NEMC: Strategies for Ultra Low-level Detection and Quantification of Short- and Long- Chain Per- and Polyfluoroalkyl Substances (PFAS) by Direct Injection LC-MS/MS

Significance of the Topic

Per- and polyfluoroalkyl substances (PFAS) are persistent environmental contaminants with growing regulatory scrutiny and public health concerns.
Accurate measurement of ultra-trace PFAS down to single-digit ng/L levels is essential for environmental monitoring, drinking water safety and industrial quality control.

Objectives and Study Overview

The study aimed to develop and optimize a single direct injection LC-MS/MS method capable of quantifying 43 target PFAS compounds, including short-chain, long-chain and perfluorinated ether acids.
Part 1: Establish quantitative performance using 25 µL direct injection on a Vanquish UHPLC-TSQ Altis Plus system with calibration over 0.5–1000 ng/L.
Part 2: Investigate injection strategies for aqueous and high-organic samples (solvent sandwich vs aqueous loop) to maximize sensitivity across PFAS classes.

Methodology and Instrumentation

Liquid Chromatography

• Vanquish Flex Binary UHPLC with PFAS delay column (3.0×50 mm Hypersil GOLD) and Acclaim RSLC C18 analytical column (2.1×100 mm).
• Mobile phases: A = H2O/MeOH 98/2 with 2 mM ammonium acetate, 0.1% acetic acid; B = MeOH/H2O 98/2 with same additives.
• Column temp: 40 °C; injection volume: 25 µL; sample temp: 20 °C.

Mass Spectrometry

• TSQ Altis Plus in negative-mode HESI; SRM acquisition with 0.7 Da unit resolution; cycle time 0.4 s.
• Ion transfer tube temperature optimized (175 °C gave equal or improved response compared to 225 °C).

Sample Preparation

• Calibration standards prepared in 50% MeOH over 0.5–1000 ng/L and spiked with isotopically labeled surrogates (50–400 ng/L).
• Surface water samples mixed 1:1 with methanol containing labels, vortexed, and directly injected.

Injection Strategies

• Strong solvent loop with sandwich injection (3×25 µL) for high-organic long-chain PFAS.
• Aqueous large-volume injection (225 µL) for short-chain PFAS in water-rich matrices.

Main Results and Discussion

Limits of detection for 40 of 43 PFAS were ≤1 ng/L (6.25–25 fg on-column), with some LODs limited by solvent blank contamination.
Calibration curves displayed linearity (R2 > 0.995) over 3–4 log dynamic range.
Direct injection of spiked surface water demonstrated detection of PFMOAA, PFO2HxA, HFPO-DA and PFNA at 0.9–44 ng/L levels across real samples.
Blank controls revealed minor background for short-chain PFAS, underscoring the need for rigorous consumable and solvent selection.

Benefits and Practical Applications

Direct injection LC-MS/MS simplifies sample workflow by avoiding solid-phase extraction, reducing analysis time and consumable costs.
Single method covers broad PFAS classes, enabling high-throughput environmental monitoring, QA/QC in water and industrial process streams.
Flexible injection strategies allow tailoring sensitivity for both hydrophilic and hydrophobic PFAS.

Future Trends and Applications

Integration of online SPE and trap columns for automated preconcentration and cleanup.
Development of dedicated PFAS-free liquid handling hardware and kits to minimize background.
Expansion of targeted panels to emerging PFAS (e.g., chlorinated ethers, polymeric species).
Coupling with high-resolution MS for non-target screening and suspect analysis.

Conclusion

The direct injection LC-MS/MS method using a Thermo Scientific Vanquish UHPLC and TSQ Altis Plus system delivers robust, sensitive quantitation of a wide range of PFAS at sub-ng/L levels with minimal sample preparation.
Optimized injection strategies and stringent blank controls ensure reliable data for environmental and regulatory applications.