Frans_M_Everaerts_Isotachophoresis_378342.pdf

126 DETECTION SYSTEMS
30 4'0
T
Fig.6.4. Relationship between the temperature inside a narrow-bore tube and the output signal of a
thermocouple mounted on the outside of the tube. 1 = Theoretical relationship; 2 = practical
relationship, found when water at a known temperature flows through the narrow-bore tube; 2%
of the surfactant Mowiol (polyvinyl alcohol) was added to the water; 3 = as 2, with no surfactant;
4 = as 2, with methanol instead of water. The experiments with methanol were difficult to perform
because a suction pump was used, and the methanol began to boil.
These temperatures were compared with the actual temperatures inside the narrow-
bore tube as follows. Water was pumped through the narrow-bore tube until the signal
derived from the thermocouple reached a constant value. Simultaneously, the water
temperature before entering the narrow-bore tube was measured. The results indicated
that in the region of the temperature of the terminating electrolyte (35-45"C), mistakes
can be expected in the qualitative recording, especially if surfactants are present.
6.2.4. Resolution
If the concentration of an ionic species in the narrow-bore tube is about 0.01 g-equiv./l
and the cross-section is about 1.6 - cm2, the minimum amount of that ionic species
which can be detected is about g-equiv. If the volume of the sample injected is
3 pl, the minimum concentration in the sample that can be detected is thus 2.7
g-equiv./l [20].
To illustrate the effect of the introduction of samples of different sizes, some isotachophoretic
separations are shown in Fig.6.5 of the anions oxalate, formate, acetate, and
P-chloropropionate in the operational system at pH 6*. The conditions for this system are
listed in Table 12.1. The concentrations of the various anions were: oxalate, 0.005 N;
formate, 0.01 N, acetate, 0.01 N, and P-chloropropionate, 0.015 N. The volumes injected
were (a) 1 pl, (b) 2 1.11 and (c) 3 p1. The ameunts that can be detected are therefore
5 * 1 O+, 1 O* , 1 O-' and 1.5 - 1 0-8 moles for the anions in the order listed for the case
when 1 pl was injected. It can be stated that the isotachopherogram in Fig.6.5 for the
separation of 1 p1 of sample represents a complete separation of the anions in the mixture.
*For a more extensive description, see Chapter 10.