Organic molecule characterization?G-SIMS

Abstract

Static SIMS (SSIMS) is a powerful technique for the analysis of complex surfaces. However, many have viewed SSIMS as a dubious and unreliable analytical method but this is now changing. Modern instruments have superb repeatability and reliability. In the VAMAS 2002 SSIMS interlaboratory study, the average repeatability of 27 instruments was 2%. Accessibility to SSIMS measurements is increasing rapidly. However, the complexity of mass spectra makes identification and quantification far from straightforward, even for the experts! This is a significant barrier to the wider take-up of SSIMS, especially in new fields. Gentle SIMS (G-SIMS) is a library independent method providing a straightforward way to interpret SSIMS data. SSIMS spectra are composed of parent fragment ions amongst a large number of high intensity degradation products. In G-SIMS, this fragmentation may be quantified in terms of the partition functions of the fragments emitted from a surface plasma with effective temperature, Tp. By extrapolation of the data to low Tp, the intensity of the degradation products rapidly reduces revealing the parent fragments. The latter peaks are directly characteristic of the material without rearrangement and can enable direct interpretation and identification. This is fine for smaller molecules but, within the plethora of possible larger molecules for which a measured total mass is insufficient to provide adequate characterization, an extension of G-SIMS has exciting prospects to elucidate the required structure. Here we develop the G-SIMS method to re-assemble parent molecules using the fragmentation pathways that are mapped out as Tp is varied, exampled by the folic acid molecule. This extension of G-SIMS is therefore called G-SIMS-FPM standing for fragmentation pathway mapping.