Paper AS-ThA9
Reducing Matrix Effects in Organic Secondary Ion Mass Spectrometry

Thursday, November 10, 2016, 5:00 pm, Room 101B

Secondary ion mass spectrometry (SIMS) is a powerful tool for surface analysis. With the development of cluster ion beams, this technique now has the capability to analyze organic materials while maintaining molecular information of the analyte. A challenging aspect in SIMS applications in multi-components systems are so called matrix effects in the ionization probability of sputtered material. Unfortunately, there is no theory to date which is capable of predicting these matrix effects. Results are presented here to show one physical and one chemical approach to address and reduce matrix effects in an organic model systems of Irganox 1010 and 1098.

The first, and perhaps most straightforward approach in overcoming matrix effects is to decouple the ionization from the sputtering process. For this purpose, the plume of sputtered material above the sample, mainly consisting of neutral species, is intersected with an intense ultrafast laser pulse in the near infrared region with power densities up to several 10¹⁵ W/cm². This pulse ionizes neutral particles within the plume and makes their detection feasible

The second approach presented is based upon the idea of enhancing protonation at the bombardment sites. HCl doped Ar_n⁺ gas cluster ions, in which the HCl molecules are incorporated into the gas clusters, are used as the primary ion beam. During analysis, H₂O is precisely leaked into the analysis chamber while the sample is cooled with liquid nitrogen. In this way, a thin ice overlayer is formed at the sample surface to facilitate the dissociation of the HCl molecules incorporated in the Ar_n gas clusters. The free hydronium ions become available at the impact site, which aid the protonation of intact molecules. During the entire analysis, a dynamic equilibrium between deposition and sputtering of ice is well maintained to yield a quantitative depth profile.