Microchip Columns

Capillary electrophoretic separations are performed in small diameter tubes, made of Teflon, polyethylene, and other materials. The most frequently used material is fused silica. Fused silica capillaries are relatively inexpensive and are available in different internal and external diameters. An important advantage of a fused silica capillary is that the inner surface can be modified easily by either chemical or physical means. The chemistry of the silica surface is well established due to the popularity of silica surfaces in gas chromatography (GC) and liquid chromatography (LC). In capillary electrophoresis, the silica surface is responsible for the EOF. Using surface modification techniques, the zeta potential and correspondingly the EOF can be varied or eliminated. Column fabrication has been done on microchips.13... 

HPLC with microchip electrophoresis. Capillary RPLC was used as the first dimension, and chip CE as the second dimension to perform fast sample transfers and separations. A valve-free gating interface was devised simply by inserting the outlet end of LC column into the cross-channel on a specially designed chip. Laser-induced fluorescence was used for detecting the FITC-labeled peptides of a BSA digest. The capillary HPLC effluents were continuously delivered every 20 s to the chip for CE separation. 

S. Zhang, X. Huang A microchip electrospray device and column with affinity adsorbents and use of the same, PCT Int. Appl. 2002, 59 pp,... 

A dual electrochemical microchip detection system, based on the coupling of conductivity and amperometric detection schemes, was developed for simultaneous measurements of both nitroaromatic and ionic explosives [34], The microsystem relied on the combination of a contactless conductivity detector with an end-column thick-film carbon amperometric detector. Such ability to monitor both redox-active nitroaromatic and ionic explosives is demonstrated in Figure 13.7, which shows typical dual-detection electropherograms for a sample mixture containing the nitroaromatic explosives trinitrobenzene (TNB) (4), TNT (5), 2,4-DNB (6), and 2-Am-4,6-DNB (7), as well as the explosive-related ammonium... 

Jacobson, S. C., R. Hergenroder, L. B. Koutny, R. J. Warmack, and J. M. Ramsey, Effects of injection schemes and column geometry on the performance of microchip electrophoresis devices. Anal. Chem., 66, 1107-1113 (1994). 

Saito, Y., Jinno, K., and Greibrokk, T., Capillary columns in liquid chromatography Between conventional columns and microchips, Journal of Separation Science 27(17-18), 1379-1390, 2004. 

Different possibilities for conducting enzymatic assays on microchip platforms including pre-, on- or post-column reactions have been reviewed [156]. An enzymatic assay (employing creatininase, creatinase and sarcosine oxidase) has also been developed in a microchip for analysing renal marks such as creatinine and creatine as well as p-aminohippuric acid and uric acid [157]. 

D.M. Osbourn and C.E. Lunte, On-Column electrochemical detection for microchip capillary electrophoresis, Anal. Chem., 75 (2003) 2710-2714. 

Pushing detection limits of nitroaromatic explosives into the parts per trillion (ppt) level requires sample preconcentration. Collins and coworkers used solid-phase extraction (SPE) of explosives from sea water which was followed by rapid on-chip separation and detection [18]. Explosives were eluted from SPE column by acetonitrile and were injected in the microchip separation channel. Lab-on-a-chip analysis was carried out in nonaqueous medium. The mixed acetonitrile/methanol separation buffer was used to produce the ionized red-colored products of TNT, TNB and tetryl [27,28]. The chemical reaction of the bases (hydroxide and methoxide anions) with trinitroaromatic explosives resulted in negatively charged products, which were readily separated by microchip... 

The column length and inner diameter are the two most important features required in column generation on microchips. The column separation capacity is measured in terms of number of plates, which is proportional to column length. But back pressure and analysis time are raised proportionally as column length increases. For gradient separations, column length is less a factor for resolution as separation is controlled by gradient rather than... 

Figure 6.3 The chromatograms of a mixture of 0.5 p,m MFITC-derivatized amino acids in a 2.5 cm x 100 xm x 5 xm LC microchip column. (A) Aspartic acid, (B) glycine, and (C) phenylalanine mobile phase acetonitrile-50 mM acetate (pH 5.45) (70 30, v/v) [45].

FIGURE 6.33 Schematic of the microchip with integrated pre-column reactor. The reaction chamber is 2 mm long and 96 im wide (at half-depth). The separation column is 15.4 mm long and 31 pm wide. The channels are 6.2 pm deep [106]. Reprinted with... 

FIGURE 10.21 (a) Cross-sectional view of the microchip, heating element, and the thermocouples. (b) Schematic of the microchip used for on-chip reactions, separations, and post-column labeling. The fluid reservoirs are (1) substrate, (2) enzyme or DTT, (3) buffer, (4) sample waste, (5) NDA, and (6) waste [1058]. Reprinted with permission from Elsevier Science. 

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