Metrohm 817 Bioscan - Manual (Page 56)

4 Basic principles
817 Bioscan
48
4.2 Measuring conditions
Amperometric detection takes place with a flowing current and there-
fore with a chemical conversion of the analyte. The course of a chemi-
cal reaction depends directly on various physical parameters, among
other things. In order to obtain optimum measuring conditions (e.g.
stable baseline or reproducible signals) it is necessary to take the fol-
lowing points into consideration:
• Temperature The reactions occurring at the working electrode
(oxidation and reduction) are influenced by the tem-
perature. However, this applies not only to the con-
version of the analyte, but also for interfering reac-
tions that produce the background current. This is
the reason why a constant temperature is a neces-
sary precondition for obtaining a stable baseline
and reproducible signals. For the determination of
carbohydrates, lower temperatures (30 °C – 35 °C)
are suitable. Furthermore, the flowcell should not be
operated above 45 °C over a longer time period.
• pH
Just like the temperature, the pH of the eluent also
has a direct influence on the electrochemical reac-
tions at the working electrode. pH alterations cause
a displacement of the characteristic current/poten-
tial curves (voltammograms). Possible results are
the reduction of the signal intensity and lower sig-
nal/noise ratios. In order to ensure that a stable
baseline and reproducible measuring conditions are
obtained care should be taken that the pH of the
eluent is correct.
• Pulsation
Electrochemical reactions at the electrode surfaces
depend on the transport of the reacting substances
to the electrode. This is why a constant eluent flow
is crucial, both for a stable baseline and also for re-
producible signals. This is why pulsation-free eluent
supply must be ensured. You should use the pulsa-
tion absorber provided (see Section 2.3.2).