Determination of Fluoride in Water Residues by Proton Induced Gamma Emission Measurements

176 Fluoride Vol. 35 No. 3 176-184 2002 Research Report

DETERMINATION OF FLUORIDE IN WATER RESIDUES BY PROTON INDUCED GAMMA EMISSION MEASUREMENTS

AKM Fazlul Hoque,a M Khaliquzzaman,b MD Hossain,c AH Khand Dhaka, Bangladesh

SUMMARY: A multielement proton induced gamma emission (PIGE) method has been developed to analyze fluoride in water residues obtained by evaporation. In this method, 200 mL of water sample mixed with 100 mg of cellulose powder is evaporated, and the residue is made into standard pellets that are then irradiated with a 2.9 MeV proton beam. The emitted γ-rays from the decay of the excited fluorine nuclei are detected with a high resolution, high purity germanium (HPGe) detector and analyzed using a commercial gamma ray spectrum unfolding software. For concentration calibration, synthetic fluoride standards of different concentrations, as NaF in a CaCO3 matrix, were prepared and homogenized by dispersing them in methanol. The method thus developed was applied to determine the concentration of fluoride in 85 water samples collected from different city supplies of Bangladesh. The concentration ranged from 0.03 to 1.10 mg/L with the mean of 0.32 ± 0.21 mg/L. Keywords: Bangladesh, Fluoride analysis, Proton induced gamma emission, Water fluoride concentration, Water residue.

INTRODUCTION Pollution of the biosphere with fluoride is a human health concern in many parts of the world. The fluoride content of drinking water is a very important factor from the health point of view. Dental researchers have reported that the supplementation of fluoride in drinking water improves the resistance to dental caries.1-3 However, recent studies by public health dentists in New Zealand, Canada, the United Kingdom, and the United States have indicated that no such benefit from water fluoridation is observed.4-7 On the other hand, there are reports that excessive fluoride in- take causes fluorosis, cancer, arthritis, and other diseases.8-12 It has also been observed that fluorine in excess affects human intelligence, especially in children, who are most susceptible to early fluoride toxicity.9,12 Chronic fluoride intoxication (fluorosis) has been reported not only in humans but also in domestic animals, such as, cattle, buffaloes, sheep, and goats.13 Different analytical methods using nuclear reactions have been reported for determination of fluoride in samples related to geology, odon- tology, medicine, metallurgy, environment, etc.14-19 Determination of fluoride using photon activation analysis is very sensitive but it requires pyrolysis or hydropyrolysis or distillation to separate fluoride from inter-

——————————————— aFor Correspondence: Dr AKM Fazlul Hoque, Accelerator Facilities Division, Atomic En- ergy Centre, P.O. Box 164, Dhaka - 1000, Bangladesh. E-mail: [email protected] bWorld Bank Dhaka office, Consultant, Environment Team, 3A Paribagh, Dhaka; cPhys- ics Department, Jahangirnagar University, Savar, Dhaka; dChemistry Department, Dhaka University, Dhaka, Bangladesh. Fluoride analysis by proton induced gamma emission 177 fering substances.15 In fast neutron activation analysis using 19F(n, α)16N, 19F(n, γ)20F, etc, the sensitivity is not sufficient to determine more than a few µg/g (ppm) of fluoride in samples.15 In ion beam analysis with 19F(p, - + 16 α1 e e ) O de-excitation takes place mainly through internal pair production, by which an electron and a positron are created and the inelasti- cally scattered particles are detected in coincidence with the emitted photon.16 A signal with very low background is obtained, but the yield is very low because of the coincidence condition. Proton induced gamma emission (PIGE) is also a nuclear reaction based analytical method. Initially, it was developed at the 3 MeV Van de Graaff Accelerator Laboratory of the Atomic Energy Centre, Dhaka (AECD) for non-destructive analysis of fluoride in human teeth.19 Subse- quently, knowing the multielement capability of the method, it was em- ployed for analyzing the light elements in environmental samples in- cluding fluoride in drinking water. The present work is thus an expansion of the scope of the application of PIGE in environmental and human health studies, where it is necessary to analyze water, food, etc. for specific information of human health significance. Generally, fluoride ions in water are analyzed by spectrophotometry, ion sensitive electrode (ISE), and ion-chromatography. Each of the methods has its own limitations and advantages.20 Spectrophotometry of fluoride in water using alizarin is not interference free, and for low concentrations it requires distillation, which is time consuming. Ion chromatography is costly, and although it can analyze multianionic samples, high concentrations of chloride ion interfere with the determination of fluoride. ISE is a very useful method for quantitative analysis of fluoride ions in water down to the concentration of 0.1 mg/L. However, the ionic strength of the solution has to be adjusted for different samples to mini- mize interference. If organic acids and boron are present in water, distillation is necessary. Compared to these methods of fluoride analysis in water, the PIGE method has the advantage of being isotopic in nature with virtually no interference; sensitivity is high, and it has the multielement capability like ion chromatography. Sample preparation steps are mostly physical in nature, and thus sample contamination is minimal. Moreover, since the same experimental setup is used for determining fluoride in teeth, with the only expense being that of sample preparation, the method provides a nondestructive procedure for analysis that is advantageous for large-scale screening where sensitivity is important. PIGE analysis of fluoride reported in this work is based on the detection of the gamma rays emitted on proton interaction with 19F. The en-

