Abstract/Summary

Over the last three decades, electrospray ionisation (ESI) and matrix-assisted laser desorption/ionisation (MALDI) have been the most prevalent types of ion source fitted to commercial instruments for biological mass spectrometry (MS) analysis. In particular, the qualitative and quantitative analysis of peptides and proteins has been revolutionised by these techniques. Also, the routine analysis of many other molecule types including DNA, sugars, carbohydrates, synthetic polymers and even virus capsids has been facilitated by ESI and MALDI. Despite the commercial success of these ionisation methods, there are relative advantages and limitations inherent to both techniques. For example, conventional solid-state crystalline MALDI generates mainly singly charged analyte ions. As the ion of interest increases in mass, the mass spectrometer analysis becomes compromised. In contrast, liquid atmospheric pressure (AP) MALDI source (the focus of this work) generates multiply charged ions and is more suited for coupling with high-performance MS systems. ESI also produces a spatially diffuse beam of ions from a flowing liquid inlet whereas the source of ions from liquid APMALDI can be precisely controlled by the laser, both in time and space. Ultimately, flexibility of control will enable ultrahigh-throughput sample rates. Liquid AP-MALDI MS is the focus of this thesis and includes details of the design and optimisation of the new ion source and coupling interface. Significant advances in the performance of the ionisation source are reported with attention to the optimisation of the pressure regime and temperature of the ion inlet interface. Focusing on multiply protonated peptides, instrumental developments have been applied to increase the sensitivity (ions detected per mole), signal persistence, as well as the speed of analysis using fast laser repetition rates of up to 5 kHz. Charge state enhancement through ion mobility offers signal- to-noise increases of up to two orders. Electron-mediated fragmentation of multiply charged ions from liquid AP-MALDI provides new tandem MS functionality relative to conventional singly charged MALDI ions (that would be neutralised by electrons in ECD). Although the new MALDI technique produces ESI-like spectra, the advantage of a discrete spatial ionisation point and time offers the distinct possibility of ultra-high-throughput sample acquisition from high-density sample arrays at rates theoretically limited to greater than 100 samples/second. It is therefore concluded that the liquid AP-MALDI source provides powerful enhancements over both ESI and conventional MALDI.