Analysis of PFAS Extractables in Filtration Products Using Modified EPA 1633
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Contaminants in PFAS testing
EPA 1633 tests for 40 PFAS compounds in matrices with higher degrees of particulates - wastewater, surface water, groundwater, soil, biosolids, sediment, landfill leachate, and fish tissue. Particulates in solution must be removed prior to LC-MS/MS as they can be detrimental to sample analysis, column longevity, and overall instrument function. EPA 1633 specifically calls for filtration using a syringe filter format after the extraction and clean-up process for aqueous, solid, and tissue samples.
EPA Method 1633 was used as a guide method to demonstrate that Millex® polyethersulfone (PES) and nylon syringe filters and Millipore® hydrophilic polypropylene filter membranes do not impart detectable levels of PFAS contamination and allow for acceptable analyte recovery.
Testing Millex® PES and Nylon Syringe Filters with Methanol Samples using Modified EPA 1633
Materials and Methods
To investigate PFAS extractables in methanol solvent, PFAS contamination of Millex® syringe filter devices was tested according to a modified version of EPA 1633 in collaboration with SGS North America (Orlando, FL location).
EPA 1633 requires the detection of a large number of PFAS compounds in high-particulate matrices - either aqueous, solids, or tissues - and thus requires filtration. However, different sample processing and extraction methods are required by the method depending on the percent solids of the samples. Despite this, every sample matrix requires a filtration step after the addition of non-extracted internal standards (NIS) and/or carbon cleanup with a 0.20 µm nylon membrane, which is performed in methanol solvent.
An overview of the method is described in the workflow below, with the LC-MS/MS conditions in Table 1. Briefly, a 5 mL methanol sample was spiked with C-13 labeled extracted (EIS) and non-extracted (NIS) internal standards ranging from 1.25 - 10 ppb, depending on the compound, according to EPA 1633. To determine if sample filtration media contributes to PFAS contamination, the entire sample was passed through the filter. The filtrate was collected and analyzed with LC-MS/MS using a C18 column. Analysis was performed using internal standards. The filters tested included: two lots of Millex® -GP syringe filters (non-sterile, 33 mm filter with 0.22 µm PES membrane, Product No. SLGP033N), two lots of Millex® nylon syringe filters (non-sterile, 33 mm filter with 0.20 µm nylon membrane, Product No. SLGN033N), and two lots of Millex® nylon-HPF syringe filters (non-sterile, 25 mm filter with 0.20 µm nylon membrane and glass fiber prefilter, Product No. SLGNM25).
Workflow for Method Used for Modified EPA 1633
Prepare samples
Filter
LC-MS/MS
Results
There were no detectable PFAS contaminants in any of the Millex® syringe filters for modified EPA 1633 above the reporting limit (RL) for the 40 compounds in any of the replicates and lots tested in methanol. However, in the category of perfluoroalkyl carboxylic acids (PFCAs), the first replicate device in Lot 1 of Millex® nylon-HPF syringe filter devices demonstrated hits that were below the RL but above the minimum detection limit (MDL) of the instrument (Table 2). Because of this, one additional replicate device was tested and PFCAs were not detected above RL or MDL in that device. None of the other devices or lots tested for this material demonstrated hits for any PFCAs or other compounds. Regarding membrane material selection, researchers should always investigate whether levels of chemical extractables that may come after exposure to organic solvents are at appropriate levels.
These results suggest that PES and nylon Millex® syringe filters are reliable and appropriate to use in the filtration with EPA 1633 for analysis of PFAS compounds. Nylon-HPF Millex® filtration devices should be considered when samples require pre-filtration, and researchers should always be aware of the required reporting limits for the method.
Chemical extractables (apart from PFAS compounds listed in Table 2) were not tested in this study. To get the best possible data quality, researchers should always investigate the levels of chemical extractables that may come after certain membrane materials are exposed to organic solvents to make sure that they are at acceptable levels.
Recovery
For EPA Method 1633 testing in methanol solvent, the percent recoveries for filtrates from nylon-based syringe filter devices versus PES-based syringe filter devices were at similar levels (Figure 1). This supports what has been reported in literature for methanol helping to dissociate PFAS molecules from filtration media.
Figure 1.The average percent recovery (mean ± standard deviation (STDEV), n=6 replicates over 2 lots) of C-13 labeled standards for PFBS, PFBA, PFOA, PFOS and PFNA after filtration with nylon Millex® syringe filters (purple, mean±STDEV, n=9 replicates over 3 lots), nylon-HPF Millex® syringe filters (blue with checked pattern) and PES Millex® syringe filters (green with horizontal slash marks). The acceptable QC range for recovery of internal standards is demonstrated by the solid black vertical line to the left for each compound.
Testing Millipore® Polypropylene Membrane Filters with Methanol Samples using Modified EPA 1633
Syringe filter devices are the most recommended and preferred format for filtering samples for LC-MS/MS analysis of PFAS due to ease of use and the range of small volumes that can be processed (10-100 mL). There are instances, however, when syringe filters may not be the best option for filtration; for example, when there are no commercially available syringe filters suitable for a specific application. In these instances, an alternative must be considered. A syringe filter-like device, such as a Swinnex® holder, is a viable alternative. This pressure-driven device holds any cut disc membrane filter of a specific size (13 mm or 25 mm diameter) and is operated in the same way as a traditional syringe filter, thus converting any membrane material into a syringe filter format.
