A Rapid and Precise Determination of Boron Isotope Ratio in Water and Carbonate Samples by Multiple Collector ICP-MS

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1 ANALYTICAL SCIENCES JUNE 2018, VOL The Japan Society for Analytical Chemistry Original Papers A Rapid and Precise Determination of Boron Isotope Ratio in Water and Carbonate Samples by Multiple Collector ICP-MS Masaharu TANIMIZU,*, ** Kazuya NAGAISHI,*** and Tsuyoshi ISHIKAWA** * Department of Chemistry for Environment, School of Science and Technology, Kwansei Gakuin University, 2-1 Gakuen, Sanda , Japan ** Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and Technology, 200 Monobe-Otsu, Nankoku , Japan *** Marine Works Japan Ltd., 200 Monobe-Otsu, Nankoku , Japan We developed a method for rapid and precise determination of B isotope ratios by MC-ICP-MS through an optimization of washout method, mass-discrimination correction and chemical separation. Resultant reproducibility of δ 11 B values was ±0.4 (2 SD) when a simple standard-sample bracketing technique was used, and it was improved to be better than ±0.2 by a mass discrimination correction with a 7 Li/ 6 Li isotopic reference. A mixed solution, which consists of HNO 3 HF mannitol, allowed a rapid washout of B memory in the sample introduction line. The validation of this technique to a wide range of δ 11 B value and various B signal intensities was confirmed from a series of B reference solutions with δ 11 B values of 20 to +40 and 25 to 125 ng/g B. Analyses of seawater standard (BCR-403) and carbonate standard (JCp-1) with sample sizes of less than 50 ng B gave δ 11 B values consistent with those determined by TIMS as Cs 2BO 2+. The simple and high-precision technique developed here is applicable to various types of commercially supplied multiple collector ICP mass spectrometers without any modification of the sample introduction system from their original instrumental setting. Keywords Isotope ratio, boron, mass discrimination, memory effect, multiple collector ICP-MS (Received January 31, 2018; Accepted April 2, 2018; Published June 10, 2018) Introduction Boron has two stable isotopes, 10 B and 11 B, and their approximate isotopic variation in nature is from 30 to +60 in δ 11 B scale relative to an isotopic reference material, l NIST-SRM From this wide δ 11 B variation among geological and environmental samples, B isotope ratios are used as an isotopic tracer in wide areas of geochemistry and environmental chemistry. Precise B isotope ratios have been traditionally determined by thermal ionization mass spectrometry (TIMS), but inductively coupled plasma mass spectrometry (ICP-MS) has been used widely in the last ten years. The advantage of ICP-MS is its high instrumental sample throughput, because sample solutions are directly nebulized into its atmospheric pressure plasma for ionization. In the case of TIMS, sample solutions must be dried to a solid state before introduction into a mass spectrometer in which the samples are heated carefully under vacuum conditions to obtain stable ion beams. Relative insensitivity against matrix elements remaining in sample solutions is another benefit of ICP-MS through its high ionization efficiency, while rigorous chemical purification is required for TIMS analysis due to its matrix element sensitivity, as reported for B and Li, for example. 