The Analytical Method Is Not Just a Technical Detail
In PFAS filter performance testing, the choice of analytical method determines which compounds are measured, at what detection limits, and therefore which removal capabilities are documented. EPA Method 537.1 and EPA Method 533 are both validated liquid chromatography-tandem mass spectrometry (LC-MS/MS) methods for PFAS in drinking water, but they target different compound spectra and achieve different detection limits for overlapping analytes. A test report generated using Method 537.1 and a report from the same filter generated using Method 533 will appear to show different performance profiles — not because the filter behaves differently, but because the two methods measure different things.
This analytical distinction has direct implications for filter certification. NSF/ANSI 53 tests for PFOA and PFOS have historically been conducted using Method 537.1 or its predecessor, Method 537, which were developed with long-chain PFAS as the primary targets. Method 533, finalized by EPA in 2019, was specifically designed to extend PFAS drinking water monitoring to short-chain ionic PFAS that Method 537.1 does not reliably quantify. For a procurement officer comparing test reports from different manufacturers, verifying which method was used is not optional — it determines whether the performance data is comparable.
EPA Method 537.1: Design, Scope, and Limitations
Method 537.1 uses solid-phase extraction (SPE) with polymeric sorbent cartridges followed by LC-MS/MS analysis. It is validated for 40 PFAS compounds with carbon chain lengths from C4 to C14. However, its practical performance for very short-chain carboxylates (C4-C5) and some short-chain sulfonates is limited by matrix interference and recovery efficiency. The method reporting limits for PFOA and PFOS are approximately 1 ppt, which is adequate for demonstrating removal to well below the 4 ppt MCL from typical challenge concentrations. For PFBS (C4 sulfonate) and PFBA (C4 carboxylate), Method 537.1 reporting limits may be in the range of 2-5 ppt, which is insufficient to verify removal to sub-MCL concentrations for hazard index compounds.
The practical consequence is that a filter tested only under Method 537.1 has documented PFOA and PFOS performance but has uncertain or absent analytical coverage for short-chain PFAS. Manufacturers who test under Method 537.1 are not being deceptive — they are using the method that NSF/ANSI 53 requires and that is appropriate for the compounds covered by that standard. The limitation is structural: the standard was written around the method, and the method was optimized for long-chain compounds.
EPA Method 533: Design, Scope, and Advantages for Short-Chain PFAS
Method 533 was developed to address Method 537.1’s limitations for short-chain and ionic PFAS. It uses a different SPE sorbent chemistry (weak anion exchange), an alternative LC column, and modified gradient conditions that improve retention and separation of C4-C7 PFAS carboxylates and sulfonates. Method 533 is validated for 25 PFAS analytes with reporting limits as low as 0.3 ppt for many compounds, significantly below Method 537.1’s performance for the same compounds. For PFBS, PFBA, PFPeA, and PFHxA — compounds that appear in contaminated water systems with increasing frequency as long-chain PFAS sources are removed — Method 533 provides the analytical resolution needed to assess filter performance at regulatory-relevant concentrations.
For filter performance testing, Method 533’s key advantage is the ability to quantify short-chain PFAS in filter effluent at concentrations that matter for hazard index compliance. A filter effluent sample from a GAC system treating water with mixed short-chain PFAS contamination may contain PFBS at 3-5 ppt — a concentration that Method 537.1 would flag as a non-detect (below reporting limit) but that Method 533 can quantify precisely. This difference in analytical resolution translates directly into different apparent filter performance and different compliance conclusions.
Selecting the Right Method for Your Testing Purpose
For filter manufacturers seeking NSF/ANSI 53 certification, Method 537.1 is the required method for the current standard. For manufacturers seeking to document performance for EPA 2024 MCL compounds beyond PFOA and PFOS, Method 533 testing is technically necessary. For procurement officers evaluating filter products for facilities subject to the 2024 rule, the relevant question is: does this product’s test report include Method 533 data for PFNA, PFHxS, and the hazard index compounds? If it does not, the compound coverage is incomplete relative to the regulatory requirement.
The ideal test documentation package for a PFAS-reducing filter intended for EPA 2024 MCL compliance includes: Method 537.1 data for PFOA and PFOS (for NSF/ANSI 53 certification); Method 533 data for PFNA, PFHxS, PFBS, PFHpA, and HFPO-DA (for 2024 MCL compound coverage); and ideally KCL or equivalent compound-specific certification for PFNA and PFHxS. Any single method alone is insufficient for the full regulatory scope. Procurement specifications should require this multi-method documentation rather than accepting any single analytical method as definitive.