Metrohm 757 VA Computrace - Manual (Page 175)

8.5 General rules for VA trace analysis
757 VA Computrace – Software
165
Samples
1. The amount of sample depends on the concentration of the
element to be determined.
2. If the sample matrix is known, a better assessment of the
analysis can be made (organic components?).
3. A digestion must be carried out on contaminated samples
and on samples where contamination is suspected (see
Metrohm Monograph «Sample preparation for techniques in
voltammetric trace analysis»).
4. A lot of errors are made during sampling and when storing
the sample. Caution and a critical approach are required.
5. The sample should have a good solubility in the electrolyte
and be mixable with it.
Blank values, contamination
The following points should be checked if the results are too
high:
1. Have the dilutions been made correctly?
2. Have contamination risks been excluded?
3. Contamination risks are very high at low concentrations:
measuring vessels should be conditioned with dilute HNO
3
solution.
4. Are the chemicals pure enough?
"Suprapure" grade reagents should be used at low concentra-
tions.
5. Very high concentrations were measured in the previous
analysis:
electrodes and measuring vessels must be carefully cleaned
and conditioned (memory effects).
6. 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:
1. Concentration too high?
HMDE overloaded, use DME/SMDE instead?
2. Buffer not correct?
Make up new one if necessary.
3. Addition ratio too low?
4. Addition ratio too high?