3,4 ICP-MS is therefore more suitable for rapid routine To whom correspondence should be addressed. tanimizum@kwansei.ac.jp isotopic analysis. ICP-MS has several potential problems however, such as spectral interferences, memory effect, and instability of mass discrimination effect, that make it difficult to acquire high reproducible isotopic data with high sample throughput. In the case of B isotopic analysis, 40 Ar 4+ on 10 B + is the most serious spectrometric interference. This can be solved by the use of multiple collector (MC) ICP-MS. The interference of 40 Ar 4+ could not be fully separated from 10 B + with quadrupole ICP-MS and single collector sector field ICP-MS in the low resolution mode, 5 but could be separated with double-focusing MC-ICP- MS even in the low resolution mode. 6 Another problem with B isotopic analysis by ICP-MS is the large memory effect of B in its sample introduction line. Several washout protocols have been proposed to eliminate B memory effectively from the sample introduction line. 7 One recent approach to solve this problem was to reduce B vaporization under high ph conditions in a PFA spray chamber. Ammonia gas introduced from an additional gas port kept the inner surface of the spray chamber in a high ph condition, which stabilizes less volatile B(OH) 4 than B(OH) 3. 6 Otherwise, a direct injection nebulizer was used to minimize the inner surface volume of the sample introduction line without a spray chamber. 8 Here, we present an alternative method for a rapid and effective rinse of B memory, in which a mixed solution of hydrofluoric acid, nitric acid, and mannitol is used for sample washout. We use a conventional sample introduction system of MC-ICP-MS instead of the special sample introduction systems

2 668 ANALYTICAL SCIENCES JUNE 2018, VOL. 34 Table1Typical operational settings of MC-ICP-MS ICP ion source RF frequencies MHz RF power 1.2 kw forward, <1 W reflection Fassel-type torch Ar gas flow rates Outer 15 L/min Intermediate 0.70 L/min Central 1.05 L/min Nebulizer Glass Expansion OpalMist PFA concentric nebulizer Spray chamber Dual cyclonic/double Scott glass chamber (ambient temperature) Sample uptake 50 μl/min (free aspiration) Injector Quartz injector Interface cones Nickel sampler and X-skimmer Mass spectrometer Ion energy Extraction Faraday cups Typical transmission B for NH 3 gas addition or direct injection nebulization, which enables precise B isotopic analysis without supply of toxic and flammable NH 3 gas in the analysis room and with minimum risk of ICP torch melting compared to direct nebulization. We show a protocol for instrumental optimization for precise B isotopic analysis using a conventional ICP-MS sample introduction setting, universally applicable to MC-ICP-MS facilities in various fields. We also monitor 7 Li/ 6 Li ratios during 11 B/ 10 B analysis to understand the degrees of the mass discrimination in the light mass range for a better application of the Li-doping technique, which was tried previously. 9 We compare the δ 11 B values obtained for reference materials including seawater and carbonate to those obtained by TIMS operated in positive mode (P-TIMS), and evaluate the accuracy, rapidity, and optimum sample size for the MC-ICP-MS. Experimental V 2000 V L4( 6 Li), L1( 10 B), H1( 11 B), and H4( 7 Li) A/ppm 11 B at low resolution Li A/ppm 7 Li at low resolution Uptake time 80 s Number of cycles 30 Integration time 8 s (each B, Li) Idle time 3 s Pressures during operation ESA Analyzer Pa ( Pa during standby) Pa ( Pa during standby) Instrumentation The instrument used in this study is a multiple collector ICP mass spectrometer, Thermo Finnigan Neptune (Thermo Instruments, Bremen, Germany) in Kochi Core Center (KCC), Kochi, Japan. This Nier Johnson-type double-focusing mass spectrometer achieves an effective mass dispersion of 17% and produces flat-topped, symmetrical peak shapes. 