Metrohm 757 VA Computrace - Manual (Page 179)

8.6 Voltammetric problems
757 VA Computrace – Software
169
Standard addition curves are not reproducible
With all types of electrodes:
1. Check method parameters (stirring time, etc.).
2. Check and test the pipetting process: Pipetting the standard
solutions must be carried out by one and the same person or
with the same instrument or the same pipette. Was the pipet-
ting unit used properly? When were the pipettes last cali-
brated (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 or replace
sealing 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.