Electrokinetic detection techniques

Research stiU continues into the development of enhanced electrokinetic injection techniques for p-chips. Modifications made to the outlet end of the separation channel, such as its connection to a length of fused silica capillary to facilitate chip interfacing with off-chip detection systems for example, can be adopted equally well for chip inlets and sample introduction. The use of conventional capillary to interface between macroscale sample reservoirs and p-chip platforms has been successfully... 

The instrumentation of HPCE is uncomplicated (see the schematic drawing in Figure 1). Briefly, both ends of the narrow-bore fused silica capillary are immersed into reservoirs containing a buffer solution that also fills the capillary. The reservoirs also contain electrodes that provide electrical contact between the high-voltage power supply and the capillary. The sample is loaded onto the capillary by replacing one of the buffer reservoirs by a sample reservoir and applying external pressure (hydrodynamic injection) or an electric field (electrokinetic injection). After the injection, the reservoir is replaced, the electrical field is applied, and the separation starts. The detection is usually performed at the opposite end of the capillary (normal polarity mode). UV/vis detection is by far the most common detection technique in HPCE. Other techniques include fluorescence, amperometry, conductivity, and mass spectrometry. Modem HPCE instruments are fully automated and thereby allow easy operations and precise quantitative analyses. 

Recently a new method was developed for the complete liquid chromatographic separation and diode array detection of standard mixtures of the 14 most frequently used synthetic colorants. Protocols for RP-HPLC - " and IP-HPLC techniques have been extensively described and the techniques were compared with micellar electrokinetic capillary chromatography, - which has been shown to be suitable for the analysis of synthetic colorants. 

Sample preconcentration techniques are used with two purposes (1) to increase concentration in order to achieve detection and (2) to eliminate disturbances of the electrophoretic system during hydraulic or electrokinetic sample introduction when the conductivity of the sample is significantly different from that of the analysis buffer. It is important to keep sample manipulations and modifications to a minimum, and a rule of thumb is to prepare the sample so that its composition is at the same pH as the analysis buffer. It is also advantageous... 

Koezuka, K., Ozaki, H., Matsubara, N., and Terabe, S. (1997). Separation and detection of closely related peptides by micellar electrokinetic chromatography coupled with electrospray ionization mass spectrometry using the partial filling technique.. Chromatogr. B 689, 3—11. 

Dedicated applications of capillary zone electrophoresis (CZE) coupled to MS are discussed, particularly in the field of drug analysis. Development of other capillary-based electrodriven separation techniques such as non-aqueous capillary electrophoresis (NACE), micellar electrokinetic chromatography (MEKC), and capillary electrochromatography (CEC) hyphenated with MS are also treated. The successful coupling of these electromigration schemes with MS detection provides an efficient and sensitive analytical tool for the separation, quantitation, and identification of numerous pharmaceutical, biological, therapeutic, and environmental compounds. 

The separation of phospholipids by micellar electrokinetic capillary electrophoresis (MEKC) has been described (17-19). In this technique, solutes are separated based on their distribution between a mobile (usually aqueous) and a pseudostationary (micellar) phase. Szucs et al. found that the major soybean phospholipids were fully resolved in only 7 minutes using deox ycholic acid for micelle formation in combination with 30% n-propanol at 50°C (18). However, quantification of the separated compounds remains troublesome. This is due first of all to the fact that only UV detection can be used, thus making the response highly dependent on the degree of unsaturation of the phospholipids. Besides, the comparison of peak areas in MEKC is more complicated than in HPLC, because all compounds are moving with different velocities. 

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