Sample processing considerations for organics include analyte properties, soil interactions, and processing tools.
As previously discussed, ISM VOC sampling involves soil increment collection directly into methanol. In addition to the logistical sampling, shipping and laboratory processing issues, the analytical detection levels of the VOC analytes of concern should be evaluated. Methanol preservation dictates higher analytical detection levels (DLs), MDLs, limits of detection (LODs), limits of quantitation (LOQs), RLs, or PQLs, due to the required methanol dilution into water prior to purge and trap analysis. In general, PQLs and MDLs may be elevated by a factor of 20–50. The use of alternative VOC trap(s) which retain methanol to a lesser degree thus allowing reduced methanol dilution, e.g. “J” or “BTEX” traps, should be considered if appropriate for the volatile contaminant. Reduced methanol dilution will result in lower analytical limits.
If the detection limits from the ISM VOC sample will not meet project DQOs, alternative VOC sampling, such as low-level discrete sampling, should be considered. In some cases, the use of SIM may be necessary to achieve adequate analytical sensitivity in methanol-preserved samples.
In general, the sample processing and drying issues previously discussed should be considered for SVOCs. Analyte loss of the more volatile (e.g., chlorinated benzenes, naphthalene-range PAHs) or less stable SVOCs (e.g., phenols, benzidines, etc.) during the ISM sample processing has not been extensively studied. Large losses of the lower-boiling-point SVOCs are possible during the air-drying step if they are not strongly sorbed to the soil matrix (Bruce 2003). If, on the other hand, the soil is a highly weathered surface soil with many years of air exposure, then weakly sorbed SVOCs have already been lost, and air-drying at temperatures no higher than what the sample has already been exposed to in the field should not result in additional analyte losses. This principle may not apply to subsurface and other samples that have had limited air exposure in situ or to site-specific soils not normally exposed to indoor air temperatures. See Section 188.8.131.52 for a headspace test description to evaluate the potential loss risk due to volatilization.
Sample processing tool materials can contribute contaminants to the sample, particularly in abrasive operations. The composition of plastic tools should be considered, and soft plastics should be avoided when phthalates are of interest. ISM sample processing should evaluate and consider all these process limitations on a chemical-specific basis.
Milling is not recommended for general purpose application with organic contaminants other than energetics (see USEPA SW-846 Method 8330B). Excessive milling may lead to destruction of organic contaminants, as demonstrated with mechanochemical dehalogenation (or mechanochemical destruction) soil remediation process. See Reference Guide to Non-Combustion Technologies for Remediation of Persistent Organic Pollutants in Stockpiles and Soil (USEPA 2005) for additional information. The usefulness of particle size reduction by milling for organic COPCs is usually small because of the larger mass (10–30 g or more) normally extracted and analyzed and the particulate “nugget” effect is often minimal.