By Nick Nigro 

The U.S. Environmental Protection Agency (EPA) has set its sights on PFAS, the class of chemicals that has come under intense scrutiny for its negative impact on human and environmental health. PFAS were once prized for durability, but now that durability lends them the ominous nickname “forever chemicals.” The EPA’s 2021-2024 PFAS Strategic Roadmap calls for an increased focus on the research, restriction, and remediation of PFAS across a variety of matrices. However, to date, the only EPA-validated test methods are focused exclusively on drinking water, leaving a need for the agency to develop and validate new methods to support the strategy. Arguably, the most essential of these is Draft Method 1633. 

Compared to previous methods, Draft Method 1633 has been on the development fast track. The first draft was published in August 2021, and by October of that year, the multilab validation study had begun. The current version, 3rd Draft Method 1633, was published in December 2022. This version includes the “final” quality control (QC) limits for wastewater. 

The fourth draft is anticipated in 2023 and is expected to include QC limits for groundwater and surface water matrices. Once the draft is finalized, expected by the end of 2023, Method 1633 will likely include QC limits for all non-potable waters, including landfill leachate. It will also cover a variety of solids, including soil, sediment, biosolids, and biota (tissue). Hopefully, it will include further editorial revisions and clarifications of lingering questions and issues. 

Draft Method 1633 vs. Legacy Methods
Before Draft Method 1633 was written, labs that wanted to test for PFAS in non-potable water and other solid matrices had to develop their own methods. Many labs utilized the core technical elements for the EPA-published drinking water method—EPA Method 537—into a method universally called 537M or 537 Modified. 

The use of the label Method 537M has caused some confusion. One of the most common questions asked is whether Method 537 is suitable for analyzing PFAS in non-potable water, specifically wastewater. To be clear, Method 537 is not designed for liquids with suspended solids. So many modifications need to be made that the modified method should not be labeled 537.  

Other labs developed methods that incorporate the requirements in the U.S. Department of Defense Quality System Manual (QSM), Table B-15. QSM Table B-15 isn’t a method but a table of quality control protocols for the testing of PFAS in non-potable water, solids, aqueous film-forming foam, and biota.  

In many cases, the protocols in Draft Method 1633 resemble the protocols used in previous methods.  

The chart below is a high-level comparison of Method 1633 against legacy modified methods, such as 537M. Two other methods that are sometimes used for wastewater projects and solid samples are included. Both EPA Method 8327 and ASTM D8421 are relatively low-cost methods. However, Draft Method 1633 is being developed to meet the needs of the EPA’s PFAS Strategic Roadmap. 

What’s New in the 3rd Draft Method 1633?
The EPA’s website describes the third draft as making only minor updates to the method, though the term “minor” is no doubt relative. Below is a summary of the key changes in the third draft, with section references directly from the method itself. 

Section 1.6 acknowledges that some PFAS analytes do not perform well. During the single lab validation phase, we noted potential problems with the perfluorooctane sulfonamide (FOSA) and perfluorooctane sulfonamidoethanol (FOSE) compounds, particularly in biota samples.  

Section 7.1.17 includes a note specifying that stacked solid phase extraction (SPE) cartridges will be allowed in place of SPE plus dispersive carbon for wastewater matrices. The note explains that this flexibility may be extended to other matrices in subsequent revisions, so we will have to wait for future drafts to see if that’s the case.  

Section 11.0 now requires labs to prescreen in the absence of source-specific knowledge. This leaves a lot of unanswered questions. Are labs required to report the prescreening data? If not, how will this requirement be enforced? Also, does an educated guess on the part of the client constitute source-specific knowledge? Appendix A contains a suggested screening protocol but notes that other procedures can be used. The lack of defined protocols raises the question of how reliable the data coming from other approaches may be. Hopefully, this will all be cleared up in future drafts. 

Section provides for a trained analyst to omit the total suspended solids analysis in aqueous samples by simple visual inspection.

Section 11.2.4 suggests centrifugation may be used, but it is not explicit. This will need to be clarified in future drafts, or it will be a point of contention with the data and likely lead to considerable variability between lab standard operating procedures. In the world of environmental compliance, variability between lab
methods is problematic.

Table 5 sets acceptance limits for laboratory control samples (LCS), referred to as OPR (ongoing precision and recovery standard) targets in the draft, and Table 8 sets the extracted internal standards (EIS) recovery acceptance limits for waste water. Once again, we will have to wait for future drafts for these limits in other matrices.

Despite wide acceptance ranges for many compounds, the third draft states that some laboratories will still struggle to achieve results within the specified ranges. In some cases, EIS recovery limits for real-world wastewater samples are narrower than the EIS recovery limits for LCS prepared from clean reagent water. It doesn’t make sense for sample acceptance criteria to be wider for clean water than for presumably more-complicated wastewater samples. We anticipate changes in Tables 5 and 8 before finalization.  

What’s Next for Draft Method 1633?
Although Draft Method 1633 has been on a fast track to validation, the single and multilab validation processes have been as thorough as ever. As is often said, Draft Method 1633 is designed to be the “one method to rule them all” (so long as “all” doesn’t include drinking water). Defining protocols across such a wide array of matrices is understandably challenging. Nevertheless, there are many questions left to be answered before this method is finalized. Once it is, the industry will have a powerful tool for better understanding the prevalence of PFAS and the path these compounds travel through the environment. 


About the author
Nick Nigro is a product manager at Pace Analytical, responsible for PFAS operations at seven emerging contaminant centers of excellence within Pace’s nationwide network of environmental testing laboratories.

About the company
Pace® has over four decades of experience in advancing science and informing decision-makers. Pace® people support our partners with data, expertise, and local-level service backed by a national laboratory network. To read more about our commitment to making the world a safer, healthier place through science, visit


Comments are closed.