Determination of selenomethionine in yeast using CNBr derivatization and species specific isotope dilution GCICP-MS and GC-MS

Abstract

A comparison of several published and in-house methods for quantitation of selenomethionine (SeMet) in yeast was undertaken using species specific isotope dilution (ID) with a ⁷⁴Se enriched SeMet. An in-house method was based on digestion of samples by refluxing for 16 h with 4 M methanesulfonic acid, derivatization of SeMet with cyanogen bromide and extraction into chloroform for determination by GC ICP-MS. Concentrations of 3434 ± 19 and 3419 ± 15 μg g⁻¹ (one standard deviation, n = 6) with relative standard deviations of 0.55% and 0.42% for SeMet were obtained in yeast based on measurement of ⁷⁸Se/⁷⁴Se and ⁸²Se/⁷⁴Se ratios, respectively, in agreement with a value of 3417 ± 27 μg g⁻¹ (one standard deviation, n = 6) obtained using ID GC-MS detection based on the ratio of 106/100 of SeCN⁺ ion. The SeMet accounts for 67% of the total Se in the sample. A method detection limit (three standard deviations) of 0.9 μg g⁻¹ was estimated for SeMet based on a 0.25 g subsample. Significantly lower concentrations of 2220 ± 7 and 2215 ± 9 μg g⁻¹ (one standard deviation, n = 4) with RSDs of 0.30 and 0.41% were obtained by ID GC ICP-MS using ⁷⁸Se/⁷⁴Se and ⁸²Se/⁷⁴Se ratios, respectively, following digestion with 2% SnCl₂ in 0.1 M HCl and reaction with CNBr to form volatile CH₃SeCN. Similar SeMet concentrations of 3415 ± 200 and 3447 ± 198 μg g⁻¹ (one standard deviation, n = 6) and significantly degraded precisions of 5.86 and 5.74%, respectively, were obtained in yeast using ⁷⁸Se/⁷⁴Se and ⁸²Se/⁷⁴Se ratios, respectively, following digestion with 4 M methanesulfonic acid and derivatization with methyl chloroformate.