Integration of Microfabricated Devices to Capillary Electrophoresis−Electrospray Mass Spectrometry Using a Low Dead Volume Connection: Application to Rapid Analyses of Proteolytic Digests

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Abstract
This report describes the development of a compact and versatile, micromachined chip device enabling the efficient coupling of capillary electrophoresis to electrospray mass spectrometry (CE−ESMS). On-chip separation provides a convenient means of achieving rapid sample cleanup and resolution of multicomponent samples (typically 2−5 min) prior to mass spectral analysis. A low dead volume connection facilitating the coupling of microfabricated devices to CE−ESMS was evaluated using two different interfaces. The first configuration used disposable nanoelectrospray emitters directly coupled to the chip device via this low dead volume junction, thereby providing rapid separation of complex protein digests. The performance of this interface was compared with that of more traditional configurations using a sheath flow CE−ESMS arrangement where a fused-silica capillary of varying length enabled further temporal resolution of the multicomponent samples. The sensitivity and analytical characteristics of these interfaces were investigated in both negative and positive ion modes using standard peptide mixtures. The separation performance for synthetic peptides using a chip coated with amine reagent ranged from 26 000 to 58 000 theoretical plates for a sheath flow CE−ESMS interface comprising a 15-cm CE column. Replicate injections of a dilution series of peptide standards provided detection limits of 45−400 nM without the use of on-line preconcentration devices. The reproducibility of migration time ranged from 0.9 to 1.5% RSD wheras RSDs of 5−10% were observed on peak areas. The application of these devices for the analysis of protein digests was further evaluated using on-line tandem mass spectrometry.