Differentiation and quantification of linear alkyl benzenesulfonate isomers by liquid chromatography-ion-trap mass spectrometry

Discrimination and quantitation of the 20 positional isomers of C10–C13 linear alkyl benzenesulfonates (LASs), based on the use of reversed-phase liquid chromatography-electrospray ionisation in negative ion mode ion-trap mass spectrometry, was undertaken. Discrimination was achieved by LAS MS-MS analysis into the ion trap, by monitoring specific fragment ions resulting from the benzylic cleavage of the carbon alkyl chain on both side of the LAS phenyl group. Parameters affecting the electrospray ionisation source and the ion-trap operation were optimised. Calibration curves for the different isomers were established and this permitted their quantitation by mass spectrometry for the first time. MS-MS responses were dependent on both the position of the phenyl group on the alkyl chain and the length of this alkyl chain, these responses being higher for the external isomers and the longer alkyl chain homologues. The precision, expressed as relative standard deviation ranged between 9 and 13%. Detection limits for LAS isomers were between 0.03 and 0.07 mg/l and therefore the method is sensitive enough to be applied to environmental samples.