Mobility of analyte

Electroosmotic flow in a capillary also makes it possible to analyze both cations and anions in the same sample. The only requirement is that the electroosmotic flow downstream is of a greater magnitude than electrophoresis of the oppositely charged ions upstream. Electro osmosis is the preferred method of generating flow in the capillary, because the variation in the flow profile occurs within a fraction of Kr from the wall (49). When electro osmosis is used for sample injection, differing amounts of analyte can be found between the sample in the capillary and the uninjected sample, because of different electrophoretic mobilities of analytes (50). Two other methods of generating flow are with gravity or with a pump. 

The UV-absorbing co-ion or probe may be the co-ion of the buffer or a specific co-ion that is added to the buffer. As already described in the previous paragraph, the probe should ideally show mobility close to that of the analyte of interest. The absorptivity of the probe is a parameter of secondary importance. Both Pacakova et al. and Macka et al. propose an extensive table matching the ionic mobilities of analytes and probes. Others use the same probe for a large range of analytes. 

The best known CE buffer ingredient is sodium dodecyl sulfate (SDS) proposed by Terabe [10,11]. SDS forms micelles and the separation of neutral analytes is achieved by their partitioning between the buffer and the SDS micelles, that is, by their hydrophobicity. This is the basis of MEKC and the mobility of analytes correlates linearly well with logP values, where P is the octanol/water partition ratio. Many other buffer ingredients have been proposed. Most of them implement hydrophobic interactions between the analytes and the buffer ingredients but also ciral selectors have been used as well as various affinity probes. Interest in the ILs used as buffer additives in capillary electromigration methods is due to the fact that they could provide an alternative separation mechanism to two currently implemented mechanisms in CE which are based either on the charge to mass ratio or on the hydrophobicity of the analytes. 

Chromatographic and electrophoretic separations are truly orthogonal, which makes them excellent techniques to couple in a multidimensional system. Capillary electrophoresis separates analytes based on differences in the electrophoretic mobilities of analytes, while chromatographic separations discriminate based on differences in partition function, adsorption, or other properties unrelated to charge (with some clear exceptions). Typically in multidimensional techniques, the more orthogonal two methods are, then the more difficult it is to interface them. Microscale liquid chromatography (p.LC) has been comparatively easy to couple to capillary electrophoresis due to the fact that both techniques involve narrow-bore columns and liquid-phase eluents. 

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... 

Capillary zone electrophoresis is another technique which has been used to separate products such as organic acids.26 Separation is based on differences in the mobility of analytes exposed to an electric field. Resolution and separation time in such systems depends on factors including electroosmotic flow (EOF), and a number of approaches for adjusting the EOF have been examined. While some of the approaches (pretreatment of capillaries) are not useful as means of process control, adjusting buffer pH and the electric field27 seem to be possible handles for true feedback control of the separation, although closed-loop operation does not seem to have been attempted. 

Electroklnetlc Injection In electrokinetic injection, the sample is introduced in the capillary by applying a voltage (in general, lower than that used for the separation), while the injection end is dipped in the sample (Fig. 3.6). Under these conditions, the analytes contained in the sample are injected by electromigration as well as by electroosmotic flow. The amount of sample loaded increases with the electrophoretic mobility of analyte, the electroosmotic flow mobility, the inner radius, the voltage, the sample concentration, and the injection time. The amount loaded will decrease with the capillary length. 

The effect of the EOF on migration time and selectivity depends on the mutual signs of the mobilities of analytes and EOF, respectively. Concerning the change in separation selectivity, we refer to the expression of the selectivity term in the resolution equation. The difference between the mobilities of the two separands, i and j, will not be influenced by the EOF. However, the mean mobility is larger for the case of comigration. This means that the selectivity term in the expression for the resolution is always reduced in this case. In practice, selectivity is lost for cation separation when the EOF is directed, as is usual in uncoated fused-siUca capillaries, toward the cathode. For this reason, cationic additives are applied in the BGE to reverse the EOF direction. 

For the case of countermigration, the situation is more complicated, because the overall effect depends on the magnitude of the mobility of analyte and that of the EOF. Roughly, it can be concluded that the res-... 

In capillary electrophoresis (CE), several criteria can be applied to classify solvents [e.g., for practical purposes based on the solution ability for analytes, on ultraviolet (UV) absorbance (for suitability to the UV detector), toxicity, etc.]. Another parameter could be the viscosity of the solvent, a property that influences the mobilities of analytes and that of the electro-osmotic flow (EOF) and restricts handling of the background electrolyte (BGE). For more fundamental reasons, the dielectric constant (the relative permittivity) is a well-recognized parameter for classification. It was initially considered to interpret the change of ionization constants of acids and bases according to Born s approach. This approach has lost importance in this respect because it is based on too simple assumptions limited to electrostatic interactions. Indeed, a more appropriate concept reflects solvation effects, the ability for H-bonding, or the acido-base property of the solvent. 

The EOF contributes significantly to the mobility of analytes in CE. The EOF is considered to be a nonse-lective mobility. However, for enantiomers, both the EOF and the electrophoretic mobility of the analyte are inherently nonenantioselective. The stereoselective analyte-selector interactions may turn both of these mobilities into a selective transport with equal success. This is the principal difference between the roles of the EOF in true electrophoretic separations and in chiral CE separations. 

Indirect detection techniques with UV-absorbing buffer components and non-absorbing analytes are widely applied in CZE of small molecules. To diminish electrodispersion, i.e. to achieve symmetrical peaks, the mobilities of analyte ions and background electrolyte should match closely [2]. In this case the sieving effect will dominate and peak broadening is only due to polydispersity. When mobility differences are large, electrodispersion rules... 

The capillary temperature can affect both peak migration time and peak area (Fig. 2a, b). The effects are due to temperature-mediated viscosity changes in the buffer. The electrophoretic mobility of analyte (p,e) is proportional to its charge (q) to mass ratio, and inversely... 

Figure 3 Ionic mobilities of analytes and absorbing co-ions.

The molar fractions of the ionic species are dependent on pH, so the effective mobility of the weak electrolyte is dependent on the pH of the solution as well. This fact shows that changing pH is a very powerful tool that allows one to tune the effective mobilities of analytes to obtain their best separation. 

As the difference in electrophoretic mobilities of analytes is of key importance for obtaining good separation, additional ways are used to influence it. 

In addition to these techniques, applications of capillary electrophoresis for lantharride analysis have appeared recently (Corr and Arracleto 1996, Vogt and Comadi 1994). Capillary electrophoretic separations rely on differences in the electrophoretic mobility of analyte species in an electrolyte buffer while tmder the influence of an applied electric field. For lanthanide analysis, the mobilities of the solvated cations are not adequately differentiated for an effective mutual separation, though separation from trarrsition metals or alkali/alkaline-earth metals should be readily accomplished. Introduction of chelating agents that form complexes with the ions leads to improved separation efficiency through... 

For many years the elecBophoretic mobility of analyte (/rep) was considered to be a selective transport able to differentiate between enantiomers, while the electroos-motic mobility (/reo) was considered to be a non-selective transport. This is not correct for chiral EKC, although it applies without any limitation for true electrophoretic separations, i.e., for the separations which are based on a different electric charge density of the sample components [2],... 

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