Paper AS-WeM12
An Evolution of TOF-SIMS for Biological Analysis: From 2D Imaging to 3D FIB-TOF Tomography

Wednesday, October 31, 2012, 11:40 am, Room 20

TOF-SIMS has become an important tool for 2D and 3D imaging mass spectrometry of biological and complex material specimens due to it’s unique capability to detect molecular and elemental ions at a spatial resolution of ≤ 300 nm, a mass resolution of ~ 15,000 m/Δm, and without the sample treatments or labeling required by e.g. MALDI or fluorescence microscopy. Among the advantages of TOF-SIMS are ~ 2 nm sampling depth, parallel detection and collection of the entire mass spectrum at every image pixel, and sensitivities in the ppm to ppb range. The ability to image surfaces having a large degree of topography while maintaining artifact-free chemical imaging is also highly desired; the resulting elemental and molecular images provide important information regarding the composition of biointerfaces, tissues and cells, and of materials such as oxide fuel cells and OLEDs.

Characterization of specimens to a depth of several microns below the sample surface has become somewhat routine with the use of a sputter ion beam to remove multiple layers of atoms and molecules between analysis (chemical imaging) cycles. Nevertheless, there are practical limitations to the use of ion beam sputtering for probing both organic and inorganic specimens beyond the surface region. Among the difficulties and limitations is the fact that the various matrix components sputter at different rates, called preferential or differential sputtering, which results in a distortion or complete loss of the true 3D chemical distribution as a function of depth. Many specimens also contain void spaces that are impossible to preserve in 3D images obtained by sputter depth profiling.

An alternative approach to achieve 3D chemical imaging of chemically complex specimens is to utilize in situ FIB milling and sectioning in conjunction with TOF-SIMS chemical imaging… what we have called FIB-TOF tomography. With FIB milling, the interior of a specimen is revealed to depths of more than 100 μm. 3D chemical imaging with a z-dimension of greater than 10 μm in tomographic increments of ≤ 0.5 μm may be achieved within a reasonable analysis time. The advantage of the FIB-TOF approach is that artifacts caused by sputter depth profiling such as differential sputtering and accumulated ion beam damage are avoided. 3D imaging by FIB-TOF tomography will be illustrated first with organic / inorganic composite materials. Applications in biological and clinical cancer research will also be presented with an emphasis on the conditions required to achieve FIB-TOF tomography.