Fractionation Correction Methodology for Precise and Accurate

Aug 3, 2009 - The validity of the method has been demonstrated by analyzing a NIST-SRM-951 boron isotopic certified standard, two synthetic B mixtures...
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Anal. Chem. 2009, 81, 7420–7427

Fractionation Correction Methodology for Precise and Accurate Isotopic Analysis of Boron by Negative Thermal Ionization Mass Spectrometry Based on BO2- Ions and Using the 18O/16O Ratio from ReO4- for Internal Normalization Suresh K. Aggarwal,†,‡ Bo-Shian Wang,† Chen-Feng You,*,† and Chuan-Hsiung Chung† Earth Dynamic System Research Centre, Department of Earth Sciences, National Cheng Kung University, Tainan, Taiwan, Republic of China, and Fuel Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085, India A novel approach to obtain a fractionation free 11B/10B isotope ratio based on oxygen isotopes determined in situ from the same filament loading by N-TIMS is described. The method uses only a few nanograms of B to produce BO2- ions. First, the oxygen isotopes are determined at a lower filament temperature using ReO4- ions and employing 187Re/185Re for internal normalization. Subsequently, the filament temperature is increased to get sufficient BO2- ions and predetermined 18O/16O isotopes from the same filament loading is used to correct for boron mass fractionation. The validity of the method has been demonstrated by analyzing a NIST-SRM-951 boron isotopic certified standard, two synthetic B mixtures, and two coral reference materials. An average analytical precision of 0.6‰ (n ) 6) has been demonstrated. This is an important and crucial step forward in making the application of BO2- ions by N-TIMS routine in coral, foraminifera, and other samples where only limited amounts of boron are available. This new method does not require any additional effort in loading or in carrying out the mass spectrometric analysis but eliminates the need of assuming a fixed 18O/16O ratio and thus provides higher accuracy for applications in paleo-oceanography, geochemistry, and cosmochemistry.

respectively.1-4 Second, 10B has a high thermal neutron absorption cross-section, and therefore, enriched 10B is highly useful in nuclear technology. These different applications demand the availability of methods to determine precise and accurate isotopic composition of boron. On one side, the studies depend upon measuring small variations (+50‰ to -50‰), compared to natural isotopic composition, in coral and foraminifera samples containing a few nanograms of B. On the other side, measurements are needed in a variety of boron samples containing different enrichments (20-90 atom %) of 10B from enrichment plants for nuclear technology. A number of mass spectrometric techniques are available which can be used for determining the isotopic composition of boron. These include thermal ionization mass spectrometry (TIMS),5-21 inductively coupled plasma source mass spectrometry (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11)

Precise and accurate isotopic analyses of boron (B) are required in many branches of science including oceanography, environmental sciences, isotope hydrology, cosmology, and nuclear technology. This is due to the interesting properties of the two isotopes, 10B (19.9 atom %) and 11B (80.1 atom %) in natural boron. First, since there is a relative large mass difference between the two isotopes, the high mobility of B leads to significant 11B/10B fractionation in nature with 10B and 11B existing as B(OH)4- tetragonal and B(OH)3 trigonal phase,

(12) (13) (14) (15) (16) (17) (18) (19)

* To whom correspondence should be addressed. E-mail: cfy20@ mail.ncku.edu.tw. † National Cheng Kung University. ‡ Bhabha Atomic Research Centre.

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(20) (21)

