A Combined MS/MS and IMS Study Into the Fragmentation Pathway of Nifedipine

Collision-induced dissociation (CID) of small molecules, typically with a molecular mass below 1500 Da, is widely applied for the structural identification of drug metabolites, synthetic by-products and unknown compounds. Nonetheless, the interpretation of MS/MS spectra derived from protonated or sodiated molecules mainly relies on the comparison with literature data and proposed structures usually lack rigorous mechanistic justification and conclusive evidence. The commonly applied even-electron rule, as implemented in most literature and prediction software, does not support radical loss from protonated molecules. Similarly, fragmentation pathways of sodium adduct ions remain insufficiently explored and lack mechanistic rationalization. To improve the understanding of radical loss from even-electron nitro-containing compounds, a mechanistic study of fragmentation patterns of nifedipine, m-nifedipine, and related analogues was performed. Fragment ions generated from [M+H]⁺ and [M+Na]⁺ of nifedipine revealed distinct mechanisms compared with its positional isomers, while showing similarities with analogues like nisoldipine and aranidipine. The nitro group at the ortho position significantly influences fragment stability, leading to a unique fragmentation mechanism described as ortho effect. The mechanism was further investigated using isotope labelled analogs, ion mobility spectrometry (IMS), precursor ion scan (PIS), and density functional theory (DFT) calculations. Different approaches in combining IMS with MS/MS demonstrated strong capability for elucidation of fragmentation pathways.