Jeon, Junho (2013-10). Development and Application of an Electrospray Ionization Ion Mobility-mass Spectrometer Using an RF Ion Funnel and Periodic-focusing Ion Guide. Doctoral Dissertation.
Thesis
A novel ion mobility-mass spectrometer was designed and built in order to achieve high transmission and high resolution for observing desolvated ion conformations of chemical and biological molecules in the gas phase. The instrument incorporates a home-built electrospray ionization (ESI) source and rf ion funnel(IF) as an ion transfer interface, modular periodic-focusing ion guide (PFIG) as a drift tube (DT) for arrival time measurement, and orthogonally accelerated linear time-of-flight mass spectrometer to maximize analytical figures-of-merit of the instrument including sensitivity and separation of gas-phase ions. The rationales for implementation of each device aforementioned are discussed as enhanced ion focusing to reduce data acquisition time of the instrument, high mobility resolution (R > 56) to separate ions having a small collision cross section (CCS) difference at low-to-intermediate field strength (10 V cm-1 torr-1~19 V cm-1 torr-1), and observation of multiple conformers from isobaric ions at various operation conditions. Gate and transfer ion optic design needed to avoid detrimental gas dynamic effect is presented during hybridization of PFIG into ESI mass spectrometer. The details of instrumentation and electrode/voltage schematics are also discussed along with SIMION calculation results. The characteristic drift motions of ions inside the PFIG require a normalization procedure to compare CCS values from the instrument with the results using conventional uniform field DT ion mobility-mass spectrometry (IM-MS). Through experimental measurements and ion trajectory calculations from well known model peptides and proteins having multiple charge states, a correlation factor ? which links the results from the two different types of instrumentation was derived and successfully applied (within +-4% difference). Ion heating was also investigated using ubiquitin, insulin, and substance P. Using ESI PFIG-MS, A significant variation of the arrival time distribution at different heated capillary temperature, rf amplitude, and electric field strength was observed for ubiquitin (+7) and (+8) and substance P (+3), while a substantial change was not observed for insulin owing to structural rigidity of disulfide bond cross-linking between two chains. The results imply that ESI PFIG-MS provides a window to monitor conformational variation of gas-phase ions and measure energy barriers of peptide/protein folding processes, which can allow us to reveal pathways in structural energy landscape.