Barth, S. Geol. Rundsch. 1993, 82, 640–651. Palmer, M. R.; Pearson, P. N.; Cobb, S. J. Science 1998, 282, 1468–1471. Pearson, P. N.; Palmer, M. R. Science 1999, 284, 1824–1826. Klochko, K.; Kaufman, A. J.; Yao, W.; Byrne, R. H.; Tossell, J. A. Earth Planet. Sci. Lett. 2006, 248, 276–285. Aggarwal, J. K.; Palmer, M. R. Analyst 1995, 120, 1301–1309. Aggarwal, J. K.; Mezger, K.; Pernicka, E.; Mieixner, A. Int. J. Mass Spectrom. 2004, 232, 259–263. Spivack, A. J.; Edmond, J. M. Anal. Chem. 1986, 58, 31–35. Ramakumar, K. L.; Parab, A. R.; Khodade, P. S.; Alamula, A. I.; Chitambar, S. A.; Jain, H. C. J. Radioanal. Nucl. Chem. 1985, 94, 53–62. Nakano, T.; Nakamura, E. Int. J. Mass Spectrom. 1998, 176, 13–21. Xiao, Y. Y.; Beary, E. S.; Fassett, J. D. Int. J. Mass Spectrom. Ion Processes 1988, 85, 203–213. Wei, H.; Xiao, Y.; Sun, A.; Zhang, C.; Li, S. Int. J. Mass Spectrom. 2004, 235, 187–195. Rao, R. M.; Parab, A. R.; Sasibhushan, K.; Aggarwal, S. K. Int. J. Mass Spectrom. 2008, 273, 105–110. Rao, R. M.; Parab, A. R.; Sasibhushan, K.; Aggarwal, S. K. Int. J. Mass Spectrom. 2009, 285, 120-125. Shen, J. J.-S.; You, C.-F. Anal. Chem. 2003, 75, 1972–1977. Foster, G. L.; Ni, Y.; Haley, B.; Elliott, T. Chem. Geol. 2006, 230, 161–174. Zeininger, H.; Heumann, K. G. Int. J. Mass Spectrom. Ion Phys. 1983, 48, 377–380. Duchateau, N. L.; De Bievre, P. Int. J. Mass Spectrom. Ion Processes 1983, 54, 289. Duchateau, N. L.; Verbruggen, A.; Hendrickx, F.; De Bievre, P. Talanta 1986, 33, 291–294. Vengosh, A.; Chivas, A. R.; McCulloch, M. T. Chem. Geol. 1989, 79, 333– 343. Hemming, N. G.; Hanson, G. N. Chem. Geol. 1994, 114, 147–156. Sonoda, A.; Makita, Y.; Ooi, K.; Hirotsu, T. J. Nucl. Sci. Technol. 2002, 39, 295–302. 10.1021/ac901224m CCC: $40.75  2009 American Chemical Society Published on Web 08/03/2009

(ICPMS),22 glow discharge mass spectrometry (GDMS), secondary ion mass spectrometry (SIMS),23 laser based mass spectrometry,24,25 spark source mass spectrometry,26 etc. Among these, TIMS and ICPMS are the two most popular mass spectrometric techniques which are used for B isotopic analysis. In view of the high ionization potential of B, it is not possible to generate B+ ions in TIMS, and therefore, alkali metaborate ions (M2BO2+) are commonly used for positive-TIMS.5-13 This requires mixing the purified B with a suitable amount of the alkali metal compound so as to produce an optimum B/alkali metal ratio for obtaining a good yield of M2BO2+ ions and high precision. Negative-TIMS using a special loading procedure to reduce the electron work function of the filament and employing BO2- ions has been shown to provide high sensitivity for B isotopic measurements.14-21 The P-TIMS requires a boron amount of 50-1000 ng, whereas a few picograms of boron are sufficient to perform isotopic analysis using N-TIMS. Routinely, however, a few nanograms of boron are used for N-TIMS to circumvent the problem of contamination from reagents and laboratory blank. Both P-TIMS and N-TIMS demand that B should be available in highly purified form. In addition, a variable systematic error due to isotope fractionation in TIMS is one of the limitations which degraded the external precision achievable in isotope ratio measurements. P-TIMS using Cs2BO2+ has been developed and shown by different research groups7-9 to provide the best precision and accuracy. Studies have also been reported recently to show that equally comparable precision and accuracy can be achieved in B isotope ratios by Na2BO2+ ions.12,13 In addition, there is no memory effect in TIMS allowing the isotopic analysis of samples containing a widely varying % of 10B. Since more time is required per analysis in MC-TIMS compared to magnetic sector based multicollector ICPMS, the latter has become quite popular for B isotopic analysis, particularly for samples with isotopic composition close to natural boron (i.e., δ11B values of +100‰ to -100‰ which means 10B abundance varying from 18 to 22 atom %). However, about 50 ng of boron is essential for isotopic analysis by MC-ICPMS. In addition, there is an inherent memory problem which prevents the determination of B isotope ratios in unknown samples containing widely different atom % of 10B. Also, there is comparatively significant background encountered in ICPMS, and this restricts handling samples containing only a few nanograms or subnanogram amounts of boron. Extensive research is going on to further enhance the capabilities of both TIMS as well as MC-ICPMS for achieving high precision and accuracy in B isotopic analysis using a few nanogram amounts of B. Recently, total evaporation in N-TIMS using 300 pg of B has been shown to minimize the effect of mass fractionation.15 However, a time of 2 h was necessary for complete exhaustion of the sample, and this required a strict control of the laboratory and sample loading procedure blank. Interlaboratory (22) Foster, G. L. Earth Planet. Sci. Lett. 2008, 271, 254–266. (23) Rollion-Bard, C.; Vigier, N.; Spezzaferri, S. Chem. Geol. 2007, 244, 679– 690. (24) Manoravi, P.; Joseph, M.; Sivakumar, N.; Balasubramanian, R. Anal. Sci. 2005, 21, 1453–1455. (25) Manoravi, P.; Joseph, M.; Sivakumar, N. Int. J. Mass Spectrom. 2008, 276, 9–16. (26) Lukaszew, R. A.; Marrero, J. G.; Cretella, R. F.; Noutary, C. J. Analyst 1990, 115, 915–918.