10,11 The instrument is equipped with a low flow PFA nebulizer attached with a quartz spray chamber (cyclonic/scott double pass), and aerosols are introduced to a typical Fassel-type quartz ICP torch; the current instrumental setting is detailed in Table 1. In this instrumental setting as commonly used for conventional ICP-MS, a 10 V signal was achieved for 11 B ion signal current for 1 μg/g B solution through Ω amplifier register with X-type skimmer cone. Low resolution mode (M/ΔM 500 at 10% height) was enough to resolve 40 Ar 4+ from 10 B + as shown previously, 6 and the width of the defining slit at the end of the ion focusing unit was set to do that. Among nine Faraday collectors, ion signals of 10 B and 11 B were simultaneously determined with L1 and H1 collectors. Further, L4 and H4 collectors were arranged to detect 6 Li and 7 Li ion signals to monitor 7 Li/ 6 Li isotope ratios during 11 B/ 10 B analysis to evaluate temporal drift of mass discrimination effect during B isotopic analysis. Boron isotope data were cross-checked by P-TIMS using Cs 2BO 2 + ions. A thermal ionization mass spectrometer Thermo Finnigan Triton (Thermo Instruments, Bremen, Germany) in KCC was used for this purpose. Detailed procedures for P-TIMS are described previously. 12 Isotope standard material, reagents, and samples A 1000 μg/g B standard solution (Merck, USA, Lot. HC893994, CertiPUR) was used as a shelf standard in this study, and its 11 B/ 10 B ratios were described as relative deviation from isotopic standard in the form of boric acid, NIST-SRM 951, in parts per 10 3 ( donation; δ-scale). The reagents were diluted by 18.2 MΩ cm Milli-Q water (MILLIPORE, USA) to prepare a 10 μg/g stock solution that was subsequently adjusted to 75 ng/g with several solutions (see below). A Li 1000 μg/g solution (SPEX Certiprep., USA, Lot Li) was also prepared in a similar manner, and was added to the diluted B solution so as to contain 100 ng/g Li. Three recently certified B isotopic reference materials (ERM-AE120, AE121, and AE122; 100 mg/kg B in 20 ml PFA bottles) provided from BAM were additionally prepared to validate our analytical method in a wide δ 11 B range ( 20, +20, and +40). 13 Determination of δ 11 B values was also performed for a seawater reference sample BCR-403 and a coral geochemical standard JCp-1 issued by IRMM and Geological Society of Japan, respectively. Extraction of B from marine samples The chemical procedure of B separation from the seawater and carbonate samples used in this study follows that developed for P-TIMS. 12 Seawater samples were prepared by mixing of 0.05 ml seawater, 10 μl 1% mannitol solution, and 1 ml 0.1 M HCl. For carbonate sample, 4 mg powder was dissolved in 1.0 ml 0.1 M HCl with 10 μl 1% mannitol. The sample solutions were then loaded onto a 0.1 ml cation exchange resin column (Biorad AG 50W-X12, mesh, 3.4 mm diameter 11 mm length). Subsequently, 0.2 ml H 2O was loaded onto the column, and the total 1.2 ml eluate was collected. After adding 38% HF to yield a HF concentration of 1 M, the boron-containing solution was loaded onto the second 0.03 ml anion exchange column (Biorad AG1-X4, mesh, 2.2 mm diameter 8 mm length). Boron fraction was collected with 0.15 ml of 6 M HCl after 0.12 ml elution of matrix elements with a mixed acid composed of 0.5 M HF and 2 M HCl. If necessary, the B fraction was subdivided into two aliquots, one for MC-ICP-MS, and the other for P-TIMS. As the rapid method, an aliquot of the boron containing solution obtained after the first column was used for MC-ICP-MS without further purification of boron. All these sample solutions were evaporated to dryness at 45C with mannitol and dissolved with an adequate solvent for the isotopic analysis (see below).

3 ANALYTICAL SCIENCES JUNE 2018, VOL Fig.1Boron washout profiles for various HNO 3 HF mannitol solutions. Combinations of the three compounds are shown in (a), and 0.15 M and 0.5 M HNO 3 based solutions in (b), and required amount of mannitol in (c). A washout profile for a HNO 3 solution with NH 3 gas addition to the spray chamber 7 is shown for comparison. All acid reagents used in this study were commercially supplied high-purity reagents TAMAPURE AA-100 (Tama Chemical, Japan), and all chemical procedures were carried out in a class-1000 clean room and class-100 clean benches equipped with a boron-free ULPA or a low-boron HEPA filter. Total procedural blank of B was less than 200 pg, typically around 100 pg, which was negligible in our routine analysis. Results and Discussion Optimization of washout solution Several chemical compositions of washout solutions were suggested to be effective for removal of B memory from the sample introduction line of ICP-MS (HF, mannitol, NH 3, and so on) as summarized previously. 