Fluoride 35 (3) 2002 178 Hoque, Khaliquzzaman, Hossain, Khand ergy of the gamma ray indicates the isotope that is present, and the intensity is a measure of the concentration of the isotope in the sample.

MATERIALS AND METHODS Sample preparation: A 200 mL water sample together with 100 mg of ash- less cellulose powder (Whatman) was evaporated to dryness in a porcelain dish on a steam bath and then under an infra-red lamp. The residue was cooled at room temperature in a desiccator for 1 hr. The powdered sample was then weighed and sealed in a polythene bag and stored in a desiccator. A 50 mg sample was pressed into a 10 mm diameter pellet with 3 tons of pressure in a graduated hydraulic press. The pellet was mounted on a 35 mm slide frame with adhesive tape and preserved in a desiccator until irradiated. A 200 mL de-ionized water mixed with 100 mg of cellulose was prepared in the same way as the blank and analyzed for any contamination in sample preparation. Concentration calibration: In order to determine the concentration of fluoride in water samples for concentration calibration shown in Figure 1, AnalaR grade NaF in the concentration range of 10-500 mg/kg in a CaCO3 matrix was used. The nuclear reaction 19F(p, p′ γ)19F is used to construct the calibration curve.

600

t y = 1.0469x 2 450 R = 0.9983

300

150 Fluoride (mg/kg) in sample pelle 0 0 100 200 300 400 500 Counts per µC

Figure 1. Calibration curve for the determination of fluoride in water residue.

NaF standards were homogeneously dispersed in 100 mg of CaCO3 with methanol, and the resulting matrices were dried under an infrared lamp.

Fluoride 35 (3) 2002 Fluoride analysis by proton induced gamma emission 179

Weighed 50 mg samples were then pressed into pellets. As a test for homo- geneity, irradiation of three 50 mg pellets gave reproducible results within ± 5%. The calibration curve was correct within this uncertainty. In real sample analysis, this calibration was used to determine the concentration of fluoride in water residues on dry weight basis (mg/kg). These data were then con- verted to mg F/L of water samples with respect to 200 mL of each analyte sample. Method of analysis: The schematic diagram of both the internal/external beam PIGE setup is shown in Figure 2. Two tantalum collimators, each of 2 mm diameter, and a 4 mm diameter cleanup aperture, were used to obtain a finely collimated beam. In internal beam PIGE, sample excitation and gamma-ray emission are performed within the vacuum chamber. A high purity germanium (HPGe) detector in the first position in Figure 2 is used for γ-ray analysis.

Figure 2. Schematic diagram of the external/internal beam of the PIGE experimental system.

In the external beam technique, samples are irradiated in air, where a wide variety of samples can be irradiated easily by extracting the proton beam through the Be window sufficiently strong to hold vacuum in the accelerator tube. The great advantage of the external beam technique is the reduction of charge build up on the sample during irradiation. Polyimide films (Kapton brand) of 1.12 mg/cm2 thickness were used to extract the proton beams from the beam port into the air. The set up is de-

Fluoride 35 (3) 2002 180 Hoque, Khaliquzzaman, Hossain, Khand signed to hold a 35 mm slide frame for solid samples at an angle of 45° relative to the beam direction, and the characteristic γ-rays are detected at 90° with respect to the beam. The total proton charge on the sample and the kapton window, from the beam port and the collimators, was integrated with a charge integrator. The external beam is most useful for irradiation of samples of different shape and size. Because of limitations in geometry, internal beam irradiation is more restrictive. The analyzed proton beam energy was 2.9 MeV. The proton energy on the target after absorption at the exit window and the air between the window and the sample is estimated to be 2.3 MeV. Each sample was irradiated for a preset charge of 20 µC with a beam intensity of 15 nA. The characteristic γ- rays were detected with a high purity germanium (HPGe) detector having a resolution of 1.75 keV at 1332 keV γ-rays. Standard NIM electronics and a 4095 channels pulse analyzer were used for data acquisition and analysis. The inelastic scattering of protons on 19F leads to the emission of gamma quanta from the first and second excited states of the 19F nucleus with energies of 110 and 197 keV, which have very high production cross sections that do not interfere with other γ-rays. These gamma lines can also be produced in the 18O(p, γ)19F reaction, but because of relative low abundance (0.2%) and low proton interaction cross section of 18O, the contribution from this reaction can be ignored without much error. A typical PIGE spectrum of a water residue sample is illustrated in Figure 3. The gamma ray spectrum was analyzed using a commercially available γ-ray unfolding software obtained from APTEC Nuclear Inc. USA.

