Frans_M_Everaerts_Isotachophoresis_378342.pdf

Appendix B
Diameter of the narrow-bore tube, applied for separation
In Chapters 6 and 7, equipment is described in whch a narrow-bore tube with an I.D.
of 0.45 mm was used. The current density in the equipment discussed was about
500 pA/mm2. From experiments conducted in 1970, it was well known that, if glass
narrow-bore tubes with 1.D.s of 0.2 and 0.1 mm were used for isotachophoretic experi-
ments, very small temperature differences could be measured between the various zones
with the micro-sensing thermocouples (section 6.2). Moreover, the increase in electro-
endosmotic flow even made analyses with dyes, as described in Chapter 17 (Fig.17.2),
impossible. No additives, e.g., Mowiol (polyvinyl alcohol) were applied at that time.
As soon as the equipment and detectors described in Chapters 6 and 7 had been
developed and tested, some research was carried out so that the I.D. of the narrow-bore
tube could be decreased. Three main reasons for desiring this reduction can be given:
(a) With a smaller I.D. of the narrow-bore tube, the total detectable amount of the
ionic species to be separated can be decreased, because the length of the zones increase on
decreasing the diameter of the narrow-bore tube.
(b) If comparable current densities are applied, the temperature differences between
the successive zones is less if the narrow bore-tube has a smaller 1.D. A smaller profile of
the zone boundary is thus obtained, especially between zones with ionic species that have
very low effective mobilities.
(c) If higher current densities can be permitted, the time of analysis will decrease.
A conductimeter (Fig.6.10) was therefore constructed, with the electrodes (10 pm
Pt-Ir) glued in a manner similar to that discussed for the probe (Fig.6.16). In ths
instance, the linear conductimeter, as discussed in section 6.4.4, can still be used. The I.D.
of the probe was made to be 0.2 mm. A PTFE narrow-bore tube, with an I.D. of 0.2 mm
and an O.D. of 0.45 mm, was mounted between the injection block (Fig.7.5) and the
conductivity probe. A similar narrow-bore tube was mounted between the probe and the
counter electrode compartment (Fig.7.9). The slit of the W absorption detector
(Fig.6.30) was also adapted. Although the diameter of the narrow-bore tube is much
smaller, the UV absorption detector can still be used because the wall thickness of the
narrow-bore tube is much smaller. It is well known that PTFE has a great W absorption.
Experimentally, it was found that the electroendosmotic flow could be decreased by
addition of, e.g., Mowiol (polyvinyl alcohol). The current density applied could be
increased up to at least 1500 pA/mm2. The sharpness of the zones increased by decreasing
the diameter of the narrow-bore tube, partly owing to the small differences in temperature
between the adjacent zones.
The main advantage of decreasing the diameter is the small heat production. Even
between the leading electrolyte and terminating electrolyte the increment is small,
compared with experiments in whch a narrow-bore tube of I.D. 0.45 mm was used. This
effect can easily be seen if terminating ions that have a very low effective mobility are
applied and the electric current is switched off. Because the conductivity is a function of
temperature, the shift in the linear signal of the conductivity detector (a.c. method) gives
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