Brigham Pope; Daniel Joaquin; Jacob Hickey, Brigham Young University
CRAFTI is an emerging method for measuring molecular collision cross sections using the ultra-high resolving FTICR-MS instrument. Collision cross sections are important measurements to understanding size, tertiary structure, and ligand binding. CRAFTI has been shown to give accurate collision cross sections comparable to those from drift ion mobility, the leading cross section measurement method. Because CRAFTI measurements are carried out under single-collision conditions, CRAFTI can obtain cross sections for non-covalently bound complexes with interactions that are far too weak to address using drift ion mobility, where multiple thermal collisions can disrupt the complexes. However, CRAFTI’s requirement of single-collision ion dephasing is a hindrance to the measurement of larger mass ions because for ions that are massive compared to the neutral collision partner it is difficult to reach sufficient center-of-mass kinetic energy to cause dephasing (which usually occurs via collision-induced dissociation). In this study, we characterize these limitations by measuring different species (tetraalkylammonium, cryptand-metal ion, and cucurbituril complexes) at a range of kinetic energies. For polyatomic ions, CRAFTI cross sections are observed to increase with increasing center-of-mass kinetic energy until they reach a limiting value that is generally similar to cross sections computed from the expected molecular structure. The CRAFTI measurement reaches the calculated cross-section at a specific center-of-mass kinetic energy related to the energy of the bond being broken. We also performed MultiCRAFTI, a new experiment where we overcome the kinetic energy limit through simultaneous measurements of multiple species for internal comparison. Two analytes in a MultiCRAFTI experiment, though both are beneath the minimum kinetic energy requirement, give correct relative cross sections. These MultiCRAFTI measurements represent a new step toward extending the FTICR-MS to vital new areas of measurement in macromolecules and organo-metal complexes.