APPLICATION OF A PORTABLE TXRF SPECTROMETER TO DETERMINE TRACE AMOUNTS OF TOXIC ELEMENTS

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1 18 APPLICATION OF A PORTABLE TXRF SPECTROMETER TO DETERMINE TRACE AMOUNTS OF TOXIC ELEMENTS Shinsuke Kunimura 1 and Jun Kawai 2 1 Materials Fabrication Laboratory, RIKEN (The Institute of Physical and Chemical Research), 2-1 Hirosawa, Wako, Saitama, , Japan 2 Department of Materials Science and Engineering, Kyoto University, Sakyo-ku, Kyoto, , Japan ABSTRACT Using a portable total reflection X-ray fluorescence spectrometer, a sample containing 1 pg each of 3d transition metals, 5 pg of As, and 1 ng each of Sr, Y, and Zr is measured. Although non-monochromatic X-rays from a 5 W X-ray tube (Tube voltage: 25 kv) are used, the trace amounts of these elements are detected. This portable spectrometer is applied to analysis of river water, a leaching solution of a toy, and a lipstick. Several ppb concentrations each of elements in river water are detected. Lead is detected from the leaching solution of the toy. Transition metal elements such as Ti, Mn, and Fe are detected from the lipstick. This portable spectrometer is possible to be applied to an evaluation of environmental pollution and screening for toxic elements in products for daily use. INTRODUCTION The total reflection X-ray fluorescence (TXRF) spectrometry [1,2] is a well-established method for trace elemental determination. Iida et al. [3] reported in 1984 that monochromatization of incident X-rays reduces the spectral background due to the scattering of the incident X-rays from a sample holder and sample itself. Since this report, it has been believed that monochromatization is essential to improve detection limits. Using a high power X-ray source with a monochromator leads to an increase in the signal to background ratio, and the use of a monochromatic synchrotron radiation has made it possible to obtain femtogram detection limits [4-6]. On the other hand, we developed a portable TXRF spectrometer with a 1 W (9.5 kv, 15 A) X-ray tube and waveguide, and we reported in 27 that a detection limit of 1 ng for Cr is achieved with the continuum X-rays from the low power X-ray tube [7]. Nanograms of various rare earth elements as well as 3d transition metals were detected [8]. The weak continuum X-rays were useful for simultaneous analysis of sub-ppm elements when a sample was measured at an optimum glancing angle of the incident X-ray beams [9]. The detection

2 181 limit was improved with the increase of counting time, and the detection limit of.1 ng was achieved when a measurement was performed for 15 s [1]. We constructed an X-ray refractive lens consisting of two sections cut from a gramophone record, and we reported that this refractive lens is possible to be used for collimating the incident X-ray beams [11]. Detection limits ranging from sub-nanograms to a few nanograms were obtained with the refractive lens as well as the waveguide. This portable TXRF spectrometer was applied to analysis of ppb or ppm concentrations of elements in rain water [8], a leaching solution of screws [1], leaching test solutions of soils [12], drinking water [13], a puddle of rainwater [14], tea [14], and a leaching solution of a plastic toy [14]. The 1 W X-ray tube is used for a commercially available electrostatic remover for avoiding electrostatic charge of electric circuits during the processing of semi-conductors, and the tube voltage and current are fixed at 9.5 kv and 15 A. Because of the fixed tube voltage of 9.5 kv, fluorescent X-rays from toxic elements such as the As K-lines and Pb L-lines could not be detected. Since 28, we have developed a portable TXRF spectrometer with an X-ray tube of which the maximum tube voltage is 5 kv. We presented that using an X-ray source without a monochromator achieves lower detection limits compared with using an X-ray source with a monochromator (i.e. a4 m thick Cu absorber) [15]. Although the spectral background increased when non-monochromatic excitation was used, the enhancement of excitation efficiency for elements led to the improvement of detection limits. Even when the X-ray tube was operated at 1 W (2 kv, 7 A), detection limits of several tens of picograms for 3d transition metals and As were achieved with non-monochromatic excitation. Detection limits of several hundred picograms for rare earth elements and Pb were achieved with the 1 W X-ray tube [16]. The contribution of the scattered X-rays from the sample dry residue to the spectral background decreases with the decrease of the amount of a sample. We reported that detection limits obtained with the weak non-monochromatic X-rays are improved when a smaller amount of a sample is measured although this sample is needed to contain elements of which amounts are large enough to detect [17]. When an optimum target material of an X-ray tube, tube Figure 1. Portable TXRF spectrometer.

