Analysis of PFAS Extractables in Filtration Products Using Modified EPA 537.1

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• Contaminants in PFAS Testing
• Testing Millex^® PES and Nylon Syringe Filters With Water Samples Using Modified EPA 537.1
• Testing Millipore^® Polypropylene Membrane Filters With Water Samples Using Modified EPA 537.1
• Recommended Filtration Products

Contaminants in PFAS testing

A key consideration for any PFAS method is to avoid contamination that can impact the accuracy of data, including those coming from sample preparation techniques such as filtration. Currently, most PFAS analytical methods are for “clean” matrices, such as drinking water, and often do not require filtration as a part of sample preparation. However, methods such as SW-846 Method 8327, ASTM D7968, ASTM D7979, EPA 1633, and ISO 21675 involve matrices that could have a higher degree of particulates, such as wastewater. 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. These methods identify the need for filtration using membranes in a syringe filter format.

A modified EPA 537.1 method was used to demonstrate that Millex^® polyethersulfone (PES) and nylon syringe filters and Millipore^® polypropylene membranes do not exhibit detectable levels of PFAS contaminants resulting from fluorinated extractables.

Testing Millex^® PES and Nylon Syringe Filters with Water Samples using Modified EPA 537.1

Materials and Methods

Three lots of polyethersulfone (PES), three lots of nylon, and two lots of nylon-HPF (nylon membrane with a glass fiber prefilter for high-particulate filtration) Millex^® syringe filter devices were tested for PFAS extractables according to EPA Method 537.1. Some PFAS compounds not required by the method, including next-generation PFAS compounds and fluorotelomer sulfonates, were also included. EPA 537.1 does not require sample filtration, but filtration was used to provide a clean sample to test extractable contamination levels from the syringe filter.

An overview of the modified method EPA 537.1 is described in the workflow below, with the LC-MS/MS conditions in Table 1. Briefly, a 250 mL PFAS-free DI water sample was spiked with surrogates. The internal standard spike of 0.08 ppb was used for QC blanks. To determine if sample filtration media contributes to PFAS contamination, the entire sample was passed through the filter and into a styrene divinylbenzene (SDVB) SPE cartridge. The sample bottles and tubes were rinsed with basic methanol, then passed through the filter and into the cartridge. The entire sample was then subjected to SPE and concentrated to 1 mL in 96:4% (v/v) methanol:water prior to LC-MS/MS analysis using a C18 column. Analysis was performed using internal standards. C-13 labeled standards were used in this study. The filters tested included: three lots of Millex^®-GP syringe filters (non-sterile, 33 mm filter with Millipore Express^® PLUS PES membrane) in both 0.22 µm (Cat. No. SLGP033N) and 0.45 µm (Cat. No. SLHP033N) pore sizes, three lots of Millex^® nylon syringe filters (non-sterile, 33 mm filter with nylon membrane) in 0.20 µm pore size (Cat. No. SLGN033N), and two lots of Millex^® nylon-HPF syringe filters (non-sterile, 25 mm filter with nylon membrane and glass fiber prefilter) in 0.20 µm pore size (Cat. No. SLGNM25).

Workflow for method used for modified EPA 537.1:

Results

There were no detectable PFAS contaminants in any of the Millex^® devices tested using modified EPA 537.1 above the reporting limits (RL) or the minimum detection limits (MDL), even given the very low thresholds of 0.0020-0.0080 ppb and 0.0010-0.0020 ppb, respectively, depending on the compound (Table 2). The results were the same for the three different lots of the 0.22 and 0.45 µm PES membranes, three different lots of 0.20 µm nylon membranes, and two different lots of 0.20 µm nylon-HPF membranes. These results suggest that PES, nylon and nylon-HPF Millex^® filtration devices are reliable and appropriate to utilize in the filtration of water samples in preparation for the analysis of these PFAS compounds.

Recovery

In this study, sample tubes and sample bottles were rinsed with basic methanol. The recoveries of the C-13-labeled standards were within the acceptable QC range for the method. However, recovery varied from filter material to filter material (Figure 1) and from compound to compound. For example, nylon-based filter devices demonstrated lower recoveries than those with PES. In the case of nylon membranes, non-specific adsorption of internally spiked standards and sample can be reduced using a methanol rinse. Similar results were found using PES membranes alone (without housing and filtered using a 25 mm Swinnex^® filter holder device).

