Studies of Trace Elements in Canadian Duplicate Diets by Neutron Activation

Abstract

Various types of instrumental neutron activation analysis (INAA) methods have been developed for the determination of thirteen major minerals and trace elements in samples of Canadian duplicate diets. The list of elements includes Al, Br, Ca, Cl, Fe, I, K, Mg, Mn, Na, Rb, Se, and Zn. Most of the elements have been analyzed by conventional INAA methods. A number of INAA methods has been evaluated for Se and I. The methods with the best sensitivity and detection limits were found to be pseudo-cyclic INAA (PCINAA) for Se and epithermal INAA (EINAA) with anti-coincidence gamma-ray spectrometry for I. Aluminum has been analyzed using INAA and a combination of INAA and EINAA for the correction of the interfering reaction 31P(n,a)2s Al.

In order to assure the quality of measurement, quality control and quality assessments have been performed. Precision and accuracy have been evaluated through the analysis of reference materials. The measured values have been found to be in good agreement, generally within 10%, with the certified values. The precision of analysis of the diets has been evaluated by triplicate analyses of each of three diet samples. Precision, as assessed by relative standard deviation has generally been found to be < 10%. Detection limits have been calculated and found to vary from 30 ppb for Se to 1200 ppm for Mg. Control reference materials have been analyzed with the diet samples in order to prepare external quality assessment (EQA) charts. The traceability has been ensured by developing a laboratory information management system and documentation procedures. The EQA charts have demonstrated the analyses to be in statistical control.

The median daily intakes (mg d-1) of 14 duplicate diets have been determined using INAA are 4.8 for Al, 5.07 for Br, 1080 for Ca, 4750 for Cl, 21 for Fe, 0.280 for I, 3800 for K, 350 for Mg, 3.6 for Mn, 3230 for Na, 3.0 for Rb, and 0.130 for Se. These results are discussed in relation to the recommended Canadian as well as the U.S. intakes and previously reported data in the literature.

Exploratory data analysis has been performed on the elemental data obtained using INAA. The utility of the statistical methods has been demonstrated by revealing associations between various elements and food groups.

A dialysis procedure has been used to determine the fraction of elements in the diets which were insoluble or bound to macromolecules. It has also been found to significantly reduce the levels of interfering elements such as Na and Cl, allowing two elements which have been previously undetected, namely V and Ti, to be analyzed.

An enzymolysis method has been adapted in order to estimate the potentially bioavailable fraction of the elements. This fraction has been calculated by difference, leading to large uncertainties. Results are reported for Al, Ca, Fe, Mn, and Zn. The percent available is generally low for all of these elements, except for Mn, for which it varied from near 0 to 59%.