A novel sheathless and electrodeless microelectrospray interface for the on-line coupling of capillary zone electrophoresis to mass spectrometry

Abstract

A novel microelectrospray interface for the on-line coupling of capillary-zone electrophoresis (CZE) and mass spectrometry is described. The outlet of the fused-silica CZE capillary is tapered and serves as the microspray tip. The spray does not contain a sheath flow nor does the capillary tip contain an electrode or a conductive coating. The capillary tip is placed directly in front of the mass spectrometer inlet. The electrical field for the CZE separation as well as for the electrospray is delivered by the CZE high-voltage power supply. The electrical contact at the capillary outlet is established through the air between the spray tip and the inlet of the mass spectrometer, which is at ground potential.The performance of this interface is investigated using several -agonists, which are low-molecular-weight, cationic drugs, as test compounds. The combination of capillary electrophoresis (CZE) and mass spectrometry offers high separation power with a highly compound-selective and universally applicable detection technique.1,2 CZE/MS interfacing is generally performed using electrospray ionization (ESI) and its application has been successfully demonstrated for a broad range of compounds. 3,4 Since the introduction of ESI as a CZE/MS interface, 5 the interface has mainly been operated with a coaxial sheath liquid to establish the electrical contact and adjustment of the spray and ionization conditions. However, the presence of additional ions in the sheath flow can reduce the detectability of the analytes. 6 The sheath liquid is delivered with a flow of several microlitres per minute via a stainless steel capillary, which surrounds the CZE capillary terminus. A different approach has been presented, in which a liquid junction was used to establish the electrical contact and to add a make-up flow before the capillary terminus. The diameter of the formed droplets depends on the flow of the sheath liquid 8,9 and influences the evaporation efficiency of the droplets and, therefore, affects the amount of ions entering the mass spectrometer. Reduction of the flow can be obtained by removing the sheath flow and spraying the CZE capillary content directly into the mass spectrometer as has already been demonstrated. [10][11][12][13][14][15][16] In this approach, the electrical contact is made either via a metal coating at the capillary tip [10][11][12][13][14][15] or by inserting a narrow metal wire into the outlet of the capillary. 16 Although a metal coating functions properly as an electrode, a coating apparatus is necessary and in several cases flaking of the coating is reported during electrospraying. The use of a metal wire as an outlet electrode, although reported to be successful by Zare et al., 16 in our experience does not always result in a stable spray.Micro-or nanospray systems have gained recently a lot of interest, because of the low flow rate and the high electrospray efficiency. 8,9,17,18 Moreover, a microspray is very stable due to the small surface from which the spray originates and allows the use of...

An atmospheric pressure ion lens that improves nebulizer assisted electrospray ion sources

“…The ion lens (8) is located near the tip of the inner stainless steel tube (7). The ion lens has a mounting bracket (9) and an adjustable arm (10). Pivots (11 and 12) allow the lens to be positioned in a range of different locations.…”

“…Shortly thereafter, Emmett et al described a packed electrospray needle for interfacing chromatography with mass spectrometry at low flow rates (300 nL/min and above) [7]. Following these initial experiments, there have been many studies with electrospray operating in this flow regime for applications involving direct infusion [8,9], or the coupling of CE [10,11] and LC [12,13] with ESI-MS. Typically these reduced flow rate ESI sources provide improved signal stability and sampling efficiency when compared to higher flow rate ESI sources.…”

Sheathless interface to match flow rate of capillary electrophoresis with electrospray mass spectrometry using regular‐sized capillary

“…While for sheathless interfaces the electrical contact was established by direct application of high voltage (HV) via a conductive coating at the capillary tip, [8][9][10][11] through a liquid junction by inserting a narrow metal wire into the capillary outlet or utilization of a stainless steel tip, [12][13][14][15] or just through the CE high voltage. [16][17][18] However, problems such as coating damage and formation of air bubbles [19] may occur at the physical interface between the electrode and the buffer solution.The other challenge for the CE-ESI-MS interface is that the flow rate of conventional ESI is not compatible with CE electroosmotic flow (EOF). For commonly used capillaries with 50 μm i.d., the EOF is about 50-250 nL/min.…”

“…Nevertheless, the constriction of the pulled tip would lead to a pressure buildup, which would cause peak broadening and reduce the resolution. [16] Furthermore, the pulled tips would be easily clogged which would limit their applications in the aspects of lifetime and reproducibility. Thus, a flow rate match between CE and ESI without compromising either capillary i.d.…”

“…This is often accomplished by the introduction of a spray emitter with smaller diameters, in which the CE capillary outlet is often pulled to ca. 10-15 μm or less [16,17] to accomplish a microspray or nanospray (20-200 nL/min) [22][23][24] to meet the EOF. Nevertheless, the constriction of the pulled tip would lead to a pressure buildup, which would cause peak broadening and reduce the resolution.…”