATION AND STANDARDIZATION
6.1 Calibration of the 2ir Alpha Counter and the Alpha
Spectrometer (Sill. 1974).
6.1.1 The windowless, 2ir alpha counter is standardized
by counting the NBS americium-241 source to approximately
5 x 10^ total counts.
6.1.2 The efficiency of the 2ir alpha counter is calculated
by dividing the observed counts per minute (cpm) by the
certified disintegrations per minute (dpm) of the NBS
americium-241 source.-
6.1.3 Correct the,counting efficiency for the difference
in backscatter between platinum and stainless steel by dividing
the calculated efficiency (from 6.1.2) by 1.023, (Hutchinson,
36
-------
1968), (Sill, 1975a).
6.1.4 Because a point-source standard electrodeposited
on platinum -- the NBS americium-241 source -- cannot be used
to calibrate an alpha spectrometer with an external detector
for use with diffuse sources electrodeposited on stainless
steel, a secondary standard must be employed. Prepare a
secondary standard containing about 1 x 10^ dpm of americium-241
electrodeposited on stainless steel under the exact conditions
subsequently described for electrodeposition of samples.
6.1.5 Standardize the secondary standard by counting in
the 2ir counter until at least 2 x 10 counts have been
collected.
•s
6.1.6 Use the secondary standard to calibrate the alpha
spectrometer and to periodically check the initial calibration
of both the spectrometer and the 2ir counter.
6.2 Purification of the Plutonium-236 Tracer, (Sill. 1970)
(U.S. Atomic Energy Commission Regulatory Guide 4.5, May, 1974)
In order to accurately calibrate the plutonium-236 tracer
by 2ir counting and alpha spectrometry, it will be necessary to
ensure the absence of plutonium-236 daughters (uranium-232,
thprium-228, radium-224, etc.) in the tracer. The following
purification must be performed just prior to the initial
calibration and annually thereafter if additional calibrations
are desired.
6.2.1 Add approximately 2.5 x 10^ dpm of plutonium-236
to a 250-ml beaker containing 25 ml of 16 M nitric acid.
37
-------
6.2.2 Evaporate the solution on a hotplate to a volume of
approximately 10 ml.
6.2.3 Remove from the hotplate and add 25 ml of 8 M nitric
acid and 10 ml of distilled water to adjust the nitric acid
concentration to 8 M.
6.2.4 Add ^200 mg of sodium nitrite crystals and stir with
a glass stirring rod. Bring the solution to a quick gentle boil
on a hotplate, and cool. Avoid prolonged heating.
6.2.5 Pass through an anion-exchange resin column (nitrate
form) prepared as described in 4.2.9 at maximum flow rate.
6.2.6 When the solution just drains to the top of the
resin bed, add two column volumes of 8 M nitric acid to the
column reservoir and wash the resin column at the maximum flow
rate. Discard the effluents from the sample and washes. NOTE:
The exact volumes of reagents used in the ion exchange separa-
tions are critical and will vary according to the column size
and the quantity of resin used. Determine the volume of the
10 cm resin bed and use the appropriate number of column
volumes of reagents in all steps.
6.2.7 Repeat step 6.2.6 twice until the resin column has
been washed with a total of six column volumes of 8 M nitric acid.
6.2.8 Wash the resin column at maximum flow rate with six
column volumes of 12 M hydrochloric acid using the same technique,
Discard the hydrochloric acid washes.
6.2.9 Elute the plutonium with four column volumes of
freshly-prepared ammonium iodide-hydrochloric acid solution
(1 part 1 M NH4I to 20 parts 12 M HC1) and collect in a 150-ml
beaker.
38
-------
6.2.10 Evaporate the solution to approximately 5 ml on a
hotplate. Rinse down the sides of the beaker dropwise with
1-2 ml of 16 M nitric acid. Add six drops of 12 M hydrochloric
acid and evaporate just to dryness on the hotplate.
6.2.11 Add 5 ml each of concentrated hydrochloric and
nitric acids to the beaker and evaporate to about 2 ml.