intercomparison experiments are also conducted periodically to assess the capabilities of new developments from time to time.27-29 From the latest interlaboratory results,29 only 15 laboratories out of a total of 28 laboratories reported the results and 5 of them using N-TIMS. It was reported29 that both MC-ICPMS and P-TIMS using Cs2BO2+ ions are the two best approaches for achieving the best precision and accuracy in the isotopic analysis of boron. It is also recognized that though N-TIMS has the potential to analyze samples containing much smaller amounts of B than with P-TIMS and MC-ICPMS, isotope fractionation remains one of the major problems preventing widespread use of the methodology. The isobaric interference at m/z 42 from CNO- can, however, be monitored using CN- ion at m/z 26 prior to mass spectrometric analysis. Since boron contains only two isotopes, it is not possible to employ other methodologies like internal normalization based on an invariant isotope ratio (e.g., in Sr and Nd) or the double spike methodology (e.g., for Pb). An interesting approach based on the 18O/16O isotope ratio was reported14 for NIST boron standard (SRM-951) after normalizing 11B16O18O/11B16O16O to a fixed 18O/16O value. A 10-fold improvement in the precision of boron isotopic analysis was reported.14 However, the authors reported that different activators or different chemical forms of boron yield the fractionation lines with different slopes. Thus this methodology can be used by developing empirical correlation between the 18 O/16O isotope ratio (observed from 11B16O18O/11B16O16O) and the 11B16O18O/10B16O16O ratio, assuming a fixed value for the 18 O/16O ratio. However, in actual practice, a fixed value for the 18 O/16O ratio cannot be assumed. Further experiments conducted using BO2- ions showed that though in each filament loading there is a good correlation between the two ratios mentioned above, the slopes varied considerably in different filament loadings, preventing the widespread use of this approach to samples with a different % of 10B. In this paper, we report a novel approach for precise and accurate isotopic analysis of B using BO2- in N-TIMS. It involves internal normalization based on the 18O/16O isotope ratio which is determined in situ from each filament loading using ReO4ions at m/z values of 253 and 251 corresponding to 187Re16O318O and 187Re16O4, respectively. The mass fractionation correction to the observed 18O/16O ratio is applied for determining the mass fractionation from the observed 187Re/185Re isotope ratio obtained using 187Re16O4 and 185Re16O4 at m/z values of 251 and 249. The mass fractionation for the 187Re/185Re isotope ratio can be obtained in each scan by comparing the observed ratio with that reported (1.6738) for natural Re.30-32 The intensity of ReO4- ions is quite large, and the mass spectrometric analysis (27) Tonarini, S.; Pennisi, M.; Adorni-Bracessi, A.; Dini, A.; Ferrara, G.; Gonfiantini, R.; Wiedenbeck, M.; Groning, M. J. Geostand. Geoanal. 2003, 27, 21–39. (28) Gonfiantini, R.; Tonarini, S.; Gro ¨ning, M.; Adorni-Braccesi, A.; Al-Ammar, A. S.; Astner, M.; Ba¨chler, S.; Barnes, R. M.; Bassett, R. L.; Cocherie, A.; Deyhle, A.; Dini, A.; Ferrara, G.; Gaillardet, J.; Grimm, J.; Guerrot, C.; Kra¨henbu ¨hl, U.; Layne, G.; Lemarchand, D.; Meixner, A.; Northington, D. J.; Pennisi, M.; Reitznerova´, E.; Rodushkin, I.; Sugiura, N.; Surberg, R.; Tonn, S.; Wiedenbeck, M.; Wunderli, S.; Xiao, Y.; Zack, T. J. Geostand. Geoanal. 2003, 27, 41–57. (29) Aggarwal, J.; Bo ¨hm, F.; Foster, G.; Halas, S.; Ho¨nisch, B.; Jiang, S. Y.; Kosler, J.; Liba, A.; Rodushkin, I.; Sheehan, T.; Shen, J. J.-S.; Tonarini, S.; Xie, Q.; You, C.-F.; Zhao, Z.-Q.; Zuleger, E. J. Anal. At. Spectrom. 2009, 24, 825831.

