External Ionization Source Improvements For Fourier Transform Ion Cyclotron Resonance Mass Spectrometry PDF Download

Abstract: Mass spectrometry imaging (MSI) is an emerging experimental methodology whose primary objective is the investigation of spatial variation of molecular composition within and across selected biological tissues to enable biomarker discovery, molecular diagnosis, and studies of drug metabolism, among other applications. The two major challenges, therefore, are the unambiguous identification and precise localization of biologically relevant compounds. These challenges can be recast as a problem of improving the accuracy and resolution of mass analyzers as well as the accuracy of the sample positioning robotics. The first part of this work reports on the progress and outlines the strategies of application of a recently developed high resolution spectral analysis technique, called the Filter Diagonalization Method (FDM), for the investigation of space-charge related phenomena inside the detection cell of a Fourier Transform Ion Cyclotron Resonance Mass Spectrometer (FT-ICR MS), understanding of which lies at the heart of the quest for further improvements in mass accuracy and resolution. The FDM spectrographic analysis revealed previously unobserved rapid isotope- beat space-charge induced ICR frequency modulations, shown to reach up to +/- 400 ppm even on high quality spectra. The application of this methodology to the investigation of a frequently observed but previously unexplained phenomenon in FT-ICR MS, the Spontaneous Loss of Coherence Catastrophe, conclusively demonstrated that it is tied directly to the space charge effect and magnetron expansion. The second part of this work reports on the development of the ionization source and vacuum compatible high precision sample positioning robotics for biological MSI applications, purpose built for FT-ICR MS. The complete design and implementation is reported herein, along with the demonstration of its performance and biological application. The XY-positioning stage capable of operating under 10 -8 mbar vacuum with submicron positioning accuracy along the entire ranges of motion of 100x100 mm was designed, built, and installed into the ionization sources of three MALDI FT-ICR MS instruments. Two dimensional chemo-spatial maps of rat brain tissue selections were constructed with 150 micron spatial resolution, identifying multiple ionic species with their distinct and discreet spatial localizations. These demonstrated performance characteristics greatly surpass current state-of-the-art robotics available for MALDI MSI and shift the effort of further improvements in spatial resolution to the ionization methodologies, and other ion source design issues, such as laser optics.