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Title: Vapor signatures from miltary explosives, part 1 : Vapor transport from buried military-grade TNT
Authors: Jenkins, Thomas F.
Leggett, Daniel C.
Ranney, Thomas A.
Keywords: 2,4-DNT
Partition coefficients
Chemical detection
SPME (solid phase microextraction)
Vapor signatures
Explosives detection
Publisher: Cold Regions Research and Engineering Laboratory (U.S.)
Engineer Research and Development Center (U.S.)
Description: Special Report
Abstract: Crystals of military-grade TNT were placed beneath 2.5 cm of soil in enclosed 40-mL amber vials and the accumulation of signature vapors in the headspace above the soil was determined as a function of time. Three different soils—sand, silt, and clay—were investigated at three different moisture contents: air dry, low moisture, and high moisture. Two replicates of each combination of soil type and soil moisture were equilibrated at three temperatures (23, 4 and –12°C) over periods ranging from 63 to 173 days. The headspace was sampled by a polyacrylate solid phase microextraction (SPME) fiber for periods ranging from 5 to 20 minutes, and analytes were desorbed in the injection port of a gas chromatograph equipped with an electron capture detector. Mass detection limits using this method were below 1 pg (1 x 10^–12 g) for the major signature chemicals 2,4-dinitrotoluene (2,4- DNT), 1,3-dinitrobenzene (1,3-DNB), and 2,4,6-trinitrotoluene (2,4,6-TNT). At the end of the experiment, the top 5 mm of soil was carefully removed, extracted with acetonitrile, and the extracts were analyzed using RP-HPLC-UV according to SW846 Method 8330. Both the qualitative and quantitative nature of the chemical signature above buried TNT is strongly a function of temperature. At 23 and 4°C, 2,4-DNT was present at the highest concentration in the headspace vapor, 2,4,6-TNT being only a minor component. At –12°C, the more volatile 1,3-DNB predominated. Vapors penetrate the soils in the order sand > silt > clay, with vapor concentrations in the same order. Dry soils are very retentive of TNT vapors, while soil moisture facilitates movement of vapors to the headspace. Soil–air partition coefficients, computed for these three soils at 23 and 4°C for 2,4,6-TNT, ranged from 1.6 x 10^4 mL-air/g-soil for moist sand at 23°C to 3.0 x 10^7 for moist clay at 4°C. Partition coefficients for 2,4-DNT were about an order of magnitude lower. Vapor concentrations for several of the air-dried soils were too low to measure and hence the partition coefficients for dry soils could not be estimated, but were much higher than for the same soil with higher moisture. These results indicate that for detection of buried mines, the largest mass of signature chemicals will be present in the surface soil rather than the overlying air.
Rights: Approved for public release; distribution is unlimited.
Appears in Collections:Special Report

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