COMPARISON OF SAMPLE DECOMPOSITION PROCEDURES FOR THE DETERMINATION OF ZINC IN MILK BY INDUCTIVELY COUPLED PLASMA ATOMIC EMISSION-SPECTROMETRY

Dry ashing and wet dissolution procedures for the decomposition of milk samples were compared based on zinc recovery and residual carbon content. Dry ashing of the samples at temperatures from 400 to 700-degrees-C with and without additions of H3BO3, Mg(NO3)2 and H2SO4 was studied. Without an ashing aid zinc losses were observed at high temperatures, especially in the presence of chlorides. At a temperature of 500-degrees-C zinc recovery was >95% and residual carbon content was <0.05%. Wet digestion procedures including hot-plate, high-pressure asher and open and closed microwave heated systems were applied for sample dissolution. Zinc concentrations measured by inductively coupled plasma atomic emission spectrometry were not affected by the residual carbon content and recovery was >95%. The residual carbon varied and could be reduced through optimization of the digestion parameters (pressure, temperature and time) or by the addition of H2SO4 and/or H2O2 to HNO3.