Polypropylene is a durable material compatible with a broad range of solvents and temperatures and demonstrates low extractables, making it appropriate specifically for PFAS-related sample and mobile phase preparation. A challenge with polypropylene is that it is naturally hydrophobic, which makes it challenging to filter aqueous samples. Most commercially available polypropylene disc filters are hydrophobic, such as Millipore® polypropylene membrane filters (Product No. PPTG04700 and Product No. PPTH04700). Though appropriate for solvents such as methanol, it can be challenging to filter aqueous samples. In a few cases, polypropylene can be found in a hydrophilic format (Millipore® hydrophilic polypropylene membrane filters, Product No. PPHG04700 and Product No. PPHH04700). These filters are suited to handle aqueous samples. Thus, realizing the potential for polypropylene material to be used within the context of a variety of PFAS workflows, including mobile phase filtration, we determined the level of PFAS extractables that these filter discs release.
Materials and Methods
Swinnex® filter holder assembly
Hydrophilic Millipore® polypropylene (PP) membrane filters of 0.2 µm pore size were tested for PFAS extractable content. Swinnex® devices (25 mm diameter) were used to convert the disc membrane filters into a Luer-lock based syringe filter device, according to Figure 2. Once assembled, the Swinnex® device can be connected to a Luer-lock syringe barrel with the material to be filtered. Filtration was then performed as with other syringe filter devices. For every disc replicate, a new and clean Swinnex® device was used.
Figure 2.Assembling Swinnex® device with a polypropylene cut disc membrane filter.
Modified 1633
Three lots of the hydrophilic 0.2 µm polypropylene cut disc membrane filters were tested, using n=3 filters per lot. EPA 1633 outlines slightly different strategies of extraction and cleanup for each high-particulate matrix tested. Thus, this study focused on just the filtration step required for all matrices after the addition of non-extracted internal standards (NIS) and/or carbon cleanup performed in a methanol solvent. This was done to focus specifically on whether the cut disc membrane filters tested contained any contamination of the 40 PFAS compounds outlined in the method. This modified method is depicted in the workflow above.
Once each cut disc membrane filter was placed securely into a Swinnex® device, 5 mL methanol samples spiked with C-13 labeled extracted (EIS) and non-extracted (NIS) internal standards according to the Method were passed through each filter. The filtrate was collected for LC-MS/MS using a C18 column using conditions described in Table 1 and analyzed by internal standards. Since the filtration was performed in methanol, none of the cut disc membrane filters required pre-wetting.
Results
As was found for Millex® syringe filter devices, there were no detectable PFAS contaminants in any of the hydrophilic polypropylene cut disc membrane filters tested according to modified EPA 1633 above the RL, ranging from 0.2-5 ppb, or the MDL, ranging from 0.05-1 ppb (Table 3). This indicates that these membranes do not have PFAS extractables at these limits and could be used for PFAS applications where filtration is needed for sample preparation. There were no challenges observed with recovery of any of the internal standards for EPA 1633 in methanol, which is unsurprising. Our studies, and previous studies1 with nylon membranes indicated that exposure to methanol solvent reduced non-specific adsorption of PFAS compounds and internal standards.
Recovery
There are nuances in binding and retention of PFAS compounds leading to different recoveries in the filtrate, as observed with polypropylene, nylon, and polyethersulfone membrane materials (Figure 3). Recovery in methanol samples filtered through 0.2 µm hydrophilic polypropylene is similar to that using 0.20 µm nylon and 0.22 µm polyethersulfone. Together, this indicates that polypropylene cut disc membrane filters can be paired with Swinnex® devices to offer an alternative sample filtration method to syringe filter formats. For PFAS methods where the mobile phase needs to be filtered, polypropylene membrane filters may be used with the appropriate filter holder. Recovery of certain PFAS compounds should be carefully considered as part of this workflow.
Figure 3.The average percent recovery of select C-13 labeled standards in methanol according to EPA 1633 for 0.20 µm nylon syringe filters (purple), 0.22 µm PES syringe filters (blue with checked pattern), and 0.2 µm hydrophilic polypropylene cut disc membrane filters (green with horizontal slash marks). All values are mean ± standard deviation across three lots, for a total of nine discs.
Water versus Methanol Samples
Using syringe filter devices, a trend was observed of increased recovery for nylon devices when filtering methanol versus water, which has also been observed in published studies.1 However, not all membrane materials may respond in the same way to filtering in water versus methanol. For example, hydrophilic polypropylene demonstrated similar recovery within the acceptable QC range of all internal standards for both methanol and water, and in some cases (for example, shorter chain PFCA and PFSA compounds) even higher recovery in water than methanol, which was not observed with either PES or nylon materials (Figure 4).
Figure 4.The average percent recovery of all C-13 labeled standards for hydrophilic polypropylene cut disc membrane filters in water vs. methanol. For water, values are mean ± standard deviation, n=3 replicates of one lot tested. For methanol, values are mean ± standard deviation, n=3 replicates across three lots, for a total of nine discs.
Filters for Modified EPA 1633
Using a modified EPA 1633 method, this study demonstrated that PES and nylon syringe filters, as well as hydrophilic polypropylene membranes held in a Swinnex® device, are reliable and appropriate to use in the analyses of PFAS compounds.
- There were no detectable PFAS compounds in any of these filtration devices.
- The percent recoveries for filtrates from all these devices were at similar levels.
Recommended Filtration Products
References
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