Frans_M_Everaerts_Isotachophoresis_378342.pdf

VALIDITY OF THE ISOTACHOPHORETIC MODEL 15
effect can be neglected only if the zone boundary width that results from it is very small
in comparison with the zone length.
Several workers [ 10, 13, 15, 201 have given an approximation for this effect and
showed that the width of the zone boundary due to diffusion is less than 0.1 mm; for
long zone lengths, this can be neglected.
4.4.3. Influence of axial and radial temperature differences
During electrophoretic experiments, radial differences in temperature exist in the
zones and axial differences in temperature between the different zones. Several quantities,
such as mobilities and pK values, depend on temperature and the concentrations and pH
values of the zones are also affected by temperature. HjertCn 1211 and Routs [I51 studied
the influence of temperature in the radial direction and found that a parabolic shape of the
zone boundary can be expected. Another important point is the difference in pK values
of the ionic species due to the different temperatures of the zones. In Fig.4.14, the pK
values of some ionic species are shown as a function of temperature.
From Fig.4.14, it can be concluded that particularly the positively charged ionic
species such as imidazole, tris and histidine, which are used as buffering counter ions for
T 1 80
60
40
20
Fig.4.14. Relationship between temperature (r) and pK values of some ionic species. 1 = pK, of
glutamic acid; 2 = pK, of glycine; 3 = pK of formic acid; 4 = pK, of glutamic acid; 5 = pK, of
oxalic acid; 6 = pK of acetic acid; 7 = pK, of histidine; 8 = pK of imidazole; 9 = pK, of citric acid;
10 = pK of tris (hydroxymethy1)aminomethane; 11 = pK of 2-amino-2-methyl-l,3-propanediol;
12 = pic of orthoboric acid.
PK