Metrohm 746 VA Trace Analyzer - Manual (Page 532)

7.4 Troubleshooting
746 VA Trace Analyzer / 747 VA Stand
7-33
7.4 Troubleshooting
7.4.1 General rules for VA trace analysis
In order to get correct results in VA trace analysis, a number of general rules must
always be observed:
Chemicals and equipment
• The purity of the reagents plays an important role in determining the re-
sults. Extremely pure chemicals should be used for determining lower
concentrations (see VA Application Note V-49).
• Measuring vessel, electrodes and all other equipment in contact with the
sample solution must be clean and free of contamination substances.
Electrolytes
• The pH during a determination plays an important role (e.g. for Zn, Cd, Pb,
Cu it should be approx. 4.5). Acetate, Ammonium acetate or PIPES buffer
are often used. For more information see the Application Bulletins.
• The electrolyte must be sufficiently conductive and concentrated.
• The purity of the electrolytes and the cleanliness of the reagent bottles is
very important.
• The working life of the electrolytes is limited, particularly for organic addi-
tives (buffer substances, complex formers). It may be necessary to make
up fresh solutions every day.
Standard solutions
• The standard solutions should be made acidic (approx. pH = 1 … 2) and
stored in plastic bottles.
• Diluted standard solutions (ppb range) are very unstable and must be
freshly made. They must also be made sufficiently acidic.
• The concentration of the standard solutions must be arranged so that a
volume between 20 and 500 µ L has to be added.
• Standard additions are recommended. The peak height after the last addi-
tion should be 2 … 5 times higher than the sample peak.
• 1000 ppm solutions (self-made or commercially available) are often used
as stock solutions. They are stable over long periods of time. Dilutions
have to be made with dilute acids.
Samples
• The amount of sample depends on the concentration of the element to be
determined.
• If the sample matrix is known, a better assessment of the analysis can be
made (organic components?).
• A digestion must be carried out on contaminated samples and on sam-
ples where contamination is suspected (see Metrohm Monograph
«
Sample preparation for techniques in voltammetric trace analysis»).
• A lot of errors are made during sampling and when storing the sample.
Caution and a critical approach are required.
• The sample should have a good solubility in the electrolyte and be mixable
with it.