Frans_M_Everaerts_Isotachophoresis_378342.pdf

362 SEPARATIONS IN NON-AQUEOUS SYSTEMS
16.2. SEPARATION OF ANIONIC SPECIES IN METHANOL USING A THERMO-
METRIC DETECTOR
For experiments with anionic species in methanol, the pH, and pK values of the
substances involved in the separation are very important. Some pK values are presented
in Chapter 5 and from these data we chose as the leading electrolyte a mixture of Tris
and hydrochloric acid in methanol at pHC 9, the concentration of chloride ions being
0.01 N(Tab1e 16.1). This means that a combination of a ‘separation based on pK values’
and a ‘separation based on mobilities’ was used, as most acids have pKz values of 8-9.
The step heights measured in this system are given in Table 16.2.
Simultaneous separations are possible in this system if the differences in step heights
are about 7% (relative to 0 PA). As discussed in Chapter 12, some fatty acids have been
measured in aqueous systems, but many of them have similar effective mobilities and some
are not sufficiently soluble for them to be separated. Their solubility in methanol is much
better and also the differences in mobility seem to be greater. A separation of fatty acids
in methanol has already been shown in Fig.5.8.
In the separation of some dicarboxylic acids, it is remarkable that often different step
heights were obtained when they were measured after various times. In particular, for
fresh and old solutions of oxalic acid different step heights were obtained. It was found
that fresh solutions of oxalic acid gave a step height of 300 mm, whereas a 2-day-old
solution gave a step height of 112 mm. Between these times, isotachopherograms were
obtained showing two step heights, one at 112 and one at 300 mm, where the zone lengths
were different according to the time of preparation. These steps were stable, i.e., when a
mixture of oxalic acid and another substance, with a step height between the two for
oxalic acid, was introduced the electropherogram showed three peaks in accordance with
those of oxalic acid and the other substance.
In Fig.16.1A the isotachopherogram of dl-malic acid is shown, in Fig.16.lB that
of oxalic acid (two steps) and in Fig.16.1C that of a mixture of dl-malic acid and oxalic
TABLE 16.1
OPERATIONAL SYSTEM AT pH* 9 SUITABLE FOR ANIONIC SEPARATIONS
Solvent: CH OH.
Electric current (PA): Ca. 50-70.
Purification: Methanol (95%, technical grade) was purified by running it through a column
filled with a mixed-bed ion exchanger (Merck V).
Electrolyte
Leading Terminating
Anion
Concentration
c1-
0.01 N
E.g., (CH,),AsOO-,
C, H ,, COO-
Ca. 0.01 N
Counter ion Tris’ Tris’
PH* 9 Ca. 8
Additive None None