In situ thermal desorption of VOCs in vadose zone soils

Abstract

A volatile organic compound (VOC) sampler developed for the site characterization and analysis penetrometer system (SCAPS) program to provide semi-quantitative field screening data was evaluated in the laboratory and in the field. The device is based on thermal desorption principles and is capable of sampling in the vadose and capillary zones. Approximately 5 g of soil is desorbed in situ, and the volatilized compounds transferred to the surface, where they are trapped and analyzed by ion trap mass spectroscopy (ITMS). Ex situ validation samples from a Mostap™ core taken 9 in. away from the in situ sample are of two types. One sample is placed in the sampler and desorbed and analyzed in the same manner as the in situ sample. The second validation sample is preserved in methanol and analyzed off site by GC/MS. Comparisons of in situ data to ex situ validation data show that subsurface heterogeneity strongly affects correlation, especially in zones of bedded sand and silt. When the two ex situ samples from the Mostap™ core are compared, the subsurface heterogeneity factor is removed and the co-efficient of determination (r2) increases to 85%. © 1998 John Wiley & Sons, Inc. Field Analyt Chem Technol 2:163–171, 1998