Metrohm 746 VA Trace Analyzer - Manual (Page 536)

7.4 Troubleshooting
746 VA Trace Analyzer / 747 VA Stand
7-37
No addition
• Has the correct standard solution been used or is the concentration of the
solution too low: increase the volume of the standard addition or use a higher
concentration or reduce the sample amount accordingly.
• If organic components are present: carry out a UV digestion or similar.
• Could it be possible that the peak is not the peak which is being looked for?
• Concentration of the analyte is too high: dilute.
7.4.3 Problems with HMDE
Low background current or unstable baseline
• The electrode drops or the drops do not remain hanging: check MME.
Change capillary if necessary (see section 3.4.8) or replace sealing needle
(see section 3.4.9).
• Has the sample been properly degassed?
• Check the concentration of the electrolyte and the pH of the solution.
• Check the initial potential 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 or change the mode (e.g. from HMDE to SMDE or
DME).
• Electrolyte solution too old: make up a new one. Its working life with organic
additives may be as short as 1 day or less.
Curves look like a "starry night"
• Check MME. Check for break in contact between needle and capillary. If nec-
essary, clean the MME (see section 3.4.9).
• If the electrode surface is overcharged: check deposition potential and time.
• No drops at the capillary: change the capillary (see section 3.4.8) or replace
sealing needle (see section 3.4.9).
• Have stirring and degassing been switched off during the measurement?
• Check tapping mechanism on stand. If tapping strength is too weak, turn
slotted screw 71 71 on the valve block during operation in an anticclockwise di-
rection until a drop falls each time the tapper is triggered.
• Electrolyte solution too old: make up a new one. Its working life with organic
additives may be as short as 1 day or less.