Frans_M_Everaerts_Isotachophoresis_378342.pdf

EXPERIMENTS IN AQUEOUS METHANOLIC SYSTEMS 313
between the various ionic species and the resolution of the thermometric detector, as
discussed in Chapters 6 and 10, is low.
16.3.3. The operational system MTMAAC
In the preceding system, the leading ion is K’ and cationic species with mobilities
less than that of K' cannot be determined. Because many ions are more mobile than K’,
we carried out some experiments with the leading electrolyte tetramethylammonium
acetate, the tetramethylammonium ion being the most mobile cationic species used in
our experiments. In Table 16.7 it can be seen that most step heights agree with those in
the system MKAC. All divalent cations are slightly slower, possibly owing to the higher pH.
16.4. EXPERIMENTS IN AQUEOUS METHANOLIC SYSTEMS USING A CONDUCTI-
METRIC DETECTOR (a.c. METHOD) AND UV ABSORPTION DETECTOR (256 nm)
More detailed research must be carried out with methanol-water mixtures as solvents
before conclusive results can be given; it is always difficult to recommend specific propor-
tions of these two solvent components because they depend mainly on the substances to
be analyzed. Therefore, only four experiments will be briefly discussed here, carried out
in 100% water and 9: 1,4: 1 and 7:3 water-methanol mixtures. The methanol (95%,
technical grade) was purified by running it through a mixed-bed ion exchanger (Merck V).
Double-distilled water was applied. The experiments were carried out in the operational
system at pH 5.0 (Table 11.3), except for the solvent. The current has been stabilized at
70 PA, the amplifications of the conductivity detector (a.c. method) and the UV absorp-
tion detector were not changed and the speed of the recorder paper was 6 cm/min for all
analyses.
Fig.16.9 shows the results of the analyses for 100% water and 9:1 water-methanol,
and Fig.16.10 shows those for 4:1 and 7:3 water-methanol.
From the isotachopherograms shown, it is clear that the effective mobilities of the
various constituents of the sample are influenced in different ways. Moreover, it can be
seen that the resolution of both detectors is sufficient.
Fig.16.9. Isotachophoretic separation of a standard mixture of cations (Fig.ll.7) carried out in the
operational system at pH 5 (Table 11.3) in (A) water and (B) 9: 1 water-methanol. 1 = K'; 2 = Ba2+;
3 = Na+; 4 = (CH,),N*; 5 = PbZ+; 6 = Girard reagent D+; 7 = Tris'; 8 = histidine'; 9 = creatinine+;
10 = benzidine'; 11 = e-aminocaproic acid+; 12 = y-aminobutyric acid'. The amplifications of the
detectors were not changed. A = Increasing UV absorption; R = increasing resistance; t = time.
Fig.16.10. Isotachophoretic separation of a standard mixture of cations (Figs.11.7 and 16.9) in
water-methanol systems (A, 4:l; B, 7:3) carried out in the operational system at pH 5 (Table 11.3).
A = Increasing UV absorption; R = increasing resistance; t = time.