Metrohm 746 VA Trace Analyzer - Manual (Page 539)

7 Safety, Errors, Troubleshooting, Diagnosis, GLP
746 VA Trace Analyzer / 747 VA Stand
7-40
7.4.4 Problems with RDE
There are various different types of RDE:
GC: Glassy carbon electrode, can be polished with aluminium oxide, glossy
surface.
Au: Gold electrode, can be polished with aluminium oxide, mat surface.
UT: Ultra-trace graphite electrode, can be scraped and polished with alumin-
ium oxide, mat surface.
Ag: Silver electrode, can be polished with aluminium oxide, mat surface.
Pt:
Platinum electrode, looks very similar to Ag, mat surface.
Low background current or unstable base line
• The electrode surface must be repolished.
• Has the correct RDE been used?
• Exchange RDE.
• Check concentration of electrolyte and pH of the solution.
• Has the electrode been conditioned (e.g. with
CONDC
) ?
• Check initial and final potential of the analysis.
• If the ion concentration in the solution is too high: dilute the electrolyte.
• If the concentration to be determined is considerably higher than expected:
reduce sample volume.
• The background current is normally higher if RDE is used in place of MME.
• A background current of several 100 nA is entirely possible.
• Electrolyte solution too old: make up a new one. Its working life with organic
additives may be as short as 1 day or less.
Standard addition curves are not reproducible
• Check program in the instrument (stirring time, etc.).
• Check RDE (see section 3.5).
• 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?
Peak displacement
• Check and adjust 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.