Metrohm 797 VA Computrace - Manual (Page 277)

9.6 Voltammetric problems
797 VA Computrace – Software
265
2. Check and test the pipetting process: Pipetting the standard so-
lutions must be carried out by one and the same person or with
the same instrument or the same pipette. Was the pipetting
unit used properly? When were the pipettes last calibrated
(GLP)?
3. Organic components interfere with the analysis: carry out a UV
digestion or other suitable sample preparation.
4. Are the calibration solutions too old?
5. Would a calibration curve be more suitable?
With MME:
1.
Check the MME, change capillary if necessary and replace sea-
ling needle (see Hardware Manual).
2. The linearity at the HMDE is naturally not as good as with the
DME. The linear range is in general no larger than 1 - 2 dec-
ades.
With RDE/SSE:
1. Check the RDE (see Hardware Manual).
Peak displacement
1. Check and adjust the pH of the solution.
2. Check electrolyte composition and correct if necessary. Use a
buffer solution instead of an acid.
3. Carry out a standard addition to check whether the correct
peak has been evaluated.
4. Organic components interfere with the analysis: carry out a UV
digestion or other suitable sample preparation.
5. Enter a new half-wave potential in the instrument and recalcu-
late the results.
6. Check reference electrode (see Hardware Manual).
7. Electrolyte solution too old: make up a new one. Its working
life with organic additives may be as short as 1 day or less.
No peak found
With all types of electrodes:
1. The peak is only displaced: adjust the half-wave potential and
recalculate the results.
2. The sample concentration is too low: increase the sample volu-
me or the amount of sample.
3. Are the Start potential and End potential correct?
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.