6.2.12 Add 15 ml of concentrated nitric acid and boil
down to about 5 ml to ensure complete dissolution of the
plutonium and complete oxidation of chlorides as indicated by
the absence of color or fumes of chlorine and/or nitrogen oxides,
6.2.13 Cool, add 25 ml of distilled water and filter the
solution through a DM-450 membrane filter in a filtering chimney.
Wash the flask and filter with enough distilled water to give a
final volume of 50 ml.
6.2.14 Dilute aliquots of the ^500 dpm/ml stock solution
with 2 M nitric acid to give concentrations desired for use.
Store all tracers in tightly capped Teflon FEP bottles.
6.3 Standardization of the Plutonium-236 Tracer (Sill,
October, 1974)
6.3.1 Transfer a 1-ml aliquot of the purified plutonium-
236 stock tracer ('v-SOO dpm/ml in 2 M nitric acid) onto a
stainless steel planchet with a calibrated silicone-treated
pipet and slowly evaporate to near dryness under an infrared
lamp to minimize any loss. Keep the activity in the center
of the planchet in an area limited to approximately 2 cm
(3/4 inch) in diameter by alternately adding the tracer a few
drops at a time and evaporating. The partially-filled
39
-------
silicone-treated pipet can be placed on its side between
additions with no loss of solution. To ensure quantitative
transfer of the tracer, carefully blow out the last few drops
with a rubber bulb.
6.3.2 When the last of the tracer*has been transferred
to the planchet and evaporated nearly to dryness, add 2 or 3
drops of concentrated nitric acid to help keep the activity
spread as uniformly as possible and evaporate to complete
dryness.
6.3.3 Heat the dry planchet over a blast burner just to
the first dull red glow. Then quickly lower the temperature
by placing the planchet on a cold steel surface to minimize
oxidation of the plate.
6.3.4 Count in the 2if alpha counter immediately after
cooling to avoid any possibility of absorption of water vapor
from the air. Collect at least 5 x 10^ counts for the standard
to ensure adequate statistical precision.
6.3.5 Prepare and count a duplicate source by repeating
steps 6.3.1 through 6.3.4.
6.3.6 The 2ir counting rate of the plutonium-236 sources
must be corrected by determining the fraction of the total
alpha activity due to plutonium-236.
6.3.6.1 Transfer 2 ml of the purified ^500 dpm/ml
plutonium-236 tracer to a 150-ml beaker and add 2 ml each of
concentrated nitric acid and hydrochloric acid.
6.3.6.2 Evaporate carefully on a hot plate just to
40
-------
dryness. Rinse down the sides of the beaker with a few ml of
12 M hydrochloric acid and evaporate to approximately 1/2 ml.
6.3.6.3 Treat and electrodeposit as described in
Sections 7.5.1 through 7.5.8.
6.3.6.4 Count the electroplated source on an alpha
spectrometer for 250 minutes over an energy range of 3-8 MeV.
Determine the fraction of the total number of counts in the
alpha spectrum that is due to plutonium-236 in the source. This
fraction is the correction factor to be applied to the counting
rate of the plutonium-236 evaporated source in the 2ir propor-
tional counter. NOTE: Prolonged and repeated counting of
high level plutonium-236 sources on the alpha spectrometer
should be avoided to minimize daughter recoil contamination of
the alpha detector. Alternatively, such contamination can be
virtually eliminated by leaving a small amount of air in the
counting chamber and applying a small negative potential to
the source plate. (Sill, November 1970)
6.3.7 Calculate the activity concentration of the
plutonium-236 tracer (dpm plutonium-236/ml) by multiplying the
observed 2ir counting rates of the evaporated sources by the
correction factor and dividing by the 2ir counter efficiency
and the volume of tracer used to prepare the evaporated sources.
7- STEP BY STEP PROCEDURE FOR ANALYSIS (U.S. Atomic Energy
Commission Regulatory Guide 4.5, May 1974)
7.1 Sample Decomposition
7.1.1 Weigh a representative 10.0 ± 0.1 grams of the
41
-------
-100 mesh soil sample and transfer to a 250-ml PTFE beaker.
