Determination of methionine and selenomethionine in selenium-enriched yeast by species-specific isotope dilution with liquid chromatography-mass spectrometry and inductively coupled plasma mass spectrometry detection.

Selenomethionine (SeMet) and methionine (Met), liberated by acid hydrolysis of selenium-enriched yeast, were quantified by liquid chromatography-mass spectrometry (LC/MS) using standard additions calibrations as well as isotope dilution (ID) based on species-specific (13)C-enriched spikes. LC inductively coupled plasma mass spectrometry (ICPMS) was also employed for the quantification of SeMet, and (74)Se-enriched SeMet was used for ID calibration. The results were evaluated to ascertain the feasibility of using these methods in a campaign to certify selenized yeast. Good agreement was found between the methods, which, when averaged, gave concentrations of 5482.2 +/- 101 and 3256.9 +/- 217.4 microg/g for Met and SeMet, respectively. This corresponds to a 1.68:1 Met-to-SeMet ratio in the yeast. Quantification by ID LC/MS and LC ICPMS yields the most precise sets of results with relative standard deviations in the range 0.5-1.3% (n = 6). A total selenium concentration of 2064.6 +/- 45.4 microg/g was obtained for this yeast material. The extraction efficiency and a mass balance budget were determined. Acid hydrolysis liberated 81.0% of the total selenium present. SeMet comprised 79.0% of the extracted selenium and 63.9% of the total selenium present in the yeast.