Metrohm 746 VA Trace Analyzer - Manual (Page 534)

7.4 Troubleshooting
746 VA Trace Analyzer / 747 VA Stand
7-35
7.4.2 Problems with DME/SMDE
Low background current or unstable baseline
• The electrode drops irregularly: 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).
• Check electrolyte concentration and pH of the solution.
• Check 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 lower than anticipated:
increase sample volume or change mode (e.g. HMDE).
• Has the sample been degassed? Degassing with nitrogen for at least 5 min is
recommended, for alkaline solutions approx. 10 min is recommended.
• Check tapping mechanism on VA 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.
• Is the gas pressure correctly set (1 bar)?
• Is the reference electrode sufficiently full (inside and outside, see section
3.6)?
Curves look like a "starry night"
• The electrode drops irregularly: 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).
• Check contact between needle and capillary. If necessary, clean the MME
(see section 3.4.9).
• Have stirring or degassing been switched off during the measurement?
• If the electrode drops much too quickly: reduce t.step in program.
• 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.
Standard addition curves are not reproducible
• Check program in the instrument (stirring time, etc.).
• Check MME, change capillary if necessary (see section 3.4.8).
• 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 or the internal standard method be more suitable?