For ultrashort-chain, alternative, and legacy PFAS
As interest in monitoring a wider range of PFAS in both potable and non-potable waters grows, efficient methodology becomes more important. Here, a team from Restek describes a unique approach that provides concurrent ultrashort-chain PFAS analysis along with alternative and legacy PFAS, allowing C2, C3, C4, C6, C8, and alternative compounds to be tested together instead of through separate methods. Results from verification experiments are presented.
Introduction
Ultrashort-chain, or C2 and C3, per- and polyfluoroalkyl substances (PFAS) are small and very polar compounds that contribute to at least 40% of the total PFAS detected in environmental waters (e.g., rain, river, and groundwaters) [1, 2, 3]. Ultrashort-chain PFAS include trifluoroacetic acid (TFA), perfluoropropanoic acid (PFPrA), perfluoroethane sulfonate (PFEtS), and perfluoropropane sulfonate (PFPrS), with TFA being the most abundant and difficult to analyse chromatographically. Current practices for PFAS monitoring do not address the analysis of these newly trending ultrashort-chain compounds owing to their insufficient retention on typical reversed-phase (RP) columns. On the other hand, analytical methods implementing anion-exchange chromatography often show too much retention and poor chromatographic performance for ultrashort-chain PFAS. The challenge becomes even greater for simultaneous monitoring of ultrashort-chain, alternative, and legacy PFAS in a single method.
To overcome this limitation, we used a unique hybrid HILIC/ion-exchange column (Raptor Polar X) to develop a fast and simple LC-MS/MS method for comprehensive analysis of C2, C3, C4, C6, C8, and alternative PFAS. Because the column employs balanced, multimode retention mechanisms, ultrashort-chain PFAS and long-chain PFAS can all be analysed in a single isocratic run. This direct injection method was evaluated by precision and accuracy analysis of fortified water samples, including tap water, river water, groundwater, and water from publicly owned treatment works (POTW, sewage effluent). As demonstrated here, the method provides convenient setup and high-throughput conditions for water testing labs interested in adding ultrashort-chain PFAS analysis to the same workflow used to measure alternative and legacy PFAS.
Experiment details
Chromatographic method:
- Column: Raptor Polar X (2.7 µm, 50 mm x 2.1 mm ID [cat.# 9311A52])
- Column temp.: 40 ºC
- Injection volume: 10 µL
- Mobile phase A: Water, 10 mM ammonium formate, 0.05% formic acid
- Mobile phase B: Acetonitrile:methanol (60:40), 0.05% formic acid
- Time (min) %B
- 0.00 85
- 8.00 85
- Flow rate: 0.5 mL/min
- Ion mode: Negative ESI
- Mode: MRM
Sample Preparation
In a polypropylene vial (used to mitigate background contamination), 250 µL of each water sample was mixed with 250 µL of methanol and 5 µL of internal standard solution (10 ng/mL of 13C2-PFHxA, 13C2-PFOA, 13C3-PFBS, 13C4-PFOS in methanol). The vial was capped with a polyethylene cap (again, to reduce background contamination) for injection and analysis.
Calibration standards were prepared by using deionised water (generated by a Thermo Scientific Barnstead E-Pure system) and fortifying it with 14 analytes at a range of 10–800 ng/L. The calibration standard solutions were then diluted 1:1 in methanol following the sample preparation procedure above.
A tap water sample from the Restek facility and three water samples (Chicago river water, groundwater, and POTW effluent water) supplied by the United States Environmental Protection Agency (U.S. EPA) were fortified at 40 and 160 ppt. Blank and fortified water samples were diluted 1:1 in methanol as above for chromatographic analysis and quantified with the calibration standards. For TFA measurement in groundwater, the sample was diluted fivefold with deionised water before fortification at 40 and 160 ppt due to its high TFA concentration.
Chromatographic performance
An isocratic elution was established that produced a fast, simple ultrashort-chain PFAS analysis along with alternative and legacy PFAS in water samples. All analytes eluted in four minutes with balanced retention and good peak shapes (Figure 1). No matrix interference was observed in any of the water samples using an eight-minute cycling time. As will be discussed below, the approximately four-minute hold after the last eluting compound was shown to be necessary to avoid possible matrix interferences.
Conclusion
A simplified isocratic method was developed and verified for ultrashort-chain PFAS analysis along with alternative and legacy compounds in water samples. Due to the balanced, multimode retention of these analytes on a Raptor Polar X (2.7 µm) 50 x 2.1 mm column, the analytical method was demonstrated to be fast, rugged, and sensitive with acceptable accuracy and precision. This method is suitable for analytical labs wanting to expand their existing PFAS assays for potable or non-potable water to include C2 and C3 compounds.
For the full details of this application note including information on transition times and internal standards, please visit https://www.restek.com/articles/a-novel-approach-for-ultrashort-chain-pfas-analysis-in-water-samples.
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