Use of noble metal substrates

Preparation of monolayers of organic materials on noble metal substrates leads to high secondary-ion yields and can even allow the detection of organic materials at levels that would otherwise remain below the detection limits. Three properties can be identified that make noble metals such exceptional SSIMS and SNMS substrates (I) enhanced desorption efficiency, (2) cation-ization and (3). stability and nonreactivity. 

Enhanced desorption efficiency. The sputter yields of metals are distinctly higher than those of organic materials. If the collision cascade takes place principally in the metal substrate (i.e., after monolayer preparation), the sputter yields of the organic species are up to factors of 3 higher than those 

Canonization. Noble metals provide a very efficient ionization mechanism in SSIMS resulting in the formation of (M + NMe)+ ions (NMe = noble metal). These quasimolecular ions are formed almost independently of chemical structure, the noble metal cation attachment being therefore an universal ionization process. (Note that cagelike structures may not accommodate the size of the metal cation, e.g., polyethyleneglycol.) If silver is used, the isotopic pattern of Ag is an additional help in identifying the (M + Ag)+ isotopic peak patterns. In other cases the differentiation between (M + H)+, (M + Na)+, (M + K)+, and (M + NMe)+ requires careful evaluation, in particular if only one peak group is observed. 

For the detection of contaminants a noble metal preparation can easily be obtained by either rubbing suspect surfaces over the sub.strate or subjecting the substrate to the suspect environment for some time. For more details see 141]. 

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