7.1.2 Add 16 M nitric acid a few drops at a time as fast
as the frothing and vigor of the reaction will permit until the
entire sample is covered.
7.1.3 Add an appropriate quantity of plutonium-236 tracer.
NOTE: If the activity is expected to be less than 1 dpm/g,
or is unknown, add 10 dpm of the tracer. For higher levels
add as much plutonium-236 tracer as the estimated activity of
plutonium-239 or plutonium-238 in the sample.
7.1.4 Add an additional 60 ml of 16 M nitric acid and
30 ml of 48% hydrofluoric acid and digest on a hotplate with
frequent stirring (Teflon stirring rod) for about 1 hour.
CAUTION: Hydrofluoric acid is an extremely hazardous liquid.
Use gloves to avoid contact with skin and work in a fume hood
to avoid breathing vapors. NOTE: For organic soils, first
add the nitric acid only, in small portions, with stirring. If
the solution threatens to overflow as a result of froth genera-
tion, add a few drops of octyl alcohol and stir. Digest on a
hotplate until the evolution of heavy reddish-brown fumes is
reduced to a barely visible level. Cool to room temperature
before carefully adding the hydrofluoric acid and digesting for
the one-hour period.
7.1.5 Remove from the hotplate and cool somewhat before
adding 30 ml each of 16 M nitric acid and 48% hydrofluoric acid,
Digest on the hotplate with intermittent stirring for an addi-
tional hour-
42
-------
7.1.6 Remove from the hotplate and cool. Carefully add
20 ml of 12 M HC1 and stir. Heat on a hotplate for 45 minutes
with occasional stirring.
7.1.7 Add 5 g of powdered boric acid and digest for an
additional 15 minutes with occasional stirring.
7.1.8 Add approximately 200 mg of sodium bisulfite
crystals and continue heating until the solution has evaporated
to a liquid volume of approximately 10 ml.
7.1.9 Add 50 ml of distilled water and digest on a
hotplate with stirring for 10 minutes to dissolve the soluble
salt s .
7.2 Sodium Hydroxide Precipitation
7.2.1 Cool and transfer approximately equal parts of the
total sample into two 220-ml centrifuge bottles with a minimum
of distilled water from a wash bottle. NOTE: If equipment
for large volume centrifugation is not available, the following
two precipitations may be performed in a beaker, allowing the
precipitate to settle, decanting the supernate, and then com-
pleting the separation by centrifugation on a smaller scale.
7.2.2 Add 1 ml of iron carrier solution (10 mg Fe3+/ml)
to each centrifuge bottle|and stir.
7.2.3 Add 50% sodium hydroxide with stirring to each
bottle to a pH of ^9. Add 5-10 ml excess of the sodium hydroxide
and stir for 1 minute.
7.2.4 Centrifuge for approximately 5 minutes, decant and
discard the supernate(s).
43
-------
7.2.5 Dissolve each precipitate with about 30 ml
(60 ml total) of 8 M nitric acid saturated with boric acid.
Digest on a hot water bath for 10 minutes.
7.2.6 Cool and centrifuge for 5 minutes. Decant the
supernates into the original 250-ml PTFE beaker and save.
7.2.7 Wash each residue with approximately 10-20 ml
(20-40 ml total) of 8 M nitric acid saturated with boric acid.
Centrifuge for 5 minutes and combine the supernates with those
in step 7.2.6.
7.2.8 Heat the supernates on a hotplate and evaporate
to near dryness.
7.3 Ammonium Hydroxide Precipitation
7.3.1 Add approximately 30 ml of water and heat to
dissolve the salts. Cool, and transfer equal portions into
centrifuge bottles.
7.3.2 Add 15 M ammonium hydroxide dropwise with stirring
to a pH of ^9.
7.3.3 Centrifuge and discard the supernate.
7.4 Ion Exchange Separations
7.4.1 Dissolve the precipitate(s) with a volume of 16 M
nitric acid approximately equal to the volume of the precipi-
tate^) and transfer using 8 W[ nitric acid into a 250-ml beaker.
Add 8 M nitric acid to a total volume of approximately 75 ml.
