Capillary Isotachophoresis

Table 1 Development of isotachophoresis


Development of:

Attributed to:

1897 Regulation function

1930 Moving boundary electrophoresis

1942 Displacement electrophoresis

1968 Capillary tube apparatus for isotachophoresis

1970-1989 ITP in closed systems, 200-500 ^m i.d. narrow-bore plastic capillary with minimized EOF Column-coupling ITP

1970-1980 Thermometric, conductometric, potentiometric and UV detection 1981 Refractometric detection

1981 Offline ITP-MS

1983 Radiometric detection

1984 Fluorimetric detection

1984 Amperometric detection

1985 Absorption spectra

1989 ITP in open system, 100 ^m i.d. fused-silica capillary with EOF, online ITP-MS

1990 ITP in open system, 25-50 ^m i.d. fused-silica capillaries

1990 Online ITP-CZE (column coupling)

1991 Offline ITP-PIXE 1993 Bidirectional ITP

1995 Raman spectroscopic detection




Verheggen, Everaerts














Walker sign as that of the sample ions, but with an effective mobility higher than that of the fastest moving sample ion. The second solution (the terminating electrolyte) contains an ion (the terminating ion) with the same charge sign, but with an effective mobility slower than that of the slowest moving sample ion. The polarity of the electric field has to be such that the leading ion migrates to the electrode that is placed on the same side of the sample as the leading electrolyte. After application of an electric field to the system, each ionic species will have a different migration velocity according to eqn [1] and hence the iso-tachophoretic process starts.

The process of isotachophoresis may be divided into two parts. In the first part, the separation of the ions proceeds and the migration velocity of the individual ions in the mixed zones is different. In the second part (in the steady state) the ions have already separated from one other and all move with the same velocity, v:

ary will not broaden further, which is in contrast to zone electrophoresis, where the peaks are broad owing to adsorption and diffusion. This effect can easily be explained. If an ion diffuses into a preceding zone, where the electric field strength is lower than the value that corresponds to its velocity, its velocity will decrease according to eqn [2], and it will be overtaken by its own zone. If an ion diffuses into a zone with a higher electric field strength, then it will obtain a higher migration velocity according to eqn [2], until it reaches its own zone.

It is characteristic for the steady state that the concentration of each component is adjusted to the value following from the Kohlrausch regulation function in the form:

where L is the leading ion, i is the ith ion and T is the terminating ion.

A schematic representation of the cationic and ani-onic modes in ITP experiments without EOF is given in Figure 1(A,B). As the ionic species are arranged in order of decreasing effective mobilities, the electric field strengths increase on the terminating ion side.

The increase in the electric field strength in the consecutive zones induces the zone-sharpening effect. When a zone has attained the steady state, the bound where R is the common counterion, Zi is the ionic charge. In the steady state, the concentration Ci of the ith ion is always adjusted to a certain value depending only on the concentration of the leading electrolyte CL and on the mobility of the ions i, L and R. From the analytical point of view this is a very important feature of ITP. It can be concluded that for a given set of experimental conditions, the zone length is a direct measure of the amount of an ion present in the zone. Another important consequence of these properties is the concentration effect of isotachophoresis. In fact, a species more concentrated in the original sample is diluted during the separation and, a species originally too dilute is concentrated during the separation.

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