How are negative ions in an ICPMS formed?

Abstract

Interest in determination of halogens containing compounds, especially fluorine, increased exponentially over the last decade. Nevertheless, the development of instruments and methodologies for direct determination of fluorine has not yet reached a state where this is possible in routine laboratories. We revisited negative ion ICPMS using a modern commercial ICPMS with few modifications on the detector and the ion optics to test whether fluorine detection with reasonable sensitivity would be possible with such an instrument. The aim of the study was to identify the processes behind the production of negative ions in a commercially available ICPMS. Using all halogens as a diagnostic tool, many parameters such as water content, forward power, gas flows, and ion optics parameters were studied. Negatively charged bromine, chlorine and fluorine ions are generated in the interface, not the plasma and their sensitivities are mainly depending on the atomic radius and not on electron affinity. This knowledge is important for building a sensitive fluorine specific instrument which is capable to tackle the needs in environmental and medical science.