using acetonitrile (Table 4). Also, concentrations obtained in the field using acetone extraction and
the EnSys colorimetric method ranged from 64% to 109% of the value obtained after subsequent
laboratory homogenization and determination by SW846 Method 8330. These field determinations
utilized a three-minute shaking period with acetone followed by a 24-minute rest period and anoth-
er three-minute period of shaking.
These kinetic extraction experiments indicate that a short three-minute extraction of TNT and
RDX from highly contaminated soils is incomplete, whether using acetone or methanol. For the
soils we tested, however, extract concentrations after a three-minute extraction were at least 70% of
those attained after 18 hours of shaking. The rate of extraction into acetone and methanol appear to
be comparable.
It is possible that the rate of extraction of TNT and RDX from other highly contaminated soils
could be slower, resulting in a smaller percentage extracted after the short three-minute extraction.
However, contaminants present at these high (%) concentrations far exceed the amount required to
form a mono-layer on soil surfaces, and so most of the contaminant must be present in crystalline
form. Differences in the rate of extraction of crystalline material from soil to soil should be small
since it is controlled by the rate of dissolution of the crystalline material. It appears that a value of
70% can be used to represent the percentage of TNT or RDX that would be extracted from a high-
concentration sample in a short three-minute extraction. Therefore, field analyses that result in con-
centrations above 7% should be considered potentially explosive.
Extract dilution study
Because all of the current on-site methods for TNT and RDX in soil were developed to detect low
levels (low g/g) in soil, all these methods, when used for high concentration samples, require that
the extracts be diluted substantially before they can be analyzed by colorimetric, immunoassy, or
IMS methods. For the EnSys colorimetric method, for example (Table 1), the concentration of TNT
in acetone extract from a soil containing 10% TNT is 20,000 mg/L. The linear range for the EnSys
method extends from about 0.2 to 10 mg/L, so a dilution of at least 1:2000 is necessary to allow
accurate determination. Similar dilutions are necessary for the other colorimetric and immunoas-
say methods and are probably required for the IMS method as well, although this is yet to be deter-
mined. In the laboratory, dilutions of this magnitude are generally conducted using volumetric
pipets and serial dilution techniques. In the field, however, this approach is cumbersome and pro-
duces a significant amount of solvent waste that requires costly disposal. A simple one-step ap-
proach is possible using glass syringes that can deliver microliter quantities of extract. The follow-
ing experiments were conducted to assess whether the precision and accuracy of this approach is
acceptable for this application.
Three experiments were conducted to assess the precision of the syringe dilution. Acetone soil
extracts from three field-contaminated soils from Volunteer AAP were used. Five replicate dilutions
for each extract were made using 10-L glass microliter syringes (Hamilton #701, Reno, Nevada).
For one sample, a 1:2000 dilution was made by diluting 10.0 L of the filtered acetone extract into
20.0 mL of 1:1 methanol/water. In another, a 1:10,000 dilution was made by diluting 2.0 L of the
acetone extract to 20.0 mL of 1:1 methanol/water. For the third, a 4.0-L aliquot was diluted with
20.0 mL of 1:1 methanol/water (1:5000). The methanol/water diluent was selected because analyte
concentrations for the diluted samples were determined using RP-HPLC.
The analyte concentrations obtained in the 1:5000 syringe dilution study were compared with a
1:5000 dilution obtained using a two-step serial dilution with glass volumetric pipets and volumet-
ric flasks to assess the accuracy of the syringe dilution approach. This was a two-step dilution: 5.00
mL of extract diluted to 100.0 mL with acetone followed by a 1.00 mL dilution to 250.0 mL with
methanol/water. The diluted extracts were determined using RP-HPLC.
Results of the syringe dilution tests are presented in Table 5. For Volunteer sample #1, a 2.0-L
volume of filtered extract was diluted to 20 mL and the precision, as measured by the relative stan-
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