Visiting Research Fellow Department of Chemistry and Molecular Biology Gothenburg University 412 96 Göteborg Sweden tel: ++46 (0)317869105 john.fletcher @ chem.gu.se
PhD on ToF-SIMS and Infrared Studies of Sulfuric Acid Hydrates, UMIST MChem in Chemistry, UMIST

My research is focused on the development of methodologies and instrumentation for imaging mass spectrometry. Particularly I have been focused on the development of secondary ion mass spectrometry (SIMS) for 2 and 3D imaging of biological cells and tissue. SIMS uses an energetic ion beam to bombard a sample leading to the ejection (sputtering) of atomic, fragment and molecular species of which a small portion are ionised and can therefore be extracted and analysed by mass spectrometry. The probing ion beams can be focused to sub-micron spot sizes and by scanning the beam over the sample chemical maps can be generated like a chemical microscope.

The introduction cluster and polyatomic ion beams has improved the ability of molecular species to be imaged by SIMS. Polyatomic ion beams particularly C60 have provided the ability to not only image the sample surface but to generate 3D molecular images. The polyatomic ions deposit most of their energy in the uppermost surface layers of the sample and hence continued analysis no longer results in the rapid accumulation of sub-surface chemical damage that results in a loss of molecular signal from the sample.

To fully exploit the properties of these new beams instrumentation has been developed that allows the mass spectrometry to be de-coupled from the sputtering process. The J105 –3D Chemical Imager has been developed in collaboration with Ionoptika Ltd and provides a 10³ increase in duty cycle while maintaining mass resolution with high resolution imaging.

Ionoptika J105 — 3D Chemical Imager
(click for animation)

Current research themes

Biological imaging

A number of tissue and cell imaging projects are currently under way. Sample preparation for analysis under the vacuum conditions required for SIMS is critical to generating useful data and optimisation of this aspect of the experiment is a continuing challenge.

Accurate data interpretation and visualisation is also crucial if biological questions are to be answered, particularly the SIMS data can be quite complex (and computationally taxing). Methods are being developed for 3D data reconstruction/visualisation along with ways of efficiently extracting important information from the large data sets.

Instrumentation

• Characterising the interaction of new ion beams with surfaces, particularly for the application to 3D molecular imaging.
• Investigating new gas cluster ion beams (GCIB) such as Ar_n that may provide ‘softer’ ejection of material from the sample and therefore reduce fragmentation of molecular species and improve depth resolution.

Publications