Determination of selenomethionine and seleno-methyl-selenocysteine in biota by ultrasonic-assisted enzymatic digestion and multi-shot stir bar sorptive extraction–thermal desorption–gas chromatography–mass spectrometry

A method based on stir bar sorptive extraction (SBSE) and thermal desorption (TD)–gas chromatography–mass spectrometry (GC–MS) has been optimized for the determination of seleno-methyl-selenocysteine (SeMetSeCys) and selenomethionine (SeMet) in biota samples. Aliquots of freeze-dried tissue, a mixture of protease XIV-lipase and water were sonicated for 2 min. After extraction, the extract was separated by centrifugation and subjected to derivatization and SBSE–TD–GC–MS. The parameters affecting derivatization, absorption and desorption steps were investigated. The optimized conditions consist of a derivatization with 40 μL of ethyl chloroformate (ECF) in 400 μL of a water:ethanol:pyridine (60:32:8) mixture, followed by dilution to 1.5 mL of 70 g NaCl L−1 in water at neutral pH and an extraction step using 10 mm × 1 mm PDMS stir bar, stirring at 800 rpm for 20 min at room temperature (23 ± 1 °C). Three stir bars were used for the extraction of three different aliquots of the same sample and then placed in a single glass desorption liner and simultaneously desorbed for GC–MS analysis. The desorption step required the following conditions: 300 °C (desorption temperature), 6 min (desorption time), 50 mL min−1 (vent flow) and −5 °C (cryotrapping temperature). The method provided precise (8.1%) and accurate results in the mg Se kg−1 range (using the selected-ion monitoring-SIM mode) against certified reference material SELM-1 yeast, with recoveries higher than 80% for spiked algae and clams samples.