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, amongst other areas. 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 such as photosynthesis.
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 distribution are well defined. Reaction is initiated by crossing the molecular beam with a laser or electron beam, the products to be detected are ionised if required by a second laser pulse, and the complete three-dimensional scattering distribution of the resulting ionised photoproducts is measured using a technique called velocity-map ion imaging (VMI). VMI combines time-of-flight mass spectrometry with imaging, separating ions of different mass and mapping their velocity distributions onto a position sensitive detector. You can see an animation of a velocity-map imaging experiment here. Having controlled the initial velocity of the reactants and measured the final velocities of the products, we can 'join the dots' to learn about the forces and energetics that drive the chemical reaction under study. Such experiments yield great insight into the physics underlying chemical reactivity.
Velocity-map imaging has been used for some time to study the photochemistry of small molecules, and we are currently exploring the capabilities of the technique for studying larger chemical systems of interest to the broader chemical community. Larger molecules have many more fragmentation pathways than smaller molecules, requiring the development of multimass imaging techniques. Multimass imaging requires a universal ionization technique (we use either a VUV 118 nm laser source or an electron beam) and an ultrafast imaging detector capable of recording multiple images on the microsecond timescale of a time-of-flight mass spectrum (see Pixel Imaging Mass Spectrometry below).
Examples of the types of photochemical systems we have studied recently include the photolysis of neutral and ionic ethyl bromide and ethyl iodide, which play a role in the marine boundary layer of the earth's atmosphere, and photolysis of N,N-dimethylformamide (DMF), a model for peptide bond fragmentation. We plan to extend the latter measurements to study fragmentation of a number of dipeptides and tripeptides in order to understand the role of different amino acid residues in determining the fragmentation dynamics. A multimass velocity-map imaging data set recorded for DMF using the PImMS ultrafast imaging sensor is shown below.
Our electron-molecule crossed-beam instrument can be used to study a range of energy-dependent electron-impact ionization and fragmentation processes, all of which are interesting from a fundamental point of view. In addition, we are planning a variety of more applied studies, including investigations into the use of imaging data to separate contributions from different analytes in the mass spectrometric analysis of mixtures, for molecular fingerprinting, and to understand the role of low-energy electrons in radiation damage to DNA.
The experiments described above rely on mapping the velocities of the ions at their point of formation onto a position-sensitive detector. By adjusting the potentials applied to the ion optics it is also possible to map the positions of the ions at their point of formation onto the detector, opening up a huge range of potential applications in the chemical imaging of surfaces.
The computer simulations below show that for an initial ion distribution having both a spread of positions and a spread of velocities, the ion optics may be tuned either for spatial mapping (in which the positions of the ions are recorded, regardless of their initial velocities) or for velocity mapping (in which the velocities of the ions are recorded, regardless of their initial positions).
In spatial-map imaging mode, imaging mass spectrometry has 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 mass analysis of spatially-resolved arrays of samples. "Soft" laser-based ionization methods such as MALDI (matrix assisted laser desorption ionization) are prime candidates for our imaging modality. Such techniques yield gas-phase ions from molecular species on a surface without sigificant fragmentation, making spatial-map imaging mass spectrometry a promising avenue for the molecular analysis of biological 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 (DHB) through an electroformed nickel mesh 'stencil'. The wire diameter (dark lines on the images) was 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.
Ultra-fast detectors for imaging mass spectrometry
We are part of the PImMS (Pixel Imaging Mass Spectrometry) consortium, a group of researchers who are working to develop ultra-fast imaging sensors suitable for applications in 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. Many of the experiments described above will eventually employ these new detectors in place of conventional CCD cameras. At present the PImMS consortium is led by Dr Renato Turchetta (Rutherford Appleton Laboratory), Dr Claire Vallance, and Prof. Mark Brouard (Oxford Chemistry), and Dr. Richard Nickerson (Oxford Physics), and we also collaborate with Prof. Andrei Nomerotski (Brookhaven). Further details on the PImMS detectors are available here.
Cavity-enhanced spectroscopies for microfluidics
Cavity ringdown spectroscopy is the most sensitive absorption spectroscopy technique currently available, with detection sensitivities down to the ppt level for gas-phase samples. At the heart of the technique is an optical cavity formed from a pair of highly reflecting mirrors. A pulse of laser light is injected into the cavity through one of the mirrors and bounces back and forth within the cavity, slowly decaying exponentially in intensity due to losses associated with the mirrors and absorption by a sample present in the cavity. The achievable sensitivies are a result of the vastly increased optical path length relative to a single-pass measurement, with effective path lengths of several kilometres achieveable in a benchtop experimental setup. Analysing the exponential decay constant, or 'ringdown time' in the presence and absence of a sample yields the absorption coefficient or concentration of the sample.
We are focusing on liquid-phase applications of cavity ringdown spectroscopy, and its sister technique, cavity-enhanced absorption spectroscopy. The high detection sensitivity makes these techniques ideal for probing small liquid volumes contained in a flow cell or microfluidic chip within the optical cavity. An area of particular interest at present is the development of methods for tracking a variety of marine nutrients. A variety of nutrients (nitrites, carbonates, trace metals etc) are needed by photosynthetic phytoplankton to produce the organic molecules making up their cellular machinery. The carbon source for these organic synthesis reactions is from dissolved CO2, and it is estimated that these processes consume several tens of billion tonnes of CO2 from the atmosphere each year (some of which is offset by respiration processes). Understanding the relationship between dissolved nutrients and micronutrients and phytoplankton growth and metabolism will play an important role in models of global warming.
To develop a detection scheme for a particular nutrient, we use a colour reaction to convert the analyte selectively into a strongly absorbing species, and then employ cavity-enhanced spectroscopy for sensitive detection. While we are currently at the stage of laboratory-based studies, the end goal is to develop remote sensors suitable for deployment on marine research buoys, which would monitor a number of chemical species present in the oceans at nanomolar to picomolar concentrations. Remote sensors will most likely be based on a microfluidic platform, in which the colourimetric reaction and spectroscopic detection are carried out automatically within a miniaturised 'lab on a chip'.