Characterization of microdialysis acidification for capillary isoelectric focusing microelectrospray ionization mass spectrometry

A microdialysis junction, based on a microdialysis membrane connecting a se paration capillary and a short, sharply tapered microelectrospray emitter c apillary, is demonstrated for on-line combination of capillary isoelectric focusing (CIEF) with electrospray ionization mass spectrometry (ESI-MS). Th e microdialysis junction provides the necessary electrical connection acros s the dialysis membrane for defining the electric fields needed for the CIE F separation and the electrospray process. Additionally, postseparation aci dification of focused protein zones eluted from the CIEF capillary is achie ved using the microdialysis junction while separation efficiency and resolu tion is maintained. A microelectrospray emitter produces a stable electrosp ray of protein analytes without the need for a makeup liquid flow and elimi nates any subsequent sample dilution and reduction in MS sensitivity. The m icrodialysis junction is advantageous over the coaxial liquid sheath interf ace as evidenced by the simplicity in operation procedures, the enhancement in detection sensitivity, and the linear correlation between protein migra tion time and isoelectric point in CIEF-ESI-MS.