Chiral nano-liquid chromatography–mass spectrometry applied to amino acids analysis for orange juice profiling

Determination of amino acid enantiomers is a very important topic in food analysis, since the presence of d-isomers may indicate, e.g., adulteration, microbiological contamination, uncontrolled fermentation processes, etc. In fact, the d- and l-enantiomers contents can be a useful marker for several elements such as quality control, contamination detection, processing monitoring, etc. Here we studied the potentiality of nano-liquid chromatography (nano-LC) coupled with mass spectrometry for the enantiomeric separation of several d- and l-amino acids that can be found in food products. Analytes were derivatized with fluorescein isothiocyanate (FITC). The mixture was injected and compounds focused on a C18 cartridge, then nano-LC analysis was carried out in a capillary column (75 μm i.d.) packed with vancomycin-modified silica–diol particles. The effect of some experimental parameters, such as pH and buffer concentration on enantioresolution and retention factors, was studied for method optimization. The chromatographic separation system was coupled with an ion-trap mass spectrometer through a nano spray interface. It provided a final evaluation on analytes detected in all investigated samples with LOD values as low as 8 ng/mL. That method was applied to the comparative analysis of two different orange juice samples (fresh natural vs. commercial one). Obtained profiles confirmed expected high quality standards. In fact, they mainly contained l-amino acids forms and not their antipodes.