Frans_M_Everaerts_Isotachophoresis_378342.pdf

EXAMPLES 103
solvent, hoping that a different solvation will affect the ionic mobilities in a favourable
way. As the ions concerned do not show any proton interaction, we must seek a
suitable solvent in the class of amphiprotic solvents, preferably with a high dielectric
constant. In that case, we can try methanol. The ionic mobilities of the alkali metals in
methanol are given in Table 5.4. It can be seen that the differences in the ionic mobilities
are much more favourable in methanol, so that we can use methanol for the separation of
the alkali metals.
The second problem is the use of a leadtng ion. In nearly all solvents, the alkali metals
have higher ionic mobilities than other positive ions except H’. Therefore, we should
choose H’ as the leading ion (in methanol there are some positive ions with a higher
effective mobility than CS', e.g., the tetramethylammonium ion). As the terminating ionic
species many positive ions can be used; in this example we tried Cu2+ ions.
In general, it is preferable to use a buffering counter ion, but for the separation af the
alkali metals the pH is not important and, because no disturbances can be expected, we
chose a non-buffering ion (chloride) as the counter ionic species.
We carried out some experiments in order to check this system, The step heights
measured with a thermocouple are given in Table 5.9. As the leading electrolyte we used a
solution of 0.OlNhydrochloride in methanol (95%) (MHCl) and as the terminator a solution
of 0.1N coppel(I1) chloride in methanol. Firstly, all step heights of the alkali metals
were measured and then a separation of a mixture of the alkali metals was carried out.
The isotachopherogram for this separation is shown in Fig.5.5. A rapid and complete
separation was easily obtained.
In Chapter 16, more data about cationic separations with both water and methanol as
solvents are given.
Example D. In Chapter 14 the separation of some nucleotides is considered; in this
example, we shall discuss how to choose a suitable electrolyte system for the separation
of nucleotide diphosphates.
As the solvent we use water as it dissolves all ionic species easily. As not all pK values
and mobilities of the diphosphates are known, we have to use the experimental method
in order to find a suitable electrolyte system. We have chosen some electrolyte systems
with pH, values of 3.4,3.7,4.2,4.6, 6.0 and 7.0 (these values were chosen because
several nucleotides have pK values between 2 and 5).
As the diphosphates will be separated as negative ions at the chosen pH values, we use
chloride as the leading ion and positively charged ionic species as the buffering counter
ionic species. It can be seen that the pH, values of the systems do not differ much from
the pK values. The conditions for the chosen electrolyte systems are given in Table 5.10
and the measured step heights are given in Table 5.1 1 and are shown graphically in
Fig.5.6. It can be seen from Fig.5.6 that maximal differences in effective mobilities are
obtained at lower pH values. For the separation of a mixture of diphosphates, we chose a
pH, of 3.7. The isotachopherogram of this separation is shown in Fig.5.7. The detection
was performed with a thermometric detector (copper-constantan thermocouple).
Example E. Fatty acids, especially the higher fatty acids, are slightly soluble in water,
so that water cannot be used as the solvent for their separation. Methanol is a better