John S. Fletcher

2012-09-07 23.22.40
Contact
john.fletcher @ chem.gu.se
(remove spaces to use)

Bio
PhD on ToF-SIMS and Infrared Studies of Sulfuric Acid Hydrates, UMIST, 2004

MChem in Chemistry, UMIST, 2000

Research
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.

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 Ar2000 that may provide ‘softer’ ejection of material from the sample and therefore reduce fragmentation of molecular species and improve depth resolution.

Latest Publications

Peptide structural analysis using continuous Ar cluster and C60 ion beams
Satoka Aoyagi, John S. Fletcher, Sadia Sheraz (Rabbani), Tomoko Kawashima, Irma Berrueta Razo, Alex Henderson, Nicholas P. Lockyer, John C. Vickerman
Analytical and Bioanalytical Chemistry (2013)

Enhancing Secondary Ion Yields in Time of Flight-Secondary Ion Mass Spectrometry Using Water Cluster Primary Beams
Sadia Sheraz née Rabbani, Andrew Barber, John S. Fletcher, Nicholas P. Lockyer, and John C. Vickerman
Analytical Chemistry 85 (2013)

Evaluating the challenges associated with time-of-flight secondary ion mass spectrometry for metabolomics using pure and mixed metabolites
John S. Fletcher, Helen L. Kotze, Emily G. Armitage, Nicholas P. Lockyer, John C. Vickerman
Metabolomics 9 (2013)

Secondary Ion Mass Spectrometry: Characterising complex samples in two and three dimensions.
John Stephen Fletcher and John C. Vickerman
Analytical Chemistry 85 (2013)

The inherent problem of transflection-mode infrared spectroscopic microscopy and the ramifications for biomedical single point and imaging applications
Paul Bassan, Joe Lee, Ashwin Sachdeva, Juliana Pissardini, Konrad M. Dorling, John S. Fletcher, Alex Henderson and Peter Gardner
Analyst 138 (2013)