7,14 In this study, we re-evaluated HNO 3 HF mannitol-based rinse solutions in detail, because this combination of solutions matches our sample preparation procedure using HF and mannitol, 12,15 and also does not require any modification of sample introduction system for the routine isotopic analysis of other elements. Washout profiles of 0.15 M or 0.5 M HNO 3 solutions with or without HF and mannitol are shown in Fig. 1. It is clear that a mixed solution of HNO 3 HF mannitol yields the effective washout of boron (Fig. 1a) by making stable B complexes with F and mannitol in solution, and both a HF concentration of 0.05 M and a mannitol concentration of 0.1% are required for the most rapid washout (Figs. 1b and 1c). Thus the rinse solution composed of 0.15 M/0.5 M HNO 3, 0.05 M HF and 0.1% mannitol was optimum. The washout time, for which the 11 B signal decreases to 0.1% from a 75 ng/g B solution signal, was about 1 min. The effectiveness was better than that achieved by NH 3 HNO 3 (Fig. 1b), 7 and the washout profile of Pb signals by HNO 3 with the same sample introduction system. 16 We used this optimized HNO 3 HF mannitol solution not only for wash out but also for the dilution of standard and sample solutions for B isotopic analysis. Precision and reproducibility of δ 11 B value with and without 7 Li/ 6 Li-based correction The shelf B standard solution (Merck B solution) and NIST- SRM 951 solution were diluted to 75 ng/g with the optimized HNO 3 HF mannitol solution for the isotopic analysis. After a Li solution (SPEX Li) was added to the sample solution to monitor degrees of mass discrimination, δ 11 B value of the Merck B solution was determined against NIST-SRM 951 based on their 11 B/ 10 B isotope ratios by means of standard sample bracketing technique. On-peak background subtraction protocol was applied to remove the remaining B signals after the washing out time of 5 min. One measurement run consisting of 30 cycles (8 s integration time per cycle), required about 4 min to obtain one 11 B/ 10 B ratio. About 30 min were therefore required to complete one set of standard-sample bracketing (standardsample-standard) analysis to obtain one δ 11 B value with sample consumption of about 45 ng B. The repeated isotopic analysis of the Merck B solution is shown in Table 2 and Fig. 2 (open symbols). The average value was ± 0.41 (2 SD, n = 26), which was consistent with the P-TIMS value (+0.15 ± 0.09; 2 SD, n = 6). Ion signals of 6 Li and 7 Li were monitored during the B isotopic analysis described above, and a strong positive correlation between 7 Li/ 6 Li and 11 B/ 10 B ratios was observed. An example of temporal variations in raw 11 B/ 10 B and 7 Li/ 6 Li ratios observed during analytical sessions for three successive days are shown in Figs. 3a and 3b. We observed a short-term ( mins), irregular fluctuation of isotope ratios superimposed on a regular longterm ( hours) drift. It is difficult to correct the former component of the mass discrimination precisely by simple standard-sample bracketing. However, the strong positive correlation between 7 Li/ 6 Li and 11 B/ 10 B ratios was maintained throughout the analytical session (Fig. 3d), which indicated that the short-term fluctuation resulted from a temporal massdependent change in the mass discrimination effect in the instrument. Therefore, we examined the SPEX 7 Li/ 6 Li-based mass discrimination correction as applied previously. 9 Here, raw 11 B/ 10 B ratios obtained were first corrected by the exponential law (Fig. 3c) on the basis of the SPEX 7 Li/ 6 Li value of (pre-determined against L-SVEC Li; NIST RM8545), and then the δ 11 B values were calculated from the Li-corrected 11 B/ 10 B ratios by means of standard-sample bracketing method in the same manner described above (the details of the exponential law are shown in Supporting Information). Resultant δ 11 B values for Merck B (Fig. 2, closed symbols) gave the average value of ± 0.16 (2 SD, n = 26). Thus, analytical reproducibility was clearly improved compared to the non-li-corrected runs without the average δ 11 B values being changed. In the case of the run with the Li correction, it takes

