Coaxial liquid-sheath interface

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... 

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... 

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 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. 

Fig. 2 CE/MS interfaces, (a) coaxial sheath liquid, (b) liquid junction, (c) sheathless, and (d) direct electrode. See text for further explanation. (Adapted from Ref.

The first ESI design at the end of the 1980s proved to work properly as the HPLC interface with mobile phase flow rates between 1 and lOpL/min. Meanwhile, the development of the HPLC instrumentation and columns was oriented in the mL/min flow rate mode. In addition, the nebulization process based only on the application of an electrical field does not produce a stable spray from aqueous mobile phases. A modified ESI source, called ionspray, was then introduced [39], in which the nebulization of a liquid solution is pneumatically assisted by a coaxial flow of nitrogen (sheath gas) that allows the formation of a stable aerosol at mobile-phase flow rates between 10 and 500 pL/ min and the use of aqueous mobile phases. When working at higher flow rates (500-1000 pL/min), an additional nittogen flow rate can be used (auxiliary gas) to assist the desolvation of the droplets. This modified source is called turboionspray. 

In the arrangement of the most widely used sheath liquid interface illustrated in Fig. 8.6, the column is introduced into the atmospheric region of the ESI source through a coaxial, narrow metal tube, which delivers the sheath liquid to the column... 

The application of CE for preparative separations of peptides and proteins is limited due to the low preparative capacity of capillary columns. In addition, adaptation of an analytical capillary system to a preparative one is not straightforward and requires certain modifications of CE instruments [2,22], Several procedures for fraction collection from a capillary have been developed recently, as reviewed in Ref. 22. For continuous fraction collection in CE it is necessary to modify the capillary outlet to complete the electrical circuit. Karger and coworkers achieved this by using a coaxial sheath liquid interface to transport the sample components leaving the capillary exit into the collection microcapillary... 

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... 

In some systems, the restriction apparatus is a simple transfer tube restriction used for low-flow experiments. In others, a variable restrictor or back-pressure regulator is used between the outlet and the source. In the configuration used by Pinkston and Baker [7], the restriction apparatus is a series of tees to add a coaxial flow of nebulizing gas, electrospray buffer sheath flow, and another to introduce liquid from a syringe pump to regulate mobile-phase pressure. In Fig. lb, a direct interface is shown in which a split directs a fraction of the effluent flow toward the API source while the remainder is sent to waste through a back-pressure regulator or some controlled restriction to maintain system pressure. 

The main problem encountered with PFT with a neutral selector remains the prevention of the chiral selector from entering the ionization source. This problem becomes particularly important at a high pH where EOF is important. Therefore, an acidic buffer pH or a coated capillary to minimize EOF is of the utmost importance. In addition, it has to be noted that the electrospray ionization process is pneumatically assisted when a sheath-liquid interface is used. The coaxial sheath gas induces an aspirating phenomenon in the capillary which may considerably affect the separation quality. This can be due to the decrease in interaction between analytes and the chiral selector and to a hydrodynamic flow induced by the Venturi effect at the capillary end [33, 34]. 

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.)... 

There are many advantages of sheathless interfaces compared to those that require a sheath flow. The main difference between sheath and sheathless interface designs is that sheathless interface does not require the external flow of a coaxial sheath liquid to establish electrical contact with the CE... 

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