signal. To improve the precision of the cadmium determination in wine, it was necessary to separate the cadmium determination from the copper determination.
Only with some samples was it possible to determine all three elements simultaneously with similar precision.
Deposition potential
A further parameter which can influence the peak area (analytical signal) is the deposition potential.
Figure 4 demonstrates the change in the analytical signal with deposition potential.
For lead and copper, a stable signal for each metal was observed at a depostion potential around –700 mV (vs. SCE). At this potential no cadmium peak was observed.
The cadmium peak appears at a potential more negative than –900 mV (vs. SCE).
For all determined elements, a maximum in the peak area was observed at a deposition potential of –1200 mV.
At more negative potentials, a decrease in the analytical signal was observed, probably due to hydrogen volution and subsequent destruction of the mercury film.
Signal stability
For the accurate determination of an element, the reproducibility of the analytical signal is an important parameter which can influence the analytical result.
Figure 5 demonstrates the change in peak areas for Cd, Pb and Cu with the number of repeat determinations.
For cadmium and lead, the second and following peaks show good repeatability.
For 36 measurements the RSD of the peak area for Cd was 7.9% and for Pb 3.9%. A stable copper peak was observed after 7 repeat measurements.
After this pretreatment the RSD of the Cu peak area was 3.1% (n =36).
For this reason, it was difficult to use the sample changer for the copper determination or, alternatively, the
first 3 determinations with two standard additions were not used for data evaluation.
Determination repeatability
With the sample changer, it is possible to reduce the man power needed for the element determinations and the human influence on the analytical data.
Table 1 presents the data for 10 determinations for one red and one white wine sample using the sample changer.