4 670 ANALYTICAL SCIENCES JUNE 2018, VOL. 34 Table2Boron isotope ratios for standard samples MC-ICP-MS Sample Li-corrected Bracketing only N P-TIMS N References δ 11 B, 2SD, δ 11 B, 2SD, δ 11 B, 2SD, δ 11 B, 2SD, Method Merck B CertiPUR ERM AE Vogl and Rosner (2012) (2se) Foster et al. (2013) 21 N-TIMS Roux et al. (2015) 22 MC-ICP-MS ERM AE Vogl and Rosner (2012) 13 Certificate (2se) Foster et al. (2013) 21 N-TIMS Roux et al. (2015) 22 MC-ICP-MS ERM AE Vogl and Rosner (2012) 13 Certificate GSJ JCp-1 (Coral, Ishigaki Isl., Japan) IRMM BCR-403 (Seawater, Atlantic) DI-MC-ICP-MS b (2se) Foster et al. (2013) 21 N-TIMS Normal method (1st and 2nd columns) a Wang et al. (2010) 9 P-TIMS (2se) Foster et al. (2013) 21 N-TIMS Douville et al. (2010) 23 Rapid method (1st column only) Wang et al. (2010) 9 MC-ICP-MS Dissard et al. (2012) 24 DI-MC-ICP-MS b Shinjo et al. (2013) 25 MC-ICP-MS McCulloch et al. MC-ICP-MS (2014) 26 Normal method (1st and 2nd columns) Seawater samples Seawater source c a Lemarchand et al. (2002) 27 P-TIMS A (CASS-2) DI-MC-ICP-MS b P Aggarwal et al. (2004) 28 Rapid method (1st column only) Foster (2008) 6 MC-ICP-MS P, A, M, S Foster et al. (2010) 29 MC-ICP-MS P, A, M, S Louvat et al. (2011) 8 DI-MC-ICP-MS b A (NASS-5) Douville et al. (2010) 23 DI-MC-ICP-MS b A (NASS-2) Roux et al. (2015) 22 MC-ICP-MS P Dissard et al. (2012) 24 DI-MC-ICP-MS b A (NASS-5) a. Data from Ishikawa and Nagaishi (2011). 12 b. DI-MC-ICP-MS: Direct Injection-MC-ICP-MS. c. P: Pacific, A: Atlantic, M: Mediterranean, S: Southern Ocean. which results in one sample/hour sample throughput for the δ 11 B determination through standard-sample bracketing after Li correction. The reproducibility of δ 11 B observed here (±0.16) is better than those reported by MC-ICP-MS previously ( ±0.25). 6,8 Fig.2Repeated δ 11 B analyses of a 75 ng/g Merck B solution with the optimized washout protocol. 12 min to complete the 30 cycles of isotopic measurement with peak jumping between 11 B/ 10 B and 7 Li/ 6 Li ratio detections, Application to samples with a wide δ 11 B range Application of the optimized washout and mass spectrometric protocols to samples with a wide isotopic variation was tested using a series of B isotopic reference materials: ERM AE120, AE121, and AE122 with δ 11 B values of 20 ( 20.2 ± 0.6), +20 (+19.9 ± 0.6), and +40 (+39.7 ± 0.6), respectively. 13 The δ 11 B values obtained by MC-ICP-MS with Li-correction were ± 0.19 (2 SD, n = 20), ± 0.14 (n = 19), and ± 0.20 (n = 56) for ERM AE120, AE121, and AE122, respectively. These values are consistent with the certified values and our P-TIMS values (Table 2 and Figs. 4a 4c). For ERM AE122, the δ 11 B values were also determined in a wide B concentration range from 25 to 125 ng/g against a constant 75 ng/g NIST-SRM 951 solution. Resultant δ 11 B

5 ANALYTICAL SCIENCES JUNE 2018, VOL Fig.3An example of temporal change in mass discrimination in (a) raw 11 B/ 10 B, (b) raw 7 Li/ 6 Li, and (c) Li-corrected 11 B/ 10 B ratios without bracketing, observed during analytical sessions for three successive days. The elapsed time means cumulative analysis time for the three days. The correlation of isotopic drift between raw 11 B/ 10 B and 7 Li/ 6 Li ratios is also shown in (d). Fig.4Repeated δ 11 B analyses of (a) 75 ng/g B solution of AE120, (b) 75 ng/g B of AE121, (c) 75 ng/g B of AE122, and (d) ng/g B of AE122. Both the data with/without Li-correction are shown.

