Metrohm 746 VA Trace Analyzer - Manual (Page 541)

7 Safety, Errors, Troubleshooting, Diagnosis, GLP
746 VA Trace Analyzer / 747 VA Stand
7-42
7.5 Hardware/software faults in the instruments
or simply the vagaries of the chemistry
7.5.1 General
The hardware and software of the 746 and 747 VA instruments are constructed of
tested, reliable and checked components. While this does not exclude faults, it en-
sures they appear only rarely. What you should do in such cases is described in
sections 7.3 and 7.4.
Much more likely than genuine hardware/software faults however, are peculiarities
and vagaries of the chemistry which simulate instrument faults and thus give
grounds for inquiries. Such faults manifest themselves primarily in the fact that the
automatic peak evaluation no longer functions properly and results on the dialog
page ”RESULTS” (see section 5.5.2) are either missing completely or are marked
by appropriate comments as having errors or difficulties associated with the peak
evaluation.
In both cases, the first move is to check whether the faults are caused by undesired
chemical processes occurring at the electrode or in the analysis solution. Here,
such faults due to the chemistry can usually only be rectified in whole or at least in
part by chemical countermeasures (e.g. different supporting electrolyte concentra-
tion, different supporting electrolyte, different pH, longer enrichment time, modifica-
tion or change of the measurement technique). In what follows, such ”vagaries of
chemistry” are explained in more detail with the aid of a few interesting and oft-
occurring examples.
7.5.2 Oxygen in the analysis solution
Oxygen can be electrochemically reduced and produces two waves in the voltam-
mogram, one of which is characterized by the appearance of a pronounced maxi-
mum. 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 oxygen
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 1 st step of the oxygen re-
duction can react further with certain substances.
For these reasons, oxygen must be removed from the analysis solution 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 747 VA Stand in the factory, a purging time of
3...5 min usually suffices.