Excellent choices for environmental applications - Water

Optimized Trace Analysis of PFOA and PFOS by LC/MS/MS

Introduction

Perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS) are persistent environmental contaminants used in manufacturing Teflon® and stain¬resistant materials. Both compounds are regulated at low parts-per-billion levels in drinking water due to human health concerns.

Analytical Challenges

• Branched and linear isomers occur in real samples, affecting ionization and fragmentation patterns.
• Matrix effects in complex samples (e.g., plasma, soil, water) can cause signal suppression or enhancement.
• Low detection limits (sub-ng/mL) require highly sensitive, selective methods.

Experimental Approach

Instrumentation: Agilent 1200 Series LC coupled to Agilent 6410 Triple Quadrupole MS. Chromatography on a 2.1×50 mm, 1.8 µm Eclipse Plus C18 column with a 10 min gradient from 0% to 100% methanol in 2 mM ammonium acetate at 0.4 mL/min. MS/MS in negative-ion mode with optimized source conditions (350 °C, 9.5 L/min drying gas, 45 psi nebulizer).

Optimization of MS Parameters

• Fragmentor Voltage was ramped to maximize precursor ions: 120 V for PFOA (m/z 413) and 200 V for PFOS (m/z 499).
• Collision Energy was optimized for key product ions: 6 V for PFOA m/z 369, 16 V for m/z 169; 45, 50, and 70 V for PFOS m/z 169, 99, and 80.

Quantitation Strategy

• Multiple Reaction Monitoring (MRM) transitions used for quantitation and confirmation. Best transitions: PFOA 413→369 and 413→169; PFOS 499→169.
• Calibration range 0.4–400 pg on-column yields correlation coefficients ≥0.998 over three orders of magnitude.
• Isotopically labeled internal standards ([1,2-13C]-PFOA and [1,2,3,4-13C]-PFOA) correct for matrix effects and improve accuracy.

Isomer Effects on Quantitation

• Branched isomers exhibit different ion ratios than linear standards, causing up to 40% quantitation errors if unresolved.
• Chromatographic separation of linear and branched isomers is essential for accurate quantitation when using a linear standard.

Matrix Effects and Internal Standards

• Direct comparison of PFOA responses in methanol vs. plasma extract shows signal suppression in plasma.
• Isotopically labeled PFOA standards provide linear calibrations (0.02–20 ng/mL) with correlation coefficients ≥0.9994.

Detection Limits, Precision, and Accuracy

• Method detection limits (MDL) of 0.002 mg/L for PFOA and 0.003 mg/L for PFOS were achieved (EPA 504 procedure).
• Repeatability of 5–6% RSD and recoveries near 100% were obtained for 0.2 mg/L spikes.
• Continuous calibration stability was demonstrated over 27 days with ≤10% response deviation.

Conclusions

By optimizing fragmentor voltages and collision energies, and by using isotopically labeled internal standards, the Agilent 1200 LC/6410 QQQ system achieves sub-ng/mL quantitation of PFOA and PFOS in complex matrices. Accurate ion transitions (PFOA 413→369/169, PFOS 499→169) and chromatographic separation of isomers are critical for reliable results. Matrix-matched or isotope-dilution calibrations mitigate matrix effects and ensure accuracy across diverse sample types.