Frans_M_Everaerts_Isotachophoresis_378342.pdf

CONDUCTIVITY DETECTION 145
contact the 0.4-mm hole opposite to it, because Cyanolite is applied again in a subsequent
step and if it penetrates into the 0.4-mm hole, this hole will be rough.
With alancet under a microscope, excess of the Pt- 10-30% Ir foil is cut away such that
the profile shown in Fig.6.16b remains. A new piece of insulating material is now glued
to the first one with Cyanolite, applying a high pressure for at least 5 min. The pieces
are glued well if the entire piece is completely clear again, which can easily be checked
by immersing the acrylic in kerosene; if TPX is used, immersion in glycerol is necessary
because TPX dissolves in kerosene.
If the two pieces are glued satisfactonly (Fig.6.16c), they are placed in alathe and,
after turning the piece, the 0.4-mm hole can now be drilled through the entire piece. On
both sides, collars are made for mounting the brass pieces with a screw-thread (Fig.6.16d).
These pieces are also glued to the Perspex with aid of Cyanolite. Finally, the brass pieces
are futed stably with a lock pin, which penetrates the brass cylinder that covers the whole
piece (Fig.6.16e).
The cables that provide the electrical contact are supplied with extra insulation;
generally the PTFE narrowbore tube can be used so as to minimize the chance of
contact of the brass housing with the cables. The cables are fixed to the micro-sensing
electrodes with a suitable metal paint, which is covered with a small amount of Cyanolite
for optimal solidity. The connection with the interrupted narrow-bore tube is made in a
simple manner. A piece of Perspex is provided with a hole with an inside diameter equal
to the outside diameter of the PTFE narrowbore tube. The narrow-bore tube is first
drawn out over a certain distance and is then pulled through the piece of Perspex. After
a few minutes, the narrow-bore tube is cut off straight with a lancet. These clamping pieces
are pressed in the detector with a clamping screw. To prevent any leakage, on the top of
the clamping piece of perspex, where the narrowbore tube is cut, a small O-ring of soft
rubber is applied. A film of electrolyte between the clamping piece of Perspex and the
Perspex of the detector may cause a leak current to pass through towards the brass
detector housing, which may render the analysis useless*. The basic principle of the
electronic circuit is shown in Fig.6.17.
This conductimeter is the result of the latest research and gives a linear response as a
function of the resistance to be measured. Some of the isotachopherograms, however,
were obtained with conductimetric equipment in which this linear relationship did not
hold. This will be discussed separately because the isotachopherograms obtained by the
older electronic measuring circuits differ. Later, the linearities of various conductimeters
are compared.
If the electric resistance of both coils of the transformer can be neglected and the
coupling of both coils is assumed to be unity, the transformer can be considered to be an
ideal transformer with a resistance R, and a coil L in parallel (Fig.6.17). The losses in
iron and copper are responsible for this resistance R,. If the material of the core is not
saturated, then R, can be considered to be a constant. The capacitance C and the coil
L together form a resonance circuit with a resonance frequency given by
*According to this principle, also a probe for potential gradient measurements (d.c. method) has been
made with the electrodes mounted axially.