Evaluation of silica-gel activator in order to find the optimal silica-gel activator for lead isotope measurement by thermal ionization mass spectrometer (TIMS) Takashi Miyazaki 1, Tomoyuki Shibata 2, Masako Yoshikawa 2, Tatsuhiko Sakamoto 3, Koichi Iijima 3 and Yoshiyuki Tatsumi 4 1 Research Program for Data and Sample Analyses, Institute For Research on Earth Evolution (IFREE) 2 Beppu Geothermal Research Laboratory, Kyoto University 3 Research Program for Paleoenvironment, Institute For Research on Earth Evolution (IFREE) 4 Research Program for Geochemical Evolution, Institute For Research on Earth Evolution (IFREE) 1. Introduction Lead isotope analyses using the thermal ionization mass spectrometer (TIMS) have been performed at many laboratories, and the silica-gel activator method [Akishin et al., 1957] has been generally adopted for about 20 years. Up to the present, hydrolysis of silicon tetrachloride has usually been used for the synthesis of the silica-gel activator [Koide and Nakamura, 1990; Nohda, 1999]. However, the silicon tetrachloride has intense reactivity during reaction, making its handling very difficult. Therefore, it is not easy to make each silica-gel activator with constantly high ion yield efficiency. Recently, other synthesis methods for the silicagel activator using a silicic acid colloidal solution and tetraethoxysilane (TEOS) were developed [Gerstenberger and Haase, 1997; Miyazaki et al., 2003]. These synthesis methods can be much easier than that which employs silicon tetrachloride because of its more gentle reactivity. Moreover, ion yields in the case of using the silica-gel activators synthesized by these method were improved relative to the case of using previous silica-gel activator synthesized from silicon tetrachloride [Gerstenberger and Haase, 1997; Miyazaki et al., 2003]. Improving the reproducibility of an isotope ratio measurement is very important for improving the reliability of the isotope analysis. The careful selection of the silica-gel activator is critical, because the physicochemical conditions such as particle size, size distribution and specific surface area of silica-gel may have large effects on not only the ionization efficiency but also stability of ion beam and condition of mass fractionation. Therefore, detail investigation of physicochemical conditions of the silica-gel is necessary for developing the optimal silica-gel activator for lead isotope measurement. Recently, it was supposed that the finer size of silica particles in silica-gel suspension is advantageous as activator to the higher yield of lead ion [Gerstenberger and Haase, 1997]. Moreover, Miyazaki et al. [2003] indirectly supported the assumption of Gerstenberger and Haase [1997] by the comparison of the integrated ion currents measured with the two kinds of silica-gel activators, which were expected to have different size of silica particles. However, detailed and direct investigation about the physicochemical conditions of the silica-gel activator has not been carried out in previous study, although the amount of silica-gel and the rate of mixture with phosphoric acid have been considered [Kuritani and Nakamura, 2002; Doucelance and Manhès, 2001]. In this work, we evaluated several silica-gel activators synthesized by different methods, in order to find the optimal silica-gel activator for lead isotope measurement, especially for a conventional lead isotope measurement method. Because the conventional method do not use spikes and special correction method for normalization of isotope fractionation, it is necessary to find the silica-gel which can restrain the in-run mass fractionation and obtain the higher ion beam current during measurement. Therefore, in evaluation, we focused on the ion beam current and reproducibility of isotope ratio. Moreover, we also focused on the particle size of silica in silica-gel as a kind of physicochemical condition, and examined the relation to the ion beam current. For that purpose, we conducted the size distribution analysis of silicagel activators and the measurement of integrated ion current using each silica-gel activator. Moreover, we also examined the reproducibility of isotope ratio and evaluated the most suitable silicagel activator for conventional lead isotope measurement method. 