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Mass spectrometric studies of molecules using intense femtosecond laser ionisation
[Thesis]. Manchester, UK: The University of Manchester; 2012.
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Abstract
Time-of-ight secondary ion mass spectrometry (ToF-SIMS) is a sensitive techniqueused 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 detectedby the mass spectrometer. To increase the sensitivity and efficiency of the SIMStechnique, post-ionisation above the surface can be used. Lasers have been widelyemployed for molecular mass spectrometry due to the available high intensity, shortpulse width, high spectral purity and spatial coherence that allow them to be highlyfocused. For molecular samples the challenge is to achieve efficient post-ionisationwithout inducing extensive fragmentation, which limits the diagnostic value of theresulting mass spectrum. An investigation was performed into the ionisation anddissociation characteristics of a series of organic molecules under the action of intenselaser fields. This study is directed towards the analysis of biomolecules usinglaser post-ionisation. Here is reported progress towards the calibration of the experimentalset-up and mass spectral data from representative biomolecules in the gasphase. In this work a Ti:Sapphire laser was used with fundamental wavelength of800 nm and non-linear optical methods (OPAs) are used to extend the wavelengthinto the mid-IR region. System calibration is achieved using the ionisation of xenonatoms and comparing the results to established atomic tunnelling theory. This wasfollowed by the analysis of representative organic and biological molecules to studyionisation-dissociation characteristics. The molecules chosen were toluene, acetone,nitroaniline and histamine. A clear transition in behavior is observed favoringmolecular ion production. This behavior is discussed in the context of the underlyingmechanisms, and the implications for molecular post-ionisation analysis usingfocused ion beams.