Abstract
The determination of free amino acids is important for quality evaluation and nutritional studies of strawberries. In this study, an ultra performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) method using hydrophilic interaction chromatographic column was established for the simultaneous determination of 18 free amino acids in strawberries, and the pretreatment, chromatography, and mass spectrometry conditions were optimized. Specific pretreatment processes include grinding, extraction with water, centrifugation, and membrane filtration. The treated samples were then analyzed by hydrophilic interaction liquid chromatography (HILIC)-tandem mass spectrometry. The separation was performed on an ACQUITY UPLC Glycan BEH Amide column (150 mm×2.1 mm, 1.7 μm), with a water-acetonitrile system containing 5 mmol/L ammonium formate and 0.1% (v/v) formic acid as the mobile phase for gradient elution. A triple quadrupole mass spectrometer was used in positive-ion electrospray ionization (ESI) scanning mode, with target amino acids quantified using the matrix-matched standard-curve method. Eighteen free amino acids were determined with good linearities in the range of 0.5-40.0 μmol/L, along with r(2) greater than 0.992. The intra-day and inter-day precisions of the method were 1.0%-14.8% and 3.6%-17.6%, respectively. The limits of detection (LODs) were in the range of 50-250 nmol/L. The recoveries of the 18 amino acids were 75.0%-114.6% with relative standard deviations (RSDs) of 0.3%-13.5%. The contents of free amino acids in strawberries at different ripening stages were statistically analyzed, and a total of seven differentiated free amino acids (phenylalanine, isoleucine, glutamine, 4-aminobutyric acid, arginine, glutamic acid, and aspartic acid) were statistically screened. The method is rapid, accurate, reproducible and stable, and can quantitatively determine the content of amino acids in strawberries.