Navigating the 537Mod to EPA 1633 Transition

Why 1633? 

One of our most frequently asked questions is around why we recommend EPA 1633 ahead of the EPA537Mod internal methods we have offered for many years and thousands of successful samples?  

The short answer is that EPA 1633 is the reference method, published by the EPA, validated by multiple labs using a wide variety of all sample types. EPA 1633 likely represents the largest method validation effort in Office of Water history, and it is now final pending promulgation. Using a reference method that is on the promulgation path is the safe route to protect future liability.“EPA537Mod” is not a method, it’s a designation used by labs to affirm a connection of their inhouse PFAS method SOP to what used to be the only EPA PFAS method. EPA537Mod methods have been validated at a single lab and have not been through extensive peer review. They will also never achieve official or reference status. It is much easier to stand behind data from an EPA reference method than from a lab-specific SOP! 

Continuity? 

The next question is, what about continuity? I want to use lab X’s “EPA537Mod” method because that’s what I’ve used to generate all data previously? 

We created this helpful graphic to track the evolution of PFAS methods since PFAS analysis became commercially available in the early 2000s. It’s an interesting history (Read on below the summary and stay or the much longer history!) 

Summary 

The greatest increase in standardization occurred when the Department of Defense added PFAS to their Quality Systems Manual (QSM) scope in version 5.1 (Table B15). This ensured that all labs running 537Mod under the accreditation umbrella followed Table B15’s list of PFAS best practices including the use of isotope dilution, whole sample analysis, multiple transitions and more. The DoD and EPA then worked together to ensure that EPA 1633 was a logical continuation and codification of Table B15 into a well-validated reference method. Due to this continuity, the innards of a 537Mod DoD method and 1633 are similar enough to ensure overall continuity in data.  The target list will be longer typically (40 vs 24), there are more quality controls and solid/biosolid samples get additional weak anion exchange cleanup, but overall, there is not much change. Accreditation involves the routine running of performance evaluation samples, and the use of batch quality controls ensure that within method accuracy is always maintained.  

So, switch now, as it’s easier to defend data and explain it to stakeholders when the data comes from an EPA reference method rather than from a lab-specific SOP.  If needed, a small comparison study can always be carried out. If you would like a more detailed checklist table that goes into specific PFAS best practices and their impact on data quality between 537Mod methods and 1633, reach out and we will be glad to share that with you. 

The history of PFAS methods  

Now that you’ve read this far, read on for history! 

  • PFAS analysis predates EPA 537 by many years. PFAS was originally run using GC-MS before LC-MS/MS systems became more widely available! Commercial labs such as SGS AXYS started running PFAS in the early 2000s, and there were still several pathways for PFAS measurement including techniques such as ion-pairing extractions. 
  • The bones of the default PFAS method and therefore EPA 1633 are based on an article by Taniyasu et al. (2005) that centered PFAS extraction/cleanup around the Weak Anion Cartridge (WAX). We switched to this approach circa 2006 and were performing isotope-dilution/surrogate standard analysis of PFAS in aqueous, solid and tissue samples on a short list of 13 targets well before EPA 537 was released! 
  • When EPA 537 came around, it took a different route. It was focused solely on drinking water, used a styrene-divinylbenzene cartridge that excluded short-chain PFAS (<C6) and did not incorporate isotope dilution/recovery correction. This was an approach designed to maximize simplicity and minimize costs for drinking water testing, but the side effect was that the approach could not be used for any type of sample other than drinking water. The lack of isotope dilution meant that there are no robust ways to account for losses and interferences in more complex samples using EPA 537.  
  • This led to a lab conundrum, the availability of an EPA method meant data users wanted their data referenced to this method. But EPA 537 was clearly not designed to run groundwater, or soil or waste samples. Thus, was born “EPA537Mod”, which was a handy designator labs used to tell their clients that they were getting a PFAS method, and connecting it to EPA 537, the only PFAS method in the books. In practice, the designation did not carry any meaning as the method itself had no connection to 537 other than potentially an analyte list and the use of LC-MS/MS. The extent of modification was well beyond the allowed 537 scope. These methods had no standardization, some were direct injection methods, some used cleanup, some used isotope dilution, the extent of labeled standard usage would vary, this was truly the wild wild west! The EPA even issued a memorandum (I can’t seem to find this on the internet anymore) discouraging the practice of using EPA 537Mod as a method designation! 
  • When the Department of Defense started tackling PFAS contamination at their sites, the lack of standardization led to the DoD releasing an update to their QSM (Table B15 in Version 5.1) to add a set of PFAS best practices that needed to be followed to obtain accreditation. While this led to much work at labs to ensure compliance, it also was the biggest step in PFAS measurement standardization in the absence of a reference method. But the understanding was always that this was an interim measure and the ultimate goal was always a standard reference method. Table B15 methods were the basis of much of the PFAS site characterization at military sites and performed quite well.  
  •  When the EPA and DoD collaborated to take the SGS AXYS inhouse DoD QSM method and turn it into 1633, it had already been essentially accredited under the DoD QSM. So, the jump to 1633 was mostly filling some target analyte list gaps, and a massive validation exercise that put the method through the Office of Water wringer of multiple sample types per matrix, multiple spiking levels and lots of replicates. This single-lab validation was, as is well known, followed by a large multi-lab validation. So, what you see in 1633 replicates the push to standardization of the isotope dilution-WAX-LC-MS/MS approach we had been running since 2006 or so with target lists expanding over time and best practices such as whole sample analysis, transition ratio checks and more being added as we learned more about the complexities of PFAS analysis. It also represents the alignment of the DoD QSM PFAS requirements into an EPA reference method.   

References 

Taniyasu, S. et al. Analysis of fluorotelomer alcohols, fluorotelomer acids, and short- and long-chain perfluorinated acids in water and biota. Journal of Chromatography A 1093, 89–97 (2005).