Detector electrophoresis

In recent years, mercury film ultramicroelectrodes have received wider application in electroanalytical practice [51,54,55]. Such electrodes are especially useful in analytical determinations, since they combine the features of ultramicroelectrodes (Chap. 12) with those of mercury film electrodes. In this case, the mercury can be deposited on carbon fibers, but many prefer a metallic support which is wetted by mercury. The solubility of the supporting metal in mercury should be low. Iridium [54] and silver [55], as well as platinum and nickel, have been used as supporting metals. Surprisingly, even gold fibers wetted by mercury have been very successfully used as electrodes in microchromatography and capillary electrophoresis detectors (Chap. 27). 

Because the separated analytes move past a common point in most types of capillary electrophoresis, detectors are similar in design and function to those described for HPLC. Table 33-3 lists several of the detection methods that have been reported for capillary electrophoresis. The second column of the table shows representative detection limits for these detectors. 

The basic instrumentation for capillary electrophoresis is shown in Figure 12.41 and includes a power supply for applying the electric field, anode and cathode compartments containing reservoirs of the buffer solution, a sample vial containing the sample, the capillary tube, and a detector. Each part of the instrument receives further consideration in this section. 

Detectors Most of the detectors used in HPLC also find use in capillary electrophoresis. Among the more common detectors are those based on the absorption of UV/Vis radiation, fluorescence, conductivity, amperometry, and mass spectrometry. Whenever possible, detection is done on-column before the solutes elute from the capillary tube and additional band broadening occurs. 

Solutes that do not absorb UV/Vis radiation or undergo fluorescence can be detected by other detectors. Table 12.8 provides a list of detectors used in capillary electrophoresis along with some of their important characteristics. 

Limits of detection become a problem in capillary electrophoresis because the amounts of analyte that can be loaded into a capillary are extremely small. In a 20 p.m capillary, for example, there is 0.03 P-L/cm capillary length. This is 1/100 to 1/1000 of the volume typically loaded onto polyacrylamide or agarose gels. For trace analysis, a very small number of molecules may actually exist in the capillary after loading. To detect these small amounts of components, some on-line detectors have been developed which use conductivity, laser Doppler effects, or narrowly focused lasers (qv) to detect either absorbance or duorescence (47,48). The conductivity detector claims detection limits down to lO molecules. The laser absorbance detector has been used to measure some of the components in a single human cell (see Trace AND RESIDUE ANALYSIS). 

We have developed the method for quantitative analysis of urinary albumin with CE. A capillary electrophoresis systems Nanophor 01 (Institute of Analytical Instmmentation, Russian Academy of Sciences, Saint-Petersburg) equipped with a UV-detector was used to determine analyte. Separation was achieved using 45 cmx30 p.m I.D. fused silica capillary column with UV-detection at 214 nm. 

A capillary electrophoresis systems CAPEE 103R ( Eumex Etd, Saint-Petersburg) equipped with a UV-detector was used to separate and quantify analytes. 

Identification of stmctures of toxic chemicals in environmental samples requires to use modern analytical methods, such as gas chromatography (GC) with element selective detectors (NPD, FPD, AED), capillary electrophoresis (CE) for screening purposes, gas chromatography/mass-spectrometry (GC/MS), gas chromatography / Fourier transform infra red spectrometry (GC/FTIR), nucleai magnetic resonance (NMR), etc. 

A computer program was compiled to work out the ray-tracing of UV detector of high performance capillary electrophoresis at the investigation of 5 and 6 (98MI59). The capacity factor of 5 at different temperature and at different mobile phase compositions was experimentally determined in bonded-phase chromatography with ion suppression (98MI15). 

Stalcup aiid co-workers [14] adapted this method to a continuous elution mini-prep electrophoresis apparatus shown in Fig. 11-3. In this apparatus, the end of the electrophoretic gel is continuously washed with elution buffer. The eluent can then be monitored using an HPLC detector (Fig. 11-4) and sent to a fraction collector where the purified enantiomers, as well as the chiral additive, may be recovered. In this system, the gel configuration was approximately 100 mm x 7 mm, and was aircooled. The number of theoretical plates obtained for 0.5 mg of piperoxan with this gel was approximately 200. A larger, water-cooled gel was able to handle 15 mg of... 

FIGURE 3-23 Schematic of a carbon-fiber amperometric detector for capillary electrophoresis A, fused silica capillary B, eluent drop C, stainless steel plate RE, reference electrode WE, working electrode, AE, auxiliary electrode. (Reproduced with permission from reference 58.)... 

Describe clearly the challenges of interfacing electrochemical detectors to capillary electrophoresis separation systems. How can these challenges be overcome ... 

AMPA = aminomethylphosphoric acid, CZE = capillary zone electrophoresis, DAD = = diode array detector. MEKC =... 

For many applications, diode array detection has become routine. A photodiode array was used for simultaneous detection of 100 capillaries in zone electrophoresis and micellar electrokinetic chromatography (MEKC).1516 Deflection of a laser beam by acoustic waves was reported as a means to scan six capillary channels on a microchip.17 The design of a low-noise amperometric detector for capillary electrophoresis has been reported.18... 

Microwave plasma detection has been reviewed [351], also in relation to GC [352,353], Coupling of chromatography (GC, SFC, HPLC) and capillary electrophoresis (CE) with ICP-MS and MIP-MS detectors has also been reviewed [181,334,335]. Various specific GC-ICP-MS reviews have appeared [334,337,345,346,354,355]. 

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