Cluster secondary ion mass spectrometrys

In principle, secondary ion mass spectrometry (SIMS) molecule-specific imaging has vast implications in biological research where submicrometer spatial resolution, uppermost surface layer sensitivity, and chemically unmodified sample preparation are essential. Yet SIMS imaging using atomic projectiles has been rather ineffective when applied to biological materials. The common pitfalls experienced during these analyses include low secondary ion yields, extensive fragmentation, restricted mass ranges, and the accumulation of significant physical and chemical damage after sample erosion beyond 1 % of the surface molecules. Collectively, these limitations considerably reduce the amount of material available for detection and result in inadequate sensitivity for most applications. In response, polyatomic (cluster) ions have been introduced as an alternate imaging projectile. Cluster ion bombardment has been observed to enhance secondary ion yields, extend the spectral mass range, and decrease the incidence of physical and chemical damage during sample erosion. The projectiles are expected to considerably increase the number of molecules available for analysis and to significantly improve the overall sensitivity. Hence, the objectives of this chapter are to describe the unique physical basis for the improvements observed during polyatomic bombardment and to identify the emerging biological applications made practical by the introduction of cluster projectiles to SIMS.