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PFAS in Air, Part 1: OTM-45

21 Sep 2021

If you’ve been following the news about PFAS, you’ll know they’re everywhere. They’re in the water, they’re in your clothes, and now they’re in your air lab. Not in the sense that your lab has some PFAS background but earlier this year the EPA published OTM-45, a method covering the sampling and analysis of stack gasses for PFAS compounds.

If you’re not familiar with OTM (Other Test Methods), they are methods published which have not been subject to the federal rulemaking process. Per the EPA, “OTM 45 is a draft method under evaluation that will be updated as more data from stakeholders becomes available. Due to the urgent need for consistency, this method is being released as an “Other Test Method (OTM)” by EPA’s Emission Measurements Center to promote consistency with what we believe is the current best practices to sample and analyze the PFAS targets from stationary sources. We solicit any and all feedback, comments, and additional data coming from the application of this method as we work to adjust this method in anticipation of developing a reference method for PFAS from air emission sources.” In short, this is a draft method that the EPA hopes to get feedback on before finalizing, so if your lab is doing or considering PFAS work for stationary sources you may want to consider kicking the tires on OTM-45 so you can have input on the final version.

So what does OTM-45 entail? On the sample collection side, if you’re familiar with SW-846 0010 the sampling train will likely look familiar (see Fig. 1). Following that the various sections of the sampling train are extracted, resulting in 4 final samples. The front half rinse and filter (sections in red and yellow) are combined and extracted via shakeout. The back half rinse and first XAD-2 (green and blue) are combined and also extracted by shakeout. The condensate, impinger water, and impinger rinse (purple and grey) are combined and extracted by Weak Anion Exchange (WAX) SPE,  similar to EPA method 533.  Finally, the breakthrough XAD-2 (pink) is extracted via shakeout.


Fig. 1 – OTM-45 sample train

After extraction the samples are analyzed by LC-MS/MS using an isotope dilution calibration similar to EPA Method 533, though with a much broader compound list. With a lack of data showing what PFAS compounds are likely to be present the EPA seems to have taken the approach of casting the widest net possible. The number of compounds and isotope analogs for pre- and post-extraction internal standards unfortunately means that implementing OTM-45 is more difficult than just throwing new samples at your existing 533 LC-MS/MS method.

As you’ve probably been spoiled by the “Part 1” in the blog title, I’ll be covering OTM-45 in multiple blogs, breaking things down to cover the LC-MS/MS analysis, calibration, sample prep, and background and cleanliness issues. There’s a lot to cover, so stay tuned.


  1. EPA OTM-45 (
  2. EPA Method 533 (