Indirect determination of trace amounts of fluoride in natural waters by ion chromatography: a comparison of on-line post-column fluorimetry and ICP-MS detectors

An alternative method for the determination of trace levels of fluoride in drinking and sea-water samples is presented. It is based on the formation of the aluminium monofluoride complex in excess of Al3+ and separation of the two species formed (AlF2+ and Al3+) in a small (5 cm long, CG2) ion exchange guard column. The final determination is accomplished by both ICP-MS specific detection and post column derivatisation with fluorimetric detection. Fundamental studies on the formation kinetics of the complex, ion chromatographic separation and optimum aluminium concentration were carried out using spectrofluorimetric detection by post-column reaction of the species with 8-hydroxyquinoline-5-sulfonic acid in a micellar medium of cetyltrimethylammonium bromide. Fluorimetric detection showed good detection limits, but interferences from cations such as Mg2+ and Zn2+ required the use of the longer CS2 ion exchange column. Iron interfered in relatively large amounts but adding EDTA to the sample solution eliminated the interference. A similar separation methodology was applied using ICP-MS detection for the indirect determination of fluoride, by monitoring aluminium at mass 27. In this case, a detection limit of 0.1 ng ml-1 was obtained using 0.45 M HNO3 as eluent and no interference caused by high concentrations of iron was observed. The proposed method was applied to the determination of very low levels of fluoride in natural waters.