Selenized yeast is used as a source of selenium in most food supplements.  Yeast grown in a broth containing inorganic selenium incorporates the selenium in place of sufur in amino acids such as methionine and cysteine. Yeast will incorporate 2000-3000 ppm of selenium in this manner.  Selenomethionine has been found to be the major form in selenized yeast.  This form of selenium has been found to be more bioavailable and less toxic than inorganic selenium (selenite or selenate).  

Selenium is regulated by both EPA and FDA.  It is also on the California Prop. 65 list as the disulfide; however, no "safe harbor limit" has been established.  Therefore it becomes important to be able to analyze for the various forms of selenium in food supplements.

FYI

Selenium, Web Elements

Selenium as an Essential Nutrient

EPA Drinking Water Fact Sheet

EPA: Selenium Hazard Information

CA Prop 65

Total Selenium by ICPMS

Traditional tests for trace Se include hydride generation-atomic absorption (HG-AA) and graphite furnace atomic absorption (GFAA).  Modern techniques include ICPMS  with reaction and collision cells.  

Selenium has 6 naturally occurring isotopes, 3 of which are commonly used in ICPMS: 77 (7.6%), 78 (23.8%), 80 (49.7%), and 82 (9.2%).  The major isotope ⁸⁰Se is isobaric with the ⁴⁰Ar₂⁺ from the argon plasma.  A reaction or collision cell is needed to reduce this interference.  Even so, ⁷⁸Se or ⁸²Se have the best signal to noise ratio than either the ⁷⁷Se or  ⁸⁰Se.

Interferences include ⁴⁰Ar³⁷Cl⁺ on ⁷⁷Se and Br⁺ on ⁸⁰Se and ⁸²Se.  These interferences can easily be removed in either of two ways: (1) using hydride generation to introduce the sample into the ICPMS or (2) using the reaction or

 collision cells to remove the interfering ions from the plasma.  Both methods are very efficient, so much so that sea water can be analyzed without any interference.  Detection limits for Se using normal ICPMS are generally 2 ug/L;.  With hydride or cell technology, the detection limit is generally <0.5 ug/L.

A recent analysis of total selenium in a selenized yeast certified reference material along with a wheat flour reference material gave the following results:

Selenomethionine
by LC-ICPMS

Selenium can exist in various ionic and covalent, organic and inorganic forms.  Some of the more chemically important forms are listed in the adjoining table.  Various forms of liquid chromatography (HPLC) have been used to separate these ions and compounds.  

The chromatogram below shows the ion chromatographic separation of selenium species found in a selenized yeast  hydrolyzate.  The yeast was hydrolyzed by sonicating with a protease enzyme¹  The species are separated using a Hamilton PRP X-100 column with 5 mM citrate with pH adjusted to 5.0 with ammonium hydroxide.²  The ⁷⁸Se isotope was monitored using hydrogen in the reaction cell.  In this way, both the selenomethioine as well as the more toxic inorganic forms can be determined.

Note that recent studies cast doubt upon the identity of selenocysteine.³

¹ Capelo, J.L., et al, Anal Chem 2004, 233-237
² Huerta, V.D., et al, JAAS 2003, 1243-1247
³ Dernovics, M and Lobinski, R, JAAS 2008, 744-751

Se Species

Inorganic forms: 

• selenite (SeO₃^-2)  

• selenate (SeO₄^-2)

• selenocyanate (SeCN^-)

• disulfide (SeS₂, antidandruff)

•  oxide (SeO₂) 

• various selenides (e.g. PbSe)

Organic forms:  

• seleno-amino acids (selenomethionine)

• seleno-proteins

Selenomethionine

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