NOTE: If the volume of the hydroxide precipitate is consider-
ably greater than should be expected from the 10 mg of Fe added,
the final volume should be brought up to approximately 100 ml
44
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with 8 M nitric acid or, alternatively, the dissolved hydroxides
should be evaporated to salts before addition of the 8 M nitric
acid solution. The final molarity of the HN03 is not extremely
critical, but should be in the range of 7-9.
7.4.2 Add approximately 200 mg of sodium nitrite (NaN02)
crystals and stir with a stirring rod. Bring to a quick gentle
boil on a hotplate, and cool. Avoid prolonged heating.
7.4.3 Pass the sample (at maximum flow rate) through an
anion-exchange resin column (nitrate form) prepared as described
in 4.2.9.
7.4.4 When the solution just drains to the top of the
resin bed, add two column volumes of 8 M nitric acid to the
column reservoir and wash the resin column at maximum flow rate.
Discard the effluents from the sample and washes.
7.4.5 Repeat step 7.4.4 twice until the resin column has
been washed with a total of six column volumes of 8 ^ nitric
acid. NOTE: See 6.2.6 note.
7.4.6 Wash the resin column with six column volumes of
12 M hydrochloric acid using the same technique. Discard the
hydrochloric acid washes.
7.4.7 Elute the pluitonium with four column volumes of
freshly-prepared ammonium iodide-hydrochloric acid solution
(1 part 1 M NH^I to 20 parts 12 M HC1) and collect in a 150-ml
beaker.
7.4.8 Evaporate the solution to approximately 5 ml on a
hotplate. Rinse down the sides of the beaker dropwise with
45
-------
1-2 ml of 16 ^ nitric acid. Add six drops of 12 M hydrochloric
acid and evaporate to near dryness.
7.4.9 Add 50 ml of 8 M nitric acid and repeat steps
7.4.2 - 7.4.8, using a fresh anion-exchange resin column
(nitrate form)-
7.4.10 Continue heating the final plutonium elution just
to dryness on the hotplate. Rinse down the sides of the beaker
with a few ml of 12 M hydrochloric acid and evaporate to
approximately 1/2 ml.
7.5 Electrodeposition
7.5.1 Add 1 1/2 to 2 ml of 4 M hydrochloric acid into the
beaker and using a disposable pipet (2-ml glass eyedropper type,
with 2-ml bulb), rinse down the sides of the beaker with the
sample solution. Transfer the solution into a plating cell.
7.5.2 Add another 1 1/2 to 2 ml of 4 M hydrochloric acid
into the beaker, rinse as above and add to the plating cell.
7.5.3 Repeat using approximately 1 ml of distilled water.
7.5.4 Add 3 drops of thymol blue indicator solution and
then add 15 M ammonium hydroxide dropwise until the color
changes to yellow.
7.5.5 Add 2 M hydrochloric acid dropwise to a salmon-pink
end point.
7.5.6 Electroplate at 1.5 amps for 20 minutes. CAUTION:
The electrodeposition should be performed in a fume hood due to
the chlorine gas evolved during the electrodeposition.
7.5.7 At the end of 20 minutes, quickly add 2-3 ml of
46
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concentrated NH^OH and leave the current on for another 20
s econds .
7.5.8 Turn the current off, rinse out the solution into a
beaker with distilled water, and dismantle the cell. Rinse the
disc with distilled water and dry it in a clean planchet on a
hotplate at medium heat for 5 minutes.
7.5.9 Count the sample in an alpha spectrometer to resolve
the isotopes of plutonium. For samples containing less than
1 dpm/g of plutonium a minimum of 1.5 x 1(H counts should be
collected for the plutonium-236 tracer. For higher levels,
4
count for 1000 minutes or until 10 counts have been collected
in each of the plutonium-236 and the plutonium-239 and/or
plutonium-238 energy regions.
7.6 Quality Assurance Program
For any analytical procedure a rigorous quality assurance
program must be followed to ensure accurate and precise results.
