Time-of-flight secondary ion mass spectrometry (ToF-SIMS) is a sensitive technique used to analyse the molecular composition of solid samples using keV ion beam sputtering. However only a small fraction (typically < 10⁻³) of the desorbed material issecondary ions - the majority species (neutrals) cannot be extracted and detected by the mass spectrometer. To increase the sensitivity and efficiency of the SIMS technique, post-ionisation above the surface can be used. Lasers have been widely employed for molecular mass spectrometry due to the available high intensity, short pulse width, high spectral purity and spatial coherence that allow them to be highly focused. For molecular samples the challenge is to achieve efficient post-ionisation without inducing extensive fragmentation, which limits the diagnostic value of the resulting mass spectrum. An investigation was performed into the ionisation and dissociation characteristics of a series of organic molecules under the action of intense laser fields. This study is directed towards the analysis of biomolecules using laser post-ionisation. Here is reported progress towards the calibration of the experimental set-up and mass spectral data from representative biomolecules in the gas phase. In this work a Ti:Sapphire laser was used with fundamental wavelength of 800 nm and non-linear optical methods (OPAs) are used to extend the wavelength into the mid-IR region. System calibration is achieved using the ionisation of xenon atoms and comparing the results to established atomic tunnelling theory. This was followed by the analysis of representative organic and biological molecules to study ionisation-dissociation characteristics. The molecules chosen were toluene, acetone, nitroaniline and histamine. A clear transition in behavior is observed favoring molecular ion production. This behavior is discussed in the context of the underlying mechanisms, and the implications for molecular post-ionisation analysis using focused ion beams.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:566547 |
| Date | January 2012 |
| Creators | Longobardo, Alessia |
| Contributors | Lockyer, Nicholas |
| Publisher | University of Manchester |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | https://www.research.manchester.ac.uk/portal/en/theses/mass-spectrometric-studies-of-molecules-using-intense-femtosecond-laser-ionisation(21d80e3c-65fa-449d-9b13-77c0c7695956).html |
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