Vapor Signatures from Military Explosives. Part 1. Vapor Transport from Buried Military-Grade TNT

摘要

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 deg 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(exp- 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 deg 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 deg C, the more volatile 1,3-DNB predomin ated. 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 deg C for 2,4,6-TNT, ranged from 1.6 x 10(exp 4) mL-air/g-soil for moist sand at 23 deg C to 3.0 x 10(exp 7) for moist clay at 4 deg C. Partition coefficients for 2,4-DNT were about an order of magnitude lower.