Excellent choices for environmental applications - Water

Introduction Perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS) are C8 perfluorinated surfactants widely used in industrial processes, nonstick coatings, and food packaging. Trace levels of these compounds in environmental and biological samples pose analytical challenges because they often co-exist as mixtures of linear and branched isomers. Quantitation by LC–MS/MS requires sensitive, selective detection of deprotonated [M–H]⁻ precursor ions and optimal collision-induced dissociation (CID) to characteristic product ions. LC–MS/MS Methodology • Instrumentation: Agilent 1200 Rapid Resolution LC coupled to Agilent 6410 triple quadrupole MS in negative electrospray ionization (ESI) mode. • Chromatography: ZORBAX Eclipse Plus C18 RRHT, 50×2.1 mm, 1.8 µm; 0–100% MeOH (2 mM NH₄OAc) in 10 min; 0.4 mL/min. • MS/MS: MRM transitions for PFOA [M–H]⁻ at m/z 413 → 369 (loss of CO₂H) and 413 → 169 (C₃F₇⁻); PFOS [M–H]⁻ at m/z 499 → 80 (SO₃⁻), 499 → 99 (C₃F₅⁻), and 499 → 169 (C₇F₁₅⁻). • Optimization: Fragmentor voltages (120 V for PFOA, 200 V for PFOS) and collision energies (6 V/16 V for PFOA, 45 V/50 V/70 V for PFOS) were established by ramp experiments to maximize ion intensities. Calibration and Linearity MRM calibration curves covering 0.02–20 ng/mL (0.4–400 pg on column) yielded linear correlation coefficients ≥ 0.998 for all transitions. Limits of detection (MDLs) using seven replicates at 0.02 mg/L were 0.002 mg/L for PFOA and 0.003 mg/L for PFOS, surpassing EPA Method 504 requirements. Isomeric Effects on Quantitation Real samples often contain branched isomers, whose MRM ion ratios differ substantially from the linear standards. In chromatograms with only 3 min gradients, branched and linear peaks coelute, leading to quantitation errors up to 40% if only linear standards are used. Separation of branched isomers using longer gradients (10 min) and a C18 column avoids this error by resolving linear from branched compounds. Matrix Effects and Isotopic Standards Matrix suppression or enhancement in biological or environmental extracts necessitates matrix-matched calibration, which may be impractical. Isotopically labeled PFOA ([¹³C]₄ or [¹³C]₂) and PFOS standards produce highly linear calibration in plasma matrix (linear r² ≥ 0.9994) over the same 0.02–20 ng/mL range, correcting for matrix effects and ensuring accurate quantitation of unlabeled analytes. Conclusions Optimized MRM transitions, fragmentor voltages, and collision energies on an Agilent LC–QQQ platform enable sensitive, selective detection of PFOA and PFOS at sub-ppb levels. Use of a C18 column and longer gradients is required to resolve branched isomers and avoid quantitation bias. Isotopically labeled internal standards compensate for matrix effects, yielding accurate results in complex samples.