Such a program must include the evaluation of all variables in
the final calculation for their degrees of uncertainty and for
any significant systematic errors. Precautions must be taken to
eliminate any cross contamination between samples, especially if
high and low level samples are run concurrently. Standard
samples should be analyzed both to check out initial capabilities
and to provide for a continuing quality control program.
7.6.1 The internal laboratory precision of the method is
evaluated by considering the uncertainties in all the variables
in the final calculation. These include the counting uncer-
47
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tainties associated with counting the sample and the standards
for calibration, uncertainties associated with pipettings and
tracer dilutions and weighing the original sample, and any
uncertainty in the timing of the 2ir count during the calibration.
All uncertainties should be evaluated and if significant, propa-
gated to the final result. Variability between laboratories is
expected to be greater than that for a single laboratory due
to the variability in NBS standards used for calibration, slight
differences in calibration procedures, etc. The interlaboratory
precision of the method can be adequately estimated only on the
basis of collaborative testing. Systematic errors in the method
will be minimized by calibrating all pipets, volumetric flasks,
and balances used for the tracer calibration and sample analysis,
and by calibrating the 2ir counter timing mechanism. The
systematic error introduced by the ± 1% uncertainty in the NBS
standard and the error in the backscatter correction factor
cannot be compensated for.
7.6.2 Cross contamination of samples may be avoided with
good housekeeping and by either segregating apparatus used for
high- and low-level samples, or by carefully decontaminating
glassware and Teflon ware between analyses. Contamination of
stock reagents must be avoided. This can best be accomplished
by employing intermediate containers to which small portions
of the stock reagents can be transferred before adding to the
sample. The excess reagent is then discarded and the inter-
mediate container rinsed before reuse. Reagent blanks using the
48
-------
same reagents, tracer, glassware, Teflon ware, electrodeposition
equipment and detector must be run initially and periodically
thereafter to determine the radiochemical background for the
method and ascertain that contamination of these items has not
occurred.
49
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8. CALCULATION OF RESULTS (Overman 1960, Sill 1975a)
8.1 CALIBRATION OF THE 2fT ALPHA COUNTER
8.1.1 The counting efficiency of the 2ir counter is
determined by counting an NBS certified americium~241
source electrodeposited on a platinum disk,
8.1.2 The 2ir counting efficiency (E ) is calculated
as :
_
E2ir ~ (a) (t)(1.023) (8,1.1)
in which c. = the net counts of the americium-241
source,
a = the certified activity of the
americium-241 source (dpm) ,
t = the duration of the count (min)
and 1.023 = the backscatter factor correcting the
counting efficiency of a source on
platinum to that on stainless steel.
8.2 STANDARDIZATION OF THE PLUTONIUM-236 TRACER
8.2.1 The purified plutonium-236 stock tracer is
standardized by counting evaporated sources on the 2ir
counter and an electrodeposited source on the alpha
spectrometer. The 2ir count which represents total activity
is corrected by multiplying by the plutonium-236 fraction of
the total activity as determined by alpha spectrometry .
50
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8.2.2 The activity concentration (AC in dpm
plutonium-236/ml) of the stock tracer is calculated from:
(c )(f )
AC • (E2ir)(v)(t) O^2-1'
in which c_ = the net counts of the evaporated source
on the 2ir counter,
E- = the counting efficiency of the 2ir
counter,
v = the volume of stock tracer used to
prepare the evaporated source (ml),
t = the duration of the count for the
evaporated source on the 2ir counter
(min)
and f., = the ratio of the net counts in the
D
plutonium-236 energy region to the net
counts in the entire 3-8 MeV energy
region in the alpha spectrum of the
electroplated tracer source.
8.2.3 The plutonium-236 activity (T in dpm) added to
the sample to trace the plutonium recovery through the
analysis is calculated as:
T = (AC)(D)(V)(e"Xt) (8.2.2)
in which AC = the activity concentration of the stock
tracer solution (dpm plutonium-236/ml),
D = the dilution factor in preparing the
51
-------
working tracer from the stock tracer,
V = the volume of working tracer added to
the sample (ml),
and e = the decay correction for plutonium-236
for the time interval between the date
of tracer calibration and date of
sample analysis.
