ZINC DETERMINATION IN HUMAN-MILK - COMPARISON OF DIFFERENT METHODS

Three flameless atomic absorption spectrometric methods for zinc determination in human milk, have been examined. Optimum analytical conditions were selected. The first was a direct method, using 1:99 milk dilution in Triton X 100 0.1% (w/v). Within-run and between-run precision were found to be 4.5% and 6%, respectively. Recovery of standard addition was found to be 103 +/- 9%. Linear range was between 0.052 and 3-mu-mol.l-1. In the second method, milk was submitted to wet ashing with concentrated nitric acid at 180-degrees-C, prior to dilution in Triton X 100 0.1% (final dilution: 1:199, final nitric acid concentration: 0.3 mol.l-1). Within-run and between-run precision were found to be 6%. Recovery of standard addition was 111 +/- 5%. Linear range was between 0.017 and 1.5-mu-mol.l-1. In the third method, milk was submitted to dry ashing at 400-degrees-C; then ash was dissolved in nitric acid 0.1 mol.l-1 (final dilution 1:99). Within-run and between-run precision were found to be 8 and 5%, respectively. Recovery of standard addition was 104.5 +/- 5%. Linear range was found to be between 0.122 and 2-mu-mol.l-1. Zinc content in different milks was determined by these 3 methods. Correlation coefficients were found to be 0.98 (methods 1 and 2) and 0.96 (methods 1 and 3). Linear regressions were Y = 0.93 X + 17 (Y = method 2, X = method 1) and Z = 0.79 X + 4 (Z = method 3, X = method 1) (figs 5 and 6). Differences in milk zinc concentrations obtained with the 3 methods were not statistically significant (match Student's t-test). In conclusion, the authors found that the direct method was the most efficient for zinc determination in milk.