2. Experimental 2.1 Reagents and preparation of Silica-gel activator TEOS (99%) and Silicon tetrachloride (99.998%) was supplied by Gelest, Inc. and Aldrich Chemical Company, Inc., respectively. The Nissan Chemical Industries, Ltd. supplied a silicic acid colloidal solution with 30% of concentration. Four types of silicic acid colloidal solution with different particle size and concentration were supplied from the Fuso Chemical Co., Ltd., which were PL-1 (12%, 0.034µm), PL-3 (19.5%, 0.069µm), PL-7 (23.2%, 0.122µm) and PL-20 (23.9%, 0.371µm), respectively. The analytical grade ethanol (>99.5%; Wako Pure Chemical Industries, Ltd.), and ultra pure reagents (TAMAPURE -AA; Tama Chemicals Co., Ltd.) of ammonia (20wt.% solution), hydrochloric acid (20wt.% solution), nitric acid (68wt.% solution) and phosphoric acid (98%) were used without further purification. Water was deionized by a Mill-Q system (Millipore ), and finally distilled at 80 C. Hydrochloric acid, nitric acid and phosphoric acid were diluted to 6 M, 0.5M and 0.075 M, respectively, with water. A standard reference material of NIST SRM 981 (99.9999% of common Pb), which was diluted with 0.5M HNO 3 to give 100 ppm, was used for lead isotope measurement. The synthesis method of silica-gel from TEOS was followed by Miyazaki et al. [2003]. We synthesized the silica-gel from TEOS using the catalysts of hydrochloric acid (SG-HCl) and ammonia solution (SG-NH 4 ). The synthesis method of silica-gel from silicon tetrachloride (SG-SiCl 4 ) in the present work was as follows. A few drops of silicon tetrachloride were dropped into a Teflon vessel (30ml) filled with water. Then the synthesized silica-gel was concentrated by centrifuge. The concentrated silica-gel 1
was rinsed several times by water. We repeated the above process until necessary amount of silica-gel was obtained. Silicic acid colloidal solutions were diluted with water added about 1ml of ethanol (SG-Col Nissan and SG-Col Fuso ). We prepared more than 30ml of each silica-gel suspension for size distribution analysis and measurement of ion beam and isotope ratio. The amounts of each reagent were adjusted so that the SiO 2 content in the suspension is 8.8 µg /µl. 2.2 Measurement of particle size, ion beam current and isotope ratio Size distribution analysis of silica in synthesized silica-gel suspension was carried out on a laser diffraction particle size analyzer (Beckman Coulter LS230 with small volume module (SVM)) at the IFREE, Yokosuka. The measurable range of size distribution using LS230 is from 0.04 to 2000 µm. Several ml of each silica-gel suspension was diluted with about 120 ml of medium (water with dispersing agent) in SVM. Six times of scan were executed in one size distribution measurement. A little silica condensation was generated in suspensions synthesized from TEOS using ammonia solution as catalyst and from silicon tetrachloride, although the mechanism of condensation is not clear. Since the size distribution is expressed with volume ratio, the existence of several huge condensed particles in suspension shift the size distribution toward course side. In order to detect the finer particles dispersed in suspension, we separated the suspension into two batches and measured the size distribution not only at 30 seconds but also at 1 hour after shaking. Lead isotope measurements (ion beam current and isotope ratio) were carried out on a multicollector mass spectrometer (Thermo Finnigan TRITON TI with 9 Faraday cup collectors) at the Beppu Geothermal Research Laboratory, Kyoto University. The measured method was described in Miyazaki et al. [2003]. The silica-gel suspension was sucked from vessel 30 seconds after shaking. A filament temperature of around 1320 C was used for the measurement. The integration of ion beam current started simultaneously with preheating. Total integration time was about 30 minutes. The acquisition of isotope ratios started at 10 minutes after preheating and beam adjustment. The data were obtained in static mode. These data were computed from 66 repeated measurements. The data acquisition time took about 20 minutes for each measurement. 