Frans_M_Everaerts_Isotachophoresis_378342.pdf

THERMOMETRIC RECORDING
T t l
Fig.6.3. Temperature profiles of zone boundaries in isotachophoretic analyses, as derived from
micro-thermocouples. In the direction X, the micro-thermocouple is mounted closer to the centre
of the narrow-bore tube. In the initial phase, the transient response of the thermocouple, mounted
closer to the centre, is more rapid, but the resolution is not improved much. T= Increasing
temperature; t = time.
of the temperature step to be attained is not changed very much; the total mass is
not changed or is changed in the wrong direction.
Calculations show that the sharpness of the temperature step approaches that of the
concentration step if the narrow-bore tube is made of ‘infinite’ thickness [ 171 .
Alist of data for thermometric detection is given in Tables 11.6 and 12.3.
The resolution and stability of thermistors are comparable with those of thermocouples,
but thermistors are not discussed further here because more complicated electronic
circuits are necessary. Optical means of detection have not been tested so far, because
the temperature differences are relatively small and the optics are very expensive,
compared with the thermocouples and thermistors [18] . The information derived from
liquid crystals [19] painted on the wall of the narrow-bore tube is poor and needs
expensive instruments for automatic recording.
6.2.3. Experimental
Thermometric detectors lack high resolution because the heat generated by the electric
current has to diffuse through the wall of the narrow-bore tube. The various heat transi-
tion coefficients influence the final recording of successively migrating narrow zones. As
a general rule, the zone length needed for a full qualitative and quantitative recording
must be about 5 mm, making use of a PTFE (or Pyrex glass) narrow-bore tube with an
outside diameter of 0.7 mm and an inside diameter of 0.45 mm. This value can vary,
depending on the heat production of the adjacent zones, the electric current applied, the
type of solvent used and some other minor factors (e.g., the addition of surfactants to
the electrolytes in order to depress the electroendosmotic flow).
In Fig.6.4, some graphs are shown of actual temperature measurements carried out
with a thin thermocouple mounted on the outside of a narrow-bore tube.
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