Metrohm 791 VA Detector - Manual (Page 26)

3 Operation
791 VA Detector
20
3.2 Electrodes
3.2.1 Working electrodes
The quality of a working electrode WE can be assessed in terms of the
following criteria:
• Background current (after adequate running-in time):
this should be small and constant
• Interference level ("noise"):
this should be low
• Response characteristics:
after switching on, the low background current should be
quickly reached
It should nevertheless be borne in mind that increases in both back-
ground current and noise levels are often due to other causes than a
poor-quality working electrode (for example, contamination of the HPLC
system, malfunction of the pulsation dampener, etc.). Generally speak-
ing, both background current and noise levels tend to increase with
higher polarization voltages, and are usually greater in eluents based
on mixed solvents than in those composed of purely aqueous solutions.
Glassy carbon electrode GCE
Ordering designation 6.0805.010
Sensor material
Glassy carbon, GC
Chemical resistance good resistance to acetonitrile, methanol, wa-
ter, etc.
Voltage range (–800) … 0 … +1200 mV (pH < 7)
Regeneration
Sprinkle a little aluminium oxide powder (Al
2
O
3
, grain size 0.3 µ m)
on the polishing cloth (ordering number of both these accesso-
ries: 6.2802.000) and moisten with distilled water. Hold the elec-
trode vertically and polish with a circular motion for about
20 … 60 s, exerting only light pressure. Rinse with distilled water
and clean with dry cloth. A glassy carbon electrode should always
be polished for a short while immediately before being fitted to
the detector cell.
Ultra-Trace graphite electrode
Ordering designations 6.1204.100 Ultra-Trace graphite tip
+ 6.2103.110 Contact pin
Sensor material Graphite (turnover is three times larger than in
GC-electrode; does not need tamping)
Chemical resistance good resistance to acetonitrile, methanol, wa-
ter, etc.
Voltage range –800 … 0 … +1200 mV (pH < 7)
Regeneration
With 6.2827.000 Trimming tool (see electrode data sheet).