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Frans_M_Everaerts_Isotachophoresis_378342.pdf

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GENERAL EQUATIONS 49<br />

Uthzone (U-llthzone (U-Zjthzone<br />

Fig.4.4. The Uth zone contains Uanionic species of the sample (A,, . u). The voltage gradient in the<br />

Uth zone is Eu: The (U-1)th and (U-2)th zones contain one and two anionic species less of the<br />

sample, respectmly. The migration velocity of the zone boundary U/(U-1) is determined by the<br />

migration velocity of the anionic species Ay which is not present in the (U-1)th zone.<br />

uniform substance with an effective mobility given by<br />

ii = aimi = $ /cimi/ct> (4.1 5)<br />

i= 0 i =O<br />

provided that the time of existence of each ionic species is small in comparison with the<br />

duration of the experiment. In this effective mobility, factors such as the relaxation<br />

effect, the electrophoretic effect and the influence of temperature are neglected.<br />

Substituting eqns. 4.7 and 4.9 into eqn. 4.1 5, we can write for the effective mobility<br />

the expression<br />

(4.16)<br />

For the separation boundary U/(V-l), this means that the ionicspecies A1 to AUp1<br />

can continuously pass this boundary, as their migration velocity is higher than<br />

EufiA (fiA, > fiA2 > . . . . > fiA > r71 ). For the anionic species A,-,, for<br />

u-1 Au<br />

exampye, we have the following situation (see Fig.4.5). An ion at point P (at time t = 0)<br />

can just reach the separation boundary S within one unit of time (at time t= 1). In this<br />

time, the separation boundary has moved from So to S1. An ion at point So (at<br />

time t=O) can just reach point M (at time r= 1). This means that all ions of the anionic<br />

species AU-l present between points P and So (at time t=O) will be found again between

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