Abstract: Design, development, and coupling of an electrospray ionization (ESI) source with a Fourier Transform Ion Cyclotron (FTICR) Mass Spectrometer (MS). Developed ESI source was used for detection and characterization of small and large molecules. Dissertation Discovery Company and University of Florida are dedicated to making scholarly works more discoverable and accessible throughout the world. This dissertation, "Internal and External Source Electrospray Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry" by Ragulan Ramanathan, was obtained from University of Florida and is being sold with permission from the author. A digital copy of this work may also be found in the university's institutional repository, IR@UF. The content of this dissertation has not been altered in any way. We have altered the formatting in order to facilitate the ease of printing and reading of the dissertation.