Table 7. ETV program GC-TID and reference laboratory precision (% RSD) for soil sample
replicates (quadruplicates).
GC-TID
Reference laboratory*
Statistic
2,4-DNT
TNT
RDX
2,4-DNT
TNT
RDX
n = 4†
n = 17
n = 13
n=3
n = 18
n = 13
Mean
15
23
14
56
29
25
Median
9.0
13
10
32
25
21
Range
931
2107
544
12123
272
4 63
*Reference laboratory used Method 8330.
†Mean is based only on sample sets where all four replicates had values reported.
of RDX and TNT value comparability between the two
cate 2), only one of the remaining false-positive values
analyses was performed by assessing the ranges of %D.
for both methods of analysis was greater than 1.1 mg/
However, it should be recognized that in addition to
kg. The carryover of TNT and other explosives, be-
variability due to sample preparation and analysis, there
cause of cold spots in the injection port of the GC-TID
is variability (heterogeneity) in the analyte distribution
system, continues to be a concern even after adding a
within the sample jar from which the subsamples were
heated injection port to the GC (Hewitt and Jenkins
removed for analysis by each participant and the refer-
1999). High concentrations of TNT could not be avoided
ence laboratory. With respect to the homogeneity of
completely during the ETV verification test because of
these analytes in each sample jar, RSDs of 20% or less
the necessity to analyze for RDX in the same sample
were estimated for five replicate measurements (ORNL
extract. Therefore, even with the addition of a heated
2000). This information does not readily lend itself to
injection port and screening samples prior to analysis,
setting an appropriate range for the %D, for judging
carryover appears to remain an issue of concern. This
acceptability. With 25 %D as the acceptance criterion,
problem is not unique to this GC system, and perhaps
65% of the RDX and 45% of the TNT results are within
with further design changes it will become less of an is-
range, whereas 96% of the RDX and 83% of the TNT
sue in the future.
results are acceptable for 50%D. Both of these com-
The experimental design also allows for compara-
parisons (regression analysis and %D) show that, in
bility testing between the GC-TID and laboratory
general, there was good agreement between the two
results for each individual sample that had analyte con-
methods of sample preparation and analysis for both
centrations estimated above 0.5 mg/kg by both meth-
RDX and TNT (Fig. 4 and 5), and poor agreement for
ods. For this comparison there were 12, 52, and 69
2,4-DNT.
comparable data points for 2,4-DNT, RDX, and TNT,
respectively (aberrant TNT laboratory value removed).
In an attempt to understand the discrepancy between
The correlation coefficients and slopes for the compari-
the GC-TID and reference laboratory results for 2,4-
DNT, the set of the samples that had been determined
son of these data points for 2,4-DNT, RDX, and TNT
were, respectively, r = 0.44 and m = 0.33, r = 0.85 and
to have this explosives analyte were reanalyzed by
m = 0.91, r = 0.95 and m = 1.32. An additional analysis
Method 8330 at CRREL (Table 9). This analysis was
Table 8. ETV program GC-TID and reference laboratory false-positive results for blank soil
samples (n = 20).
GC-TID
Reference laboratory*
Statistic
2,4-DNT
TNT
RDX
2,4-DNT
TNT
RDX
No. FP†
0
5
0
0
2
0
% FP
0
25
0
0
10
0
*Reference laboratory used Method 8330.
†False-positive value reported.
12
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