Frans_M_Everaerts_Isotachophoresis_378342.pdf

206 INSTRUMENTATION
4
A 6
Fig.7.2. Principle of the six-way valve used for rinsing and re-filling the isotachophoretic equipment
and sample introduction (J. Vacik, Prague, private communication). In position A, the narrow-bore
tube is rinsed via an open Hamilton valve (1MM1) at the side of the counter electrode compartment;
(3) is the connection towards drain. The sample is introduced via the syringe (S), while (2) again is
connected with the drain. The reservoir of the terminating electrolyte can be rinsed via (6); (1) is
the reservoir for the terminating electrolyte. In position B, the valve is shown in the ‘running’ position.
By varying the central bore inside the plunger, the volume to be injected can be varied.
The tap constructed in our laboratory had a volume of 5 pl.
The narrow-bore tube is connected with the six-way valve without the use of any
adhesive, using a piece of insulating material that has an outside diameter such that it fits
exactly in a chamber made for it in the plunger housing. The length of tlus piece of
insulating material is about 2 cm. In order to make a liquid-tight connection with the
piece of insulating material, it must be smooth and flat on top. Moreover, an extra small
O-ring made of rubber is mounted on top of this cylinder of insulating material. In the
cylinder, a hole is drilled with a diameter equal to the outside diameter of the narrow-
bore tube in which the analyses are performed. The narrow-bore tube that is to be
mounted is first stretched over a length of about 4 cm to enable it to penetrate the
cylinder of insulating material via the central bore. The narrowbore tube is then pulled
through this central bore until it fits exactly. After allowing for shrinking (this piece of
insulating material with the narrow-bore tube is inserted in hot water), the narrow-bore
tube is cut with a lancet. The cylinder of insulating material with the narrow-bore tube
can be connected to the plunger housing by fitting a screw-cap over the threaded base.
A water pressure of at least 7 atm can be applied without any visible leakage at this
clamping piece. However, a pressure no higher than 6 atm could be applied, because at
this pressure the narrow-bore tube shows its porosity and droplets appear all over it.
The pressures applied for rinsing and re-filling are, of course, much lower. The
connection of the six-way valve with the injection block (or directly with the reservoir
filled with terminating electrolyte) is achieved with a cylinder of insulating material with