Metrohm 746 VA Trace Analyzer - Manual (Page 537)

7 Safety, Errors, Troubleshooting, Diagnosis, GLP
746 VA Trace Analyzer / 747 VA Stand
7-38
Standard addition curves are not reproducible or non-linear
• Check program in the instrument (stirring time, etc.).
• Check MME, change capillary if necessary (see section 3.4.8) or replace
sealing needle (see section 3.4.9).
• The pipetting process was not correct: was the pipetting unit used properly?
• Repeat the analysis again or try out automatic standard addition with Dosi-
mat, Dosino or 6.5611.010 Pipetting equipment.
• Pipetting the standard solutions must be carried out by one and the same
person or with the same instrument or the same pipette.
• Check and test pipetting process. When were the pipettes last calibrated
(GLP)?
• Organic components interfere with the analysis: carry out a UV digestion or
other suitable sample preparation.
• Are the calibration solutions too old?
• Would a calibration curve be more suitable?
• 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 decades.
Peak displacement
• Check and adjust the pH of the solution.
• Check electrolyte composition and correct if necessary.
• Organic components interfere with the analysis: carry out a UV digestion or
other suitable sample preparation.
• Carry out a standard addition to check if the correct peak is present.
• Enter a new half-wave potential in the instrument and recalculate the results.
• 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
• The peak is only displaced: adjust the half-wave potential and recalculate.
• The sample concentration is too small: increase the sample volume.
• Has the complex former been forgotten? (adsorptive voltammetry).
• The deposition time under
Meas
in "OP.SEQUENCE" in the inverse voltam-
metry is too short: increase the time.
• No Hg drops at the capillary: check MME. Adjust sealing needle (see section
3.4.4/3.4.5). If necessary, change capillary (see section 3.4.8) or replace
sealing needle (see section 3.4.9).
• Electrolyte solution too old: make up a new one. Its working life with organic
additives may be as short as 1 day or less.
• Are organic components present? Carry out a UV digestion or other suitable
sample preparation.