Capillary electrophoresis wall effects

A number of developments have increased the importance of capillary electrophoretic methods relative to pumped column methods in analysis. Interactions of analytes with the capillary wall are better understood, inspiring the development of means to minimize wall effects. Capillary electrophoresis (CE) has been standardized to the point of being useful as a routine technique. Incremental improvements in column coating techniques, buffer preparation, and injection techniques, combined with substantive advances in miniaturization and detection have potentiated rugged operation and high capacity massive parallelism in analysis. 

Figure 4-34. Wall effects in capillary electrophoresis. The capillary walls, which are usually made of fused silica contain a small proportion of dissociated silanyl groups which bind positively charged counterions. In the region close to the wall, defining the Stern layer, these ions are relatively immobile mobility increases beyond this layer in a region which defines the Guoy-Chapman layer. The distribution of positively charged ions is termed the Zeta-potential it is fairly constant in the Stern layer, but falls off sharply with distance from the wall in the Guoy-Chapman layer.

Electrophoresis is accompanied by several phonemena which stem from the principle of the method itself or the instrumentation used. The most important accompanying effects during electrophoresis are the production of Joule heat, electro-osmosis, diffusion and, in the case of capillary electrophoresis, the interaction of samples with the wall. These dispersive processes decrease the resolution of the electrophoretic technique and it is important to minimize them in order to obtain correct results. 

There are four related electrokinetic phenomena which are generally defined as follows electrophoresis— the movement of a charged surface (i.e., suspended particle) relative to a stationary liquid induced by an applied electrical field, sedimentation potential— the electric field which is crested when charged particles move relative to a stationary liquid, eleetroosmosis—the movement of a liquid relative to a stationary charged surface (i.e., capillary wall), and streaming potential—the electric field which is created when liquid is made to flow relative to a stationary charged surface. The effects summarized by Eq. (20-23) form the basis of these electrokinetic phenomena. 

Interactions between the solutes and the capillary wall also have a negative effect on the efficiency in capillary zone electrophoresis. Both hydrophobic interactions and electrostatic interactions of cations with the negatively charged capillary wall can be the cause of solute adsorption. Significant adsorption has been found for high-molecular-weight species, e.g., peptides and proteins. Because of the increased surface-area-to-volume ratio of narrow-bore capillaries, this effect is even more pronounced. 

CEC has recently become an alternative to HPLC. A capillary is filled or its internal wall covered with a porous sorbent. The free volume remaining in the capillary is filled with an electrolyte. High voltage (on the order of ten kV) is applied across the length of the capillary. Sample plugs are introduced at one end. Sample components are carried to the other end due to electro-osmosis and - in the case of ions - also electrophoresis. In CEC the more important effect is electro-osmosis, which is essentially a flow mechanism of the electrolyte solution without the need for applied pressure. The separation of the sample components occurs mainly due to phase distribution between the stationary phase and the flowing electrolyte. Thus CEC is very similar to HPLC in a packed capillary except that the flow is not pressure driven and that ionic analytes undergo electrophoresis additionally to phase separation. 

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