Direct Implementation of MSⁿ Using Frequency Scanning Collision Induced Dissociation in a Digital Ion Trap Mass Spectrometer

Tandem mass spectrometry (MSⁿ) is one of the most effective methods to obtain the structures of organic molecules, enabling the observation of multigenerational ion fragments. Collision-induced dissociation (CID) is currently the most mature technique for mass spectrometry analysis. Ion trap mass spectrometry (ITMS) is favored for on-site detection field, due to its ability of MSⁿ analysis with a single trap and its small size. However, conventional MSⁿ analysis in ITMS requires repeated isolation and excitation processes multiple times, causing high complexity of the entire scanning process. Additionally, the fragment ion detection in ITMS is limited by low-mass cutoff (LMCO) and the weak fragmentation yield. In this study, a method named reverse scanning-collision induced dissociation (RS-CID) is proposed, which involves increasing RF and AC frequencies while maintaining RF voltage constant during the CID process. Twelve representative illegal drugs were analyzed adopting this method, enhancing the intensities of low-mass fragment ions compared to conventional dissociation method. Moreover, experimental results with ketamine and methamphetamine show that RS-CID effectively reduces the LMCO effect and slightly improves CID efficiency. It also enables direct acquisition of their multigeneration fragment ions spectra in a single sequence of ion injection, cooling, isolation, RS-CID, cooling, mass scanning and empty. The experiments to distinguish between the isomers ab-4en-pinaca and adb-3en-butinaca as well as the isomeric compounds 5f-cumyl-pegaclone and cumyl-pipetinaca were also successful by this method. In summary, RS-CID enables MSⁿ analysis in a single sequence and improves the low-mass fragment ions intensity. It can simplify workflows, achieve faster analysis and provide more valuable mass spectral information.