Evaluation of selenium in dietary supplements using elemental speciation

Abstract

Selenium-enriched dietary supplements containing various selenium compounds are readily available to consumers. To ensure proper selenium intake and consumer confidence, these dietary supplements must be safe and have accurate label claims. Varying properties among selenium species requires information beyond total selenium concentration to fully evaluate health risk/benefits A LC-ICP-MS method was developed and multiple extraction methods were implemented for targeted analysis of common "seleno-amino acids" and related oxidation products, selenate, selenite, and other species relatable to the quality and/or accuracy of the labeled selenium ingredients. Ultimately, a heated water extraction was applied to recover selenium species from non-selenized yeast supplements in capsule, tablet, and liquid forms. For selenized yeast supplements, inorganic selenium was monitored as a means of assessing selenium yeast quality. A variety of commercially available selenium supplements were evaluated and discrepancies between labeled ingredients and detected species were noted.

Keywords: Dietary supplement; LC-ICP-MS; Selenium; Speciation.

Fig. 1

Standard LC-ICP-MS chromatograms using conditions for Separation 1 with a post-column selenium marker peak (M). A) ~10 ng Se g⁻¹ selenium standard mixture in mobile phase including SeCys₂ (2a), MeSeCys (4), SeMet (5), Se(IV) (7), and Se(VI) (8), B) ~10 ng Se g⁻¹ selenium standard mixture in mobile phase with MeSeMet (1) and MSeA (6) standards added (note a SeOMet (3) impurity), C) ~10 ng Se g⁻¹ SeOMet (3) in mobile phase, D) ~10 ng Se g⁻¹ MeSeOCys (2b) in mobile phase, with the impurity MSeA (6).

Fig. 2

Using chromatographic conditions for Separation 1, Se Supplement #3 (top) was examined and determined to not contain labeled SeMet (5). Se Supplement #9 (bottom) was determined to have increased iSe levels as compared to quality yeast (>2% of total Se).

Fig. 3

Chromatogram A used conditions for Separation 1; Chromatogram B used conditions for Separation 3. The Unk-A peak eluted similar to SeCys₂ (2a) using the Separation 1 conditions.

Fig. 4

LC-ESI-MS using a Q Exactive for detection, capable of exact mass determination; chromatographic conditions were for Separation 2. The theoretical m/z for the most abundant isotope (⁸⁰Se) of each compound was used to generate each EIC trace (left). Full scan mass spectra (middle) display the measured m/z for the most abundant isotope; the measured m/z values for both samples and standards were all within 1.10 ppm of the theoretical m/z values. The spectra on the right contain the corresponding MS² data.