Plasma processes to detect fluorine with ICPMS as M-F+: an argument for building a negative mode ICPMS/MS

Detection of fluorine with commercial ICPMS is impossible due to fluorine's high ionisation potential (IP). A novel approach through the formation of fluorine-containing polyatomic ions [M-F]+ in the plasma allows the successful detection of F in sub ppm level by ICPMS/MS. Two theories behind [M-F]+ formation have been proposed, yet there is no clear understanding about this mechanism. Here, different metal solutions were tested to study the characterisation of plasma processes in the formation of [M-F]+. Three characteristics: high [M-F]+ bond dissociation energy (BDE), low [M-O]+ BDE and low IP, found to be essential to get highest sensitivity of [M-F]+. It was found that for elements with a higher [M-F]+ BDE than [M-O]+ BDE, the sensitivity decreases linearly with the element's second IP, meaning that the major process in the plasma is that M2+ harvests F- in the plasma to form [M-F]+. Barium exhibited the highest sensitivity for [M-F]+. However, the robustness of this approach was questioned due to matrix effects, hence an argument for re-developing the negative ion ICPMS/MS was discussed with which detection limits of sub-ppb could be reached.