An alternative ionization technique for LC-MS/MS analysis of perfluoroalkyl substances (PFAS) in environmental samples

Importance of the Topic

Perfluoroalkyl substances (PFAS) are persistent environmental pollutants associated with adverse health effects. Accurate and sensitive analysis of PFAS in water and soil is essential for regulatory compliance and environmental monitoring. Traditional electrospray ionization (ESI) methods are widespread but may face limitations at trace levels.

Aims and Overview

This study compares a novel UniSpray Ionization (USI) source to conventional electrospray for LC-MS/MS analysis of PFAS in diverse environmental samples. The objectives are to evaluate signal intensity, signal-to-noise ratio, quantitation accuracy, and robustness across water and soil matrices.

Methodology and Instrumentation

Water samples (reagent, surface, ground, influent, effluent) and soil extracts (sand, silt, fat clay, lean clay) were spiked with unknown PFAS concentrations and prepared following ASTM 7979 (water) and ASTM 7968 (soil). Chromatographic separation used an ACQUITY UPLC I-Class PLUS system with a BEH C18 column (2.1×100 mm, 1.7 µm) at 35 °C. Mobile phases were (A) 95:5 water:methanol with 2 mM ammonium acetate and (B) methanol with 2 mM ammonium acetate. Injection volume was 30 µL, with column effluent directed to a Waters Xevo TQ-S micro mass spectrometer.

Instrumentation Used

• UPLC System: ACQUITY UPLC I-Class PLUS
• Column: ACQUITY BEH C18 2.1×100 mm, 1.7 µm
• Mass Spectrometer: Xevo TQ-S micro
• Electrospray Ion Source: 0.5 kV capillary voltage, 350 °C desolvation, 900 L/hr gas flow
• UniSpray Ion Source: 1 kV impactor voltage, 400 °C desolvation, 900 L/hr gas flow

Main Results and Discussion

UniSpray provided increased peak areas and equal or improved signal-to-noise ratios for the majority of PFAS compounds compared to electrospray, enabling lower detection limits. A comparative plot of PFOA, PFOS, and PFNA illustrates notable S/N enhancements with USI. GenX (HFPO-DA) calibration curves exhibited higher responses across the range, facilitating robust low-level quantitation. In 30-injection robustness tests, UniSpray achieved peak area RSDs below 5 %, outperforming electrospray (mostly under 10 %). Quantification of PFAS in effluent water and fat clay extracts produced comparable concentrations between both sources, indicating minimal matrix effects with UniSpray.

Practical Benefits and Applications

• Enhanced sensitivity and lower limits of detection support regulatory monitoring.
• Improved quantitation accuracy at trace PFAS levels.
• Demonstrated robustness for routine analysis in complex matrices.
• Adaptable to both polar and non-polar analytes in a single injection.

Future Trends and Potential Applications

Potential developments include:

• Extension of UniSpray to broader analyte classes beyond PFAS.
• Integration with high-resolution MS for non-targeted screening.
• Automation of sample preparation to increase throughput.
• Development of portable UniSpray sources for field applications.

Conclusion

UniSpray Ionization is a viable alternative to electrospray for LC-MS/MS analysis of PFAS, offering enhanced ionization efficiency, improved signal-to-noise ratios, and robust performance across water and soil matrices. Its adoption can lead to lower detection limits and more reliable quantitation for environmental monitoring applications.