3. Result and discussion 3.1 Particle size distribution of silica Figure 1 shows the particle size distributions of silica in SG- NH 4, SG-HCl, SG-SiCl 4 and SG-Col Nissan, respectively. The particle size distribution of SG-NH 4 at 30 seconds after shaking was unimodal and extended from 1 µm to 200 µm. On the other hand, two large peaks of 0.60 µm and 1.83 µm and two small peaks of 0.06 µm and 0.24 µm were observed between 0.04 µm (lower measurable limit) and 3 µm at 1 hour after shaking. The particle size distributions of SG-HCl at 30 seconds and 1 hour after shaking were almost similar and extended from 0.4 µm to 3 µm with two large peaks of 0.79 µm and 1.83 µm. The particle size distributions of SG-Col Nissan at 30 seconds and 1 hour after shaking showed sharp peak of 0.11 µm with narrow range between 0.08 µ m and 0.16 µm indicating monodisperse of silica particles. The particle size distribution of SG-SiCl 4 at 30 seconds after shaking showed warp peak profile extended from 1 µm to 1000 µm. On the other hand, three broad peaks of 1.15 µm, 4.66 µm and 10.8 µm were observed between 0.2 µm to 60 µm at 1 hour after shaking. The shift of the particle size distribution toward finer size with time (SG-NH 4 and SG-SiCl 4 ) reflects the precipitation of condensed silica particles. In case of SG-NH 4 and SG-SiCl 4, particles of more than 3 µm (SG-NH 4 ) or 30 µm (SG-SiCl 4 ) were precipitated during 1hour. On the other hand, the similar size distribution patterns at 30 seconds and 1 hour after shaking observed in SG- HCl and SG-Col Nissan indicate that the silica particles less than 3 µm are keeping in dispersion during 1 hour. The existence of finer particles in the SG-NH 4 relative to the SG-HCl is consistent with the result of Esquena et al. [1997], in which silica particles formed in the presence of ammonia as a catalyst were finer particles than those formed using hydrochloric acid as a catalyst. 3.2 Relation between the particle size and the ion beam current Figure 2 shows the relation between particle size and integrated ion current ( 208 Pb). Since the silica-gel used for measuring ion current was sucked from suspension at 30 seconds just after shaking, the values of horizontal axis of Figure 2 are the particles sizes of silica in silica-gel suspension at 30 seconds after shaking. In order to simplify the discrimination of size distributions, we compared the size at 50% cumulative volume (d 50 ). The ion currents (A) were integrated and transferred to coulomb (C). The integrated ion currents measured with SG-NH 4, SG-HCl, SG-SiCl 4 and SG-Col Nissan were 4.33 10-8 C, 4.68 10-8 C, 2.87 10-8 C and 10.6 10-8 (C), respectively. In addition, the integrated ion currents measured with four types of SG-Col Fuso, which sizes were known as certified values of the Fuso Chemical Co., Ltd., were 5.41 10-8 C (PL-1), 5.53 10-8 C (PL-3), 5.49 10-8 C (PL-7) and 4.53 10-8 (C) (PL-20), respectively. Broad positive correlation between the particle size and the integrated ion current was observed. The integrated ion currents measured with SG-SiCl 4 and SG-NH 4, which particle sizes of silica were larger than 30µm, were lower than the integrated ion currents measured with other silica-gels with silica particle size less than 1µm. The integrated ion currents measured with SG-HCl and SG-Col Fuso, which silica particle sizes are less than 1µm, also generally increase with decreasing of silica particle size, excepting the extremely high integrated ion current measured with SG- Col Nissan. We also confirmed that the finer size of silica particles in suspension is advantageous to the higher yield of lead ion from the results of SG-Col Fuso. However, it is difficult to simply compare all integrated ion current measured with each silica-gel, because the effect of polycondensed silica particles in SG-NH 4 and SG-SiCl 4 for the beam emission is not clear (which may deter the emission of ion beam). Moreover, the integrated ion current measured with SG-Col Nissan was two times higher than that measured with SG-Col Fuso (PL-7) in spite of similar particle size of silica. These results suggest that other physicochemical conditions (such as surface condition of silica particle etc.) may also influence the emission of ion beam. 3.3 Comparison of reproducibility measured with silica-gels synthesized by different methods In order to examine the reproducibility of the isotope ratio measured with each silica-gel activator described above, repetition measurements of lead reference material (NIST SRM 981) were conducted and results are shown in Figure 3. The uncertainties for the 208 Pb/ 204 Pb ratio of NIST SRM 981 (100 ng) were different 2
