Determination of mineral and trace elements in total diet by inductively coupled plasma atomic emission spectrometry: comparison of microwave-based digestion and pressurized ashing systems using different acid mixtures

Abstract

The effectiveness of different acid mixtures for the de-mineralization of total diet samples of different origin and composition by means of a closed microwave-based technique and a pressurized-ashing technique were investigated. The aim was to obtain complete acid solubilization for the subsequent accurate determination of Al, Ca, Cu, Fe, K, Mg, Na, P and Zn by means of inductively coupled plasma atomic emission spectrometry (ICP-AES). The sampling and sample preparation steps for the trace element analysis are described. The special care taken in the sampling design to ensure the representativeness of the sample for the population studied is also mentioned. The regions and population groups were selected from the high mountain valleys, where a high incidence of stomach cancer exists, and a low altitude valley at the foot of the Andes, which is a low stomach cancer incidence region, both located in the state of Tachira, Venezuela. Total diets including breakfast, lunch, supper, snacks and drinks from the whole-day intake of 140 adults were collected. The collection of the total diet was made according to the double-portion technique. The most reliable digestion procedure for the determination of Al, Ca, Cu, Fe, K, Mg, Na, P and Zn in total diet by means of ICP-AES was a microwave-based technique using HNO₃ alone or with the mixture HNO₃+ H₂O₂(2 + 1). In comparison, a pressurized-ashing technique is faster although lower values for K (13 and 11%) were obtained. Despite low sensitivity for the ICP-AES measurements, the microwave-based digestion using HNO₃+ H₂SO₄(4 + 1) also gave good agreement for the determination of Al, Ca, Cu, Fe, K, Mg, Na, P and Zn by means of ICP-AES. Because of the simple and rapid procedure, the pressurized-ashing technique using the mixture HNO₃+ H₂O₂(2 + 1) was selected to perform the digestion of the total diet samples collected. The results of the mean daily intake of the population investigated showed for the high mountain valleys (high stomach cancer incidence area, n= 77) as compared with the low altitude valleys (low stomach cancer incidence area, n= 33) significantly higher daily intake with respect to Na (2082 versus 1471 mg), K (1190 versus 731 mg), P (640 versus 381 mg) and a significantly lower daily intake with respect to Ca (925 versus 1379 mg) and Cu (2.78 versus 4.66 mg). No differences were obtained with respect to Al, Fe, Mg and Zn.