Frans_M_Everaerts_Isotachophoresis_378342.pdf

256 PRACTICAL ASPECTS
acid. The current was 70 PA. The ratios fL/tCi are taken as a check*.
In Fig.9.2, the relationship between the pH of the terminator and the rL/tU ratio is
given for theoretical (solid line) and experimental (individual points) values. Good agreement
is obtained, showing that a moving-boundary model provides a better description
than isotachophoresis.
If the influence of background electrolytes such as H' is too great, elution phenomena
will appear after a certain time. The zone boundaries become less and less sharp and after
a long time they release each other. The elution effects are often caused by electrode
reactions when the electrode compartments are not renewed in time; using C1- as a
counter ion in methanol (95%, wlw), the following reactions can be expected:
2C1- * Clz + 2e
+HZO HOCl
HC1+
+CH30H CHjOCl
In experiments with an unbuffered system, the H+ ions produced disturb the analyses.
As an example, the separation of caesium, sodium and lithium with the leading electrolyte
0.01 N hydrochloric acid and terminator cadmium chloride is shown in Fig.9.3a.
In Fig.9.3b, the separation of the same mixture in the same system after the terminator
Fig.9.2. Theoretical (line) and experimental (points) relationship between the pH of the terminator
solution and the relative detection times for solutions of KCl applied as terminator, in a moving-
boundary system.
*t~' Time of appearance of a sample zone if no ionic species with the same charge as the species
studied passes the separation boundary; tU = time of appearance of a sample zone if an ionic species
with the same charge as the species studied passes the separation boundary. If tL/ru = 1, an isotacho-
phoretic zone is obtained; if tL/tu< 1, a moving-boundary zone is obtained.