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Table 1. Faraday Cup Configuration Used for Monitoring ReO4- and BO2- Ions in TIMS Faraday cup number cup configuration

L3

L2

-

L1 185

forReO4

16

Re O4 (m/z 249)

C 187

Re O4

185

for BO2-

10

B16O2 (m/z 42)

11

B16O2

10

B16O17O (m/z 43)

H1 187

16

16

16

H2 18

Re O3 O (m/z 253)

18

Re O3 O (m/z 251) 10 16 18 B O O

11

B16O18O (m/z 45)

11

B16O17O (m/z 44)

a The bold-italicized species are the ones which give small contributions due to the existence of different oxygen isotopes, viz., 17O and 18O with abundances of 0.039% and 0.20%, respectively.

requires only an independent method for data acquisition from the same filament loading, prior to acquiring data for B isotope ratios. The methodology was applied to NIST-SRM-951 isotopic reference material of B, reference coral samples, and to samples used recently in an interlaboratory intercomparison experiment. It is shown that high precision and accuracy can be achieved using the present approach by N-TIMS for B isotope ratio measurements using nanogram amounts of boron. EXPERIMENTAL PROCEDURES Instrumentation. The thermal ionization mass spectrometer used in the present work is a latest generation TRITON TI instrument equipped with nine Faraday cups and a secondary electron multiplier (SEM) in the central channel along with a retarding potential quadrupole (RPQ) to achieve high abundance sensitivity. The positions of the Faraday cups can be adjusted depending upon the configuration required during mass spectrometric analysis. A single or a double filament assembly can be used for loading the sample. The amplifier of each Faraday cup is connected to a resistance of 1011 Ω. Data can be acquired in the static mode of multicollection, dynamic peak jumping, or multidynamic mode. Reagents and Apparatus. All the reagents and apparatus used for diluting the NIST-SRM-951 B isotopic reference material were cleaned with nitric acid, Milli-Q-water, hydrochloric acid, and again with Milli-Q-water. A solution of B standard containing 7 µg of B/g was prepared by diluting the master B solution with diluted HCl, which was purified by sub-boiling distillation. Single filament assembly made up of high-purity rhenium (Re) filament was used for sample loading. Prior to sample loading, the filament was outgassed at 2 A for 8 min, 3.5 A for 7 min, and 4.5 A for 30 min at a pressure of 10-7 mbar and was then left for 1 week at ambient atmosphere under a laminar flow hood for reoxidation. Boron free seawater (BFSW) was used as an activator during B sample loading on the filament for enhancing the yield of BO2- ions during TIMS analysis. For sample loading, 1 µL of solution containing about 7 ng of B was mixed with 0.5 µL of BFSW prior to loading. The solution was loaded on a filament preheated to about 0.6 A and was dried in air at this heating current. (30) Wachsmann, M.; Heumann, K. G. Int. J. Mass Spectrom. Ion Processes 1992, 114, 209–220. (31) Walczyk, T.; Hebeda, E. H.; Heumann, K. G. Int. J. Mass Spectrom. 1994, 130, 237–246. (32) Liu, Y.; Huang, M.; Masuda, A.; Inoue, M. Int. J. Mass Spectrom. 1998, 173, 163–175.

