Frans_M_Everaerts_Isotachophoresis_378342.pdf

390 SIMPLIFIED MODEL OF MOVINGBOUNDARY ELECTROPHORESIS
A.4. EXPERIMENTAL
With the equations presented above, a computer program was developed and experiments
were carried out in order to check this model. To this end, all concentrations
should be determined in all zones. However, as this is difficult, another possibility is to
measure the velocities of the zones by means of a detector.
Each zone has a specific constant velocity, vu = EumAU; for practical reasons, we use
relative velocities instead of absolute velocities* :
If the distance between the point of injection and the point of detection is P, the time
needed for a particular ionic species to be detected will be
or
P = VUtU
The correlation between vu and vL will be
P= VUtU = V LtL
and hence
v = VU/VL = tL/tU
U
The times of detection can be measured from the time of the starting point of the
analysis up to the time of the appearance of the zone of a particular ionic species. Because
the velocity of the leading electrolyte is equal to the velocity of the first sample zone
(isotachophoretic condition), we use the relationship
v' =tL/tU=tJt"
U
In this way, the measured ratio tl/tU from the electropherograms can be used to check
the computed ratio vu/vL (vh).
In order to check the model, some experiments were carried out. The values of tl ItU
were measured from the electropherograms for different mixtures of P, Na+, Li+, (CH3)4NC
and (CzHS)4M. The leading electrolyte was 0.01 Nhydrochloric acid and the current was
stabilized at 70 PA. Experimental and theoretical values are given in Table Al.
In Fig.Al these values are represented graphically (the broken lines represent the
experimental values). The experimental values agree very well with the calculated values
and it can be concluded that this model is suitable in many instances.
Because the relative time of detection for a mixture of two ions of known concentrations
is constant in a given system and depends only on the mobilities, it can be used
for the determination of the effective mobility of an ion. In order to demonstrate this
*The relative velocity of a moving-boundary zone is related to the isotachophoretic velocity of the
leading zone (isotachophoretic condition).