CE-MS Detection

Detection of the effluent in a 2D system is carried out at the end of the second dimension s column. UVand LIF are the most widely used and the simplest methods of detection for CE separations because they are performed on-column. MS detection, unlike UV and LIF, is carried out on the effluent as it exits the CE column. The direct coupling of CE with mass spectrometry has shown great potential in proteomic research (Janini et al., 2004). The method of choice for detection of peptides is MS-electrospray ionization (ESI). However, ESI requires a special interface between the CE column and the mass spectrometer that has proven not to be a simple matter (Issaq et al., 2004). 

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 16.1 Schematic diagram of the CE capillary with on-column mSPE cartridge-sheathless ESI-MS interface, (reprinted with permission from Analytical Chemistry). 

In this design, on-column sample enrichment is incorporated into the sheathless interface (Janini et al., 2003). A miniaturized solid-phase extraction (mSPE) cartridge, made of reversed-phase material, was attached to the CE capillary near the injection end as shown in Fig. 16.1. 

FIGURE 16.2 Representative base peak electropherograms from CZE runs of RPLC fractions, (a) Fraction 15 (5 peptide identifications) and (b) fraction 20 (19 peptide identifications). Column, bare fused silica capillary, 60 cm x 180 pm ODx30pm i.d. separation voltage, 15 kV observed CZE current, 1.91 p.A running electrolyte, 200 mm acetic acid + 10% isopropanol temperature, 22°C injection time, 10 s at 2 psi ( 4 nL total injection volume) supplementary pressure, 2 psi flow rate, 25nL/min spray voltage, 1.5 kV (reprinted with permission from Electrophoresis). 

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