Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/15/2003
Publication Date: 7/1/2003
Citation: Roberge, M.T., Borgerding, .J. and Finley, J.W. Speciation of selenium compounds from high selenium broccoli is affected by the extracting solution. J. Agricul. Food Chem. 51: 4191-4197, 2003
Interpretive Summary: Diets high in selenium have been shown to be protective against several types of cancer. The type of selenium compounds ingested determines the amount of protection. Plants, such as broccoli, can incorporate selenium into many different molecules, however, many of these selenium compounds are unstable. In extracting selenium compounds for analysis there may be chemical reactions of the selenium compounds with the solvent or with other compounds from the broccoli. When a chemical reaction takes place, the original selenium compound is changed to another compound that may have very different biological activity than the original compound. We extracted selenium compounds from broccoli using 27 different common extracting solutions and found that each extracting solution gave a different distribution of selenium compounds. Reports of the identities of selenium compounds present in natural materials should be approached with care because of changes that may occur in the analysis process itself.
Technical Abstract: The speciation of selenium compounds from high selenium broccoli (876 ¿g/g) depends on the extraction conditions. Twenty-seven extraction conditions were explored involving nine different buffering systems between pH 1 and pH 9. In non-buffered extractions of broccoli, more than 40% of the spiked Se-methylselenocysteine was not recovered in the filtered solution. However, in buffered extractions, losses for Se-methylselenocysteine ranged from 10-20%. Mass balance indicated that approximately 30% of naturally occurring selenium in broccoli samples was volatilized and lost to the atmosphere when buffered extractions were made. Solid Phase Extractions (SPE) indicated that the polarity of selenium compounds in solution was also dependent on the extracting solution. High Pressure Liquid Chromatography coupled to an Inductively Coupled Plasma Mass Spectrometer (HPLC-ICP-MS) was used to show that selenium compounds extracted from broccoli reacted with the extracting solution. Compound identities were assigned by matching retention times to standards of selenite, selenate, methylseleninic acid, Se-methylselenocysteine, selenomethionine and the selenonic acids of Se-methylselenocysteine and selenomethionine. Changes in speciation were analyte, pH and buffer dependent, but generally higher pH resulted in more highly oxidized selenium compounds. For valid conclusions to be drawn from the analytical data, the extraction conditions should match the conditions present in the matrix or be specified for a particular application such as a simulated gastrointestinal digestion.