Analyte migration

As concluded by Menzinger et CE will only fully augment chromatographic techniques in the routine determination of pesticide residues when software becomes capable of compensating for the intrinsic variability in analyte migration times. This... 

Peng, X., Bowser, M.T., Britz-McKibbin, P., Bebault, G.M., Morris, J.R., and Chen, D.D.Y., Quantitative description of analyte migration behavior based on dyamic complexation in capillary electrophoresis with one or more additives, Electrophoresis, 18, 706, 1997. 

Since CE is an on-column detection technique, analytes migrate with different velocities through the detection window. Thus, slower migrating compounds will have the same peak height but a larger peak area than faster migrating compounds. Therefore, it is common to work with corrected peak areas, i.e., peak area divided by migration time. The work on a racemic mixture of enantiomers demonstrates the importance of this correction. 

MT Bowser, DDY Chen. Higher-order equilibria and their effect on analyte migration behavior in capillary electrophoresis. Anal Chem 70 3261-3270, 1998. 

In the absence of EOF and separation mechanism other than electrophoresis, each analyte migrates with its own velocity which, according to Equation 6.8, is proportional to the strength of the electric field applied across the capillary tube. The constant of proportionality of the observed velocity of the charged analyte is defined as the observed mobility (p bs) and can be directly calculated by the migration time and the other experimental parameters, according to the following equation ... 

For analytes migrating as symmetrical Ganssian peaks, the nnmber of theoretical plates can be calcnlated directly from the electropherograms nsing the following equations ... 

An analyte migrates up or across a layer of stationary phase (most commonly silica gel), under the influence of a mobile phase (usually a mixture of organic solvents), which moves through the stationary phase by capillary action. The distance moved by the analyte is determined by its relative affinity for the stationary vs the mobile phase. 

Separation is carried out by applying a high potential (10-30 kV) to a narrow (25-75 pm) fused silica capillary filled with a mobile phase. The mobile phase generally contains an aqueous component and must contain an electrolyte. Analytes migrate in the applied electric field at a rate dependent on their charge and ionic radius. Even neutral analytes migrate through the column due to electro-osmotic flow, which usually occurs towards the cathode. 

As the analyte migrates through the column, it occupies an increasing area (Fig. 1.6). This linear dispersion column length, the variance will be ... 

In the in-channel detector, the working electrode is directly placed inside the separation channel. Thus, analytes migrate over the electrode while they are still confined to the channel. The design of the working electrode would have to minimise the coupling between the high voltage and the detection potential. As commented in Section 34.1.3, this is an inconvenience in most of the cases and the electrode width and the... 

In capillary electrophoresis a sample, usually containing charged species, is introduced into the end of a capillary that has been filled with a solution of buffer (or electrolyte). Under the influence of an electric field, the analytes migrate away from the injection end of the capillary toward the detector end, where they are visualized. Three distinct separation mechanisms have been developed for the separation of analytes by CE. 

The whole area of specific migration determinations can be subdivided in two phases (i) the pre-analytical migration exposure phase, which is more or less identical to that necessary for overall migration determination and (ii) the pure analytical phase, where the specific migrant must be determined in the respective food or simulant as precisely and reproducibly as possible. This pure analytical migration test phase comprises many considerations to be made and includes so many technical possibilities that it deserves to be described in an own comprehensive section (see Section 10.2). 

Capillary isotachophoresis (CITP) — An electrophoretic separation technique (-> electrophoresis) in a discontinuous -> buffer system, in which the analytes migrate according to their -> electrophoretic mobilities, forming a chain of adjacent zones moving with equal velocity between two solutions, i.e., leading and terminating electrolyte, which bracket the mobility range of the analytes. Ref [i] Riekkola ML, Jonsson jA, Smith RM (2004) Pure Appl Chem 76 443... 

It is evident that increasing the mobility simultaneously raises the EOF, and the relative velocity for each pair of zones is reduced along with the resolution. On the other hand, if the analyte migrates in an opposite direction to the EOF, the resolution increases because the residence time in the capillary is prolonged and the effective migration distance is lengthened, so that even components with very small differences in mobility can be separated from each other. 

Different models of the retention in these complex systems do not allow for accurate prediction of the analyte retention, but they assist in the understanding of the processes governing analyte migration through the column and also help in the selection of starting conditions and intelligent optimization of a particular separation. 

---