Survey of the multi-stage mass spectrometry of common ionic liquid cations and some polyfluorinated ions

Abstract

Ionic liquids are useful for many applications and the structural diversity as a class is ever increasing. Knowledge of gas-phase dissociation pathways can inform analytical method development for the characterization of novel syntheses, degradation products, or environmental contaminants, and can inform our knowledge of the fate of ionic liquid species in the gas phase (e.g., after ejection by electrospray spacecraft thrusters). In this work, the N-heterocyclic (NHC) ions:1-butyl-1-methylpiperidinium (BMPi), 1-methyl-1-propylpiperidinium (MPPi), 1-butyl-1-methylpyrrolidinium (BMPyr), 1-ethyl-1-methylpyrrolidinium (EMPyr), 1-butylpyridinium (BPy), and 1-Butyl-1-(3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl) imidazolium (BFIm) were studied. An additional fluorinated anion, nonafluorobutanesulfonate (NFBS), was also studied. Collision-induced dissociation mass spectrometry (CID-MS), as MS2 and MS3 using an ion trap mass spectrometer and energy-resolved breakdown curves using a quadrupole time-of-flight mass spectrometer, were used to determine dissociation patterns. Assignment of proposed dissociation pathways were made based on the experimental results and complementary computational chemistry. It was observed that for aromatic NHCs, cleavage at the C–N bond was preferred, to generate alkene-based neutral losses. Non-aromatic NHCs typically generated both alkane- and alkene-based neutral losses. Breakdown curves, supported by theoretical thermodynamic calculations, suggest that the loss of the alkane is the favorable dissociation pathway. Additionally, some ring-opening and ring contraction reactions via C–C bond cleavage were observed in non-aromatic species, whereas the intact aromatic NHC is typically a stable terminal ion. The polyfluorinated imidazolium fragmented via the breaking of C–N, C–C, and C–F bonds. The fluorinated anion, NFBS, fragmented by C–C and C–S bond cleavage to generate fluoroalkene, sulfur dioxide, and sulfur trioxide neutral losses. Beyond these broad stroke findings, the dissociation pathways of the seven ionic liquid ions were mapped using evidence from MS3, breakdown curves, and computational chemistry together assigning product ions as either arising from competitive or sequential pathways and proposing tentative structures. Some of the ionic liquid ions showed class-based product ions which will be useful in analytical development for their characterization. Finally, these results were contextualized within previous literature reports on other ionic liquid species.