Frans_M_Everaerts_Isotachophoresis_378342.pdf

Fig.6.31. Isotachopherograms for the separation of some anions that lack W absorbance. (a)
Creatinine, for which the molaf absorptivity of creatinine is function of the pH. Leading electrolyte
= HCl(O.01 M, pro analysi grade) t creatinine (purified); pH = 4.5. Terminating electrolyte =
morpholinoethanesulphonic acid (MES) (re-crystallized three times). (b) Counter ion for which the
change in pH of a zone does not influence the molar absorptivity (p-alanine). Leading electrolyte
= HCl (0.01 M, pro analysi grade) t e-aminocaproic acid (purified); pH = 4.5. Terminating
electrolyte = MES (re-crystallized three times). Non-W-absorbing ions can thus be detected by the
‘indirect UV method‘. A = Increasing UV absorption; R = increasing electric resistance; t = time. The
current was stabilised in both instances at 30pA. The chart paper speed was 2 cm/min. An injection
was made of 1 pl of the sample 0.01 Mchlorate t 0.01 Macetate + 0.01 M formate + 0.01 M
glutamate. Peaks: 1 = chloride; 2 = chlorate; 3 = formate; 4 = acetate; 5 = glutamate; 6 = MES. In (a)
the lower pH of the glutamate zone with respect to the zone preceding it and following it is clearly
visible. From this isotachopherogram, the pH can easily be checked as it can be calculated with the
computer program discussed in Chapter 4. The difference in the step height as found in the linear
conductivity trace, due to the difference in the counter ion, should be noted. The conductimeter
used is discussed elsewhere (Fig.6.18); the measuring electrodes were mounted equiplanar. Various
impurities commonly present in the chemicals can be observed in the traces from both the UV and
conductivity detectors.
167