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 o f the aluminium monofluoride complex in excess of Al3+ and separation of th e two species formed (AlF2+ and Al3+) in a small (5 cm long, CG2) ion excha nge guard column. The final determination is accomplished by both ICP-MS sp ecific detection and post column derivatisation with fluorimetric detection . Fundamental studies on the formation kinetics of the complex, ion chromat ographic separation and optimum aluminium concentration were carried out us ing spectrofluorimetric detection by post-column reaction of the species wi th 8-hydroxyquinoline-5-sulfonic acid in a micellar medium of cetyltrimethy lammonium bromide. Fluorimetric detection showed good detection limits, but interferences from cations such as Mg2+ and Zn2+ required the use of the l onger CS2 ion exchange column. Iron interfered in relatively large amounts but adding EDTA to the sample solution eliminated the interference. A simil ar separation methodology was applied using ICP-MS detection for the indire ct determination of fluoride, by monitoring aluminium at mass 27. In this c ase, a detection limit of 0.1 ng ml(-1) was obtained using 0.45 M HNO3 as e luent and no interference caused by high concentrations of iron was observe d. The proposed method was applied to the determination of very low levels of fluoride in natural waters.