Short-range heterogeneity is estimated using the
the sample size study. Here, we are looking at our
RSDs from the 10 pairs (two pairs at each depth)
ability to obtain replicate analytical-size
separated by 1.0 m. These RSDs range from 7.3 to
subsamples from a carefully homogenized bulk
127%, with a pooled RSD of 71.8%. Although we
sample. Each subsample goes through the entire
expect RSDs to increase as concentrations decrease
extraction and HPLC analysis procedure. The
(Horowitz 1982), that pattern is not apparent for
RSDs for HMX for the 12 duplicates ranged from
these samples. What is clear is that close-lying
1.6 to 99.7%, with a pooled RSD of 55.4%. In this
samples can produce widely disparate results, and
case, RSDs do appear to vary with concentration,
our ability to estimate mean concentration for a
although the pattern is not completely consistent.
grid using a single discrete sample is very poor.
For samples with concentrations above 10 mg/kg,
While the concentrations of RDX, TNT, 4-AmDNT,
the pooled RSD is 32%, compared to 74% for
and 2-AmDNT are quite low relative to HMX at
samples with concentrations below 10 mg/kg.
sampling location 1-44, the magnitude of the stan-
Over all, these RSDs are considerably larger than
dard deviations are similar to their means, and
those we have observed for similarly processed
hence our ability to estimate mean concentrations
samples at other explosives sites, which ranged
from a single discrete sample for these compounds
from 4.5 to 13.5% (Jenkins et al. 1996).
is also very poor (Table 3).
We conducted a sample size study to assess the
In sampling area 1-48, there are an insufficient
reproducibility of analysis as a function of the size
number of cases where measured values were
of subsample used. This study (Table 5) yielded
above MDLs to obtain good estimates of standard
some unexpected results. We expected to see mea-
deviations (Table 4). Even so, inspection of the data
surable reduction in RSD estimates as sample size
tells us that HMX and RDX concentrations for the
increased, but that trend was not present. The RSD
four discrete samples at the same depth for a given
estimates are much lower than those observed for
grid are so disparate, that, here again, it would be
the 12 duplicates discussed earlier; they are all in
impossible to obtain good estimates of mean con-
the 1020% range, except for the 0.5-g sample size
centrations from analysis of single discrete
and one 2.0-g ground sample. The mean concen-
samples.
tration estimates from different sample sizes
Finally, the very-short-range heterogeneity is
showed excellent consistency, but they are consid-
examined using the 12 sets of duplicate samples
erably lower than the original estimates (Table 2).
from location 1-44, coupled with the results from
The initial HMX estimate for sample 1-44-8 was
Table 5. Analytical results for sample size/heterogeneity study using
surface soils (015 cm).
HMX concentration (mg/kg) by SW-846 Method 8330
Sample
20-g
7.5-g
2.0-g
2.0-g
0.5-g
(rep.)
portion
portion
portion
ground
portion
1448 a*
47.9
29.0
39.5
33.6
33.1
1448 b
41.1
26.9
51.2
67.8
57.8
1448 c
54.4
36.7
36.7
56.8
19.2
1448 d
42.9
30.0
35.6
47.5
65.9
1448 e
38.9
44.3
42.9
22.8
43.5
1448 (X%RSD)†
45.1 13.7
33.4 21.3
41.2 15.2
45.7 39.3
43.9 42.7
14411 a
168
171
132
113
73.5
14411 b
128
120
133
145
99.3
14411 c
143
155
135
118
65.2
14411 d
129
133
169
138
274
14411 e
136
139
129
124
105
14411(X%RSD)
14111.5
14313.9
14011.7
12810.4
12469.5
* Letters indicate different replicate samples.
† Mean and percent relative standard deviation.
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