Metrohm 746 VA Trace Analyzer - Manual (Page 533)

7 Safety, Errors, Troubleshooting, Diagnosis, GLP
746 VA Trace Analyzer / 747 VA Stand
7-34
Blank values, contamination
The following points should be checked if the results are too high:
• Have the dilutions been made correctly?
• Have contamination risks been excluded?
• Contamination risks are very high at low concentrations:
measuring vessels should be conditioned with dilute HNO
3
solution.
• Are the chemicals pure enough?
"Suprapure" grade reagents should be used at low concentrations.
• Very high concentrations were measured in the previous analysis:
electrodes and measuring vessels must be carefully cleaned and condi-
tioned (memory effects).
• Has the standard addition been carried out properly?
Was the volume set correctly on the pipetting unit?
The following points should be checked if the results are too low:
• Concentration too high?
HMDE overloaded, use DME/SMDE instead?
• Buffer not correct?
Make up new one if necessary.
• Addition ratio too low?
• Addition ratio too high?
Measurement modes
The following points should be considered by selecting the measurement mode:
• DPMODE (Differential Pulse) should be always the first choice. It is the
most universal and frequently used voltammetric determination method
and is equally well suited for reversible and irreversible systems. It offers a
high sensitivity down to 10 –8 mol/L and a separation ability of 1:50'000.
• DCTMODE (Direct Current) is the classic, simplest VA method with limited
sensitivity (down to 10 –5 mol/L) and a separation ability of only 1:10. It is
mainly used for the investigation of reversible redox systems.
• AC1MODE (Alternating Current, 1 st derivative) is primarily suitable for de-
terminations based on reversible redox reactions and is virtually com-
pletely insensitive to irreversible reactions.
• AC2MODE (Alternating Current, 2 nd derivative) is also primarily suitable for
determinations based on reversible redox reactions. Compared with the
AC1MODE measurements, an increase in sensitivity, resolution and
separation efficiency is often obtained.
• SQWMODE (Square Wave) is primarily suitable for investigations of re-
versible electrode processes and kinetic studies. It is used particularly for
sensitive stripping voltammetric determinations at the HMDE or RDE.
• PSAMODE (Potentiometric Stripping Voltammetry) is mainly used to de-
termine substances in an organic matrix by means of mercury film elec-
trodes without prior digestion and deaeration. Only analytes which form an
amalgam can be analyzed.