CESI-MS of Intact Proteins

There is a growing need for selective and sensitive analytical tools for the characterization of intact proteins. The effective coupling of capillary electrophoresis and electrospray ionization timeof-fl ight mass spectrometry (CE-ESI-TOF-MS) may offer an attractive option by providing both the separation and detection required to distinguish structurally-related protein species [1,2]. In order to bring CE-ESI-MS to a high performance level, novel sheathless CE-MS interfacing was studied to achieve sensitive detection of intact proteins. Based on a design of Moini [3], a prototype CESI sprayer was recently developed in the laboratories of Beckman Coulter, Inc. Flow rates in CE are very low and the initial droplets formed during the sheathless electrospray process are small, which leads to more effi cient ionization (i.e., nanospray). With sheathless [1] interfacing, the ESI [2] spray tip can be positioned close to the MS inlet, thereby improving ion sampling effi ciencies. Overall, this can lead to enhanced sensitivity and lower limitsof-detection (LODs). The CESI sprayer provides electrical contact without the need for metal conductive coating, microelectrodes or liquid junctions. In the CESI design (Figure 1), the last 3-4 cm of the bare fused-silica capillary are etched with hydrofl uoric acid until the section becomes conductive, producing an ~5-μm thick porous wall, which is conductive when in contact with an electrolyte. Capillaries with porous tips are simply inserted in a stainless steel ESI needle fi lled with a static conductive liquid.