Frans_M_Everaerts_Isotachophoresis_378342.pdf

242 INSTRUMENTATION
Fig.7.26. Principle of regulation of a counter flow of electrolyte, via a membrane pump (as shown
in Fig.7.25). Attention should be paid to the common terminal and the earth, which prevent distur-
bances to the current stabilization (electrophoretic driving current) by the counter flow regulation.
This could destroy, or at least obscure, the final result.
the isotachophoretic run. If gas is now produced in the electrolysis cell of the membrane
pump, the volume of this electrolysis cell tends to expand. The volume can expand easily
because between the electrolysis cell and a cell filled with leading electrolyte, mounted.
next to it, a thin membrane (e.g., a rubber contraceptive) is mounted. This membrane
is mounted with pre-stressing.
In Fig.7.27, the construction of the membrane pump is shown in more detail, and a
photograph is shown in Fig.7.28.
The flow of liquid caused by the production of gas in the electrolysis cell of the
membrane pump counteracts the increment in V,. This gas is produced by an electric
current I,, in an electrolyte (e.g., 0.01 NKC1). Because V, is of the magnitude of kilovolts,
V, is reduced to a value B V, with the aid of two resistors of 100 Ma and 56 ka. An
electronic circuit (Fig.7.29) compares this value B V, with an adjustable V, If BlV,l<
V,, (V,> 0), then I, = 0. If BlV,l> V&., then I, # 0 and the increment in V, is counter-
acted. The regulation is such that I. will reach a value such that BIG1 becomes and
remains approximately equal to VEf..
Thus the relationship betweenI,,, BI V,l and V, is:
BI Vc I Q Vmf.
then I,, = 0, and
BI VCl > V,f.
then I, = A (BI V, I - VXt).
The optimal value for the amplification factor, dl,/dl V,l = BA, is dependent, among
other factors, on the electrolytic system chosen (operational system) and on the cross-
(7.4)
(7.5)