Molecular recognition and quantitative analysis of free and combinative selenium speciation in selenium-enriched millets using HPLC-ESI-MS/MS

The biological activities of selenium (Se) compounds vary according to speciation. The identification of Se compounds is crucial to evaluate the potential health benefits of Se-enriched foods. Molecular recognition and quantification analysis of selenoamino acids in Se-enriched millets were developed by high-performance liquid chromatography/electrospray ionization tandem mass spectrometry (HPLC-ESI-MS/MS). Ultrasonic bath pretreatments were used to extract free Se, while the combinative ones were released using an enzymatic hydrolysis process with protease XIV and α-amylase. Derivatization by carbamidomethylation was applied to enhance the detection sensitivity of trace selenocystine and selenocysteine. Molecular structure analysis of free and combinative selenoamino acids was performed according to the MS2 cleavage rules and isotope property with the product ion scan mode. Only combinative forms of selenoamino acids were detected via the multiple reaction monitoring (MRM) mode. Combinative selenoamino acids as Se-methyselenocysteine and selenomethionine were found, occupying more than 75.74 % of the total Se. The spiked recoveries ranged from 79.76 % to 90.79 % in the matrices. The detection limits for carbamidomethyl-selenocystine, Se-methyselenocysteine, and selenomethionine were 4.759, 0.699, and 1.084 ng mL−1 in the hydrolysate matrix, respectively. The HPLC-ESI-MS/MS method was established to simultaneously analyze the free and combinative forms of selenoamino acids in millets.