Frans_M_Everaerts_Isotachophoresis_378342.pdf

20 PRINCIPLES OF ELECTROPHORETIC TECHNIQUES
2.4.4. Some isotachopherograms
In order to detect the zones in isotachophoretic separations, several detection methods
can be used, some of which are described in the section Instrumentation (Chapters 6
and 7). In order to understand the isotachopherograms shown in later chapters, some of
them are discussed here, although only a brief description will be given.
The first isotachopherograrn (see Fig.2.8) was obtained by means of a thermocouple.
As explained in section 2.4, the temperatures of the proceeding zones increase and these
temperatures can be measured by means of a thermometric detector, e.g., a thermocouple
(made of 15-pm constantan and 25-pm copper wire). A signal as shown in Fig.2.8 is
obtained. The construction of the thermocouple is described in the section Instrumentation.
In Fig.2.8, the differential of the linear trace is also given. This signal marks the zone
boundaries more clearly. The distance between two differential signal peaks is a measure
of the zone length and hence it is a measure of the amount of the ionic species in that
zone, because the concentration of the ionic species in that zone is constant for a given
operational system. The step heights to be measured on the linear trace of the thermo-
couple signal are a measure of the conductivity in that zone and are also a measure of
the effective mobilities of the ionic species in the zones. Hence the step height can be
used for the identification of the ionic species in the sample.
For recording the isotachopherogram shown in Fig.2.8, a potential recorder with
zero suppression was used, which is advantageous for the accurate determination of the
various step heights, but may confuse the information if one is not familiar with it.
Under the conditions chosen, the step heights hl and h2 are characteristic of the tetra-
methylammonium and the ammonium ion, respectively. It should be noted that hl and
h2 refer to the temperature of the chloride zone, which is also constant under the condi-
tions chosen. The data presented later (Chapters 11 and 12) are referred to the thermo-
couple signal at 0 PA. [Note that, e.g., in gas chromatography, the distances are a measure
of the identification (retention times) and the peak areas are commonly a measure of
the amounts present.]
Fig.2.9 shows an isotachopherogram for the separation of some anions. The experi-
ments were carried out in the operational system at pH 6 (Table 12.1). The analyses
were performed in equipment that is described in section 7.4.4, using the two high-
resolution detectors: a conductimeter (a.c. method)* and a UV absorption detector
(256 nm).
The linear trace from the conductivity detector, as in the linear trace from a thermo-
couple detector, is a measure of the conductivities of the zones. Hence it is a measure of
the effective mobilities of the ionic species in the zones and characterizes the ionic
species. The ‘step heights’ that can be found in the linear trace can be used for the
identification of the various ionic species in a well defined operational system. All of
the step heights, as described in the section Applications (Chapters 8-17), refer to the
conductivity of the zone of the leading electrolyte, which is-adjusted to ‘zero’ with the
electronic device described in Chapter 6 (Fig.6.18). The differential of the linear signal
*For the difference between the as. method, using a conductivity detector, and the d.c. method, using
a potential gradient detector, see Chapter 6.