Interface sheath-flow

Huggins, T. G., and Henion, J. D. (1993). Capillary electrophoresis/mass spectrometry determination of inorganic ions using and ion spray-sheath flow interface. Electrophoresis 14, 531 — 539. 

FIGURE I Experimental setup for CE ESI/MS with a coaxial sheath-flow interface. 

The sheath-flow interface can meanwhile be considered a routine tool, at least for qualitative determinations. Quantification is possible, with relative standard deviations between 5 and 10% typically achieved. Quantification problems may result from the limited lifetime of the capillaries. Due to high local field strengths near the tip, the so-called electrodrilling effect commonly occurs. Modifications of the tip can improve the lifetime (22). 

The sheathless interface (Fig. 2c) is known since the first CE/MS attempt by Olivares et al. (10). In this system the CE capillary was sleeved in a metal tube, whereas in modem sheathless interfaces the capillary exit is carefully sharpened or pulled to a fine point (14). The outer surface of the capillary tip is coated with metal, usually gold, which is readily accessible for electrical contact. This setup enables the maintenance of both electrical circuits from CE and ESI as well. The advantage of the sheathless approach over the coaxial sheath flow interface is that the eluting CE zone is not diluted by makeup flow and therefore the obtainable sensitivity can be quite high, especially when small-ID capillaries (e.g., 10 yarn) are used. Detection limits in the low fmol range have been demonstrated (13). A... 

Comparative studies indicate that a sheathless configuration offers improved sensitivity over sheath flow interfaces. However, this implies the need to use specialized, home-built CE capillaries and electrospray hardware. Therefore, it has not yet been as popular as the easily accessible sheath liquid system. 

Several research groups have presented work on the development of CE-ESI-MS interfaces. The interfaces developed can be categorized into three main groups coaxial sheath flow, liquid junction, and sheathless interfaces. A schematic of the sheath-flow interface first developed for CE-ESI-MS by Smith et al. [6] is illustrated in Fig. la. A sheath liquid, with... 

Fig.l Schematic illustration of CE-MS interfaces to an ESI source (a) a coaxial sheath-flow interface (b) a liquid-junction interface (c) a sheathless interface. 

CE/MS is being rapidly developed and is catching on as it offers different selectivity from LC. Principally, the low flow rates used with CE, typically in the 100 nl/min range, are well suited for the introduction of the effluents into a mass spectrometer through an electrospray sheath flow interface. However, a sheath liquid has to be added to the CE electrolyte in order to establish the electrical contact of the capillary and to ensure sufficient flow for the electrospray. 

Either a sheathless, coaxial sheath-flow, or liquid-junction interface is commonly used for CE-MS, Figure 9.11. The reproducible and straightforward construction of the coaxial sheath-flow interface has resulted in its general use, although the sheathless interface provides higher sensitivity. The difficulty in making low-dead... 

A suitable electric ccmnection between the CEC separatiOTi and the ESI emitter is a key for the CEC-ESl-MS coupling. Three main interfaces including coaxial sheath-flow interface, liquid junctiOTi interface, and sheathless interface have been used in CE/CEC-ESl-MS. The main advantages of the sheath-flow interface are the wide flexibility in selectiOTi of separation electrolyte solutions, its reliability, and the existence of several commercial designs which make it the most widely used interface in CE-ESl-MS routine analysis [6]. However, due to sample dilution... 

CE provides a complementary approach to HPLC separation. It is performed in several different formats, including capillary zone electrophoresis, miceller electrokinetic chromatography, capillary gel electrophoresis, capillary isoelectric focusing, isotachophoresis, and capillary electrochromatography. Of these formats, capillary zone electrophoresis is the most popular separation technique. The most successful coupling of CE with mass spectrometry is achieved via an ESI interface. The three most practical designs are sheathless interface, sheath-flow interface, and liquid-junction interface. 

The CE/MS analysis of the venom of the snake Dendroaspis polylepis polylepis, the black mamba, is reported by Tomer and coworkers.A VG 12-250 quad-rupole equipped with a Vestec ESI source (coaxial sheath flow interface) was employed for this experiment. The sheath fluid was a 50 50 methanol 3% aqueous acetic acid solution. The CE voltage was set at -30 kV during the analysis and the ESI needle was held at -h3 kV. The CE running buffer used was 0.01 M acetic acid at pH 3.5. The APS column was flushed with buffer solution for 10 min prior to sample analysis. The snake venom was dissolved in water at a concentration of 1 mg/ml and 50 nl of the analyte solution was injected into the column. They demonstrated the existence of at least 70 proteins from this venom. 

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