Analysis of PFAS Extractables in Glass Fiber Membrane Filters After OTM-45 Front-Half Extraction
OTM-45 Method for Air Monitoring of PFAS Compounds
The United States EPA PFAS Strategic Roadmap announced in 2021 includes a focus on developing methods for air monitoring of PFAS compounds (indoor, ambient, and industrial air monitoring). OTM-45 for monitoring PFAS compounds in air emissions from stationary sources (such as stack gas), modified from EPA Method 5, SW-846 Method 0010, was released in 2021 as the first draft method, to be updated as more data becomes available. The method includes sample collection train of seven stack gas fractions, one of which is a particulate fraction collected on a glass fiber particulate filter. Fractions are extracted and analyzed using LC-MS/MS. As with any PFAS method, it is critical that any consumables and sample preparation equipment, including filters, are not contaminated with PFAS analytes. To avoid this, an extensive cleaning process of the particulate filter fraction is commonly performed in basic methanol and methylene chloride prior to sampling and extraction. After fractions are extracted, laboratories may also use syringe filters to remove particulates before LC-MS/MS; syringe filter recommendations can be found on our “PFAS Testing Methods and Guidance for Sample Filtration” page.
We studied the suitability of Millipore® AP-40 glass fiber filters without binders for use in the particulate phase sampling fraction of OTM-45. Our data demonstrated that Millipore® AP-40 glass fiber filters without binder are appropriate for use as the particulate filter fraction in the sampling train for OTM-45 testing of PFAS compounds in air emissions from stationary sources.
Method for Testing Millipore® Glass Fiber Filters Using Front-half Extraction
Two lots of blank AP-40 filters were tested. PFAS analysis demands attention to cleanliness, as even trace contaminants can significantly skew results. Consequently, methods incorporate rinsing or cleaning steps, often supplemented by laboratory-specific SOPs. To align with such cleaning methods prior to sampling and front-half extraction of OTM-45, filters were first soaked in 5% (v/v) NH4OH in methanol for 30 minutes. Filters were then soaked in methylene chloride for 30 minutes (Figure 1). In this method, the on-site sampling step was skipped to isolate and study filter cleanliness. For the front-half extraction method, C-13-labeled internal standards were added to the cleaned filters. Filters were then soaked in methanol for extraction on a shaker table for 18 hours. The extract was concentrated and evaporated using hot-block concentration followed by a blow-down step. Samples were then analyzed by LC-MS/MS using internal standards (Figure 2).
Figure 1.Schematic outline for cleaning step used in testing Millipore® glass fiber cut disc membranes without binder, 90 mm diameter, for PFAS contamination using OTM-45.
Figure 2.Schematic outline for front-half extraction step used in testing Millipore® glass fiber cut disc membranes without binder, 90 mm diameter, for PFAS contamination using OTM-45.
Analysis of PFAS Extractables After Front-Half Extraction
Out of the 49 PFAS analytes tested, none were identified in concentrations above the reporting limit (RL) of 1.00 ng/sample tested (Table 1). Further, the results were the same for a competitor glass fiber disc also tested. Because OTM-45 includes many compounds of widely varying properties, including polarity, there is a wide QC range suggested for the method (25-150%). Thus, there can be a degree of uncertainty in quantifying certain compounds according to the method, particularly fluorotelomer sulfonic acids, fluorotelomer carboxylic acids, and fluorotelomer unsaturated carboxylic acids. It is expected that in the future, additional methods focused on non-polar versus polar PFAS compounds will alleviate some of these concerns.
These results suggest that Millipore® AP-40 glass fiber filters without binder are reliable and appropriate to use in the filtration with OTM-45 for analysis of PFAS compounds.
Additional compounds tested, not shown in Table 1:
- Perfluorooctane Sulfonamides: PFOSA/FOSA, N-EtFOSA, N-MeFOSAA (Result: ND, all samples)
- Perfluorooctane Sulfonamidoacetic Acids: N-EtFOSAA, N-MeFOSAA (Result: ND, all samples)
- Perfluorooctane Sulfonamido Ethanols: N-MeFOSE, N-EtFOSE (Result: ND, all samples)
- Fluorotelomer Sulfonic Acids: 4:2 FTS, 6:2 FTS, 8:2 FTS, 10:2 FTS (Result: ND, all samples)
- Per and Polyfluoroether Carboxylic Acids, Per and Polyfluoroether Sulfonic Acids, Fluorotelomer Carboxylic Acids and Next-Generation PFAS Analytes: ADONA, GenX, 9Cl-PF3ONS, 11Cl-PF3OUdS, 6:2 FTUCA, 7:3 FTCA, 10:2 FTCA, 8:2 FTCA, PFEESA, 8:2 FTUCA, PFMPA, PFMBA, 5:3 FTCA, 6:2 FTCA, 3:3 FTCA, PFECHS, NFDHA (Result: ND, all samples)
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