Metrohm 757 VA Computrace Manual do Utilizador descarregar pdf (Página 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

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?

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Metrohm 757 VA Computrace Manual do Utilizador Página 175