Ion yield improvement for static secondary ion mass spectrometry by use of polyatomic primary ions.

Static secondary ion mass spectrometry (S-SIMS) is one of the potentially most powerful and versatile tools for the analysis of surface components at the monolayer level. Current improvements in detection limit (LOD) and molecular specificity rely on the optimisation of the desorption-ionisation (DI) process. As an alternative to monoatomic projectiles, polyatomic primary ion (P.I.) bombardment increases ion yields non-linearly. Common P.I. sources are Ga+ (liquid metal ion gun (LMIG), SF5+ (electron ionisation) and the newer Au(n)+, Bi(n)q+ (both LMIG) and C60+ (electron ionisation) sources. In this study the ion yield improvement obtained by using the newly developed ion sources is assessed. Two dyes (zwitterionic and/or thermolabile polar functionalities on a largely conjugated backbone) were analysed as a thin layer using Ga+, SF5+, C60+, Bi+, Bi3(2+) and Bi5(2+) projectiles under static conditions. The study aims at evaluating the improvement in LOD, useful and characteristic yield and molecular specificity. The corrected total ion count values for the different P.I. sources are compared for different instruments to obtain a rough estimate of the improvements. Furthermore, tentative ionisation and fragmentation schemes are provided to describe the generation of radical and adduct ions. Characteristic ion yields are discussed for the different P.I. sources. An overview of the general appearances of the mass spectra obtained with the different P.I. sources is given to stress the major improvement provided by polyatomic P.I.s in yielding information at higher m/z values.