Testing Millipore^® Polypropylene Membrane Filters with Water Samples using Modified EPA 537.1

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.

ASTM D7979 (for PFAS detection in water matrices excluding drinking water) and ASTM D7968 (for PFAS detection in environmental solids) both suggest the use of polypropylene for sample preparation and filtration. 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 (Cat. No. PPTG04700 and Cat. 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, Cat. No. PPHG04700 and Cat. 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 and hydrophobic Millipore^® polypropylene (PP) membrane filters of 0.2 µm and 0.45 µm pore sizes were tested for PFAS extractable content. Swinnex^® devices (25 mm diameter) were used to convert the various 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.

Modified EPA 537.1

In this part of the study, four types of 25 mm cut disc membrane filters were tested:

• 0.2 µm hydrophobic PP (Cat. No. PPTG02500)
• 0.45 µm hydrophobic PP (Cat. No. PPTH02500)
• 0.2 µm hydrophilic PP (Cat. No. PPHG02500)
• 0.45 µm hydrophilic PP (Cat. No. PPHH02500)

Once each cut disc membrane filter was placed securely into a Swinnex^® device, 250 mL water samples spiked with surrogates were passed through each filter and into a styrene divinylbenzene (SDVB) SPE cartridge using EPA 537.1 for drinking water matrices as a guideline. The entire sample was subjected to SPE and concentration, according to procedures outlined in the workflow above. LC-MS/MS with a C18 column was performed according to conditions in Table 1 and analysis was carried out using internal standards to determine if there were levels of extractables present in the PP cut disc membrane filters. One lot (n=3 filters per lot) was tested per membrane type.

It is important to note that it is difficult to flow pure water through hydrophobic PP cut disc membrane filters; thus, for improved water flow for these samples, cut disc membrane filters were pre-wetted in methanol prior to filtering 250 mL water. There was no need to pre-wet hydrophilic PP cut disc membrane filters.

Results

As was found for Millex^® syringe filter devices, there were no detectable PFAS contaminants in any of the polypropylene cut disc membrane filters according to modified EPA 537.1 above the RL, ranging from 0.0020-0.0080 ppb, or the MDL, ranging from 0.0010-0.0020 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.

However, for only the hydrophobic polypropylene membranes, there were four compounds (perfluoro-n-dodecanoic acid (PFDoDA), perfluoro-n-tridecanoic acid (PFTrDA), perfluoro-n-tetradecanoic acid (PFTeDA), and N-ethyl perfluorooctanesulfonamidoacetic acid (N-EtFOSAA)) where there were no detectable PFAS compounds, but the associated standards of ¹³C₂-PFDoA, ¹³C₂-PFTeDA and D₅-NEtFOSAA, demonstrated recoveries outside of the control limits of 40-140%, 30-130%, and 40-140%, respectively. These compounds demonstrated average recoveries of approximately ~15-25% (for PFDoDA, PFTrDA, and PFTeDA) and 30-33% (for N-EtFOSAA). This indicates that non-specific adsorption of these compounds onto hydrophobic polypropylene could be significant in water. Considering the long chain lengths and bulky functional groups of these compounds, there is a potential for hydrophobic and steric interactions with filtration media and other consumables. Interestingly, hydrophilic polypropylene recoveries remained within control limit ranges for all compounds, indicating that hydrophilization of the membrane material reduced any non-specific interactions with PFAS standards.

Recovery

Hydrophobic Versus Hydrophilic Polypropylene Membranes

In water samples, the hydrophilic polypropylene membranes tested demonstrated recovery of internal standards within the QC range, while the hydrophobic membranes did not (Figure 3). The recoveries for ¹³C₂-PFDoDA, ¹³C₂-PFTeDA and D₅-EtFOSAA consistently demonstrated recovery outside of the QC range for hydrophobic polypropylene, making it difficult to quantify the true PFAS content for these compounds. Given that PFDoDA, PFTeDA, and PFTrDA are long-chain PFCA molecules, observed loss may be due to steric hindrance and hydrophobic interactions with the membrane material or Swinnex^® housing material.