Frans_M_Everaerts_Isotachophoresis_378342.pdf

PROCEDURE OF COMPUTATION
A.3. PROCEDURE OF COMPUTATION
For a separation boundary between the zones U- 1 and U, eqn. A1 gives
u- 1 U
~u-i * 2 (mA,+mB)cA,,U-i =E,* C (mAr+mB)cAr,U (A71
r= 1 r=l
Combining eqns. A7 and A6 gives
JI
In fact, this is a modification of the ‘Dole polynomals’ [ 1,8] and solutions for the
equations are valid if
If the composition of the leading electrolyte and the sample solution are known, all
parameters can be computed with the equations given above. The velocity of the concen-
tration boundaries can be neglected.
In the first instance, the concentrations of the ionic species in the last sample zone
are taken to be equal to the original concentrations in the sample. Although this assump-
tion is not correct, the ratio between the concentrations in the sample remains constant
in the zones, according to eqn. A2, which gives
(It is assumed that the velocity of the concentration boundary can be neglected.)
Using eqns. AS and A9, flu-] ,u can be calculated if all mobilities are known. With
flu- ,u and eqn. A6, all concentrations of the zone U- 1 can be calculated, and thus all
concentrations and values can be calculated for all zones.
The concentration of the ionic species in the first sample zone can be calculated in
two ways, either with the equations given above or using the isotachophoretic condition
as described in Chapter 4. In the first computation, we chose arbitrarily as concentrations
for the ionic species in the last sample zone those concentrations present in the original
sample solution, and all quantities could be obtained. If the parameters of the first zone
obtained in this way did not agree with those obtained by the isotachophoretic method,
we re-computed from the first to the last zone with the quantities obtained with the
isotachophoretic condition, using the 0 values from the moving-boundary procedure. By
this calculation, new concentrations for the sample zones can be proposed and new
/3 values can be calculated. This procedure of ‘iteration’ must be repeated until the
P values and concentrations fit.
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