Volatiles redeposited materials

Unfortunately, since ion milling is a purely physical process, selectivity is generally poor. Indeed, selectivity in such systems depends almost exclusively on differences in sputter yield between materials. Finally, since the etch products are not volatile, redeposition and trenching can be serious limitations (146). 

Transport of organic compounds from the solid or aquatic phases to the gas phase (and back again) is now known to be a highly important process for the dispersion of chemical compounds around the globe. Dissolution into and volatilization from the aqueous phase is an elaborate process that depends on solubility, vapor pressure, turbulence within the two phases, and other physical and chemical factors. Volatilization of materials from the earth s surface into the troposphere can result in their long-range transport and redeposition, with the outcome being that measurable quantities of such substances can be detected far from their point of release. 

For SIMS analysis, the sample can be a solid material, a thin layer or particles supported on a substrate. It must be able to withstand ultra-vacuum, since volatile components may be redeposited during analysis. Because of the erosion of the sample during ion impact, any contamination layer can be eliminated prior to analysis. The preparation of the sample is thus subject to less severe conditions than for other surface analysis techniques. For the results of composition profiles to be significant the surface must, however, be flat at the scale of the ion probe. The use of a liquid metal ion gun will enable concentration profiles to be determined on grains of around 0.1 pm in diameter. 

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