Volatile organic compounds (VOCs) are rapidly volatilized from moist soil and sediment. Losses in VOC concentrations can amount to as much as three orders of magnitude during the sampling, transportation, storage, and analytical processes. These losses usually occur during various sample handling procedures. Use of a narrow-mouth sample container such as a 40 mL volatile organic analysis (VOA) vial necessitates breaking up the soil sample, thus increasing surface area and VOC losses.
Laboratory sample preparation procedures also lead to VOC losses.There is headspace loss when the sample jar is opened. Stirring the sample promotes VOC losses, and the weighing process can lose major amounts of VOCs because of the small sample sizes recommended for purge and trap VOC analysis.
In this study to minimize the losses of VOCs due to sample handling, the soil VOCs were preserved with methanol in the field. The U.S. EPA Contract Laboratory Program (CLP) medium level VOC analytical methodology with minor changes was then followed for quantitative measurement.
In a site investigation where seven chlorinated chemicals were present, samples were taken from a split spoon using both the 40 mL VOA vial soil sampling and the methanol field preserved sampling methods. The analyses of these samples indicated a difference of approximately two orders of magnitude between the two methodologies. The methanol method gives results that are more accurate and representative of the environmental field conditions. However, the detection limit is about 250 μg/Kg. The theoretical detection limit for low level VOC analysis using CLP methodology is 5 μg/Kg, however, given the documented VOC losses of approximately two orders of magnitude this low level detection limit specification is misleading. After accounting for potential VOC losses due to sample handling the methanol preparation methodology appears to be more applicable to measurement of low levels of VOCs in soil than existing procedures.