Novel ion beams to enhance ionization in molecular secondary ion mass spectrometry

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Secondary ion mass spectrometry (SIMS) is an analytical technique with unique potential to probe the chemistry of complex materials in 2D and 3D on the micron level without the need for chemical modification or tagging. Surface chemistry is desorbed (sputtered) using a focused high energy (keV) 'primary' ion beam and the ionised fraction is subjected to mass spectrometric analysis. Our research over the last several years has focused on developing the application of SIMS to the study of biological cells and tissue, although the outcomes are equally applicable to other complex systems such as organic electronic materials. Sensitivity is the central issue in mass spectrometry particularly in the imaging mode because the desire for increasing levels of spatial resolution means that the sample to be analysed gets smaller and smaller and there can never be enough ion yield. With EPSRC's support we have very successfully introduced and demonstrated the power of new primary ion beams for SIMS, first based on gold clusters and then on C60, that have greatly increased the ion yield of the large molecular species that are chemically significant. Despite these advances spatially resolved analysis below 1 micron is problematical because the current ionisation probability in organic SIMS (and indeed in other desorption mass spectrometries such as MALDI and DESI) is less than 10-5 in most cases. Related to this problem is the operation of the matrix effect. The secondary ion formation mechanism for compound A is influenced by the chemistry of the other molecules surrounding it in the emission zone. The ion yield of A may be enhanced, reduced or even entirely suppressed dependent on the identity of the surrounding molecules. This makes analysis uncertain and quantification very difficult. In the case of organic materials there is a considerable SIMS and MALDI literature that demonstrates the influence of the relative basicity of compound A compared to its molecular neighbours on the formation of the (A+H)+ species. Thus a significant contributor to the matrix effect is probably due to competition for protons in the ion emission zone. Thus in the case of organic analysis where the majority of molecular related ions are formed by proton transfer, greatly increasing the density of proton source molecules in the emission zone is expected to increase the M+H or M-H yields. In a collaboration with a small ion beam manufacturer, Ionoptika Ltd, this project will develop a water ion beam system capable of delivering a high density of either H3O+ or giant water cluster ions (H2O)nH+ (N=50 to 10000), that on impact with the sample surface will generate a high density of proton related species to enhance the secondary ion yield of (A+H)+ by at least a factor of 10, and is expected to have the added benefit of relieving the matrix effect. The project will be a mix of instrument development and water beam characterisation followed by research into its operation. The water beam system will be built refined and interfaced with an ion optical column developed previously for a giant argon cluster beam. The optimum operational conditions for the water beam will be researched and then using model compound systems the fundamentals of the degree and mechanism of proton assisted ion yield enhancement will be researched. This will then be followed by studies of multi-component model materials to investigate the influence of the water beam on the matrix effect and the improvement of quantitative analysis in imaging mass spectrometry. Time permitting we hope to carry out proof of principle studies into the beneficial effects of the water beam in MALDI MS.The successful outcome of this project will very significantly enhance the capabilities and wide uptake of SIMS, enabling molecular sub-micron analysis and imaging, greatly ameliorating the interference of the matrix effect and significantly improving quantitative measurements.