Kinetic Method Coupled with Thermal-Assisted Paper Spray Ionization Mass Spectrometry for Direct Determination of Enantiomeric Excess of Multiple d/l-Amino Acids in Functional Foods

Amino acids are commonly used as nutritional fortification substances in functional foods, and their chiral configuration is an important determinant of food function. Rapid chiral screening methods are urgently needed in food analysis but are limited by the long-time chiral separation and matrix interference. In this study, we show a kinetic method coupled to thermal-assisted paper spray ionization mass spectrometry for direct determination of enantiomeric excess (ee) of multiple d/l-amino acids in complex food matrixes without sample pretreatment. 3-(2-Naphthyl)-l-alanine was selected as a new chiral reference ligand for the kinetic method to achieve efficient chiral differentiation (discrimination degree is 8.7 for d/l-phenylalanine and 10.2 for d/l-tyrosine). An additional thermal-auxiliary device was developed for paper spray ionization mass spectrometry to facilitate the enantiomeric purity determination. The developed method allowed a rapid simultaneous enantiomeric purity determination of multiple chiral amino acids (d/l-phenylalanine and d/l-tyrosine) within 30 s. Good linearities were achieved for the quantitation of ee (R2 = 0.9996 for phenylalanine and 0.9995 for tyrosine) with unknown amino acid concentrations ranging from 10 μM to 600 μM. The developed method was successfully applied for the enantiomeric purity determination of multiple chiral amino acids in functional capsules and beverages and showed great potential for efficient enantiomer-related food safety screening and nutrition analysis.