8.3 CALCULATION OF PLUTONIUM CONCENTRATIONS IN THE ALIQUOT
OF SOIL TAKEN FOR ANALYSIS
8.3.1 The concentration of plutonium-239 or
plutonium-238 in the aliquot of soil taken for analysis
(X. in dpm/g) is calculated from:
i (C6)(W)
in which C. = the net sample counts in the
plutonium-239 or plutonium-238 energy
region of the alpha spectrometer,
C.. = the net sample counts in the
o
plutonium-236 energy region of the
alpha spectrometer,
T = the activity of plutonium-236 tracer
added to the sample (dpm) ,
and W = the weight of the soil aliquot taken
for analysis (g) .
8.3.2 The above calculation assumes that the
plutonium-236 tracer used in the analysis is sufficiently
-------
free from plutonium-238 and plutonium-239 activities
(<0.1%) to cause negligible interference in the plutonium
determinations. Older supplies of plutonium-236 (pre 1974)
may contain appreciable amounts of plutonium-238 and/or
plutonium-239 (up to 1-2%) and should not be used. If
the poorer grade tracer is used, a freshly purified
portion must be assayed for plutonium-236, plutonium-238
and plutonium-239 by alpha spectrometry and the necessary
corrections for adding plutonium-238 and plutonium-239 to
the sample with the tracer must be made.
8.4 CALIBRATION OF THE ALPHA SPECTROMETER AND CALCULATION
OF THE PLUTONIUM RECOVERY
8.4.1 The absolute counting efficiency of the alpha
spectrometer (E ) must be determined in order to evaluate
s
the plutonium recovery through the analytical procedure.
Americium-241 electroplated in the same manner as the
samples should be used for this purpose. The spectrometer
counting efficiency may be calculated from:
_ s2TT (8.4.1)
S r
in which r = the net counting rate of the electro-
s
plated source over the entire energy
region on the alpha spectrometer (cpm),
r = the net counting rate of the same source
2ir
on'the 2ir counter (cpm),
and E? = the counting efficiency of the 2ir co-unter
53
-------
8.4,2 The plutonium recovery through the analysis (Y)
is calculated from:
Y =
(T)(Es)
(8.4.2)
R, = the net counting rate in the
in which
o
plutonium-236 energy region of the
alpha spectrum of the sample (cpm).
8.5 PROPAGATION OF UNCERTAINTIES
8.5.1 The uncertainties associated with the plutonium-236
tracer calibration and the soil analysis are estimated from the
2a- or 95% confidence level (95% C.L.) uncertainties of all
appropriate radioactivity counts, weighings, pipetings, dilutions
and measurements of counting times.
8.5.2 The 2a or 95% C.L. uncertainty in a net radio-
activity count, C = G - B, is:
in which
and
± 2/G + B
G = the gross number of counts collected
B = the expected number of background
counts during the same time interval,
The uncertainties in the other variables are determined experi-
mentally by replicate calibrations.
8.5.3 For linear addition or subtraction of independent
variables, uncertainties are propagated by taking the square
root of the sum of the squares of the individual uncertainties
54
-------
8.5.4 For linear multiplication and division of
independent variables, the fractional uncertainty in the
final result is obtained by taking the square root of the
sum of the squares of the fractional errors in each of the
independent variables.
55
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SAMPLE CALCULATIONS
1. CALIBRATION OF THE 21T ALPHA COUNTER
1.1 The NBS certified americium-241 standard
(3.23 x 10 dpm ± 1%) was counted on the 2ir alpha counter
for 10.00 ± 0.02 minutes. The total number of counts
collected was 1,564,612 ± 2500 at the 95% C.L. (2a).
1.2 The 2ir counting efficiency calculated from
equation (8.1.1) is:
1,564,612 ± 2500*
£
2ir 5
(3.23 X 10 M10.00 ± 0.02)(1.023)
= 0.474 ± 0.001
2. STANDARDIZATION OF THE PLUTONIUM-236 TRACER
2.1 The first evaporated source of 1.029 ± 0.002 ml
of the stock plutonium-236 tracer gave 61,124 ± 494 net
counts for a 250.0 ± 0.0 minute count on the 2ir counter.
