Developments in molecular SIMS depth profiling and 3D imaging of biological systems using polyatomic primary ions.

In principle mass spectral imaging has enormous potential for discovery applications in biology. The chemical specificity of mass spectrometry combined with spatial analysis capabilities of liquid metal cluster beams and the high yields of polyatomic ion beams should present unprecedented ability to spatially locate molecular chemistry in the 100 nm range. However, although metal cluster ion beams have greatly increased yields in the m/z range up to 1000, they still have to be operated under the static limit and even in most favorable cases maximum yields for molecular species from 1 µm pixels are frequently below 20 counts. However, some very impressive molecular imaging analysis has been accomplished under these conditions. Nevertheless although molecular ions of lipids have been detected and correlation with biology is obtained, signal levels are such that lateral resolution must be sacrificed to provide a sufficient signal to image. To obtain useful spatial resolution detection below 1 µm is almost impossible. Too few ions are generated! The review shows that the application of polyatomic primary ions with their low damage cross-sections offers hope of a new approach to molecular SIMS imaging by accessing voxels rather than pixels to thereby increase the dynamic signal range in 2D imaging and to extend the analysis to depth profiling and 3D imaging. Recent data on cells and tissue analysis suggest that there is, in consequence, the prospect that a wider chemistry might be accessible within a sub-micron area and as a function of depth. However, these advances are compromised by the pulsed nature of current ToF-SIMS instruments. The duty cycle is very low and results in excessive analysis times, and maximum mass resolution is incompatible with maximum spatial resolution. New instrumental directions are described that enable a dc primary beam to be used that promises to be able to take full advantage of all the capabilities of the polyatomic ion beam. Some new data are presented that suggest that the aspirations for these new instruments will be realized. However, although prospects are good, the review highlights the continuing challenges presented by the low ionization efficiency and the complications that arise from matrix effects.