3 182 voltage, and tube current (W, around 25 kv, and 2 A) were used for measurements, a detection limit down to 1 pg was achieved by using the present portable spectrometer [18]. In the present paper, river water, a leaching solution of a toy, and a lipstick are measured by using the present portable spectrometer in order to discuss the possibility of applying to an evaluation of environmental pollution and screening for toxic elements in products for daily use. A multi-element standard solution containing.1 ppm each of 3d transition metals,.5 ppm of As, and 1 ppm each of Sr, Y, and Zr is also measured. PORTABLE SPECTROMETER The details of the present portable TXRF spectrometer were reported in a previous paper [18] and are briefly summarized here. Figure 1 shows the present portable spectrometer. An X-ray tube 5 kv Magnum W anode was used. The maximum tube voltage and current were 5 kv and 2 A, respectively. A Si PIN detector -123 (Amptek Inc., Bedford, MA) was used for detection. A quartz optical flat (Flatness: /2, =632.8 nm) (Sigma Koki Co., Ltd., Tokyo, Japan) was used as a sample holder. A waveguide was placed between the X-ray tube and quartz sample holder, and it restricted the incident X-ray beams to 1 m in height and 1 mm in width. In the present paper, the X-ray tube was operated at 25 kv and 2 A, and TXRF spectra were measured at a glancing angle of.4 in air. The critical angle of total reflection for 25 kev X-rays on quartz was.8 the critical angle becomes larger with the decrease in X-ray energy. SAMPLE PREPARATION A sample containing 1 pg each of Sc, Cr, and Co, 5 pg of As, and 1 ng each of Sr, Y, and Zr (sample 1), a certified reference material of river water (JSAC 32-3) (sample 2), river water (Kamogawa river, Kyoto) (sample 3), a leaching solution of a miniature toy car (sample 4), and a lipstick (sample 5) were measured. Table 1 shows sample volumes and concentrations of elements in these samples. Pipetting and drying a 1 L portion of sample 2 was repeated five times. Pipetting and drying a 1 L portion of sample 3 was repeated three times. Elements detected from sample 3 were quantified by using an internal standard element. Yttrium was used as the internal standard because it was one of the rare elements and would not be present in the sample. The sample and a 1 ppm of Y standard solution weremixedina9:1ratio. Inpreparationofsample4,.1gofaplasticfragmentofthetoy car was immersed in 4 L of1mhno 3 for 2 hours, and then the leaching solution was pipetted and dried on the optical flat. Sample 5 was prepared by rubbing a lipstick on the quartz optical flat.

4 183 Table 1. Samples measured by using the present portable TXRF spectrometer. Sample 1 Sample 2 Sample 3 Sample 4 Sample 5 Sample volume L L L L Solid sample Concentrations of each element Sc:.1ppm,Cr:.1ppm, Co:.1ppm,As:.5ppm, Sr: 1 ppm, Y: 1 ppm, Zr: 1ppm Cr: 1 ppb, Mn: 5 ppb, Fe: 58 ppb, As: 5 ppb, Pb: 1 ppb Y (internal standard): 1ppm RESULTS AND DISCUSSION Figure 2 shows TXRF spectra of sample 1 and ultra pure water. Sub-nanograms each of 3d transition metals and As and a few nanograms each a 4 Si Ar of Sr, Y, and Zr were W detected as shown in Figure 2a. As shown in 3 Figure 2, Si was detected due to the quartz sample holder. Argon was 2 Cr detected because air Sc Co contains.9 % Ar. The 1 As characteristic X-rays (the Sr Y W L-lines) from the X-ray Zr tube were detected. Figure 3 shows TXRF b spectra of samples 2 and 3. 4 Ar W As shown in Figure 3a, Si ppb concentrations of 3 elements were detected. As shown in Figure 3b, S, K, Ca, Br, and Sr were 2 detected. The quantified concentrations of S, K, Ca, Br, and Sr were.6 ppm,.1 ppm, 4.3 ppm,.1 ppm, and 55 ppb, respectively. 1 Several ppb concentrations X-rayenergy[keV] of elements can be Figure 2. TXRF spectra of (a) sample 1 and (b) ultra pure detected by using the water. X-ray intensity [counts / 18 s]

