Analytic0 Chimica Acta, 283 (1993) 489-493 Elsevier Science Publishers B.V.. Amsterdam 489 Kinetic flow-injection spectrofluorimetric for the determination of fluoride V. Marco, F. Carrillo, C. Perez-Conde and C. CBmara method Lkpartamento de Q&mica Analitica, Facultad de Ciencias Quimicas, Universidad Complutense, 28040 Madrid (Spain) (Received 10th September 1992) Abstract A flow injection-spectrofluorimetric method is reported for the determination of fluoride, based on the ability of trace fluoride to increase the rate of formation of a fluorescent Al(III)-Eriochrome Red B complex in the presence of hexamethylenetetramine. Various chemical and physical variables affecting the reaction and its kinetics in the flow system were evaluated. The proposed method is very sensitive, with a detection limit of 10 pg I-’ and a linear calibration graph in the range 1 x 10m6-2 X 10d4 M. The method was successfully applied to the determination of fluoride in tap and mineral waters. Keywords: Flow injection; Kinetic methods; Fhtorimetry; Fluoride; Waters The monitoring of fluoride has recently re- ceived attention because of its dual role as an essential trace element and at high levels as a toxic substance. Both very high and very low fluoride intakes can cause mottling of teeth and bone disorders. Potentiometry with a fluoride ion-selective electrode (ISE) is an effective and convenient technique commonly used to determine fluoride [l], but inaccurate results may be obtained at low concentrations of this analyte. Other methods reported include molecular absorption spectrom- etry (MAS) with electrothermal atomization [2] and spectrophotometric [3] and spectrofluoromet- ric [4] methods based on ternary complex forma- tion or catalytic reactions [5], some of them using flow-injection systems [6,7]. In a previous paper the slow rate of formation of an unstable fluorescent complex by aluminium Correspondence to: C. Camara, Departamento de Qu’mica Analitica, Facultad de Ciencias Quimicas, Universidad Com- plutense, 28040 Madrid (Spain). and Eriochrome Red B (ERB) in the presence of hexamethylenetetramine (HMTA) and the use of this reaction to determine AN110 were reported zyxwvutsr Bl. This paper presents a sensitive flow-injec- tion-spectrofluorimetric method for the determi- nation of fluoride based on the ability of trace fluoride to increase the rate of formation of the Al(III)-ERB complex. EXPERIMENTAL Instrumentation The flow-injection equipment consisted of a Gilson HP4 peristaltic pump, an Omnifit injec- tion valve (six-way), PTFE segments with a 120- 120 Y (i.d. 0.8 mm) configuration, 0.5 mm i.d. PTFE connecting tubing and various end fittings and connectors (Gmnifit), a Perkin-Elmer MPF- 44A spectrofluorimeter equipped with a xenon- lamp source, a 189~1 (10~mm light path) flow cell and a Perkin-Elmer Model 56 recorder. The 0003-2670/93/$06.00 0 1993 - Elsevier Science Publishers B.V. All rights reserved