The stability of the organic standards and samples diluted in AVK was also investigated for comparison purposes, and the results are shown in Fig. 2. Except for calcium, that remained stable for four days, the analytical signal for magnesium and zinc in the standard solution and in the sample decreased about 20% in 1 h for magnesium, reaching almost half of its initial value over 6 h for both elements. These results confirm the necessity of immediate analysis of standards and samples for these elements after the dilution.