Chiral Discrimination of DL-Amino Acids by TrappedIon Mobility Spectrometry after Derivatization with (+)-1-(9-Fluorenyl)ethylChloroformate

摘要

A novel analytical method based on hybrid trapped ion mobility spectrometry-time-of-flight mass spectrometry (TIMS-TOFMS) has been developed to achieve fast enantiomeric separation of amino acids (AAs). Resolution of chiral AAs was achieved by forming diastereomers through derivatization with the chiral agent (+)-1-(9-fluorenyl)ethyl chloroformate (FLEC), avoiding the use of reference compounds. Electrospray ionization (ESI) in positive mode yielded sodiated FLEC-AAs ions of which the diastereomers could be separated by TIMS. The effect of other alkali metal ions (such as Li and K) on the enantioselectivity was studied, but chiral discrimination was only observed for Na. TIMS conditions, including voltage ramp, ramp time, and accumulation time were optimized for each AA, and collision cross sections (CCSs) were determined for all diastereomers. The migration order of the DL enantiomers was found to be dependent on the structure of the AA. The resulting TIMS resolution (K0/ΔK0) for the FLEC-AA diastereomers on average was 115, requiring a mobility (K0) difference of about 0.009 cm²/(V s) toachieve 50%-valley separation. From the 21 AAs studied, enantiomerseparation was achieved for 17 AAs with mobility differences rangingfrom 0.009 for lysine up to 0.061 cm²/(V s) for asparagine.Moreover, the presented methodology provided mutual separation ofvarious AAs, allowing chiral analysis of multiple AAs simultaneouslywhich may be challenging with previous enantioselective IMS approaches.It appeared possible to fully resolve all studied DL-AAs using threedistinct TIMS methods, resulting in a total MS run time of about 3min (1 min per method) and a total analysis time (including derivatization)of less than 15 min. The method demonstrated capable to determineenantiomeric ratios down to 2.5% with detection limits for the D enantiomersin the nanomolar range. This new TIMS-based methodology opens up possibilitiesfor easy and fast analysis of AA enantiomers.