6 672 ANALYTICAL SCIENCES JUNE 2018, VOL. 34 Fig.5Matrix dependence of δ 11 B values of Merck B solution against doped matrix elements. Its reference value with uncertainty obtained by P-TIMS is shown as a hatched line. values of ERM AE122 were consistent within analytical uncertainty compared to its certified value in the entire concentration range (Fig. 4d). These results demonstrate that the analytical protocols developed here are applicable to various geological and environmental samples with varied δ 11 B values, without strict matching of B concentration between sample and standard. Matrix robustness The relative matrix insensitiveness compared to TIMS is one of the important advantages of ICP-MS. The degree of robustness in our analytical protocol was tested by adding Na, Mg, Al, Si, Ca, and Fe, which are major matrix elements in water, carbonates, and sediment samples. The matrix elements were doped to 75 ng/g Merck B solutions to give their concentrations of 1, 5, and 10 μg/g. The MC-ICP-MS analyses revealed that even in the case of 10 μg/g, no δ 11 B drift beyond analytical uncertainty was observed for any matrix element (Fig. 5). The δ 11 B values were also insensitive to the various mannitol concentrations (Fig. 6a). In contrast to the robustness observed for matrix elements and mannitol, however, the δ 11 B values were found to be sensitive to HNO 3 concentration as reported previously. 8 In Fig. 6b, δ 11 B values of Merck B solutions with varied HNO 3 concentrations from 0.05 to 0.55 M, determined against NIST-SRM 951 dissolved in a constant 0.5 M HNO 3 solution, are shown. A slight deviation of the sample HNO 3 concentration from the matched HNO 3 concentration resulted in lower precision and accuracy in δ 11 B even after the Li correction (Fig. 6b). HNO 3 is an acid commonly used to separate boron from matrix elements with a boron-specific resin, Amberlite IRA-743. However, our result suggests that evaporation of the chemically separated B fraction is necessary to control the HNO 3 concentration of the sample solution strictly to match that of standard solution for precise B isotopic analysis. The HNO 3-sensitiveness of δ 11 B even after Li correction is rather unexpected, compared with the case of Pb isotopic analysis in which Tl-doping efficiently eliminates matrix effects induced from major element impurity and various HNO 3 concentrations. 16 This may indicate the limitation of the element doping technique in lighter mass regions, or in the elements showing quite different chemical behaviors in solutions like B and Li. B isotopic composition of reference seawater and carbonate samples B isotopic compositions determined for seawater (BCR-403)

7 ANALYTICAL SCIENCES JUNE 2018, VOL and carbonate (JCp-1) reference samples are summarized in Table 2 and Fig. 7. Their MC-ICP-MS data with Li-correction, ± 0.18 and ± 0.19 (2 SD, n = 23), respectively, obtained through the two-step column chemistry (normal method) were consistent with those determined by P-TIMS previously ( ± 0.11 and ± 0.08). 12 Moreover, precision and accuracy of the data obtained without the second column separation (rapid method: ± 0.19, n = 31 and ± 0.17, n = 15) were comparable to those obtained with the two-step column separation. This verifies the matrix robustness of our MC-ICP-MS technique and advantage over the P-TIMS that requires strict removal of matrix elements with the two-step column separation. The column chemistry of the rapid method takes only 1 h, and enables high-throughput B isotope analysis for natural water and carbonate samples. Such a rapid B isotope analysis with a precision better than ±0.2 will be very useful for the studies that require analyses of hundreds of samples, such as groundwater assessment studies. 17 Fig.6Matrix dependence of δ 11 B values of Merck B solution for mannitol and HNO 3 concentrations. In (a), mannitol concentrations of Merck B solution was varied against constant 0.10% mannitol in NIST-SRM 951, and various HNO 3 concentrations were used for the Merck B solution against a constant 0.50 M HNO 3 in NIST-SRM 951 in (b). Comments on application of MC-ICP-MS to B isotopic analysis Here, we determined 11 B/ 10 B (δ 11 B values) with the precision of about ±0.4 using a standard-sample bracketing approach without Li-correction (Table 2). This precision is comparable to ±0.39 for 7 Li/ 6 Li 18 and less precise than ±0.1 for 26 Mg/ 24 Mg 19 obtained by the same instrument at KCC with similar standard-sample bracketing techniques. This indicates that the instrumental drift of mass discrimination effect is very variable at the lightest mass ranges. The better reproducibility of δ 11 B values after Li-correction (±0.14 to ±0.19; Table 2) also suggests a mass-dependent feature of the mass discrimination effect in MC-ICP-MS in the lightest mass ranges. The spectral background in such light mass region is also noisy, and its stability is essential to obtain a better precision after on peak subtraction of the background signals. Especially around 10 B, Fig.7Repeated δ 11 B analyses of (a) BCR-403 (seawater) and (b) JCp-1 (coral).

