Simultaneous analysis of DL-Amino acids in foods and beverages using a highly sensitive chiral resolution labeling reagent

Amino acids with various functions are abundant in living organisms and foods. Recent advances in analytical technology show that trace amounts of D-amino acids exist in living organisms and foods. In addition, studies show that these amino acids are involved in various physiological functions that differ from those of L-amino acids. Thus, a technique for analyzing DL-amino acids is required. However, the simultaneous separation and highly sensitive detection of DL-amino acids are complicated; therefore, highly sensitive analytical methods that can rapidly separate and identify compounds are required. We previously developed our original chiral resolution labeling reagents for the separation and highly sensitive detection of DL-amino acids. Here, we developed a simple method for the rapid separation and highly sensitive detection of DL-amino acids in various foods and beverages by liquid chromatography–mass spectrometry (LC–MS) using an octadecyl (C₁₈) column after labeling with 1-fluoro-2,4-dinitrophenyl-5-D-leucine-N,N-dimethylethylenediamineamide (D-FDLDA; enantiomeric excess > 99.9 %). In addition, we synthesized a stable isotope (¹³C₆)-labeled D-FDLDA (¹³C₆-D-FDLDA) and established an analytical method that can accurately identify the peak of each DL-amino acid. MS sensitivity of DL-amino acids labeled with our labeling reagent was higher than that of conventional labeling reagents (Marfey’s reagents). The labeling reagent was neither desorbed from each DL-amino acid nor degraded for at least 1 week at 4 °C. Furthermore, we determined the DL-amino acid contents in foods and beverages using the proposed method, and differences in the total amino acid content and D/L ratio in each food and beverage were observed.