RESIDUES AND TRACE ELEMENTS Comparison of Partial Digestion Procedures for Determination of Ca, Cr, Cu, Fe, K, Mg, Mn, Na, P, and Zn in Milk by Inductively Coupled Plasma-Optical Emission Spectrometry CARMEN S. KIRA and FRANCA D. MAIO Instituto Adolfo Lutz, Divisno de Bromatologia e Química, Av. Dr. Arnaldo 355, CEP 01246-902, Sno Paulo, SP, Brazil VERA A. MAIHARA Laboratório de Análise por Ativaçno – IPEN/CNEN-SP, CEP 05508-000, São Paulo, SP, Brazil A fast procedure was developed for determination of Ca, Cr, Cu, Fe, K, Mg, Mn, Na, P, and Zn in milk samples. This procedure consisted of a partial di- gestion with hydrochloric acid on a hot plate. The results obtained were compared with 3 other diges- tion procedures (dry ashing and 2 microwave di- gestions). All the procedures showed similar preci- sion levels, with coefficients of variation <10% for most analyzed elements. Accuracy was evaluated by using certified reference materials, and the val- ues obtained were within the confidence intervals for these products. The results obtained were not considered statistically different. The partial diges- tion on a hot plate with HCl can be very practical for laboratories with relatively large numbers of sample analyses. A s a basic component of the human diet, milk is of great importance. It is known that milk is a good source of calcium, proteins, and vitamins A and D, which are necessary for bone and tooth development (1, 2). Milk is the main nourishment for toddlers, and an appropriate intake is recommended for adults to retain bone mass so that fractures and osteoporosis can be prevented. Many spectroscopic techniques have been used for the de- termination of inorganic elements in a range of different ma- trixes. However, these techniques generally involve a sample pretreatment, which is the most critical part of the analysis be- cause it is the slowest step in the analytical process and errors in the results obtained are often associated with this step. Tra- ditional methods of sample preparation for milk samples (powder and liquid) are well documented. These methods in- clude wet digestion and dry ashing (3–9). Wet digestion pro- cedures performed on a hot plate or in a microwave oven re- quire the use of concentrated acids, necessary for digestion monitoring (10–12). Dry ashing is time-consuming and mainly influenced by the final temperature of the muffle fur- nace as well as the programmed temperature gradient, which must be low enough in order to avoid losses of elements. The acids used for dissolving the ashes can also influence sample treatment (13–17). Conventional digestion procedures with total organic mat- ter destruction by dry or wet digestion make the routine pro- cessing of large numbers of samples difficult. Therefore, with the increasing demand of analyses required by public health programs and private companies, it is important to use diges- tion procedures that require minimum sample treatment, re- sulting in good precision and accuracy. There is growing in- terest in sample pretreatment procedures that could both avoid analyte loss and sample contamination leading to analytical costs, as well as increase productivity. In the present study, the inductively coupled plasma-opti- cal emission spectrometry (ICP-OES) method was used to de- termine Ca, Cr, Cu, Fe, K, Mg, Mn, Na, P, and Zn in milk pow- der samples. Four digestion procedures were evaluated to determine inorganic elements in milk samples, including one dry ashing procedure with total destruction of organic matter, and 3 wet digestions with partial sample destruction—one on a hot plate and the other 2 in a microwave oven. Experimental Apparatus (a) Muffle furnace.—For dry ashing of samples. (b) Focused open-vessel microwave oven.—For micro- wave digestions (Star 2; CEM, Matthews, NC). (c) Spectrometer .—For ICP-OES spectrometric measure- ments (Optima 3000 DV; Perkin Elmer Corp., Norwalk, CT). Details on ICP-OES instrumental operating conditions and measuring parameters used are given in Table 1. (d) Glassware.—All glassware was washed with detergent and water. After being rinsed with water for several times, it was soaked in 20% HNO 3 (v/v) for 24 h. This solution was dis- carded and the glassware was soaked again in 20% HNO 3 (v/v) for 24 h. This procedure was repeated. The glassware was then rinsed several times with deionized water, and dried. KIRA ET AL.:JOURNAL OF AOAC INTERNATIONAL VOL. 87, NO. 1, 2004 151 Received April 21, 2003. Accepted by JS August 19, 2003. Corresponding author’s e-mail: carmkira@ial.sp.gov.br. Downloaded from https://academic.oup.com/jaoac/article/87/1/151/5657321 by guest on 31 August 2020