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EPA Method 1633:

Analysis of Per- and Polyfluoroalkyl Substances (PFAS) in Aqueous, Solid, Biosolids, and Tissue Samples by LC-MS/MS

Environmental samples are prepared and extracted using method-specific procedures. Sample extracts are subjected to cleanup procedures designed to remove interferences. Analyses of the sample extracts are conducted by LC-MS/MS in the multiple reaction monitoring (MRM) mode. Sample concentrations are determined by isotope dilution or extracted internal standard quantification (see Section 10.3) using isotopically labeled compounds added to the samples before extraction.

1.0 Scope and Application
1.1 Method 1633 is for use in the Clean Water Act (CWA) for the determination of the per- and polyfluoroalkyl substances (PFAS) in Table 1 in aqueous, solid (soil, biosolids, sediment) and tissue samples by liquid chromatography/mass spectrometry (LC-MS/MS).
1.2 The method calibrates and quantifies PFAS analytes using isotopically labeled standards. Where linear and branched isomers are present in the sample and either qualitative or quantitative standards containing branched and linear isomers are commercially available, the PFAS analyte is reported as a single result calculated from the combined responses of the linear and branched isomers.
1.3 The instrumental portion of this method is for use only by analysts experienced with LC-MS/MS or under the close supervision of such qualified persons. Each laboratory that uses this method must demonstrate the ability to generate acceptable results using the procedure in Section 9.2.
1.4 By their very nature, many components of PFAS present analytical challenges unique to this class of analytes. For example, PFAS analytes readily adhere to the walls of the sample containers and may also stratify in the container. EPA has included procedures in the method that must be employed to address such challenges (see Section 11.0 and Appendices A and B).
1.5 This method is “performance-based,” which means that modifications may be made without additional EPA review to improve performance (e.g., overcome interferences, or improve the sensitivity, accuracy, or precision of the results) provided that all performance criteria in this method are met. Requirements for establishing equivalency are in Section 9.1.2 and include For CWA uses, additional flexibility is described at 40 CFR 136.6. Changes in performance, sensitivity, selectivity, precision, recovery, etc., that result from modifications within the scope of 40 CFR Part 136.6, and Section 9.0 of this method must be documented, as well as how these modifications compare to the specifications in this method. After promulgation, changes outside the scope of 40 CFR Part 136.6 and Section 9.0 of this method may require prior review or approval by EPA under the Clean Water Act Alternate Test Procedure program described at 40 CFR 136.4 and 136.5.
1.6 The target analytes in Table 1 were included in this method based in part on the availability of standards for both unlabeled and isotopically labeled PFAS compounds at the time that the method was first developed. Data from multi-laboratory validation study data (References 11-14) demonstrate that the method is suitable for the analysis of all 40 target analytes in all of the matrices studied, although there are some performance differences across the analyte classes, which is not surprising given the wide range of structures across the nine classes of compounds in that table. Therefore, EPA supports use of the method for those analytes, and has identified the analytes in Table 1 that that did not perform as well in the study as others in some sample matrices. (The analytes noted in Table 1 were able to achieve 50 – 150% accuracy or better in most of the analyses from the multi-laboratory validation study.) Data users and laboratories should take that information into account during project planning.