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Mass Spectrometric Analysis Procedure. The mass spectrometric analysis was performed in the static multicollection mode using different Faraday cups. Prior to carrying out mass spectrometric analysis, the Faraday cups were calibrated for their gain calibration. Preliminary studies showed that ReO4- ions are produced at a filament heating temperature lower than that required for isotopic analysis of B using BO2- ions. Therefore, two different cup configurations were selected judiciously, keeping in mind that the Faraday cup positions should not be disturbed when acquiring data in the two different mass ranges (i.e., 42-45 and 249-253). The two Faraday cup configurations used in the present work are given in Table 1. The ion source pressure and the analyzer pressure were ∼2 × 10-7 and ∼3.7 × 10-9 mbar, respectively, during the mass spectrometric analysis. First, with the analyzer gate valve kept closed, the filament was heated to 1000 mA at a heating rate of 120 mA/min. Then the analyzer gate valve was opened, and a peak of mass 251 corresponding to 187Re16O4- was monitored. The filament temperature was heated slowly at a heating rate 50 mA/min to obtain an ion current of about 2 × 10-11 A at mass 251. The focusing as well as the peak centering was performed using the available software and hardware. The filament was heated further to obtain an intensity of ∼4 × 10-11 A (4 V) for mass 251. The corresponding intensities at masses 249 (185Re16O4) and 253 (187Re16O318O) were about ∼2.4 × 10-11 A (2.4 V) and ∼3 × 10-13 A (32 mV). The filament temperature was 800 ± 30 °C (1125 ± 30 mA) during different mass spectrometric analyses. A value of 1.6738 for 187Re16O4/185Re16O4 isotope ratio based on the theoretically calculated abundance ratio of 187Re/ 185 Re was given as an input for internal normalization, in each cycle, to the 253/251 intensity ratio to obtain a fractionation corrected 18O/16O isotope ratio. Exponential law was applied to correct for fractionation during internal normalization. Data acquisition was started immediately, and 5 blocks with each block consisting of 10 cycles, with an integration period of 4 s each time, were obtained. The calculation methodology is given below since there would be contributions from 185Re16O318O to the total intensity of the peak at m/z 251 in addition to that of 187 Re16O4. Subsequently, the cup configuration was changed to that for the 42-45 mass range (Table 1) and the 18O/16O isotope ratio obtained above was used for internal normalization of the 43/ 42 isotope ratio to obtain the fractionation corrected 11B/10B ratio in each cycle (scan). The filament was heated further at a heating rate of 50 mA/min to obtain an intensity of ∼4 ×

10-11 A (4 V) for the peak at mass 43 and an intensity of ∼2 × 10-13 A (20 mV) for 11B16O18O at mass 45. The filament heating temperatures required to obtain these intensities were 150-200 mA higher than that used for obtaining data with ReO4- ions. Prior to data collection, care was taken to check for the isobaric interference of the CNO- peak at m/z 42 by monitoring the CN- peak at m/z 26. Here again data were collected for 5 blocks, with each block consisting of 10 cycles (scans), with an integration period of 4 s each time. After completion of the first set of data, the filament current was again reduced to lower heating current. After changing the cup configuration to the 249-253 mass range, data were acquired once again, first for the oxygen isotope ratio and then after increasing the filament temperature for the B isotope ratio. Since the intensity of the 187Re16O4 ion will be very high (more than 10 V) at the higher filament heating current used for data collection in the BO2- mass range, care should be exercised to first lower the filament heating current and then change the cup configuration to that required for ReO4-. This is essential to avoid the exposure of the Faraday cups to highintensity signals. Different Samples Analyzed. The validity of the analysis and calculation methodology was tested using different B samples. First, replicate analyses of NIST-SRM-951 B solution were performed using several independent filament loadings. Two synthetic mixtures prepared by mixing solutions containing enriched 10B and 11B isotopes were analyzed by N-TIMS, and the values were compared with those obtained by using MC-ICPMS using a Neptune (Thermo-Finnigan) instrument available in the laboratory. A few other samples, e.g., Japanese coral reference (JCp1) and an in-house coral reference (KTP-1), were also analyzed. Calculation Methodology. The observed intensity ratio of the peaks at masses 249, 251, and 253 can be written as R253/251 ) 4(187Re16O318O) / (187Re16O4 + 4(185Re16O318O)) (1)

(187Re16O4 + 4(185Re16O318O))/ 185Re16O4

R251/249 )

R253/249 ) 4(187Re16O318O) / 185Re16O4

(2) (3)

