Identification of selenium compounds using HPLC-ICPMS and nano-ESI-MS in selenium-enriched rice via foliar application

Se-enriched rice was prepared by foliar fertilization. In the present work, speciation and distribution of Se in Se-enriched rice and non-supplemented rice was evaluated by ion-paring reversed phase (IP-RP) and strong anion-exchange (SAX) chromatography coupled with inductively coupled plasma mass spectrometry (ICPMS) detection. Three extraction procedures: water extraction, acid extraction (0.1 M HCl) and enzymatic hydrolysis were studied with a combination of protease XIV and amylase with ultrasonic bath and the latter was found to be optimal for the extraction of selenospecies. The chromatograms obtained revealed that the major selenospecies found in Se-enriched rice was selenomethionine (SeMet), which was further identified by nanoelectrospray ionization, ion trap mass spectrometry (nano-ESI-ITMS). Approximately 86.9% of the total Se in the Se-enriched rice extract was SeMet, while only 26.7% of non-supplemented rice extract was due to SeMet. Moreover, nearly 60% of inorganic Se in the form of Se-IV was present in the non-supplemented rice, while only 6.8% of inorganic Se as Se-IV and Se-VI was found in Se-enriched rice extract with minor selenocystine (SeCys(2)) and trace selenomethionine selenoxide (SeOMet) present. The results proved that SeMet was efficiently extracted by the enzymatic hydrolysis without oxidization. The high SeMet concentrations of Se-enriched rice indicate a high inorganic to organic conversion when Se enrichment was carried out by foliar application. Since SeMet is a readily bioavailable Se species for animals, Se-enriched rice could be considered for supplementation in Se poor geographical regions.