Metrohm 797 VA Computrace - Manual (Page 275)

9.6 Voltammetric problems
797 VA Computrace – Software
263
3. Exchange the RDE.
4. Has the electrode been conditioned (e.g. by using
Conditioning
cycles and Cleaning potential ) ?
5. If the concentration to be determined is considerably higher
than expected: reduce sample volume.
6. The background current is normally higher if RDE is used in
place of MME; a background current of several 100 nA is pos-
sible.
Curves with high noise
Did you work in the SqW mode (if yes, see SqW Problems, section
9.6)? If not:
With all MME types:
1. Adjust sealing needle.
2. Change capillary and replace sealing needle (see Hardware
Manual). If necessary, clean the MME (see Hardware Manual).
3. Check tapping mechanism on stand. If tapping strength is too
weak, turn corresponding slotted screw on the valve block dur-
ing operation in an anticlockwise direction until a drop falls
each time the tapper is triggered (see Hardware Manual).
4. Electrolyte solution too old: make up a new one. Its working
life with organic additives may be as short as 1 day or less.
With DME/SMDE:
1. If the electrode drops irregularly: check the MME.
Adjust sealing needle. If necessary, change capillary and replace
sealing needle (see Hardware Manual).
2. If the electrode drops much too quickly: increase the Voltage
step time on the Voltammetric tab of the EDIT WORKING MET-
HOD PARAMETERS window.
With HMDE:
1.
If the electrode surface is overloaded : check deposition poten-
tial and time.
2. If no drops are at the capillary: change the capillary and replace
sealing needle (see Hardware Manual).
SqW Problems
In order to get good results for measurements in the “Square
wave” mode, the auto current range feature of the potentiostat
should not be used. The potentiostat parameters
Highest current
range and Lowest current range should be set on the same level.
Finding the optimal current level needs trying. It should be as small
as possible, but larger than the expected maximum peak (to mini-
mize noise and not disturb the peak shape).