From the above equations, oxygen and rhenium isotope ratios can be calculated as follows with A and B representing the experimentally measured intensity ratios R251/249 and R253/249, respectively. 18

O/ 16O ) [A - (A2 - 4B)1/2]/8

(4)

Re/ 185Re ) [A + (A2 - 4B)1/2]/2

(5)

187

The calculated 187Re/185Re ratio was used to determine the fractionation factor R1 using the exponential mass fractionation law and expected 187Re/185Re ratio in natural Re. Sensitivity test on the effect of the 187Re/185Re ratio in the calculation demonstrates only negligible δ11B changes. The calculated R1 was then used to obtain the fractionation corrected 18O/16O isotope ratio, i.e., true 18O/16O ratio in the sample present on

the filament, from the observed 18O/16O ratio from eq 4. Subsequently, this true 18O/16O ratio was used to obtain the mass fractionation factor R(oxygen) by comparing the observed 18 O/16O ratio based on the 45/43 intensity ratio during analysis using the BO2- ion and using eq 6 to correct for contribution of 17 O at mass 43. 18

O/ 16O ) 1/[2{1/I45/43 - (17O/ 18O)/(11B/ 10B)}]

(6)

The 17O/18O ratio corresponding to their natural abundances of 0.039% and 0.20% was used, and the value of the 11B/10B ratio was the I43/42 intensity ratio observed. Since the contribution of 17O is quite small, these approximations do not contribute any significant errors in the accuracy of the final results. The mass fractionation factor R(oxygen) calculated in this way was finally used to obtain the 11B/10B ratio in the sample using eq 7 as B/ 10B ) (I43/42)observed(43/42)R - 0.000 78

11

(7)

In eq 7, the constant 0.000 78 accounts for the contribution of B16O17O at mass 43. All these calculations were performed for data obtained in each cycle (scan) using the Excel spread sheet. 10

RESULTS AND DISCUSSION The use of negative ions for the Re-Os isotope system is well documented in the literature.33 This is because of the different masses of the two species, ReO4- and OsO3-, which helps in overcoming the isobaric interferences and thus allowing some flexibility in the separation chemistry of Re and Os. During isotopic analysis of B using BO2- ions, we attempted to look at the signals corresponding to negative species of ReOx- and were indeed pleasantly surprised to see the high intensity of peaks in the mass range of ReO4-, with a 2-3 orders of magnitude lower intensity of ReO3- and no signals corresponding to ReO2- or ReO- species. The existence of ReO4- and ReO3- species has already been reported in the literature30-32 when using N-TIMS for Re isotopic analysis. However, in one of these studies,32 O2 gas was introduced through a leak into the ion source to enhance the formation of ReO4-, and it was found essential to maintain very stable oxygen pressure in the source chamber to minimize oxygen isotope fractionation. We, therefore, immediately decided to exploit this interesting observation to determine the 18O/16O isotope ratio based on the ReO4- ion, since 187Re has a very long β-decay half-life (about 43 Gyr)34 and there are no reports in the literature showing significant variations in the 187Re/185Re isotope ratio. Figure 1 displays the observed 43/42 ratios in different blocks from three independent filament loadings. It is clearly seen that the isotope ratio changes with time. Further, different filament loadings show different fractionation patterns, which leads to variable systematic error in the final results. Also it was observed that in many of the mass spectrometric analyses, the observed 43/42 ratio decreases initially which corresponds to the reverse (33) Gangopadhyay, A.; Walker, R. J. Chem. Geol. 2003, 196, 147–162. (34) Krebs, R. E. The History and Use of Our Earth’s Chemical Elements; Greenwood Publishing Group: Westport, CT, 2006; p 422.

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Figure 2. The observed linear correlation between the 45/43 and 45/42 ratios in each filament during BO2- N-TIMS analysis of the NIST SRM 951 standard. There are significant differences among the different curves of three independent filament loadings, and this hampered the precise and accurate determination of the 11B/10B isotope ratio by N-TIMS. The various symbols represent the different filament runs.

