Gas diffusion with preconcentration for the determination of fluoride in water samples by flow injection

The preconcentration of fluoride is achieved on-line by converting it to trimethylsilane which then diffuses through a gas permeable membrane to be absorbed in a stationary sodium hydroxide acceptor stream. This stream is enclosed in the sample loop of an injection valve and after preconcentration, the fluoride sample is flushed into a flow injection manifold for spectrophotometric analysis by the zirconium/alizarin S procedure at 520 nm. The method is suitable for fluoride analysis in the range 0.1-10 mg/l at a sampling rate of 17/hr. Phosphate does not interfere and aluminium and iron can be tolerated at 200 and 500 times the fluoride concentration, respectively. The LOD was calculated to be 0.055 mg/l and LOQ was found to be 0.18 mg/l.