8 674 ANALYTICAL SCIENCES JUNE 2018, VOL. 34 scattered ions in the instrument potentially result in the apparent B isotopic shift observed during washout. 8 Mass-discrimination correction of 11 B/ 10 B with 7 Li/ 6 Li was already tried previously. 9 They reported Li-corrected δ 11 B values for actual samples (seawater and carbonate) with uncertainties of around ±0.3 to ±0.4 (2 SD), and here we improved the uncertainties to ±0.17 to ±0.19 (Table 2). We consider that this improvement resulted from the strict matrix matching between standard and samples owing to the highmatrix HNO 3 HF mannitol-based solution, which enabled effective reduction of additional matrix effects originated from small impurities that remained after chemical purification of the actual samples. Conclusion We optimized a washout protocol for a rapid rinse of B signal in a conventional sample introduction line, and enabled precise B isotope analyses by MC-ICP-MS for a wide range of δ 11 B values. The robustness of our MC-ICP-MS technique in terms of varied chemical impurities (up to 10 μg/g) and B concentrations (25 to 125 ng/g) was verified. The reproducibility of δ 11 B values in reagent standards was about ±0.4 and ±0.14 to ±0.19 (2 SD) without and with the 7 Li/ 6 Li correction, respectively, with a consumption of 45 ng B in a single analytical run. B isotope ratios of seawater and carbonate samples determined by MC-ICP-MS with a rapid separation chemistry were in good agreement with those determined by P-TIMS. High sample throughput capability of our MC-ICP-MS technique with an excellent δ 11 B reproducibility of ±0.17 to ±0.19 is advantageous to rapid analyses of natural water and carbonate samples. The reproducibility described above was kept throughout the analysis of hundreds of actual samples for about four years, only with the routine maintenance of the ion extraction lens in the ion optics assembly. Our B isotope analytical technique can be applied to various commercially supplied multiple collector ICP mass spectrometers under standard instrumental settings. This technique is applicable to various fields of geochemistry, including the estimation of paleo-seawater ph from δ 11 B values recorded in fossil marine carbonates (e.g. Henderson, 2002) 20 to evaluate IPCC paleo climate models, and the tracing of anthropogenic nitrate contamination sources to groundwater from the point of view of drinking-water quality (e.g. Tirez et al., 2010). 