Instrumental development of DESI imaging MS and its applications

Abstract

Desorption electrospray ionization (DESI) is one of the first ambient ionization techniques. Since its introduction in 2004, it has been widely utilized in research, clinical and industrial settings. It is a low cost, simple and non-destructive ionization technique that allows one to operate it with minimal sample preparation, and without a bulky vacuum source interface. DESI has been adopted by a diverse range of end users, but its design and setup remain virtually unchanged since its original conception. Most laboratories adjust only the geometrical parameters according to the needs of the experiment. We believe that there is room for improvement in redesigning and modifying the current DESI setup, and thus further expanding its applicability. In this vein, the coupling of DESI imaging with ion mobility is a promising application that can resolve isobaric metabolites by their difference in mobility. This is particularly important in a clinical setting for diagnostic purposes, as the identity of biomarkers need to be accurately assigned. The geometrical design of DESI can be improved by incorporating a flow focusing mechanism in an existing DESI sprayer for better control of droplets - this setup is named desorption electro-flow focusing ionization (DEFFI). Existing setup parameters for DEFFI showed promising results in its capacity for imaging non-biological samples and parameters were optimized for imaging biological samples, namely: voltage, solvent flow rate, nitrogen gas flow, orifice diameter, grounding of orifice, solvent composition, shape and inner diameter of solvent capillary, and distance between solvent capillary and orifice. In this thesis, the optimal parameters for DEFFI were shown to yield an order of magnitude improvement in sensitivity, as well as enhancing spatial resolution. DEFFI’s sensitivity was further improved by coupling it with a triple quadrupole mass spectrometer, which enabled the detection of a specific drug-dosed metabolite that was otherwise not detectable with a quadrupole time of flight mass spectrometer (QToF). Lastly, a cold stage interface was made and used for the detection of rapidly degradable and reactive metabolites, and potentially better RNA extraction protocol for post-imaging laser micro dissection. Overall, the work presented here has the potential to allow DESI users to modify their existing standard setups with multiple degrees of freedom, according to their specific requirements.