HPLC–ICP-MS speciation of selenium in enriched onion leaves – a potential dietary source of Se-methylselenocysteine

In this work chromatographic separation was coupled with mass spectrometry detection to study the speciation of selenium in selenium enriched onion leaves. The plants of onion (Allium cepa L.) were grown hydroponically for one week in standard medium containing inorganic selenium. The leaves and bulbs were separated from roots, the two types of material (leaves and bulbs) were dried and homogenized. The sodium hydroxide (0.1 mol l−1) extracts were analyzed by size exclusion chromatography (with spectrophotometric and mass spectrometric detection) showing the incorporation of selenium to the high molecular weight fraction, which was more pronounced in the leaves relative to the bulbs. This incorporation was better with Se(IV) enrichment (33% in leaves and 26% in bulbs) than with Se(VI) (3% and 5%, respectively). After extraction of the low molecular weight compounds (chloroform–methanol–water or 0.4 M perchloric acid–ethanol) and elimination of inorganic selenium (Dowex 1X8), selenium speciation was carried out by ion-pairing high performance chromatography [5 mmol l−1 citric acid, 5 mmol l−1 hexanesulfonic acid, pH 4.5: methanol (95:5)]. The primary organic selenium species found in leaves extracts was Se-methylselenocysteine, which has been reported as one of the most active chemopreventive selenium species as found in cell and animal experiments. The percentage contribution of selenium in this species was 4.0% and 1.9%, respectively, of the total selenium in leaves enriched with Se(IV) and Se(VI). It may be possible to use selenium enriched leaves as a dietary source of Se-methylselenocysteine. The important advantage of this source is the simple and fast enrichment process.