Functionalized Fused Silica Capillaries

Coating the interior surface of the capillary with an uncharged hydrophilic material reduces or eliminates electroosmotic flow. To be useful for routine analysis, however, the coating must be stable under normal operating conditions. If the capillary coating deteriorates over time, the efficiency and reproducibility of the separation will be compromised. 

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In contrast, chemical etching of the inner wall of the fused-silica capillaries was used to increase the surface area. This enables achievement of a higher phase ratio since more alkyl functionalities can be attached to the surface, thus improving both the separation process and loadability of the column. The surface morphology of the etched capillary depends on the time the methanol solution of ammonium hydrogen difluoride is left in contact with the capillary and temperature at which the reaction is carried out (Fig. 10) [77]. The surface features have been described by Pesek to range from spikes of silica material extending... 

To date, most NMR microcoils have been wound directly on to a capillary tube which functions as both sample container and coil form [26,27], According to electromagnetic field theory, a sample enclosed by a perfectly uniform and infinitely long hollow cylinder (e.g. a fused silica capillary) experiences a uniform static magnetic field [28]. In reality, the susceptibility variation of the... 

Capillary electrophoresis separations are dependent on the relative mobilities of analytes under the influence of an electric field and do not depend on mobile phase/stationary phase interactions. A fused silica capillary is filled with a buffer and both ends submerged into two reservoirs of the buffer. A platinum electrode is immersed in each reservoir and a potential difference (5-30 kV) is applied across the electrode. An aliquot of sample of a few nanoliters is injected onto the capillary by either hydrostatic or electrokinetic injection, and the components migrate to the negative electrode. Separations of analytes arise from differences in the electrophoretic mobilities, which are dependent on the mass-to-charge ratio of the components, physical size of the analyte, and buffer/analyte interactions. An electro-osmotic flow (EOF) of the buffer occurs in the capillary and arises as a result of interactions of the buffer with dissociated functional groups on the surface of the capillary. Positive ions from the buffer solution are attracted to negative ions... 

The measurement of fast interfacial reactions is very difficult, since the reaction has started just after contact of the two phases. The HSS method could measure a reaction several seconds after the contact, and the two-phase stopped-flow method could measure the reaction in the range from a few tenths of a millisecond to several hundred milliseconds. The micro two-phase sheath flow method can measure reactions as fast as < 1 ms [10]. A schematic drawing of the laser-induced fluorescence measurement in the sheath flow system is shown in Fig. 3. An inner organic phase and an outer aqueous phase flow with the same line velocity. The fluorescence at the interface is observed as a function of the distance from the end of the fused-silica capillary. The distance is converted into time. 

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