2005 International Nuclear Atlantic Conference - INAC 2005 Santos, SP, Brazil, August 28 to September 2, 2005 ASSOCIAÇÃO BRASILEIRA DE ENERGIA NUCLEAR - ABEN ISBN: 85-99141-01-5 FLUORINE IN WATER AND TEETH MEASURED WITH PIGE Fernando H. M. Medeiros, Márcia A. Rizzutto, Nemitala Added Departamento de Física Nuclear Instituto de Física Universidade de São Paulo Travessa R da Rua do Matão 187 05508-900 São Paulo - SP medeiros@if.usp.br, rizzutto@dfn.if.usp.br, nemitala@dfn.if.usp.br ABSTRACT This paper presents preliminary results of a work whose objective is the determination of fluorine concentrations in drinking water and teeth by using the PIGE (Particle Induced Gamma-ray Emission) technique. Teeth and water samples were analysed in the external beam setup of the 1.7 MV Tandem accelerator of LAMFI (Laboratório de Análise de Materiais por Feixes Iônicos) using the 19 F(p,p ) 19 F nuclear reaction. The results indicate that the technique is sufficiently sensitive for the proposed objectives. Detection limits are also discussed. 1. INTRODUCTION It is a well-known fact that fluorine plays an important role in dental caries prevention. Water fluoridation and use of fluoridated dentifrices have been pointed as the major responsible for the reduction of caries prevalence in populations of many countries during the last century [1]. On the other hand, exposure of teeth to excessive levels of fluorine, particularly during the formation period, may lead to dental fluorosis, a disease whose main effect is enamel demineralization [2]. For these reasons, monitoring the fluorine levels in the public system of water supply and correlating it with effective fluorine absorption by teeth is an important task from the public health point of view. Measurement of fluoride ions in water has been predominantly performed by electrodebased chemical methods, whose main advantage is its high sensitivity. Nevertheless, this method often requires preparation of the samples to avoid interference with other ions, which may influence the final result [3]. Particle-Induced Gamma-ray Emission (PIGE) technique is an alternative analytical method based on nuclear reactions induced by accelerated particles, particularly appropriated for the analysis of light elements (Z<~17) [4]. It is also a highly sensitive method and requires only minor manipulation of the sample. The main objective of this work is to establish an experimental setup and a procedure to determine fluorine concentrations in water and teeth samples by means of the PIGE technique. 2.1 Experimental Setup 2. EXPERIMENTAL METHODS
PIGE analysis of fluorine was based on the 19 F(p,p ) 19 F nuclear reaction, which results in the emission of both 110 and 197 kev gamma-rays. The gamma spectra were measured with a 20% relative efficiency HPGe detector (FWHM 2.7 kev at 1.33 MeV), positioned at 20 cm from the sample. Experiments were performed at LAMFI (Laboratório de Análise de Materiais por Feixes Iônicos), in the Institute of Physics of the University of São Paulo, where a 1.7 MV Tandem Accelerator is installed. An external beam setup was used to irradiate the samples. A 7.5 m kapton foil was used as the exit window of the vacuum system. The accelerating voltage used in the experiments was 1.16 MV and the proton beam current was kept below 10 na to avoid overheating of the window. The final energy of the proton beam after passing through the window and the 5 mm of air between the window and the sample was approximately 2.2 MeV. The beam charge for each measurement varied between 5-15 C. 2.2 Sample Preparation 2.2.1 Water Samples Water samples were not irradiated directly by the proton beam. Fluorine in water was preconcentrated in a filter using the procedure developed by Hoque et al. [5]. 200 ml of the water sample containing small pieces of filter paper were evaporated in an oven at 80 ºC. The resultant fluorine-enriched paper was then pressed into pellets (1 cm diameter and 3mm thick), ready for irradiation. Two water samples were analyzed in this way, one from a calibrated NaF solution (Orion standard) containing 5 ppm (mg/l) of fluorine and other from the public supply system of the city of São Paulo, Brazil. 2.2.2 Teeth Samples Extracted teeth were obtained from the Faculty of Odontology of the University of São Paulo. Standard samples with known amounts of fluorine were prepared mixing lithium fluoride (LiF) in a matrix of synthetic hydroxyapatite (HAP Ca 10 (PO 4 ) 6 (OH) 2 ). The mixture, with a uniform concentration of approximately 2000 ppm of fluorine, was pressed in a pellet. 3.1 Fluorine in Water 3. RESULTS AND DISCUSSION Figure 1 presents the gamma energy spectra of irradiated filter pellets. The standard pellet (originally 5 ppm in water) presents very well defined fluorine peaks (110 and 197 kev), as well as the 440 kev sodium peak. The beam charge for this measurement was 8.6 C. Although being much less intense, the peaks are also observable in the pellet from the
