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From eqn [2], it can be seen that the ion's effective charge, its size and the viscosity of the solution decide ionic mobility. Thus in a given system ionic mobility is an intrinsic property of an ion. Usually ionic mobility cannot be directly derived from eqn [2], as the parameters are not easily accessible quantities. Instead it can be measured based on relevant experimental data, i.e. how long an ion takes to travel through a certain distance under a definite electric field, as follows:

Leff 1 Leff

V/Lto

VXtm

where Leff and Ltot are the effective migration length (from inlet to detection window) and total migration length, respectively, V is the applied voltage, and tm is the migration time of the ion. For a CE system operated under a constant voltage, Leff, Ltot and V are all where s is the dielectric constant of the buffer solution, £ is the zeta-potential across the diffuse layer, and y is the viscosity of the electrolyte. Unlike conventional electrophoresis where EOF is regarded as unfavourable and thus usually suppressed, in CE it has several important positive implications.

First, the existence of an EOF offers a simple and highly efficient way of driving a separation system. The zeta-potential is uniformly distributed within an extremely narrow cylindrical region along the whole capillary so the bulk electrolyte solution is pumped out of the capillary with virtually no pressure drop (Figure 2). A 'plug-like' flow is obtained, which subsequently contributes to high column performance. This is advantageous over the conventional pumping methods such as in HPLC, where the pressure-based flow always introduces a parabolic profile thus adding to the loss of column efficiency.

Figure 2 The generation of electroosmotic flow (EOF) in a silica capillary.

Second, the presence of EOF affects the apparent mobilities of ions (Figure 3). In any electrophoretic separation system where EOF is not fully suppressed, the observed mobility of a charged species will be the resultant of its effective electrophoretic mobility and EOF:

Mobs Mep #

Under normal conditions, with EOF directs towards the cathode, obviously cations will be accelerated, while anions will be decelerated. If the magnitude of the EOF exceeds the mobilities of the anions, the anions will be swept towards the detection side, thus allowing the simultaneous analysis of cationic and anionic species. As the magnitude and direction of EOF will affect how long the analytes stay inside the separation capillary, manipulation of EOF often becomes a core issue for effecting a satisfactory resolution. Since the formation of EOF involves two phases (capillary wall and running buffer), any modification to their chemistries will bring about a change in EOF.

Causes of Band Broadening

As in a chromatographic process, in electrophoresis it is necessary to contain the ionic species within narrow bands while creating sufficient mobility differences. How narrow a band is depends not only on the various dispersive factors inherent to the elec-trophoretic process, but also on how well the whole process is performed. The common causes of band

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Effect of EOF on the apparent mobilities of anions and

Figure 3

cations.

Effect of EOF on the apparent mobilities of anions and broadening in CE include longitudinal diffusion, injection-related volume overloading, thermal effects, electrodispersion, wall adsorption, etc. These band broadening mechanisms are deemed to be random and independent events, so that the concept of summation of variances can be used to evaluate the contributions of individual factors to the overall band broadening effect, that is:

°tot = °diff + °mj erm + ffwall + °dectr + other [6]

A brief description of these band broadening factors is given below.

Longitudinal diffusion In the course of electrophoretic transportation of an analyte band along the capillary, the sample molecules will inevitably have a tendency to enter the surrounding buffer solution because of the apparent concentration difference, leading to a wider and more dilute sample band. According to Einstein's diffusion equation, band dispersion due to longitudinal diffusion is a function of diffusion coefficient and time:

Under an ideal situation, longitudinal diffusion becomes the only unavoidable band broadening process. Therefore it defines the maximum attainable column efficiency in CE. Based on chromatographic theory, the maximum obtainable theoretical plates (N) can be derived as follows:

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