17 Supporting Information The explanation about the mass discrimination correction of 11 B/ 10 B isotope ratios with 7 Li/ 6 Li by the exponential law is available free of charge on the Web at analsci/. Acknowledgements We thank Dr. J. Matsuoka for assistance in P-TIMS analyses. Illustration editing was helped by Ms. M. Sasaoka. This study was supported in part by the Ministry of Education, Culture, Sports, Science and Technology and the Japan Society for the Promotion of Science as KAKENHI (grants-in-aid for scientific research; Grant Numbers , , , and 17K00527 to MT and 16H04066 to TI). References 1. J. K. Aggarwal and M. R. Palmer, Analyst, 1995, 120, E. J. Catanzaro, C. E. Champion, E. L. Garner, G. Marinenko, K. M. Sappenfield, and W. R. Shields, Boric Acid; Isotopic and Assay Standard Reference Materials, US National Bureau of Standards, 1970, Nat. Bur. Stand. (U.S.), Spec. Publ E. Nakamura, T. Ishikawa, J.-L. Birck, and C. J. Allègre, Chem. Geol., 1992, 94, P. B. Tomascak, R. W. Carlson, and S. B. Shirey, Chem. Geol., 1999, 158, J. Vogl, M. Rosner, and W. Pritzkow, J. Anal. At. Spectrom., 2011, 26, G. L. Foster, Earth Planet. Sci. Lett., 2008, 271, A. S. Al-Ammar, R. K. Gupta, and R. M. Barnes, Spectrochim. Acta, Part B, 2000, 55, P. Louvat, J. Bouchez, and G. Paris, Geostand. Geoanal. Res., 2011, 35, B.-S. Wang, C.-F. You, K.-F. Huang, S.-F. Wu, S. K. Aggarwal, C.-H. Chung, and P.-Y. Lin, Talanta, 2010, 82, S. Weyer and J. B. Schwieters, Int. J. Mass. Spectrom., 2003, 226, M. E. Wieser and J. B. Schwieters, Int. J. Mass. Spectrom., 2005, 242, T. Ishikawa and K. Nagaishi, J. Anal. At. Spectrom., 2011, 26, J. Vogl and M. Rosner, Gestand. Geoanal. Res., 2012, 36, S. K. Aggarwal and C.-F. You, Mass Spectrom. Rev., 2017, 36, K. Nagaishi and T. Ishikawa, Geochem. J., 2009, 43, M. Tanimizu and T. Ishikawa, Geochem. J., 2006, 40, K. Tirez, W. Brusten, D. Widory, E. Petelet, A. Bregnot, D. Xue, P. Boeckx, and J. Bronders, J. Anal. At. Spectrom., 2010, 25, Y. Nishio, K. Okamura, M. Tanimizu, T. Ishikawa, and Y. Sano, Earth Planet. Sci. Lett., 2010, 297, T. Yoshimura, M. Tanimizu, M. Inoue, A. Suzuki, N. Iwasaki, and H. Kawabata, Anal. Bioanal. Chem., 2011, 401, G. M. Henderson, Earth Planet. Sci. Lett., 2002, 203, G. L. Foster, B. Honisch, G. Paris, G. S. Dwyer, J. W. B. Rae, T. Elliott, J. Gaillardet, N. G. Hemming, P. Louvat, and A. Vengosh, Chem. Geol., 2013, 358, P. Roux, D. Lemarchand, H. J. Hughes, and M.-P. Turpault, Geostand. Geoanal. Res., 2015, 39, E. Douville, M. Paterne, G. Cabioch, P. Louvat, J. Gaillardet, A. Juillet-Leclerc, and L. Ayliffe, Biogeosciences, 2010, 7, D. Dissard, E. Douville, S. Reynaud, A. Juillet-Leclerc, P. Montagna, P. Louvat, and M. McCulloch, Biogeosciences, 2012, 9, R. Shinjo, R. Asami, K.-F. Huang, C.-F. You, and Y. Iryu, Mar. Geol., 2013, 342, M. T. McCulloch, M. Holcomb, K. Rankenburg, and J. A. Trotter, Rapid. Commun. Mass Spectrom., 2014, 28, D. Lemarchand, J. Gaillardet, C. Göpel, and G Manhès, Chem. Geol., 2002, 182, J. K. Aggarwal, K. Mezger, E. Pernicka, and A. Meixner, Int. J. Mass Spectrom., 2004, 232, G. L. Foster, P. A. E. Pogge von Strandmann, and J. W. B. Rae, Geochem. Geophys. Geosys., 2010, 11, Q08015.