depending on the silica-gel used for measurement. Higher reproducibility were realized in the measurements with SG-NH 4 and SG-Col Fuso (PL-7). On the other hand, the uncertainties of the 208 Pb/ 204 Pb ratio in the measurements with SG-SiCl 4, SG- Col Nissan, and SG-Col Fuso (PL-20) were more than four times higher than those in the measurement with SG-NH 4 or SG-Col Fuso (PL-7). It is generally recognized that higher intensity of ion beam current improves the reproducibility of isotope ratio. However, the uncertainty of isotope ratio measured with SG-Col Nissan was six times larger than that of isotope ratio measured with SG-COl Fuso (PL-7), in spite of the integrated ion current measured with SG- Col Nissan was two times higher than measured with SG-COl Fuso (PL-7). In case of SG-Col Nissan, intense fractionation was observed during the measurements. Moreover, the reproducibility of isotope ratio measured with SG-NH 4, which integrated ion current was a little lower than that measured with SG-HCl, was more than two times higher than the reproducibility of isotope ratio measured with SG-HCl. These results indicate that the reproducibility of isotope ratio is not only controlled by intensity of ion beam, but other factors such as isotope fractionation and un-stability of ion beam during measurement also have large effect on the reproducibility of isotope ratio. Gerstenberger, H. and G. Haase, A highly effective emitter substance for mass spectrometric Pb isotope ratio determinations, Chem. Geol., 136, 309-312, 1997. Koide, Y. and E. Nakamura, Lead isotope analyses of standard rock samples, Mass Spectroscopy, 38, 241-252, 1990. Kuritani, T. and E. Nakamura, Precise isotope analysis of nanogramlevel Pb for natural rock samples without use of double spikes, Chem. Geol., 186, 31-43, 2002. Miyazaki, T., T. Shibata and M. Yoshikawa, New synthesis method of silica-gel for lead isotope analysis, Proc. Japan Academy Ser. B,79, 58-62, 2003. Nohda, S., Precise isotopic measurements of lead by fused silica-gel, Geochem. J., 33, 133-139, 1999. 4. Conclusion In this report, we have first experimentally confirmed the relation between the particle size in silica-gel and integrated ion beam current. The broad positive correlation was recognized, but there were other factors (ex. existing of condensed silica, surface condition of silica particle, etc.) controlling the emission of ion beam. Moreover, the high reproducibility of the isotope ratio is not obtained by high intensity of ion beam only, but other conditions such as suppression of isotope fractionation and stability of ion beam are also important. Therefore, in order to realize the high reproducibility of isotope ratio, the selection of silica-gel optimal for the lead isotope measurement is necessary. In this work, we found that SG-Col Fuso (PL-7), which synthesis method is most simple, is the best silica-gel activator for lead isotope measurement, realizing the high intensity of ion beam and high reproducibility of isotope ratio. Acknowledgement. We are especially grateful to Mr. B.S. Vaglarov and Ms. Y. Yonezawa for their technical support in this study. Nissan Chemical Industries, Ltd. and Fuso Chemical Co., Ltd. were kindly provided us a free sample of silicic acid colloidal solutions. Dr. K. Akabane kindly provided us a trial product of phosphoric acid. References Akishin, P. A., O. T. Nikitin and G. M. Panchenkov, A new effective ionic emitter for the isotopic analysis of lead, Geokhimiya, 5, 425, 1957. Doucelance, R. and G. Manhès, Reevaluation of precise lead isotope measurements by thermal ionization mass spectrometry: comparison with determination by plasma source mass spectrometry, Chem. Geol., 176, 361-377, 2001. Esquena, J., R. Pons, N. Azemar, J. Caelles and C. Solans, Preparation of monodisperse silica particles in emulsion media, Colloids Surf. A, 123-124, 575-586, 1997. 3
Figure 1. Particle size distributions of silica in SG-NH 4, SG-HCl, SG-Col Nissan and SG-SiCl 4 measured at 30 seconds and 1 hour after shaking. The d 10, d 50 and d 90 values indicate that 10%, 50% and 90% of the particles measured were less than or equal to the size stated. 4
Figure 2. Comparison of integrated ion current ( 208 Pb) using silica-gel activators with different particle size of silica. The particle sizes of SG-Col Fuso are certified values of products Figure 3. Variation in 208 Pb/ 204 Pb ratios for measurement using different types of silica-gel. The vertical bar with each measurement shows error in 2σ m. The vertical bar with horizontal line shows the average and range of error (2σ) in the 208 Pb/ 204 Pb ratios. 5