We use state-of-the-art imaging techniques to study photoinduced chemical reactions and electron-molecule collision processes, and to carry out chemical imaging of surfaces. We are also investigating applications of cavity ringdown and other cavity-enhanced spectroscopic techniques for the chemical analysis of small liquid volumes, with applications in microfluidics and marine sensing.
Photoinduced and electron-induced chemical reactions
Chemical reactions initiated by light or by collisions with electrons play an important role in atmospheric chemistry, astrochemistry, synthetic chemistry, and biology. Understanding the mechanisms of these reactions in detail offers new insight into a range of vital physical and chemical processes, ranging from the breaking of a single chemical bond all the way through to complex multistep processes occurring in biological systems.
We study photoinduced and electron-induced chemistry in the gas phase. The molecule of interest is prepared in a molecular beam, so that its initial velocity and internal state distributions are well defined. Reaction is initiated by crossing a laser or electron beam with the molecular beam, the products to be detected are ionised if required by a second laser pulse, and the complete three-dimensional scattering distribution of ionised products is measured using a technique known as velocity-map imaging (VMI). VMI combines time-of-flight mass spectrometry with imaging, separating ions of different masses and mapping their velocity distributions onto a position-sensitive detector. By analysing the measured scattering distributions we obtain detailed information on the forces and energetics driving the chemical process under study.
Molecules we have studied recently include neutral and ionic ethyl bromide and ethyl iodide, both of which play a role in the marine boundary layer of the earth's atmosphere, and N,N-dimethylformamide (DMF), a model for peptide bond fragmentation. A data set showing the time-of-flight mass spectrum and images following photofragmentation of DMF is shown below. We plan to use our approach to improve our understanding of a range of chemical processes, including investigating the role of different amino acid residues in determining peptide fragmentation mechanisms, and the role of low-energy electrons in radiation damage to DNA.
The experiments described above rely on mapping particle velocities onto a position-sensitive detector. It is also possible to map particle positions onto the detector. In spatial-map imaging mode, our techniques have the potential to identify, with high lateral resolution, the many molecular constituents that may be present at a surface. It therefore has potentially exciting applications in surface analysis (e.g. tissue imaging or materials characterisation) or as a high-throughput detection method in the analysis of spatially-resolved arrays of samples. The results of two sets of proof-of-concept measurements are shown below. On the left are spatial-map ion images of the grid pattern formed by sublimation coating of a MALDI matrix compound (DHB) through an electroformed nickel mesh 'stencil'. The wire diameter (dark lines on the images) is 40 microns. On the right are spatial-map images of photoelectrons emitted from the stainless steel repeller plate of a velocity-map imaging instrument. The 1 mm hole through which the molecular beam enters the interaction region is clearly seen in the centre of the image, and we can even image the molecular beam itself when the pulsed valve is turned on.
Ultrafast detectors for imaging mass spectrometry
We are part of the PImMS (Pixel Imaging Mass Spectrometry) consortium, a group of researchers working to develop ultrafast imaging sensors suitable for applications in time-of-flight mass spectrometry. The sensors allow velocity-map or spatial-map images to be acquired for each mass peak in a time-of-flight mass spectrum, opening up a range of new applications in mass spectrometry, state-of-the-art chemical dynamics studies, neutron detection, and other fields of science. Many of the experiments described above will eventually employ these new detectors in place of conventional CCD cameras. More information on the PImMS detectors is available here.
Cavity-enhanced spectroscopies for microfluidics
Cavity ringdown spectroscopy (CRDS) is one of the most sensitive absorption spectroscopy techniques currently available, routinely achieving detection limits down to the ppt level for gas-phase samples. The sensitivity arises from the extremely long absorption path lengths that may be achieved within an optical cavity, with effective pathlengths of several kilometres achieveable in a benchtop instrument. Analysing the data from a cavity-enhanced absorption measurement provides an absolute measurement of the absorption, which may be related directly to the concentration in analytical applications.
We are focusing on liquid-phase applications of CRDS and related techniques. The high detection sensitivity makes these methods ideal for probing small liquid volumes contained within a flow cell or microfluidic chip within the optical cavity. An area of particular interest is the development of methods for tracking a variety of marine nutrients, trace compounds present in sea water that are vital to the metabolism of photosynthetic phytoplankton. Phytoplankton synthesise organic compounds using dissolved CO2 as their carbon source, and remove several billion tonnes of CO2 from the atmosphere in the process. Understanding the relationship between dissolved nutrients and phytoplankton growth will therefore play an important role in models of global warming.
Our approach is based on a colourimetric assay to convert the analyte of interest selectively into a strongly absorbing derivative species, followed by a cavity-enhanced absorption measurement for sensitive detection of the coloured compound. We are currently carrying out laboratory studies to optimise our detection methods for a range of species of interest, but the eventual aim is to develop remote sensors suitable for deployment on marine research buoys. Such sensors will most likely be based on a microfluidic platform, and will be capable of monitoring a number of chemical species present in the oceans at nanomolar to picomolar concentrations.