12000 Na-23 8000 Counts F-19

4000 F-19 511

0 50 150 250 350 450 550 Gamma ray energy (keV) Figure 3. Gamma ray spectrum from water residue.

Fluoride 35 (3) 2002 Fluoride analysis by proton induced gamma emission 181

RESULTS AND DISCUSSION The bulk of the water residue obtained after evaporation consists mostly of carbonates and bicarbonates. The weight of the residue obtained from different samples varied from 0.01 to 0.30 g. Under these conditions, it was assumed that the matrix composition consists predominantly of H, C, and O. Thus, CaCO3 was chosen as the carrier for calibration with known amounts of NaF. Some water samples were analyzed by the fluoride ion selective electrode (ISE) method, considered to be the most appropriate one for fluoride in water. The results are shown in Figure 4 along with those from PIGE measurements for comparison. The two results agree within ± 11%. The correspondence between the two results indicates the accuracy of the PIGE meth- odology. The reproducibility of the results has been tested by measuring the fluoride content of the same sample several times. For 5 replicate measurements, the standard deviation was ± 0.03 mg/kg at the concentration level of 0.60 mg/kg.

1 ISE 0.8 PIGE

0.6

0.4

0.2

Fluoride concentration (mg/L) 0 12345678 Sample Figure 4. Fluoride measurements of water samples using ISE and PIGE methods.

Minimum detection limit (MDL) The MDL is defined as the amount of an element in mg/kg that yields a γ- ray intensity equal to 3 σ of the background under the peak in an interval equal to the full width at half maximum (FWHM). The MDL for water residue of 200 mL water containing 100 mg cellulose and for a 20 µC irradiation charge was 0.2 mg/kg on the dry weight basis of the water residue,

Fluoride 35 (3) 2002 182 Hoque, Khaliquzzaman, Hossain, Khand which corresponds to approximately 0.0001 mg F/L in the original water sample. Fluoride levels in city supply waters of Bangladesh The mean fluoride concentration in water supplies of 11 major cities is shown in Figure 5. The concentration range of fluoride in 85 different city water supplies as observed in this study was 0.03-1.10 mg/L, with the mean of 0.32 ± 0.21 mg/L. It appears from the results that fluoride levels in most of the city supplies are lower than the WHO recommended maximum of 1.5 mg/L for drinking water.21 The results further indicate that the external beam PIGE method is a sensitive and reliable nuclear analytical method for fluoride analysis in water residues preconcentrated on cellulose powder by evaporation. Vegetables, fruits and other food samples may also be analyzed by PIGE with higher sensitivity down to the fluoride level of ≤ 1 mg/kg dry weight basis and minimum sample preparation.

1.2

0.8

0.4

Fluoride concentration (mg/L) 0.0

Bogra Sylhet Barisal Dhaka Tangail Comilla Dinajpur Gazipur Rajshahi Chittagong Mymensigh City Figure 5. Mean concentration of fluoride in 11 city water supplies of Bangladesh.

An important advantage of the PIGE method, in addition to its high sensitivity, is its ability to determine simultaneously a number of low Z elements, especially lithium, boron, fluorine, etc. in health related environmental samples. The non-destructive analysis of human teeth for fluoride levels in large

Fluoride 35 (3) 2002 Fluoride analysis by proton induced gamma emission 183 scale screening is the distinct advantage of the PIGE method reported else- where.19 It can be used as a method that is complementary to proton induced X-ray emission (PIXE) for multielement trace analysis in different disciplines reported earlier from this laboratory.22

ACKNOWLEDGMENT The authors wish to acknowledge the cooperation of the colleagues of the Accelerator Facilities Division and the Chemistry Division of AECD during the course of this work. The partial financial support received from the Ministry of Science and Technology, Government of Bangladesh, under the special grant program for carrying out this research is gratefully acknowl- edged.

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