Use of a partial filling technique and reverse migrating micelles in the study of N-methylcarbamate pesticides by micellar electrokinetic chromatography-electrospray ionization mass spectrometry

This study describes three ways to couple micellar electrokinetic chromatography (MEKC) on-line with electrospray ionization mass spectrometry (ESI-MS) for the analysis of N-methylcarbamate pesticides. The methods involved the use of a partial filling (PF) technique under basic conditions and the use of reverse migrating micelles (RMMs) under acidic and basic conditions. The use of RMMs in basic electrolyte solutions required coated capillaries with low electroosmotic flows, and capillaries coated with anionic poly(sodium 2-acrylamide-2-methylpropanesulfonate) were selected for the purpose. Before the on-line MEKC-ESI-MS coupling, the MEKC and MS conditions were separately optimized under off-line conditions. The methods were compared in terms of detection limits and the stability of the electrospray process. The PF method offered good separation but poorer stability of the electrospray relative to the other methods. A more stable electrospray performance was obtained with use of RMMs in acidic electrolyte solutions, but some of the analytes were protonated and could not be detected due to the increase in their retention factors. However, with the use of anionic polymer-coated capillaries and RMMs at pH 8.5, all analytes were successfully separated. The high-salt stacking method was applied to improve the sensitivity of MEKC-ESI-MS and the detection limits were in the range of 0.04-2.0 microg/ml.