The electroplated tracer counted for 250 minutes on the
alpha spectrometer yielded 51,460 net counts in the
plutonium-236 energy region and 62 net counts in the rest
of the 3-8 MeV energy region giving a correction factor
(f£) of £514607(51460 + 62)3 ± 2C (51460) (62) / (51460 + 62)33?S
b
or 0.999 ± 0.000 to be applied to the 2ir count. (Sill 1975a)
* To avoid confusion, experimentally observed values were
not rounded off in all equations. The calculated results,
however, have been rounded off to the appropriate number of
significant figures for the given situation,
56
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2.2 The activity concentration (AC) of the
plutonium-236 tracer calculated from equation (8.2.1) is:
A = (61.124 ± 494H0.999 ± 0.000)
(0.474 ± 0.001)(1.029 ± 0.002)(250.0 ± 0.0)
= 501 ± 4 dpm plutonium-236/ml at the 95% C.L.
2.3 The second evaporated source yielded an activity
concentration value of 495 ± 4 dpm plutonium-236/ml.
2.4 Averaging the two values the activity concentra-
r
tion of the stock tracer is:
AC = 498 ± 3 dpm plutonium-236/ml at the 95% C.L.
3. CALCULATION OF PLUTONIUM-239 AND PLUTONIUM-238
CONCENTRATIONS IN THE ALIQUOT OF SOIL TAKEN FOR ANALYSIS
3.1 A 1.002 ± 0.002 ml aliquot of working
plutonium-236 tracer (stock tracer diluted 1.029 ± 0.002 to
49.8 ± 0.01) was added to 10.0 ± 0.1 grams of the -10 mesh
fraction of the soil sample and the analysis was performed
65 days after the calibration of the stock tracer
(e = 0.958). A 1134-minute spectrometer count of the
sample yielded the following data:
Energy
Region
Plutonium-236
Plutonium-239
Plutonium-238
Gross
Counts
1953
1394
215
Background
Counts
10
6
8
Net Counts
± 2a
1943 ±
1388 ±
207 ±
89
75
30
3.2 The amount of plutonium-236 tracer added
(T in dpm) is calculated from equation (8.2.2):
57
-------
T = (498 ± 3) ( 49 s ± 0 01 M1'002 * 0.002) (0.958)
9.88 ± 0.07 dpm plutonium-236
3.3 The plutonium-239 and plutonium-238 concentrations
in the -10 mesh fraction taken for analysis are calculated
from equation (8.3.2):
(1388 ± 75)(9.88 ± 0.07)
9 (1943 ± 89)(10.0 ± 0.1)
= 0.71 ± 0.05 dpm plutonium-239/g
(207 ± 30)(9.88 ± 0.07)
8 (1943 ± 89)(10.0 ± 0.1)
= 0.11 ± 0.02 dpm plutonium-238/g
4. CALIBRATION OF THE ALPHA SPECTROMETER AND CALCULATION
OF THE PLUTONIUM RECOVERY
4.1 The electroplated americium-241 source (Section 6.1.4)
yielded 206,741 ± 909 net counts in the 3-8 MeV energy range
for a 100.0 ± 0.0 minute count on the alpha spectrometer. A
100.0 ±0.0 minute count of the same source on the 2ir counter
yielded 465,093 ± 1363 net counts.
4.2 The counting efficiency of the spectrometer (E ) is
s
calculated from equation (8.4.1):
(2067 ± 9) (0.474 ± 0.001)
s (4651 ± 14)
= 0.211 ± 0.001
58
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5.3 The plutonium recovery for the analysis of the
soil sample calculated from equation (8.4.2) is:
= (1943 ± 87)/1134
(9.88 ± 0.07)(0.211 ± 0.001)
= 0.82 ± 0.04
59
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REFERENCES
5.
6.
7.
8.
9.
