Metrohm 791 VA Detector - Manual (Page 24)

3 Operation
791 VA Detector
18
If no signal is observed, change the pH value of the eluent (a proto-
nated or deprotonated substance may be oxidized more easily). Gen-
erally, a higher polarization voltage has to be applied with lower pH val-
ues (approx. 50 … 100 mV per pH unit).
• 1 st experiment with eluent of pH ≈ 6
• 2 nd experiment with eluent of pH ≈ 3
If there is still no signal, the analyte is "inactive", i.e. not suitable for elec-
trochemical detection.
As soon as a signal is observed, decrease the polarization voltage in
steps of 100 mV: a hydrodynamic current/voltage curve results.
For detection, select the lowest polarization voltage U
pol
at which a
good signal is still observed.
3.1.5 Practical information about ELCD
In principle the electrochemical detector can be connected to all HPLC
and IC systems. In order to carry out work successfully, particularly in
trace-level quantities, all components such as pump, dampening sys-
tem, injector, separating column, capillaries, couplings, etc. must be in
perfect condition. In electrochemical detection it is possible that inter-
ferences may occur which, for example, are not observed with UV or
conductivity detection. These can often be traced back to contaminants
from the water supply system (e.g. Fe(II) ions), contaminants from the
separating column (e.g. organic residues) or variations in pressure
caused by leaking connections.
Particular attention must be given to the following points:
Cleanliness
The general demands placed on trace analysis work also apply here.
Each possible source of corrosion of steel components must be
avoided as Fe(II) ions may interfere. The eluent should also not contain
any complexing agents.
Pulsation
The noise of the detector signal depends on the pressure variation in-
side the cell and on the back-pressure (max. 1 bar), which is deter-
mined by the length of outlet capillary 38 (i.d. 0.3 mm). The use of a
pulsation dampener (e.g. Metrohm 6.2620.150 Pulsation dampener) for
dampening such pulsations has proved itself in practice and is also
adequate if only single-piston pumps are used.
Eluent
Most of the solvents normally used in HPLC and IC can be used to pre-
pare eluents. The conductivity which is necessary for detection is
achieved by the addition of a conductive salt. Concentrations of 1 … 10
g/L are normally adequate for this purpose, this results in a conductivity
of approx. 1 … 10 mS/cm. Sulfates, nitrates, phosphates, acetic acid,