A New High‐performance Interface for Capillary Electrophoresis/Electrospray Ionization Mass Spectrometry

Abstract

A novel capillary electrophoresis electrospray ionization mass spectrometry (CE/ESI-MS) interface has been developed based upon an alternative means of establishing electrical contact at the CE capillary terminus. In this design, the terminus of the CE separation capillary and a short ESI emitter capillary are connected and sheathed by polysulphone micro-dialysis tubing. The electrical connection, to close the CE circuit and simultaneously establish the electrospray voltage, is provided outside the membrane via an electrode in a small liquid reservoir. In contrast to the sheath flow and liquid junction interfaces, the flow rate of liquid to the ESI source consists of the CE eluent and is not increased. The use of a 'micro-spray' type emitter produces a stable electrospray at the very low flow rates typical of CE, providing a highly sensitive, efficient interface. Post-separation acidification of analytes via the dialysis junction is also demonstrated.The separation technique of capillary electrophoresis (CE), in which analytes are separated in a small diameter ( c 100 pm) fused-silica capillary due to differences in their electrophoretic mobilities, is increasingly being employed, especially in biomedical and biochemical applications, where minute sample volumes and high-speed analyses are desirable. Mass spectrometric detection in conjunction with CE separations was recognized early as having significant potential, and the development of elecuospray ionization mass spectrometry (ESI-MS)' provided an attractive basis for CE/MS interfacing. The first CE/MS interface was demonstrated by Olivares et al. in 1987; who showed that the electrospray ionization process could provide an effective interface based upon an electrical contact established at or near the end of the CE capillary (or point of electrospray emission).' The electric contact serves to establish both the CE electric field as well as the ESI voltage.Several effective means for establishing this electrical contact have beenIn the initial report,2 a metal coating on the tip of the CE capillary made contact with a metal sheath capillary to which the CE terminus/ESI voltage was applied. Although high sensitivity and efficiency were achieved, problems included a dependence on the buffer system used, ESI instability under some operating conditions and the need to regularly replace the metal coating on the capillary tip. These issues led directly to the development, also in this laboratory, of the 'coaxial sheath-flow' approach: In this interface, a sheath liquid, generally with a small electrolyte content, is infused to the ESI source through a sheath capillary which surrounds the end of the separation capillary and terminates near the end of the separation capillary. This sheath liquid, generally flowing at a rate of a few microliters (pL) per minute, is then added to the CE effluent as it elutes from the terminus of the CE capillary, thus providing the necessary electrical contact. At the liquid flow rates used in this approach some organic content to the ...

Coupling CE with atmospheric pressure photoionization MS for pharmaceutical basic compounds: Optimization of operating parameters

“…On the contrary, a sensitivity decrease (factor about 2) was observed in the presence of formic acid. In contrast to ESI, where an acid in the sheath liquid is mandatory to assist positive ionization [56,[59][60][61], the presence of a protic solvent in the sheath liquid appeared sufficient for photoionization.…”

“…As expected, an increase in NebuP negatively influenced efficiency because of the pressure-induced flow in the capillary, known as "suction effect" [56,61]. Therefore, it was kept at its lowest value (10 psi) to avoid large losses in efficiency (up to 80% with 40 psi).…”

Nanoelectrospray emitters: Trends and perspective

“…One such example involves an open hole where a CE capillary is connected to the nanoelectrospray emitter, where the entire union is immersed in electrolyte in a pressurized vessel containing the electrode (Fanali et al, 2006). Another example uses microdialysis tubing as a union between the CE capillary and emitter which is immersed in liquid inside a plastic pipette tip also containing an electrode (Severs, Harms, & Smith, 1996). Edwards et al (2006) actually etched a porous hole in the side of their CE capillary upstream of the emitting end that served as a junction, this section being immersed in electrolyte in the presence of an electrode.…”

A simple method for fabrication of silver‐coated sheathless electrospray emitters

“…Among the sheathless CE/MS techniques, the gold‐coated fused‐silica capillary method is the most popular one 11–13. In general, the fabrication of gold‐coated tapered capillaries requires special equipment and is also time‐consuming 14–16. In addition, the coated tips have a limited lifetime.…”