5 184 present portable spectrometer, and this spectrometer can possibly be applied to the evaluation of environmental pollution. a 4 X-ray intensity [counts / 18 s] Mn Cr Fe As,Pb X-ray intensity [counts / 18 s] b K 1 Ca S X-ray energy [kev] Figure 3. TXRF spectra of samples (a) 2 and (b) 3. Br Sr Y Figure 4 shows TXRF spectra of samples 4 and 5. It is important for protecting health against toxic elements such as Pb to analyze products for daily use. As shown in Figure 4a, Pb, P, Ca and Fe were detected. As shown in Figure 4b, S, K, Ca, Ti, Mn, Fe, Cu, and Ba were detected. Toxic elements such as Pb were not detected from the lipstick.

6 185 4 a Ca X-ray intensity [counts / 3 s] Si P Fe Pb Pb L b Fe K Ti Ti K Fe Intensity [counts / 18 s] S Ca K Mn Ba L Cu K X-ray energy [kev] Figure 4. TXRF spectra of samples (a) 4 and (b) 5. CONCLUSIONS A sample containing 1 pg each of 3d transition metals, 5 pg of As, and 1 ng each of Sr, Y, and Zr was measured by a portable TXRF spectrometer. Using non-monochromatic X-rays from a 5 W X-ray tube was effective to detect the trace amounts of these elements. River water, a leaching solution of a toy, and a lipstick were measured by using the present portable spectrometer. Several ppb concentrations each of elements in river water were detected. Lead was detected from the leaching solution of the toy. Transition metals such as Ti and Fe were detected from the lipstick. Consequently, this portable spectrometer is able to be

7 186 applied to an evaluation of environmental pollution and screening for toxic elements in products for daily use. ACKNOWLEDGEMENT This research was financially supported by SENTAN, JST. One of the present authors (S.K.) thanks the JSPS Research Fellowships for Young Scientists and the Special Postdoctral Researchers Program of RIKEN. REFERENCES [1] Klockenk R. Total Reflection X-ray Fluorescence Analysis; Wiley: New York, [2] Wobrauschek, P. X-Ray Spectrom. 27, 36, [3] Iida, A. and Gohshi, Y. Jpn. J. Appl. Phys. 1984, 23, [4] Wobrauschek, P., G R., Kregsamer, P., Streli, C., Pahlke, S., Fabry, L., Haller, M., Knochel, A., Radtke, M. Spectrochim. Acta Part B 1997, 52, [5] Pianetta, P., Baur, K., Singh, A., Brennan, S., Kerner, J., Werho, D., and Wang, J. Thin Solid Films 2, 373, [6] Sakurai, K., Eba, H., Inoue, K., Yagi, N. Anal. Chem. 22, 74, [7] Kunimura, S. and Kawai, J. Anal. Chem. 27, 79, [8] Kunimura, S. and Kawai, J. Powder Diffr. 28, 23, [9] Kunimura, S., Watanabe, D., and Kawai, J. Spectrochim. Acta, Part B 29, 64, [1] Kunimura, S., Watanabe, D., and Kawai, J. Proceedings of the 4th International Congress on the Science and Technology of Steelmaking 28, Gifu, October 6-8, 28, (28). [11] Kunimura, S. and Kawai, J. Spectrochim. Acta, Part B 29, 64, [12] Kunimura, S., Kawai, J., and Marumo, K. Adv. X-ray Chem. Anal. Japan 27, 38, [13] Kunimura, S. and Kawai, J. Anal. Sci. 27, 23, [14] Kunimura, S., Watanabe, D., and Kawai, J. Bunseki Kagaku 28, 57, [15] Kunimura, S. and Kawai, J., Analyst, submitted. [16] Kunimura, S. and Kawai, J., Powder Diffr., submitted. [17] Kunimura, S., Ida, H., and Kawai, J. Adv. X-Ray. Chem. Anal., Japan 29, 4, [18] Kunimura, S. and Kawai, J., Adv. X-ray Chem. Anal. Japan 21, 41,