The Bristol & Oxford Chemical Dynamics Group

Multiphoton and 'universal' electron impact and/or vacuum ultraviolet ionization methods for detection of all reaction products or photofragments, and secondary ion or desorption electrospray ionization for spatial imaging.

Many of the scientific ambitions within the Programme Grant will require new or improved, efficient, sensitive and clean (i.e. minimal background) ionization schemes to generate ions for subsequent imaging detection. Four strategies are envisaged, all of which are within the scope of existing laser or electron beam equipment in our laboratories.

New and existing REMPI schemes:

Many of the reaction and photodissociation products featured in the SWPs are readily detected using established, sensitive REMPI schemes (obvious examples include halogen atoms, OH and CH3 radicals, HCl and NO molecules, etc). No such schemes have been reported fort other important targets (e.g. HCN), however, and we propose investigating both one and two colour REMPI schemes to rectify this deficiency.

One-photon VUV ionization

The ionization potentials of many of the proposed photodissociation and reaction products are sufficiently low that these radicals are amenable to 1-photon ionization using VUV radiation. Two convenient VUV sources (118 nm, the ninth harmonic of an Nd:YAG laser) and 157 nm (from an F2 excimer laser) will be investigated as 'soft' ionization sources, enabling simultaneous ionization and detection (when combined with a PImMS camera) of several reaction products arising in a single laser shot - as illustrated by the accompanying preliminary data.

Electron impact ionization

Truly 'universal' ionization of any reaction or photofragmentation product can be achieved with electron impact ionization and, as with VUV ionization methods, it is desirable to control the energy of the electron beam to minimize unwanted ion fragmentation pathways ('soft' ionization). This third strand will seek to incorporate electron impact ionization within a velocity-map imaging instrument, with the aim of providing a wealth of new and quantitative data relating to electron-molecule collision processes and to ionizing (and thus imaging) 'REMPI-challenging' molecular fragments like CH2, CN, HCN, etc.

SIMS and DESI for spatial imaging

Secondary ion mass spectrometry (SIMS) and desorption electrospray ionisation mass spectrometry (DESI-MS) methods will both be investigated extensively as routes to boosting the rate of sample throughput in spatial map imaging (SMI)-MS. SIMS employs a beam of primary ions to ionise and ablate an analyte, and is considered an excellent candidate for use in microscope mode imaging techniques in combination with the PImMS camera. The technique should particularly suit pharmacological applications where spatial distributions of low mass lipids and metabolites are of interest. DESI does not require sample preparation either, and has the added advantage of being an ambient technique, meaning that biological tissue imaging should be possible directly after sectioning.

Total translational energy (ET) distributions derived from images of HCO (m/z 29) and N(CH3)2 (m/z 44) fragments following 193 nm photolysis of N,N-dimethylformamide. The radicals were ionized by absorbing, respectively 118 nm and 193 nm photons. The vertical dashed lines indicate the maximum ET values allowed for three possible fragmentation pathways following N-CO 'peptide' bond fission.

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