Which is more important in bioimaging SIMS experiments—The sample preparation or the nature of the projectile?

Sample preparation is central to acquiring meaningful molecule-specific images with SIMS, especially when submicron lateral resolution is involved. The issue is to maintain the distribution of target molecules while attempting to introduce biological cells or tissue into the high vacuum environment of the mass spectrometer. Here we compare freeze-drying, freeze-etching, freeze-fracture and trehalose vitrification as possible strategies for these experiments. The results show that the prospects for successful imaging experiments are greatly improved with all of these methods when using cluster ion bombardment, particularly C60+ ions, not only due to increased sensitivity of this projectiles, but also since it removes contamination overlayers without insult to the underlying chemistry. The emergence of 3-dimensional imaging capabilities also suggests that sample preparation should not perturb the 3-dimensional morphology of the cell, a situation not generally possible during freeze-drying. Hence, sample preparation and projectile type are strongly coupled parameters for bioimaging with mass spectrometry.