This is a summary condensed from an article written by Alan D.
Hewitt, published in the Sept/Oct 1999 issue of Environmental Testing and
Analysis, page 18. The article’s title is “Frozen Storage of Soil
Samples for VOC Analysis.” The article reviews with the reader why proper methods of sampling and storage are matters of great concern for the analysis of VOCs, and that freezing samples within En Core samplers or VOA vials can be a highly effective means of sample preservation. It has been shown that in open and unsecured containers volatilization can decrease the VOC concentration by more than 80% within 30 minutes. VOC loss may also occur through biodegradation, which is much more pronounced for BTEX compounds than for chlorinated solvents. To minimize VOC losses during sample collection and storage, USEPA (Method 5035) and ASTM (D4547-98) have recommended the use of chemical preservation in the field by sodium bisulfate solution or by methanol. Alternatively, samples can be collected and held up to 2 days in a refrigerated (4 ±2ºC) airtight container before preservation. Once the sample is preserved, the pre-analysis holding period can be extended for 14 days at 4ºC. These options are problematic because of the labor involved, knowledge of expected contaminant concentrations is needed, shipping methanol can present DOT difficulties, and acidification is not appropriate for some matrices. The study described in this article evaluated the simplification of VOC collection and storage by freezing the sample within its storage device at -12 ±3ºC. The currently recognized collection/storage device (syringe +VOA vials) was used in this study. Data on the En Core samplers, at present, is limited. It was demonstrated that freezing the sample in its storage device immediately after collection preserves VOC concentrations in all samples matrices (including biologically active soils that would tend to degrade BTEX compounds) and for both types of VOC analytes for up to 14 days of storage. Sample integrity was maintained with less than a 5% loss of analyte concentrations even after a 14-day holding time. Freezing can be initiated in the field through the use of dry ice in well-insulated coolers. Alternatively, bags of water ice mixed with table salt may be used to achieve cooler temperatures between -12 and -4ºC. Refrigeration of samples contained within the collection devices for 48 hours at 4 ±2ºC prior to freezing the samples for the remaining 12 days of a 14-day holding time showed good preservation (less than 10% less of VOC analyte concentrations) in all soil types, with the significant exception of a biologically active soil matrix. In a garden loam soil which had been amended with manure, benzene and toluene concentrations decreased 50% or more during the 48 hours of refrigeration. The study demonstrated that BTEX compounds are also more susceptible to volatilization loss during refrigerated storage than chlorinated compounds. http://www.clu-in.org/tiopersp/hewitt.htm |