Separation of cinchona alkaloid stereoisomers and analogues by ion mobility and chemical theoretical calculation

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In this study, a simple and accurate method for the separation and quantitative analysis of cinchona alkaloid stereoisomers and analogues was developed using trapped ion mobility spectrometry (TIMS)-mass spectrometry (MS) and theoretical calculations. Stereoisomer studied were N-(9-Deoxy-epi-cinchonin-9-yl) picolinamide (N-CNP)/N-(9-Deoxy-epi-cinchonidin-9-yl) picolinamide (N-CDP), N-Benzylcinchoninium (BCN)/N-Benzylcinchonidinium (BCN), and Cinchonine (CCN)/Cinchonidine (CCD). TIMS results revealed slight mobility differences in the protonated stereoisomers, [CCD + H]+/[CCN + H]+ complexes showed the most effective separation, with a resolution (RP–P) of 0.431. When the macrocyclic antibiotics rifamycin (Rif) and natamycin (Nat) were introduced as separation reagents, the binary diastereomer complexes [Nat/Rif + CCD/CCN + H]+, [Rif + N-CDP/N-CNP + H]+, and [Rif + BCD/BCN + H]+ were formed by non-covalent interaction with the stereoisomers. The TIMS analysis revealed that the stereoisomers were separated by the formed binary complexes: CCD/CCD was separated by [CCD/CCN + Nat + H]+ (RP–P = 1.771), N-CDP/N-CNP was separated by [N-CDP/N-CNP + Rif + H]+ (RP–P = 0.914), and the BCD/BCN was separated by [BCD/BCN + Rif]+ (RP–P = 0.713). The conformations of the binary complexes were optimized using chemical theoretical calculations which revealed the microscopic interaction between the stereoisomers was different, and that the theoretical collision cross section (CCS) was consistent with the observed experimental results. An additional quantitative analysis of the stereoisomers yielded a limit of detection (LOD) ≦ 0.408 μg L−1 and satisfactory linearities (R2 > 0.99). The proposed method is simple, highly selective, highly sensitive, and without the need for chromatographic separation, and is therefore a promising option for the separation and analysis of cinchoninestereoisomers or analogues, as well as in desirable figures-of-merit for forensics.