A new instrument has been developed to perform spatially resolved molecular analysis using laser mass spectrometry. The instrument, a "laser mass microscope", incorporates a differentially pumped load-lock for rapid sample introduction, a precision computer controlled XYZ stage for accurate sample positioning, and a high resolution reflecting geometry single field time-of-flight mass analyser. The technique of two-step laser desorption laser photoionisation mass spectrometry (L2MS) is used in order to enable separate optimisation of sample desorption and ionisation. The spatial resolution of the instrument, primarily determined by the diffraction limited spot size at the desorption wavelength employed, ranges from 10 μm (532 nm) to 40 μm (10.6 μm). The mass resolution of the instrument is typically <I>M</I> ≥ 1400 for coronene (m/z = 300), with a detection limit at 266 nm of 0.7x10<SUP>-18</SUP> mole per shot. The range of molecules amenable to L2MS has been extended to include non-aromatic amino acids and aliphatic polymers, by employing femtosecond laser photoionisation as an alternative to single photon ionisation. A detailed description of the instrument and the L2MS technique is given together with the underlying background theory. The instrument has been characterised in terms of achievable spatial resolution, mass resolution, and detection sensitivity using a variety of desorption and photoionisation lasers as well as different photoionisation schemes. Desorption effects, such as any photochemical or photophysical consequences of employing UV desorption laser wavelengths as well as effects due to different methods of sample presentation have been investigated. An extensive set of experiments has been carried out (initially to explore the feasibility of employing femtosecond laser photoionisation) to investigate the possibility of achieving enhanced detection sensitivity for large molecules due to more efficient laser photoionisation. For such systems, fast intramolecular decay channels frequently inhibit the attainment of high ionisation efficiency using laser photoionisation on a nanosecond timescale. These experiments were carried out at the Lasers for Science Facility at the Rutherford Appleton Laboratory, Didcot. Several large molecules of analytical interest, such as porphyrins, biological molecules such as hemin and chlorophyll <I>a</I>, as well as amino acids and synthetic polymers have been efficiently ionised in this way using near-infrared radiation (λ = 750 nm). The mechanism of femtosecond ionisation for an aromatic amino acid tryptophan and for nickel octaethylporphyrin have been investigated.
|University of Edinburgh
|Electronic Thesis or Dissertation
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