Characterisation of a micro-plasma for ambient mass spectrometry imaging

Abstract

Results are presented on the characterisation and optimisation of a non-thermal atmospheric pressure micro-plasma ion source used for ambient mass spectrometry imaging. The geometry of the experiment is optimised to produce the most intense and stable ion signals. Signal stabilities (relative standard deviation) of 2.3–6.5% are achieved for total ion current measurements from chromatograms. Parameters are utilised to achieve MS imaging by raster scanning of PTFE/glass samples with a spatial resolution of 147 ± 31 μm. A systematic study of resolution as a function of acquisition parameters was also undertaken to underpin future technique development. Mass spectra are obtained from PTFE/glass sample edges in negative ion mode and used to construct images to calculate the spatial resolution. Images are constructed using the intensity variation of the dominant ion observed in the PTFE spectrum. Mass spectra originating from the polymer are dominated by three series of ions in a m/z spectral window from 200–500 Da. These ions are each separated by 50 Da and have the chemical formula [C₂F + [CF₂]_n]⁻, [CF + [CF₂]_n + O]⁻ and [CF + [CF₂]_n + O₃]⁻. The mechanism for the generation of these ions appears to be a polymer chain scission followed by ionisation by atmospheric ion adduction. Positive and negative ion mode mass spectra of personal care products, amino acids and pharmaceuticals, dominated by the proton abstracted/protonated molecular ion, highlight the potential areas of application for such a device. Further to this end a mass spectral image of cardamom seeds, constructed using the variation in intensity of possible fragments of the 1,8-cineole molecule, is included to reveal the potential application to the imaging of foods and other biological materials.