Figure 1. Typical variation of the 43/42 ratios in different blocks from three independent filament loadings during BO2- N-TIMS analyses.

fractionation effect. Subsequently, it becomes stable for a few blocks and then starts increasing as expected from Raleigh’s model. This behavior may be interpreted in terms of the presence of different zones of the sample layer present on the filament. As shown in Figure 2, despite the different fractionation patterns given in Figure 1, there is a linear correlation between the observed 45/43 and 45/42 ratios in each filament analysis.14 However, there are significant differences among the different curves of three independent filament loadings and this hampered the precise and accurate determination of the 11B/10B isotope ratio by N-TIMS. Table 1 gives the two different cup configurations defined in two separate methods during collection of the data for the Re and B isotopes. As can be seen, there is no need to disturb the positions of the Faraday cups for using these two configurations except for giving the suitable configuration and collection method file in the software. It may be added that we did not change the focusing conditions in moving from one cup configuration to the other. This was considered worthwhile with a view to performing 7424

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the isotope ratio measurements of Re and B under the identical mass spectrometric conditions, except for the difference in the filament temperature. It would be interesting to investigate the possibility of using ReO3- ions at the higher temperature used for B analysis, since it is reported that the ReO3-/ReO4intensity ratio increases with an increase in temperature of the filament. Also the use of a secondary electron multiplier (SEM) in the central channel can be explored but at the expense of introducing uncertainty in the Faraday cup to SEM crosscalibration. Table 2 shows the data obtained for a typical mass spectrometric analysis from one filament loading. The blocks 1A to 5A denote the data obtained by first performing the Re isotopic analysis at lower filament temperature and then raising the temperature to acquire data for the B isotopic analysis. After this sequence, the filament temperature was again lowered and the above sequence of obtaining data first for Re and then B was repeated for the same filament. This was done to see the effect of any change in the oxygen isotope ratio since the B isotopic analysis is done at a filament temperature which is higher by 150-200 °C than the temperature of analyzing Re. As can be seen, there is no significant effect on the final results of the B isotope ratios, which are measured with a precision of better than 0.05% (2σ). It may be added that the intensity of the peak at mass 253 is about 0.8% of that of the peak at mass 251, due to the presence of four oxygen atoms in the ReO4 species. Table 3 shows the results of six independent filament loadings for the NIST-SRM-951 B standard. Data are given again for the two sets A and B from the same filament loading as explained above. It is seen that the grand mean of the 11B/10B isotope ratio from these six filament loadings gives an overall precision of 0.6‰ (2σ, n ) 6) with a positive bias of 0.4%. This could be attributed to different oxygen isotopic composition in the boron solution compared to that determined using ReO4 species from

Table 2. Results of the

B/10B Isotope Ratio Determined in One of the Typical Filament Loadings for NIST-SRM-951a

11

observed

calculated 18

O/16O

block number

251/249

253/249

1A 1B 1C 1D 1E 2A 2B 2C 2D 2E

1.680 8 1.680 8 1.680 8 1.680 8 1.680 8 1.683 0 1.683 0 1.683 0 1.683 0 1.683 1

0.013 648 0.013 644 0.013 645 0.013 643 0.013 638 0.013 611 0.013 611 0.013 616 0.013 611 0.013 603

a

observed

187

Re/185Re

0.002 040 0.002 039 0.002 039 0.002 039 0.002 038 0.002 032 0.002 032 0.002 032 0.002 032 0.002 030

1.672 6 1.672 6 1.672 6 1.672 6 1.672 6 1.674 8 1.674 9 1.674 9 1.674 9 1.674 9

calculated

45/43

43/42

0.004 167 0.004 165 0.004 165 0.004 162 0.004 160 0.004 151 0.004 151 0.004 149 0.004 150 0.004 146

4.105 0 4.104 5 4.104 4 4.104 3 4.104 3 4.107 6 4.107 4 4.107 1 4.107 1 4.106 8

11

B/10B

4.059 8 4.060 4 4.060 4 4.061 7 4.062 6 4.059 5 4.059 6 4.060 2 4.059 6 4.061 6

Certified value: 4.0437 ± 0.0033.

