Frans_M_Everaerts_Isotachophoresis_378342.pdf

THERMOMETRIC RECORDING
a concentration of the leading electrolyte of 10-3N these pH limits are narrowed to
4 and 9, respectively. The upper limit is determined mainly by the interference in the
analyses by carbon dioxide from the air.
The detection limit can also be decreased by the injection of a larger sample in a
suitably constructed sample tap. Some possibilities for this approach are considered in
Chapter 7. The sample tap may have a volume of 30 pl, which decreases the minimum
detectable concentration by a factor of 10 compared with sample introduction via a micro-
syringe. An important aspect of the use of a sample tap is that the sample is already
separated from the leading and terminating electrolytes and the separation takes place in
the tap. On injection with a syringe, the sample is always mixed with the highly mobile
chloride or with the terminating electrolyte, which may decreaFe the pH of the sample
generally in anion separations and increase it in cation separations. If the average concen-
tration of the ionic species in the sample is low, a sample tap is always recommended, if
no sample pre-treatment can be applied.
The detection limit can also be decreased by using a regulated counter flow of
electrolyte, again because larger sample volumes can be used., in spite of the short length
of the narrow-bore tube available for separation. The time of analysis, of course, will
increase considerably.
Because detectors are available with a higher resolving power than that of a thermo-
metric detector, the detection limit in an isotachophoretic analysis must not be estab-
lished by using a thermometric detector, and therefore the use of a sample tap and a
counter flow of electrolyte in combination with a thermometric detector will not be discussed,
It is important, however, to establish the limit of concentration that can be detected
with a thermometric detector. For this purpose, analyses with a leading ion concentration
of g-equiv./l were carried out. Again the operational system at pH 6 was chosen. The
basic information is listed in Table 12.1. The concentration of the hydrochloric acid was
decreased to 0.001 N. Because excessive heat production may render the analysis useless
and also the driving potential available is limited, the direct driving current was decreased
to 7 pA. Fig.6.6 shows the isotachopherogram of the separation of nitrate, chlorate,
formate, citrate and adipate. Acetic acid (0.001 N) was used as the terminating electrolyte.
A complete separation could easily be obtained, but the disadvantage is shown very
clearly. Because at these low concentrations only small temperature differences can be
expected, the signals derived from the thermocouples need to be amplified too much.
6.2.5. Conclusion
Thermometric recording acts contrary to the requirement that for a complete reproducible
analysis by electrophoretic techniques, optimal thermostating is necessary. If thermometric
detection is used, all of the equipment can be optimally thermostated, but at the
position where the detector is mounted good thermostating will destroy the temperature
differences necessary for measurement. Another disadvantage is that in thermometric
detection the direct driving current plays such an important role (I2) and the detection
is only proportional to the concentration adjustment (R). This disadvantage is partly
overcome by the construction of a good current-stabilized power supply, which can now
be obtained commercially.
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