Director: Dr Pat Langridge Smith

Surface Studies using Laser Mass Microscopy: An instrument has recently been constructed for carrying out spatially resolved laser desorption mass spectrometry, using small spot-size (less than 40 um) laser desorption. The instrument, a 'laser mass microscope', consists of a sample exchange chamber, ion source and a time-of-flight mass spectrometer. The technique we employ, two-step laser mass spectrometry (L2MS), involves the use of a pulsed infrared laser for desorption of intact neutral molecules which are post-ionised using a second pulsed ultraviolet laser, then mass analysed [1]. This approach shows considerable promise for chemical analysis of the molecular composition of materials and material surfaces in such fields as geochemistry, polymer science, material science and environmental science, amongst others. For example, this technique has been used for the detection of polycyclic aromatic hydrocarbons (PAH's) in Martian meteorites [2]. Organic molecules on a surface constitute an analytical challenge. There is an increasing need to detect and characterise small quantities of organic contaminants in many application areas. Our interest is in using this instrument to perform novel microanalytical tasks such as two-dimensional mapping of molecular adsorbates on a wide range of conventionally problematic substrates.

Microseparation Science: Capillary electrochromatography (CEC) is a new microseparation technique that is rapidly attracting widespread interest, particularly in the pharmaceutical industry. In CEC analytes are transported through a packed capillary column (typically 50 um diameter) by electroosmotic flow, rather than by a mechanically pumped flow. We have recently been awarded funding by EPSRC to develop a novel laser desorption ion-trap time-of-flight mass spectrometer for coupling with microbore separation techniques, such as CEC, to provide on-line ultra-trace detection of chemical carcinogens. Such an approach promises unrivaled separation efficiencies coupled with mass spectrometric detection and structural characterisation of separated components at the femtomole level.

Cluster Science: Over the past decade cluster science has been one of the most rapidly advancing areas of chemical physics. Indeed last year’s Nobel Prize in Chemistry was awarded to the scientists who discovered C₆₀, buckminsterfullerene. One experimental difficulty in this field is that virtually all cluster sources used produce a distribution of sizes, making it difficult to carry out experiments on a particular size of cluster. We have developed a new instrument for tandem time-of-flight mass spectrometry. Here, ions, generated either directly in a cluster source or produced by photoionisation of neutral clusters or molecules, are dispersed using one time-of-flight analyser, thereby providing mass-selection. These mass-selected ions can then be studied, via processes such as photodissociation and photodetachment spectroscopy, using the second time-of-flight analyser to examine the decay channels. We also plan to use this instrument to record photoelectron spectra for mass-selected metal clusters and cluster adducts in order to investigate their size-dependent surface chemistry. Other recent work has focused on the laser chemistry of transition metal complexes, where we have used single-step laser mass spectrometry to investigate the aggregation behaviour of preformed organometallic cluster complexes [3].

Laser Photophysics: The use of laser desorption to generate supersonic beams of involatile, thermally labile compounds has also enabled us to undertake detailed studies of the photophysics and photochemistry of molecules such as buckminsterfullerene and porphyrins. We plan to extend our work to a wider range of non-aromatic molecules and other primary pollutants. This will involve the application of tunable laser radiation for photoionisation, tuned to the chromophore of interest, as well as the use of frequency tripling techniques in rare gases to generate vacuum ultraviolet radiation. We are also investigating the use of sub-picosecond (femtosecond) laser radiation for enhanced photoionisation efficiency of large molecules.

Relevant Publications

1. M.J. Dale et al., "Investigation of Porphyrins and Metalloporphyrins Using Two-Step Laser Mass Spectrometry", Anal. Chem., 31, 590 (1996)
2. D. S. McKay et al., "Search for Past Life on Mars: Possible Relic Biogenic Activity in Martian Meteorite ALH84001", Science, 273, 924 (1996)
3. M.J. Dale et al., "Laser Desorption Mass Spectrometry of [Ru₆C(CO)₁₇] and its Derivatives: Cluster Aggregation in the Gas Phase", J. Chem. Soc., Dalton Trans., 771 (1996)

Pat Langridge-Smith, 1999