Mass spectral studies towards more reliable measurement of perfluorooctanesulfonic acid and other perfluorinated chemicals (PFCs) in food matrices using liquid chromatography/tandem mass spectrometry

摘要

Liquid chromatography/mass spectrometry (LC/MS) experiments are described, leading to a reliablemethod for the measurement of perfluorooctanesulfonic acid (PFOS) and other perfluorinatedchemicals (PFCs) in foods. Separations were performed on new fluorinated stationary phases, RPOctyl (-C_8 F_(17)) or propyl-perfluorobenzene (-C_3H_6-6F_5), to ensure resolution of PFOS and interferingtaurohydroxycholate isomers. Aqueous ammonium formate (5 mM) and methanol were used as themobile phases. The mass spectrometer was operated in negative electrospray ionisation mode,recording two transitions for each analyte and one for each internal standard. The purities of theanalytical standards for the eleven target perfluoro analytes (C_7toC_(12)carboxylic acids,C_4, 6andC_8sulfonic acids, and octanesulfonamide (PFOSA)) were found to be in close agreement with thesupplied values; the lowest purity was 91%. Five candidate internal standards were investigated,~(13)C_4-PFOS, perfluorooctanoic acid, 2-perfluorodecanoic D_9-n-ethylperfluorooctane-sulfonamidoethanol (D_9-n-Et-FOSE) and D_3-methylperfluorooctanesulfonamide (3-n-Me-FOSA);the purities were all >98%. The use of tetrahydro-PFOS generated backgrounds (>1 μg/kg) forperfluoroheptanoic acid and perfluorobutanesulfonic acid. Similarly D9-n-Et-FOSE was unacceptableand D_3-n-Me-FOSA was volatile, leaving no clear candidate for normalisation of the measurement ofPFOSA. Severe matrix-induced suppression and enhancement effects influenced ionisation, makingexternal calibration and quantification problematic. This was addressed by a parallel standardaddition and matrix-matching approach, comparing ionisation in methanol, in procedural blanksand in food-based extracts. The limits of detection (LODs) of 0.001-0.01 jig/kg in solvent and 0.01-1 jig/kg in foods demonstrate that this method is suitable for the determination of PFCs in all food tothe required 1 μg/kg reporting level.