Coaxial sheath flow

Electrospray ionization (ESI) is ideally suited as a detection technique for the online interfacing of liquid-phase separations (HPLC and CE) to MS, because it facilitates the transfer of analytes from the liquid phase of the HPLC or CE column to the gas phase of the MS. Also, it allows the detection of high molecular weight species, such as peptides. Three interface designs have been developed in the past 18 years for coupling CE with MS. The first CE-MS interface, coaxial sheath flow, was introduced by Smith and his group in 1987 (Olivares et al., 1987) and was improved upon in later work (Smith et al., 1988). Coaxial sheath flow is formed using two concentric metal capillaries, whereby the CE terminus and the makeup flow line are inserted into the... 

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

FIGURE 6 Schematic representation of different interfaces for chip CE-ESI/MS (A) spray directly from the chip, (B) liquid-junction capillary interface, (C) gold-coated capillary interface, and (D) coaxial sheath-flow configuration. Reprinted from reference 410 with permission from Elsevier Science B.V. 

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... 

Because there is no ionizable groups of the coating in the neutral capillary, the interaction between charged molecules with ionic capillary surface is eliminated. Also, the electro-osmotic flow (EOF) of a neutral capillary is eliminated. However, a continuous and adequate flow of the buffer solution toward the CE capillary outlet is an important factor for routine and reproducible CE-ESI-MS analysis in order to maintain a stable ESI operation, some low pressure applied to the CE capillary inlet is usually needed, especially when the sheathless interface is employed. The disadvantage of the pressure-assisted CE-ESI-MS is the loss of some resolution because the flat flow profile of the EOF is partially replaced by the laminar flow profile of the pressure-driven system. A typical neutral capillary is a LPA (linear polyacrylamide)-treated capillary. Karger and co-workers [6] used mixtures of model proteins, a coaxial sheath flow ESI interface. 

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. 

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... 

FIGURE 7.4 Interfaces for the direct coupling of CE to ESI-MS using (A) sheathless (B) liquid-junction and (C) coaxial sheath-flow designs. (Adapted from Simo, C. et al. Electrophoresis, 26, 1306, 2005. With permission.)... 

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... 

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. 

If the liquid flow rate is increased too much, the ion signals become lower and less stable. The practical upper Umit to flow rate in pure electrospray is 10-20 pL/min, depending on the composition of the solvent and on the use of a coaxial sheath flow (see part 4.1). Pneumatically assisted electrospray has been used up to 1 mL/min. High-flow electrospray is always combined with the supply of heat to assist evaporation of solvents. In a commercial embodiment of high-flow ionspray (called Turbo-IonSpray) the spray plume... 

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