Sample preparation inconsistencies could affect reported PFAS results for aqueous samples containing particulates. Current EPA analytical methods for PFAS address solids-free, drinking water only. Sample preparation and extraction procedures of aqueous samples containing suspended solids vary amongst laboratories. Depending on the laboratory and the analyst, solids present in the sample may or may not be included in the final extract for PFAS analysis. Several laboratories centrifuge the samples and decant off the water for subsequent extraction. Some laboratories may also extract the remaining particulates and combine the extract with the aqueous extract. Laboratories that are not including the extraction of the particulates (i.e., water only) are not providing a total PFAS measurement of these aqueous samples; this may be acceptable in certain instances depending on the ultimate project objectives.
The problem with not controlling or not knowing whether solids are included in the analysis is that PFAS may partition to the solids, especially the longer-chain PFAS. If these solids settle out or are removed from the water samples, reported PFAS concentrations in the sample can be lower, possibly by an order of magnitude or more than if the solids were included in the analysis as a “total” sample. Moreover, the reported mixture of PFAS compounds (i.e., fingerprint) may differ. Thus, we need to work with the laboratory in advance to determine the appropriate procedure to be used, depending on the end use of the data (e.g., remedial action, human or ecological risk assessment, compliance, etc.). The project objectives may also vary depending on the water matrix (i.e., groundwater, wastewater, stormwater).
