EPA Method 1633: the 40 / 24 / 7 PFAS reference standard set

What EPA 1633A is, and where it stands legally

Full title: Method 1633A — Analysis of Per- and Polyfluoroalkyl Substances (PFAS) in Aqueous, Solid, Biosolids, and Tissue Samples by LC-MS/MS, document EPA 820-R-24-007, issued December 2024. It was developed by the US EPA Office of Water together with the US Department of Defense through the SERDP programme, project ER19-1409.

Before version A, the method went through four drafts (August 2021, June 2022, December 2022, July 2023) and a final version in January 2024. Method 1633A of December 2024 incorporates the errata of the earlier version and is the only version EPA still posts. If a laboratory’s documentation still cites “Method 1633 (January 2024)”, it is worth updating.

The multi-laboratory validation was led by the US Department of Defense with 10 laboratories, across 15 aqueous matrices and 9 solid matrices, published in four volumes between July 2023 and January 2024.

The 40 / 24 / 7 structure — three groups of standards the method requires

This is the part that decides what a laboratory has to buy.

EPA 1633A quantifies by isotope dilution: each target analyte is measured against a corresponding stable isotope-labelled compound (¹³C, ²H, ¹⁸O). Results are therefore corrected for any loss during extraction, cleanup and concentration. The method splits the standards into three groups with three different roles.

That is 71 distinct reference materials in total. US Department of Defense documents call the third group “IIS”; the EPA method itself uses “NIS” — the same seven compounds.

The seven NIS compounds

PFBA ¹³C₃ · PFHxA ¹³C₂ · PFOA ¹³C₄ · PFNA ¹³C₅ · PFDA ¹³C₂ · PFHxS ¹⁸O₂ (sodium salt) · PFOS ¹³C₄ (sodium salt).

Concentrations are chosen by the laboratory, not dictated by the method

This is often misunderstood at the ordering stage. The method states that the concentrations in Table 4 are those used in the validation studies and are provided as guidance; laboratories may use other concentrations and may widen the calibration range above or below them. Commercial mixtures are therefore sold at convenient stock concentrations for dilution, not at mandated values.

The method also recommends directly that commercial standard solutions or mixtures should be purchased, and that preparation from neat materials is only necessary when these are not available. For calibration, a minimum of six contiguous standards is required with a linear model, or a minimum of seven with a second-order model.

Eight matrices and three sample-preparation routes

Why 1633A does not replace the drinking-water methods, and vice versa.

EPA publishes the method for eight matrices: wastewater, surface water, groundwater, landfill leachate, soil, sediment, biosolids and fish tissue. The method itself writes its procedures for three groups, so both counts are correct — it is only worth being explicit about which one is meant.

Carbon cleanup is mandatory in all three routes. That is the significant technical difference from the two drinking-water methods, EPA 537.1 and 533, which have no mandatory cleanup step and were evaluated by EPA for drinking water only. Wastewater, sludge and fish tissue are far more complex matrices, so a drinking-water method cannot simply be borrowed for them.

Four mixtures and the full list of 40 target analytes

The Dr. Ehrenstorfer range for EPA 1633, checked against QEMIX stock.

The neat structural point of this range is its one-to-one mapping onto the method’s three groups of standards: one mixture of 40 native targets, one of 24 EIS, one of 7 NIS. The manufacturer’s table of 40 target analytes matches Table 1 of the method exactly, which is a quick way to verify that the standard set covers the full reportable list.

The 40 target analytes: neat materials and single solutions

Mixture compositions and 31 single labelled solutions

The newest addition to the range, announced on 30 September 2025.

The two labelled mixtures remove the need to blend standards in-house and to run stability studies on them. The manufacturer cites three practical benefits: fewer preparation steps and calibration runs, higher sample throughput, and compensation for matrix effects and instrument variability. Below are the full compositions with the concentration of each component — the data needed to check the mixture against the analyte list your method has to report.

The 24 components of the EIS mixture

The 7 components of the NIS mixture

31 single ¹³C and deuterated solutions

In the manufacturer’s literature the EIS group is labelled ES (Extraction Standard — added before extraction) and the NIS group IS (Injection Standard — added before injection). The EIS/NIS naming used here is the EPA method’s own terminology.

How 1633A, 537.1, 533, 8327 and UCMR5 differ

Picking the wrong method means buying the wrong standard set, and finding out after the first batch has run.

The rule of thumb: a laboratory testing drinking water to a US-style programme needs 537.1 and/or 533 standards. A laboratory testing industrial wastewater, treated effluent, sludge, soil, sediment, leachate or fish needs the Method 1633 set. They are not interchangeable, and the 1633 set is substantially larger.

Other method standard sets held at QEMIX

The last code is the mixture used in the Shimadzu study QEMIX examined in 32 PFAS in baby food by LC-MS/MS — a concrete illustration of why the analyte list has to be checked before ordering.

Why PFAS testing is growing, and what to check before you order

Three pressures from three directions, and five practical checks.

The Stockholm Convention

Vietnam ratified the Stockholm Convention on 22 July 2002. Four PFAS groups have since been listed:

The long-chain PFCA listing added in 2025 widens the list of substances Parties, Vietnam included, are expected to address. Vietnam’s National Implementation Plan records a PFOS inventory carried out in 2015 and reports PFOS, PFOA and other PFAS detected in surface water, groundwater, soil, sediment, sludge, wastewater and fish, alongside a stated need to build a network of dedicated POPs monitoring laboratories.

Domestic regulation

Decree 08/2022/ND-CP, implementing the 2020 Law on Environmental Protection, is the Vietnamese instrument covering POPs and POP-containing materials, including PFOA under Annex A.

Export-market pressure

EU Directive 2020/2184 on water intended for human consumption sets two limits: 0.10 µg/L for the sum of 20 named PFAS and 0.50 µg/L for PFAS Total. Both became applicable on 12 January 2026, the first time PFAS in drinking water is monitored systematically across the EU. For Vietnamese exporters of seafood, food and textiles, PFAS testing requirements usually arrive from customers and supply chains before they arrive in domestic regulation.

Five things to check before you order

• The right version of the method. The current one is 1633A, December 2024. Method validation files should cite that version.
• All three groups. Without the EIS mixture there is no isotope dilution; without the NIS there is no check on injection performance or EIS recovery.
• Solvent system. All three mixtures are in methanol:water, compatible with the method’s SPE and LC-MS/MS steps. Changing solvent means re-evaluating the method.
• Working concentrations. The method does not mandate them, so calculate from the calibration range you intend to use before choosing a stock concentration.
• COA matched to the lot. For a multi-component mixture the certificate gives the concentration and uncertainty of each component per lot — check the lot on the packaging before filing it.