Metrohm 817 Bioscan User Manual
Page 58
4 Basic principles
817 Bioscan
50
Measuring interval (E1, t1 and ts)
The measuring potential depends on the substance being investigated.
If no data is available in the literature that can initially be used for opti-
mization then you can determine these parameters yourself. For this
reason Bioscan has the scan mode available, among other things. Re-
cording the corresponding voltammograms is described in Section 4.4.
As each change in potential can cause a higher charging current to oc-
cur at the working electrode, the current measurement itself is only
started when the signal has stabilized itself to a large extent. This time
is defined as t1 – ts (see Fig. 11). It influences the level of the back-
ground current and should therefore be selected so that it is not too
small. In practice a time of 0.1 to 0.4 s is frequently used.
The measuring interval ts can be set to 20, 40, 60, 80 or 100 ms. These
values correspond to multiples of 50 Hertz. In this way possible interfer-
ence from the mains current supply can be avoided.
Regeneration interval (E2, E3, t2 and t3)
The potentials E2 and E3 which are required for the regeneration of the
electrode surface are primarily determined by the material of the work-
ing electrode.
With the gold electrode used the oxide layer is already formed at E2 >
+200 mV (Ag/AgCl) under alkaline conditions. Higher potentials accel-
erate oxide formation, therefore in practice E2 = +750 mV and t2 = 0.2
s are often selected.
For example, –800 mV at 0.2 s or –150 mV at 0.4 s can be selected for
E3 and t3.
Measuring frequency
In pulsed amperometric detection the total of the three individual inter-
vals (t1 + t2 + t3) represents the duration of one measuring cycle. The
reciprocal of this cycle duration (in seconds) gives the pulse fre-
quency. Please note that the measured value is outputted from the de-
tector at this pulse frequency. The sampling frequency which other-
wise has to be set under IC Net for data acquisition (
IC NET / Method /
Method setup / Measure / Sampling rate
and
Frequency divisor
) is
automatically adapted accordingly.