Electrophoretic separations fabrication and uses

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... 

Much effort has been invested in integrating mass spectrometry with on-chip CE. The flow rates typically used (nL to p,L/min) are very suitable for electrospray ionisation (ESI) prior to MS. However, the buffers used in CE tend not to be compatible with ESI and there is also a need to decouple the two electric fields (one for the electrophoretic separation, one for the electrospray). One method that has been used for interfacing electrospray with chips is to bond electrospray nozzles/needles to the outlet of the microchannel. Electrospray tips can also be incorporated onto the chip as part of the fabrication process. Electrospray detection following separation by CE has worked well for proteins, carbohydrates and many other compounds. 

A nanofluidic channel device, consisting of many entropic traps, was designed and fabricated for the separation of long DNA molecules. The channel comprises narrow constrictions and wider regions that cause size-dependent trapping of DNA at the onset of a constriction. This process creates electrophoretic mobility differences, thus enabling efficient separation without the use of a gel matrix or PEF. Samples of long DNA molecules (5000-... 

FIGURE 15.1 Schematic representation of a microchip used for high-speed electrophoretic separations. Narrow channels were etched in the injection and separation areas, while wide channels were fabricated for all other sections. These differential channel widths ensured the majority of the potential was apphed across the narrow channels. Owing to the high separation field strengths (up to 6.1 V cm per volt of applied potential), suhsecond separations were possible. (Reproduced from Jacobson, S. C. et al. Anal. Chem., 70, 3476, 1998. With permission from American Chemical Society.)... 

Richter et al. have used this process to produce electrodes (see Figure 41.5) for EC detection in CE. These Au-CDtrodes were used first in a home-made CE system, in which an electrophoretic separation of iodide, ascorbic acid, dipyrone, and acetaminophen was successfully performed. Au-CDtrodes were later applied to an electrophoresis microchip fabricated with PT, where the effectiveness of the proposed system was demonstrated with a separation of iodide and ascorbic acid. A series of 10 repetitive injections obtained in a conventional CE-EC system and one electropherogram obtained in miniaturized system are shown in Figure 41.6. 

In the past 15 years, the use of microfluidic devices for chemical analysis has increased tremendously. Indeed, a broad range of chromatographic and electrophoretic separation methods have been implemented in microchips. However, for widespread utilization of microfabricated devices in analysis applications, particularly in the field of proteomics, further efforts are needed to develop simple fabrication techniques that achieve functional integration of multiple tasks in a single device. " In this section, we describe the fabrication of microdevices using sacrificial materials and discuss some of the advantages of this approach over conventional microfabrication methods. 

For mechanical lysis, nanostructured filter-Uke contractions are employed in microfluidic channels with pressure-driven cell flow. Prinz et al. utilized rapid diffusive mixing to lyse Escherichia coli cells and trap the released chromosome via dielectrophoresis (DEP). Kim et al. developed a microfluidic compact disk platform for mechanical lysis of cells using spherical particles with an efficiency of approximately 65 % however, this method is difficult to be apphed for single-cell analysis. Lee et al. fabricated nanoscale barbs in a microfluidic chip for mechanical cell lysis by shear and frictional forces. Munce et al. reported a device to lyse individual cells by electromechanical shear force at the entrance of 10 mm separation channels. The contents of individual cells were simultaneously injected into parallel channels for electrophoretic separation, which can be recorded by laser-induced fluorescence OLIF) of the labeled cellular contents. The use of individual separation channels for each cell separation eliminated possible cross-contamination from multiple cell separations in a single channel. 

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