Determination of Essential and Non-Essential Elements in Dietary Supplements by Microwave-Induced Plasma Optical Emission Spectrometry: Method Development and Study of Non-Spectral Interferences

Dietary supplements have been used to overcome nutritional deficiencies and the knowledge concerning essential and non-essential elements is an important issue. In this work the suitability of microwave-induced plasma optical emission spectrometry (MIP OES) for the determination of essential and non-essential elements in dietary supplements was evaluated. Twelve dietary supplement samples of several classifications (vitamins/minerals, minerals, amino acids, and botanicals) were digested in their whole form for determination of essential (Ca, Co, Cu, Fe, K, Mg, Mn, Mo, Na, and Zn) and non-essential (Ag, Al, B, Ba, Be, Cd, Cr, La, Li, Ni, Pb, Sr, and V) elements by MIP OES. Potential non-spectral interferences by common concomitants (C, S, K, Na, and Ca) were evaluated, as well as those by residual acidity of digests. The study of non-spectral interferences showed that a signal suppression effect is observed with higher concentrations of Ca, Na, and K. Relatively good robustness was observed considering the presence of C and S, as well as residual HNO3. The limits of quantification (LOQs) were dependent on the sample mass used for decomposition (from 0.6 to 1.6 g in the commercial product) and on the minimum dilution factor. From the results, there was a prevalence of essential and non-essential elements in vitamins and minerals, minerals, and botanicals-based dietary supplements, whereas lower concentrations were found in the dietary supplements based on amino acids. All elements were in a concentration below the recommended dietary allowances (RDAs), exception for those with the concentration intentionally higher. Accuracy of results by MIP OES was evaluated by using standard reference materials (SRM) NIST 1572 and NIST 1575a. In addition, results showed no statistical difference by comparison with those by ICP OES. MIP OES proved to be a suitable technique for the determination of metals in dietary supplements, being a feasible alternative for the quality control of these products.