Heath, R. L. , Scintillation Sp ectrometry-Gamma Ray
Spectrum Catalogue
Idaho Falls
Phillips Petroleum Co., Report
ID, Report #100-16880-1. (1964)
Hutchinson, J. M. R., C. R. Naas, H. Walker, and
W. B. Mann, "Backscattering of alpha particles from
thick metal backings as a function of atomic weight,"
International Journal of Applied Radiation and
Isotopes, Vol. 19. pp. 517-522. (1968)
Overman, R.
Techniques,
T., and H. M. Clark, Radioisotope
McGraw Hill, New York, NY, p. 109. (1960)
Sill, C. W., "Some problems in measuring plutonium
in the environment," Proceedings of the Second
Los Alamos Life Sciences Symposium, J. W. Healy, Ed.,
Los Alamos, NM. (May 1974), Health Physics. Vol. 29.
No. 4, pp. 619-626. (October 1975b)
Sill, C. W., Private communication, Health Services
Laboratory, USAEC, Idaho Falls, ID. (January 1975a)
Sill, C. W., and Dale G. Olson, "Sources and prevention
of recoil contamination of solid-state alpha
detectors," Anal. Chem.
1607. (November 1970)
Vol. 42, No. 13, pp. 1596-
Sill, C. W., K. W. Puphal, and F. D. Hindman,
"Simultaneous determination of alpha-emitting nuclides
of radium through californium in soil," Anal. Chem.,
Vol. 46, No. 12, pp. 1725-1737. (October 1974)
Talvitie, N. A., "Electrodeposition of actinides for
alpha spectrometric determination," Anal. Chem.,
Vol. 44, No. 2, pp. 280-283. (February 1972)
U.S. Atomic Energy Commission, "Measurement of
radionuclides in the environment - Sampling and
analysis of plutonium in soil," U.S. Atomic Energy
Commission Regulatory Guide 4.5. (May 1974)
60
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA-600/7-79-081
2.
3. RECIPIENT'S ACCESSION NO.
. TITLE AND SUBTITLE ACID DISSOLUTION METHOD FOR THE ANALYSIS
OF PLUTONIUM IN SOIL: Evaluation of an interlaboratory
collaborative test and comparison with results of a
fusion method test
5. PEPORT DATE
March 1979
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
E. L. Whittaker and G. E. Grothaus
8. PERFORMING ORGANIZATION REPORT NO.
3. PERFORMING ORGANIZATION NAME AND ADDRESS
Environmental Monitoring and Support Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Las vegas, Nevada 89114
10. PROGRAM ELEMENT NO.
11. CONTRACT/GRANT NO.
12. SPONSORING AGENCY NAME AND ADDRESS
U.S. Environmental Protection Agency-Las Vegas, Nevada
Office of Research and Development
Environmental Monitoring and Support Laboratory
Las Vegas, Nevada 89114
13. TYPE OF REPORT AND PERIOD COVERED
Final
2/1/76-6/1/78
14. SPONSORING AGENCY CODE
EPA/600/07
15. SUPPLEMENTARY NOTES
16. ABSTRACT
The data from an interlaboratory collaborative test are presented. A statistical
analysis of the data is also presented. From that analysis, statements are made of
the combined within-laboratory precision, the systematic error between laboratories,
the total error between laboratories based on a single analysis, and the method
bias.
Soil samples used for the test contained plutonium in a highly refractory form,
a very insoluble form, and therefore, difficult to measure the true concentration.
Plutonium concentrations in those samples ranged from 0.1 to 10 dpm/g of soil.
A comparison is made between the acid dissolution method and a fluoride-
pyrosulfate fusion method which was tested in a similar study using the same test
samples.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS C. COS AT I Field/Group
Plutonium
Quantitative Analysis
Quality Assurance
Soil
07B, D
14D
18. DISTRIBUTION STATEMENT
RELEASE TO PUBLIC
19. SECURITY CLASS (ThisReport)
UNCLASSIFIED
21. NO. OF PAGES
68
20. SECURITY CLASS (Thispage)
UNCLASSIFIED
22. PRICE
A04
EPA Form 2220-1 (Rev. 4-77) PREVIOUS EDITION is OBSOLETE
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