Frans_M_Everaerts_Isotachophoresis_378342.pdf

306 SEPARATION OF ANIONIC SPECIES IN AQUEOUS SOLUTIONS
3 @ 2+i 1 3 0 2
Fig.12.7. Isotachopherograms with Fe(CN,)4- as the leading ion at concentrations of (a), 0.01 N;
(b), 0.005 N; (c), 0.001 N. Histidine was used as the buffering counter ion. (a) shows that C1- is more
mobile than Fe(CN,)4-, but that the original concentration is not changed if this mobile chloride
passes the first separation boundary; (b) shows that a mixed zone is obtained; (c) shows that Cl- is
less mobile than Fe(CN,)4-. The current was stabilized at 70 MA. A = Increasing W absorption;
R = increasing resistance; t = time. 1 = Fe(CN,)4-; 2 = C1-; 3 = MES-; 4 = mixture of anions.
Quantitative analyses could easily be performed, although the leading electrolyte must
be freshly prepared each time as it is unstable. It should be repeated that the concentration
change in the leading electrolyte of the operational system may give this system
completely different properties.
The concentration adjustment is not influenced if impurities with a higher mobility
than that of the leading ion pass the first separation boundary.
The test mixture of anions (Fig.12.5) was also analyzed in the mixture urea-water
(1: 1). Differences similar to those in the isotachopherograms shown in Chapter 16 were
obtained. Because of the high concentration of urea, these operational systems are
difficult to work with, for practical reasons as the eqhpment soon becomes covered
with urea. Nevertheless, this system can be used, because many organic substances are
more soluble in it and it is not aggressive.