Frans_M_Everaerts_Isotachophoresis_378342.pdf

120 DETECTION SYSTEMS
TABLE 6.1
SURVEY OF SPECIFIC PROPERTIES THAT CAN BE RECOGNIZED IN ISOTACHOPHORETIC ANALYS:
Type of electrolyte Initial conditions Steady state
Leading electrolyte (L.E.)
Sample zone(s)
Conductivity
Determined by the choice of the
operational system in which
the analysis is carried out.
Unknown, but generally adjusted
to the conditions of the L.E.
with respect to pH and the
concentration of the ionic
species (roughly).
Terminating electrolyte (T.E.) Chosen to be of approximately
the same order of magnitude
as the leading anion (anion
separation) or cation (cation
separation), to minimize the
effect of fast-moving
impurities.
(pH is adjusted approximately to
the pH of the leading elec-
trolyte to prevent sample
ions being 'missed'.)
Conductivity
Determined by the initial choice of
the operational system.
AU zones adjusted to the concen-
tration of the leading anion
(anion separation) or cation
(cation separation).
The concentration is adjusted to the
concentration of the leading
anion (anion separation) or
cation (cation separation).
The conductivity decreases from the
L.E. towards the T.E. (except
for 'enforced' isotachophoretic
systems).
An easy method of making these thin thermocouples is as follows. Copper (30 pm) and
constantan (25 pm) wires are twisted symmetrically together, after the ends of each have
been fixed in a small piece of shellac placed on a rod [ 1 ] .
The difference in the diameters of the wires is necessary because otherwise, owing to
their very different flexibilities, the twisting of the two wires of identical diameter results
in an asymmetrically wound thermocouple, the copper wire being wound around the
constantan wire. The procedure for mounting a thermocouple that is not symmetrically
wound around the narrow-bore tube is very difficult and the thermocouple easily breaks
on the copper side, especially if both diameters are 30 pm. Copper wires of 25 pm diameter
easily breaks during the twisting procedure, especially if the wires are corroded.
The tightly twisted part is cut so that a length of about 1 mm remains twisted. This
twisted end is silver soldered by inserting it into molten silver solder, protected with a
small amount of flux. This silver solder is kept molten by utilizing the heat capacity of
a thick-walled Pyrex glass tube (Fig. 6.1). Other methods of making thin thermocouples
are given in ref. 1.