How are negative ions in an ICPMS formed?†

Interest in the determination of halogen containing compounds, especially fluorine, has 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 to the detector and 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 diagnostic tools, 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 mainly depend on the atomic radius (as a proxy for collision cross-section) and not on electron affinity. This knowledge is important for potentially building an instrument capable, among other elements, of determining fluorine with the capability to address the needs in environmental and medical science.