Selenium speciation in enriched and natural samples by HPLC-ICP-MS and HPLC-ESI-MS with perfluorinated carboxylic acid ion-pairing agents

Abstract

Selenium-enriched plants, such as hyperaccumulative phytoremediation plants (Astragalus praleongus, 517 micrograms g-1 Se, and Brassica juncea, 138 micrograms g-1 Se in dry sample), yeast (1200, 1922 and 2100, micrograms g-1 Se in dry sample), ramp (Allium tricoccum, 48, 77, 230, 252, 405 and 524 micrograms g-1 Se in dry sample), onion (Allium cepa, 96 and 140 micrograms g-1 Se in dry sample) and garlic (Allium sativum, 68, 112, 135, 296, 1355 micrograms g-1 Se in dry sample) were analyzed by HPLC-ICP-MS for their selenium content and speciation after hot water and enzymatic extractions. Reference samples with natural selenium levels, such as onion and garlic controls, cooking garlic powder, baking yeast powder and a commercial garlic supplement were also analyzed. Selected samples were also examined by HPLC-electrospray ionization (ESI)-MS. HPLC was mostly carried out with 0.1% heptafluorobutanoic acid (HFBA) as ion-pairing agent in 1 + 99 v/v methanol-water solution, but 0.1% trifluoroacetic acid (TFA) in 1 + 99 v/v methanol-water solution was also utilized to permit chromatography for compounds that did not elute with HFBA. More than 75% of the total eluting selenium compounds, based upon element specific detection, were identified from retention time data and standard spiking experiments, and between 60 and 85% of compounds were identified by MS, with up to 25% of the total eluting molecular selenium species being unidentified as yet. Limits of quantification (LOQ, defined as the concentration giving an S/N of 10) for HPLC-ICP-MS were in the range 2-50 ng mL-1 Se in the injected extracts for the selenium-enriched samples and 2-10 ng mL-1 Se for the natural selenium level samples. LOQ values for HPLC-ESI-MS were ca. 100 times higher than those measured by HPLC-ICP-MS.

Speciation of selenium dietary supplements; formation of S-(methylseleno)cysteine and other selenium compounds

“…1 shows the HPLC-ICP-MS chromatogram of the Life Extension Super Selenium Complex capsules. Comparisons with results for selenium supplements utilizing these HPLC conditions [40,51,52] confirm the peak identifications and show a complex selenium speciation including inorganic and organic compounds. The former (selenate and selenite) are quantifiable since the both elute after the void volume and no other anionic selenium species have ever been observed under these conditions.…”

“…The major organoselenium species in selenized yeast is selenomethionine [38][39][40]. In 2004, we reported [51] the presence of a sulfur-selenium amino acid, S-(methylseleno)cysteine, in archived selenized yeast samples from the Clark study [42].…”

“…The operating conditions with a cross flow nebulizer and Scott spray chamber were as follows: RF power 1100 W, plasma Ar flow rate 15 L min −1 , nebulizer Ar flow rate 0.98 L min −1 , auxiliary gas flow rate 1.20 L min −1 , methane collision cell gas 0.6 mL min −1 , isotopes monitored 78 Se, 80 Se, 72 Ge, dwell time 80 ms [52]. The HPLC system which utilized a 5-m Symmetry Shield TM RP-C 8 column (3.9 mm × 15 cm) (Waters Corporation, Milford, MA) and an ion-pairing mobile phase of 0.1% heptafluorobutanoic acid (HFBA), 1% methanol and 99% water (v/v) was described previously [40,51].…”

“…Baker's yeast (Saccharomyces cerevisiae) is most commonly used for the production of supplements; selenized or 'high selenium yeasts' are prepared by growing yeast in a sodium selenite-enriched medium [35][36][37]. The principal organoselenium species produced in selenized yeast is selenomethionine [38,39], but selenoanions and other selenoamino acids including selenocystine, Se-lanthionine and Se-(methyl)selenocysteine have been determined at levels <1% of that of selenomethionine [40]. Ouerdane and Mester have reported essentially quantitative replacement of methionine by selenomethionine in yeast grown on a SeMet-containing medium [41].…”

Identification and Synthesis of a Novel Selenium−Sulfur Amino Acid Found in Selenized Yeast: Rapid Indirect Detection NMR Methods for Characterizing Low-Level Organoselenium Compounds in Complex Matrices

“…However, these compounds must generally be identified by comparison with known standards unless sufficient material is available for spectroscopic characterization. For example, following digestion of yeast samples and chromatographic separation, Se-containing glutathione S-conjugates were identified by nanoelectrospray MS/MS (8), selenoadenosylmethionine and homocysteine were identified by ICP-MS (9)(10)(11), and the selenopentapeptide SeMet-Asn-Ala-Gly-Arg was identified using matrix-assisted laser desorption/ionization timeof-flight mass spectrometry (MALDI-TOFMS) (12,13). By combining the use of chiral HPLC columns with ICP-MS, chiral speciation of D-and L-selenomethionine in selenized yeast can be accomplished (14).…”