Selenomethionine content of candidate reference materials

Abstract

Selenium has been identified as an antioxidant of importance in the diet. Accurate determination of its chemical forms depends on the availability of suitable reference materials (RMs). Two candidate reference materials for determination of selenomethionine (Semet) in food-related materials, a standard wheat gluten sample (NIST RM 8418 Wheat Gluten) and a commercial selenium enriched yeast, have been examined by use of a gas chromatography–isotope dilution mass spectrometry (IDMS) procedure, after treatment of the matrix with 0.1 mol L^–1 hydrochloric acid containing stannous chloride, addition of CNBr, and extraction with chloroform. This procedure results in cleavage of the CH₃Se group to form volatile CH₃SeCN. Addition of isotopically enriched ⁷⁴Semet to an analytical sample enables estimation of the naturally occurring protein-bound ⁸⁰Semet by IDMS without a protein-digestion process.

We found that the Wheat Gluten RM contains a significant amount of Semet as a portion of its assigned value of 2.58 μg Se_total g^–1. Commercial selenium yeast tablets are labeled as containing an elevated level of “organic selenium”, usually as Semet. The sample we investigated contained 210 μg Se_total g^–1 sample as determined separately by IDMS, measuring elemental selenium after digestion. 73% of this total (153±21 μg Se_Semet g^–1; n = 23) was present as Semet. Thus, these two materials contain significant amounts of their total selenium content as Semet and would be good candidates for further study and characterization as reference materials for determining this important food component.

The CNBr reaction used will also enable the determination of Se-(methyl)selenocysteine, the biological role of which is of recent interest. In addition to matrix RMs for Semet, it is important to have standard materials of the pure substance. We have examined a sample of a candidate standard material of selenomethionine being prepared by the USP. It was confirmed that this material is pure selenomethionine.