Abstract/Summary

Matrix-assisted laser desorption/ionisation (MALDI) is a soft ionisation technique suitable for ionising biological molecules for analysis by mass spectrometry (MS). MALDI MS is typically performed on vacuum-compatible solid samples and on instruments with intermediate to high vacuum (10-2-10-7 mBar) ion sources. Less commonly, MALDI can be performed at atmospheric pressure (AP). However, AP-MALDI presents useful advantages. For example, compatibility is expanded to include samples with components usually volatile under vacuum conditions. Liquid samples may be analysed with ease. Recent developments in the fieldusing liquid support matrices (LSMs), which incorporate a viscous liquid into a solution of a MALDI matrix compound, have demonstrated the ability to reliably generate stable ion signal over long periods of time. Interestingly, the combination of AP-MALDI, LSMs and a heated ion inlet on the mass spectrometer enables the predominant generation of multiply charged ions. MALDI MS spectra are usually dominated by signals for singly charged ions, with high charge states generally the domain of electrospray ionisation (ESI). The m/z of highly charged proteins enables AP-MALDI analysis on narrow m/z range mass spectrometers. Additionally, tandem mass spectrometry (MS/MS) experiments also benefit: collision induced dissociation (CID) results in more fragment ion signals from multiply charged precursor ions and electron transfer dissociation (ETD) is possible, since it requires a minimum 2+ charge state to be performed at all. Liquid AP-MALDI has potential for mass profiling and sample screening, due to the reliability of the signal. In this thesis, the ability to rapidly analyse liquid samples using a prototype AP-MALDI ion source was demonstrated. Applications included the identification of doubly charged phospholipid ions by MS/MS; profiling of lipids, peptides and proteins for sample classification; and the assessment of milk samples for -lactamase activity. Barium adduct ions of phosphatidylcholines were successfully analysed by liquid AP-MALDI CID and ETD MS/MS to reveal detailed structural information. Profile analysis of milk extracts was used to classify milk samples. A major section focuses on the rapid and confident detection of bovine mastitis, a disease with a considerable economic impact.