High energy gas cluster ions for organic and biological analysis by time-of-flight secondary ion mass spectrometry

There is considerable excitement surrounding the application of gas cluster ion beams (GCIBs) for SIMS analysis in order to study organic materials and biological samples such as cells and tissues. These ion beams, that often comprise several thousand argon atoms in the primary ion, have been used mainly for the etching of organic materials to remove damage from the surface allowing molecular depth profiling experiments to be performed. The energy of the ion beam is normally 2-20 keV. There have been relatively few studies reported on the use of GCIB as analysis beams, due to difficulties related to fast pulsing and focusing of the beam along with the sometimes low ionisation efficiency. In this study, we report on the use of a new higher energy (40 key) GCIB operated in a continuous mode. When compared to lower energies depth profiles on thin films of Irganox 1010 show an increase in sputter yield while fragmentation, damage accumulation and ionisation efficiency remains unchanged. Experiments on brain tissues show increased signal levels especially for higher mass secondary ions (m/z 500+) in comparison to C-60(+) at 40 keV and Ar-4000(+) at 20 key impact energy. The use of higher energies facilitates better focusing of the primary ion beam as demonstrated here on a human hair sample where we achieve a spatial resolution of <3 mu m. Even with this small spot size, we can detect enough signal from and high mass species for clear localisation. All results indicate that higher energies are beneficial for most aspects of ToF-SIMS applications in biology. (C) 2014 Elsevier B.V. All rights reserved.