Selenomethionine content of candidate reference materials

Selenium has been identified as an antioxidant of importance in the diet. A ccurate determination of its chemical forms depends on the availability of suitable reference materials (RMs). Two candidate reference materials for d etermination of selenomethionine (Semet) in food-related materials, a stand ard wheat gluten sample (NIST RM 8418 Wheat Gluten) and a commercial seleni um enriched yeast, have been examined by use of a gas chromatography-isotop e dilution mass spectrometry (IDMS) procedure, after treatment of the matri x with 0.1 mol L-1 hydrochloric acid containing stannous chloride, addition of CNBr, and extraction with chloroform. This procedure results in cleavag e of the CH3Se group to form volatile CH3SeCN. Addition of isotopically enr iched (74)Semet to an analytical sample enables estimation of the naturally occurring protein-bound (80)Semet by IDMS without a protein-digestion proc ess. We found that the Wheat Gluten RM contains a significant amount of Semet as a portion of its assigned value of 2.58 mug Se-total g(-1). Commercial sel enium yeast tablets are labeled as containing an elevated level of "organic selenium", usually as Semet. The sample we investigated contained 210 mug Se-total g(-1) sample as determined separately by IDMS, measuring elemental selenium after digestion. 73% of this total (153 +/- 21 mug Se-Semet g(-1) ; n = 23) was present as Semet. Thus, these two materials contain significa nt amounts of their total selenium content as Semet and would be good candi dates for further study and characterization as reference materials for det ermining this important food component. The CNBr reaction used will also enable the determination of Se-(methyl)sel enocysteine, the biological role of which is of recent interest. In additio n to matrix RMs for Semet, it is important to have standard materials of th e pure substance. We have examined a sample of a candidate standard materia l of selenomethionine being prepared by the USP. It was confirmed that this material is pure selenomethionine.