public supply water (charge 5.0 C). These preliminary results indicate qualitatively that the methodology is sufficiently sensitive to detect fluorine concentrations in the typical range it is found in the public supply system. The detection limit will be discussed further. Figure 1 Gamma spectra of irradiated filter pellets used in the analysis of fluorine in water. 3.2 Fluorine in Teeth The gamma energy spectrum of an irradiated tooth is presented in figure 2. The general appearance is very similar to figure 1. If the distribution of fluorine in tooth is assumed to be uniform, the concentration can be obtained directly by comparing the measurements of tooth (t) and standard (st), according to equation 1 [6],
Ct C st I I t st S (1) where C is the concentration, I is the area of the gamma peak (normalized for the beam charge) and S is a correction for the difference of stopping power between the tooth and the standard. In this case, as the standard was based on a hydroxyapatite matrix, this correction was assumed to be negligible (S=1). Substituting the experimental values in eq. 1, a concentration of 820(40) ppm was obtained. If the hypothesis of uniform fluorine distribution is not a good approximation, the quantitative analysis must take into account the depth profile of fluorine distribution in teeth. 3.3 Detection limits Figure 2 Gamma spectrum of an irradiated tooth. The quantitative analysis of the sensitivity of this method was based on measurement of the prepared standard samples. The detection limit obtained in this work for fluorine in water is 0.3 ppm. For homogeneous distribution in teeth the limit is 40 ppm. These values were calculated considering Curie s definition of detection limit [7], and relate to a
confidence interval for type I and II errors (false-positive and false-negative, respectively) of 95%. Both the fluorine signal (peak area in a width equal to ~1 ) and the background signal (area under the peak in the same width) were assumed to follow Poisson statistics. Figure 3 shows the relevant areas taken for the calculation of the detection limits. It s worthwhile to mention that the smooth behavior of the background allowed a precise discrimination between the net signal and the background. The calculated detection limits are well below the values found in typical samples. Water in the public supply system is fluoridated with 0.7 ppm and typical concentrations in teeth are of the order of 1000 ppm. Thus, the methodology is sufficiently sensitive to detect fluorine in these kind of samples. Figure 3 Details of the 197 kev peak used in the determination of the detection limit for fluorine in water. 4. CONCLUSIONS As mentioned before, the results presented in this work are preliminary. The small number of analyzed samples doesn t allow general and definitive conclusions about the suitability of the method for quantitative determination of fluorine concentrations. Nevertheless, these first results indicate that the PIGE experimental setup and methodology mounted in LAMFI has enough sensitivity to detect fluorine in teeth and
drinking water. Detection limits in these samples were evaluated in 40 and 0.3 ppm, respectively. ACKNOWLEDGEMENTS The authors thank CNPq and FAPESP for the financial support. REFERENCES [1] PC Narvai. Dental caries and fluorine: a twentieth century relation, Ciência e Saúde Coletiva, 5(2), pp. 381-392 (2000). [2] MCT Cangussu, PC Narvai, RC Fernandez, V Djehizian. Dental fluorosis in Brazil: a critical review, Cad. Saúde Pública, 18(1), pp. 7-15 (2002). [3] AO Al-Othman and JA Sweileh. Phosphate rock treatment with citric acid for the rapid potentiometric determination of fluoride with ion-selective electrode, Talanta 51(5), pp. 993-999 (2000). [4] JR Tesmer, M Nastasi. Handbook of Modern Ion Beam Materials Analysis, Materials Research Society, Pittsburgh (1995). [5] AKMF Hoque, M Khaliquzzaman, MD Hossain, AH Khan. Determination of fluoride in water residues by proton induced gamma emission measurements, Fluoride, 35(3), pp. 176-184 (2002). [6] G Demortier. Prompt gamma-ray yields from proton bombardment, J. Radioanal. Chem. 45(2), pp. 459-496 (1978). [7] LA Curie, Limits for qualitative detection and quantitative determination, Analytical Chemistry, 40(3), pp. 586-593 (1968).