Metrohm 797 VA Computrace - Manual (Page 280)

9 Troubleshooting
797 VA Computrace – Software
268
2. If organic components are present: carry out a UV digestion or
other suitable sample preparation.
3. Concentration of the analyte is too high: dilute.
4. Electrolyte solution too old: make up a new one. Its working
life with organic additives may be as short as 1 day or less.
With HMDE:
1. Standard addition solution containing metal complexing agents
need time to form the metal complex.
Spikes / signal jump in voltammogram
1. For MME: check the electrode.
2. Reduce the dynamic range of the potentiostat (see Potentio-
stat, section 3.3).
Oxygen interference
Oxygen can be electrochemically reduced and produces two waves
in the voltammogram, one of which is characterized by the ap-
pearance of a pronounced maximum. The oxygen reduction can
interfere for two reasons:
• The signals of the analytes are masked by the oxygen waves.
This becomes noticeable primarily in trace analysis as the oxy-
gen is present in a relatively high concentration in solutions
saturated with air (ca. 8 mg/L at room temperature).
• The hydrogen peroxide formed in the first step of the oxygen
reduction can react further with certain substances.
For these reasons, oxygen must be removed from the analysis solu-
tion before the polarographic analysis by saturation with inert gas
(usually nitrogen). With the inert gas flow rate of ca. 20 L/h set on
the 797 VA Computrace stand in the factory, a purging time of
3...5 min usually suffices.
Compare the curves (0.1 mol/L KNO3) before and after purging:
Before purging (Still oxygen in the solution):