Band broadening wall adsorption

A major focus of researchers creating planar CE devices is speed of analysis, in part so that the devices can be used as chemical sensors and circumvent the severe selectivity and lifetime requirements of conventional chemical sensors. To increase the speed of analysis, shorter capillaries should be used, in combination with higher electric field strengths. Optimum efficiency depends on minimization of all unavoidable sources of band broadening, in addition to the elimination of nonideal effects such as Joule heating and adsorption on capillary walls. Therefore, work to understand the contributions which limit the efficiency of the separation is continuing. 

A variety of surfaces such as metals, plastics, and glass can retain proteins during a separation process. In CE, a problem is manifested in the adsorption of proteins by fused silica capillaries (see Section IV). This problem is attributed to the adsorption of positively charged sites of proteins on negatively charged sites (silanol groups) on the capillary wall—a process that leads to band broadening and a much lower number of theoretical plates than would be expected on the basis of theory. 

Other effects may also contribute to band broadening causing reduced achievable plate counts. Besides the already-mentioned wall adsorption, temperature effects (Joule heating) may reduce plate numbers. Sample application can have a strong influence on plate count, especially when large volumes and/or high sample concentrations are injected. Mobility differences between buffer constituents and analyte ions lead to asymmetric (triangular) peaks caused by electrodispersion, which is extremely noticeable with smaller molecules. Differ-... 

The adsorption of proteins to the inner wall of the capillary is a major consideration in ACE assays (44,45). The interaction can lead to band broadening, irreproducible migration times, low resolution, and low recovery of the protein. Various approaches to control the problem include chemical modification to the inner surface of the capillary, choosing a proper buffer species and pH, and the use of buffer additives to reduce the protein interaction with the capillary wall. The buffer additive has to be cautiously chosen so as not to participate in the interaction between Ag and Ab. 

Variance Caused by Analyte-Wall Interactions The interactions of analyte and capillary wall, or components within the sample solution are numerous, complex and sample specific. The best approach to understanding the band broadening due to adsorption is the approach of McManigill and Swedberg. adsorption term... 

In reality, additional sources of zone broadening include the finite width of the injected band (Equation 23-32), a parabolic flow profile from heating inside the capillary, adsorption of solute on the capillary wall (which acts as a stationary phase), the finite length of the detection zone, and mobility mismatch of solute and buffer ions that leads to nonideal elec-... 

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