Metrohm 791 VA Detector - Manual (Page 23)

3.1 Electrochemical detection
791 VA Detector
17
current ranges > 5 nA. Please pay particular consideration to this on
changing the eluent or the sample solution: the "same" eluent may have
a different background current because eluents are never exactly the
same (ionic strength, pH value, etc.!).
3.1.3 Selection of polarization voltage
The hydrodynamic voltammograms show the voltage limit from the
point where a substance is oxidized or reduced. This voltage limit is
generally higher than the voltage predicted by thermodynamics be-
cause of inhibitions of the electrode reaction. Increasing voltage values
give rise to increasing currents up to a maximum value, the diffusion-
controlled limiting current.
In Fig. 8 for example, analytes a, b, and c are detected at voltage U
2
;
whereas at voltage U
1
, component a is detected selectively. Therefore
the polarization voltage to be set has to be optimized for sensitivity on
one hand and selectivity on the other.
The polarization voltage to be set also depends on the type and condi-
tion of the working electrode (see section 3.5) as well as on the eluent.
The latter could be illustrated by Fig. 8, saying that the curves are from
one single analyte taken in 3 eluents with different pH values.
a b c
U
1
U
2
U
pol
I
Fig. 8: Selection of polarization voltage
3.1.4 Procedure for unknown substances
The hydrodynamic current/voltage curve of an analyte with unknown
electrochemical characteristics is recorded as follows:
• Short circuit the system, i.e. connect the injector directly (without
separating column) to the electrochemical detector.
• Set current range of 791 VA Detector to 100 … 500 nA.
• Set polarization voltage of 791 VA Detector to +1200 mV.
• Inject analyte (20 … 100 ng).
Background
current
Diffusion-
controlled
limiting
current