Uncovering new challenges in bio-analysis with ToF-SIMS

The introduction of cluster ion beams for routine SIMS analysis has greatly improved the prospects for characterising biological samples. The ultimate quality of the secondary ion image remains limited by the efficiency of the primary beam. Without overcoming the low ionisation probabilities associated with SIMS, the highest lateral resolution available for molecular imaging becomes limited by the statistical probability of any ions being generated from the area of the pixel. C60+ primary ions are currently the most efficient available for routine analysis but although commercial systems have been demonstrated to produce spot sizes under 200 nm, focusing the beam produced in such electron impact sources results in rather low ion currents. The time scale for such high lateral resolution analysis can become impractical on conventional time-of-flight instruments. Molecular depth profiling capability has been revealed using SF5+ and C60+ ion beams and recent work has advanced the technique by combining the profiling and imaging abilities of these high efficiency projectiles to generate 3D molecular maps of biological systems. In this paper we discuss the challenges associated with 2D and 3D bio-analysis with emphasis on how instrumental advances aid such investigations yet demonstrating the obstacles that need to be overcome using a range of model and real world biological samples. We discuss complications with the biological matrix, challenges in manipulating and visualising the data and look at how instrumental advantages might aid the routine generation of these 3D molecular maps.