Applications and fundamental characterization of open air and acoustic-driven ionization methods
Hampton, Christina Young
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One of the most fundamental challenges in analytical mass spectrometry (MS) is the efficient conversion of neutral molecules into intact gas-phase ions. In this thesis, I investigate the capabilities of various new and established ionization techniques including (a) the Array of Micromachined UltraSonic Electrosprays (AMUSE), (b) Direct Analysis in Real Time (DART) and (c) Electrospray Ionization (ESI) for bioanalytical and biomedical analysis purposes. The AMUSE is a MicroElectroMechanical System (MEMS)-based device that was created as an alternative, and more sensitive approach for ion generation in an array format. In the AMUSE, the processes of droplet formation and DC droplet charging are separated allowing ionization of liquid samples using low charging voltages and a wide variety of solvents. Our analytical characterization work with the AMUSE showed that ion generation with this device was indeed possible, and that incorporation of a Venturi device increased signal stability and sensitivity due to enhanced droplet desolvation and increased ion transfer efficiency. A detailed investigation to determine the optimal source parameters for ionization of aqueous solutions of model compounds including reserpine, leucine enkephalin and cytochrome C was carried out and it was found that ionization was possible even without the application of a DC charging potential. Subsequent experiments using the thermometer ion method to characterize the AMUSE from a more fundamental point of view, showed that AMUSE ions are lower in internal energy than ESI ions, opening interesting possibilities for the mass spectrometric study of labile species. Furthermore, it was found that it was possible to manipulate the internal energy of the ion population by varying the parameters that most strongly affect desolvation and focusing. Our studies with DART were directed at investigating its analytical potential for application to the identification of active ingredients (AIs) in low quality combination medicines and counterfeit antimalarials that are commonly sold in regions of the world (particularly Southeast Asia) where drug resistant malaria is endemic as their use may engender increased resistance against the few remaining effective antimalarials.