Liquid development electrophoretic

Figure 20. Electrophoretic liquid development apparatus. Key 1, photoconduc-tive insulator 2, liquid toner suspension 3, toner suspension enroute to development zone and 4, developed image. (Reproduced, with permission, from Ref. 2.)...

In summary, the following interfaces are important in electrophoretic liquid development ... 

With the work of Fenn and co-workers, liquid chromatography—electrospray interfaces for mass spectrometers were developed in 1984. Subsequently, the Pacific Northwest Laboratory began work in the area of CE—ESI—MS under the direction of Richard Smith and published the initial paper describing on-line CE—MS in 1987. Initial interface designs involved removing the polyimide at the end of the capillary in favor of a layer of silver for electrical contact. This interface was limited due to below optimum flow rates and limited lifetime of the metallized capillary. The introduction of the sheath flow design dramatically improved the CE—MS results. In lieu of being connected to a standard outlet buffer, the CE—MS interface used the outlet end of electrophoretic capillary connected directly to the electrospray mass spectrometer. 

Liquid chromatography-electrospray ionization mass spectrometry (LC-ESTMS) for analysis of sildenafil has also been developed and applied to 40 botanical products. About half of the analyzed samples proved to contain undeclared additives of the three drugs sildenafil, vardenafil, and tadalafil <2004JPBA525>. A similar electrokinetic capillary chromatography method to that proposed for the determination of sildenafil 82 has been reported for vardenafil 98 and tadalafile 99. Statistical evaluation of the electrophoretic results was achieved. The described method is thought to be rapid and sensitive <2004JCH231>. 

The electrostatic stabilization theory was developed for dilute colloidal systems and involves attractive van dcr Waals interactions and repulsive double layer interactions between two particles. They may lead to a potential barrier, an overall repulsion and/or to a minimum similar to that generated by steric stabilization. Johnson and Morrison [1] suggest that the stability in non-aqueous dispersions when the stabilizers are surfactant molecules, which arc relatively small, is due to scmi-stcric stabilization, hence to a smaller ran dcr Waals attraction between two particles caused by the adsorbed shell of surfactant molecules. The fact that such systems are quite stable suggests, however, that some repulsion is also prescni. In fact, it was demonstrated on the basis of electrophoretic measurements that a surface charge originates on solid particles suspended in aprotic liquids even in the absence of traces of... 

In most electroosmotic flows in microchannels, the flow rates are very small (e.g., 0.1 pL/min.) and the size of the microchannels is very small (e.g., 10 100 jm), it is extremely difficult to measure directly the flow rate or velocity of the electroosmotic flow in microchannels. To study liquid flow in microchannels, various microflow visualization methods have evolved. Micro particle image velocimetry (microPIV) is a method that was adapted from well-developed PIV techniques for flows in macro-sized systems [18-22]. In the microPIV technique, the fluid motion is inferred from the motion of sub-micron tracer particles. To eliminate the effect of Brownian motion, temporal or spatial averaging must be employed. Particle affinities for other particles, channel walls, and free surfaces must also be considered. In electrokinetic flows, the electrophoretic motion of the tracer particles (relative to the bulk flow) is an additional consideration that must be taken. These are the disadvantages of the microPIV technique. 

The development of the electrospray ionization (ESI) source for mass spectrometry provided an ideal means of detection for capillary electrophoretic (CE) separations. The ESI source is currently the preferred interface for CE-MS, due to the fact that it can produce ions directly from liquids at atmospheric pressure and with high sensitivity and selectivity for a wide range of analytes. 

The choice of an appropriate sheath liquid and its flow rate is essential to achieve good performance. This choice is a compromise between separation (to maintain an efficient electrophoretic separation) and ionization performances (to assist droplet formation and spray stability). Most CE-ESI-MS applications described in the literature for the analysis of protonated compounds use a sheath liquid containing a mixture of organic solvent, water and formic or acetic acid. In method development, the impact of the nature of the sheath liquid on the expected chiral separation can be evaluated by placing it in the outlet vial. The solubility of the chiral resolving agent in the sheath liquid has to be carefully investigated to avoid its precipitation at the spray needle. 

---