Table 3. Results of 11B/10B Isotope Ratio Determined in Six Independent Filament Loadings for NIST-SRM-951a set A

sample JABAa δ11B (‰)b

set B

11 11 B/10B B/10B filament isotope R isotope R loading ratio uncertaintyb (oxygen) ratio uncertaintyb (oxygen)

1 2 3 4 5 6

4.062 87 4.061 55 4.062 06 4.066 08 4.061 03 4.061 27

0.000 33 0.000 57 0.000 39 0.000 25 0.000 51 0.000 23

Table 4. Results of δ11B (‰) Reported by N-TIMS in the Recent Interlaboratory Experiment

0.50 0.24 -0.60 -0.26 -0.44 -0.81

4.056 58 4.054 69 4.057 76 4.057 49 4.060 15 4.056 91

0.000 37 0.000 45 0.000 55 0.000 63 0.000 39 0.000 33

0.24 0.13 -0.63 -0.34 -0.48 -0.82

Mean of six independent loadings ) 4.059 87 ± 0.001 25 (0.62‰, 2σ), difference with respect to certified value + 0.4%. (1) A and B refer to two different sets of data acquisition from the same filament loading, each set consisting of 5 blocks with each block having 10 cycles. (2) The symbol R(oxygen) denotes the fractionation factor calculated using the observed 18O/16O isotope ratio from the intensity ratio 45/43; using the exponential law of fractionation correction given as true ratio) observed ratio(m1/m2)R, where m1 and m2 are the masses of the two isotopic peaks. b Denotes standard error of the average value. a

the oxidized Re filament. The present approach assumes that the oxygen fractionation corrected isotope ratio determined from the filament is the same as that present in the BO2species, and this could lead to a constant bias. However, the present approach allows an in situ correction for variable isotope fractionation, which has been plaguing the routine use of N-TIMS with BO2- species for B isotopic analysis. On the other hand, the present methodology does not demand strict control of the sample loading procedure, heating temperature of the filament, as well as the time of data acquisition. As shown in Figure 1a-c, the isotope ratios obtained by N-TIMS will be having large variable systematic errors (>1% or more), in the absence of the fractionation correction methodology. The constant bias in NIST-SRM-951 has to be confirmed by different researches worldwide as this could be due to a small difference in the actual 18O/16O isotope ratio value in the sample compared to that determined experimentally. However, excellent agreement on data in the coral samples shows that there is nothing wrong with the fractionation correction procedure described. Tables 4 and 5 show the data extracted from a recent interlaboratory intercomparison experiment in which 15 laboratories reported the results of two B samples with δ11B values of 9.86‰ and -23.65‰. It is seen that δ11B values range from 5.3 to 12.6 and -22.49 to -35.3 for the two samples by N-TIMS.

sample JABBa δ11B (‰)b

no.

amount loaded

mean

uncertainty

n

mean

uncertainty

n

1 2 3 4 5 6c

10 10 4 50 5 0.6

8.2 5.3 10.05 8.8 12.6 11.12

1.0 1.6 0.8 2.2 0.3 0.88

6 3 6 5 10 3

-22.9 -35.3 -22.49 -25.3 -23.8 -24.13

0.9 1.8 0.20 2.8 0.3 0.36

3 2 3 5 9 3

a The expected 11B/10B ratios on a weight basis were 4.0835 and 3.9480, for JABA and JABB, respectively, i.e., δ11B values were 9.86‰ and -23.65‰, respectively. b δ11B (‰) ) [(11B/10B)sample (11B/10B)NIST-951][1000/{(11B/10B)NIST-951}]. c Using the total evaporation technique.

Table 5. Results of δ11B (‰) Reported by MC-ICPMS, P-TIMS, and N-TIMS in Recent Interlaboratory Experiment sample JABAa δ11B (‰) sample JABBa δ11B (‰) MS technique

average

standard deviation

average

standard deviation

MC-ICPMS [5]a P-TIMS [2]a N-TIMS [5]a

10.29 11.30 8.99

1.42 1.14 2.67

-23.65 -24.45 -25.96

0.91 0.21 5.33

a

Number of laboratories which used this method.

Further, as is seen from data in Table 5, there is a significant difference in the mean values of different laboratories using MCICPMS, P-TIMS, as well as N-TIMS. It was, therefore, of interest to use the present methodology for the above two mixture samples, as well as a few other samples (e.g., an international coral reference and an in-house coral standard). Table 6 shows a comparison of the data for 11B/10B isotope ratios determined using N-TIMS and MC-ICPMS, in the two synthetic mixtures prepared in the laboratory. It is seen that there is good agreement in the results obtained by the two mass spectrometric techniques with a small positive bias of the data obtained by N-TIMS. The results of the mass spectrometric analyses of the latter samples by N-TIMS and using the internal normalization approach discussed above are given in Table 7. It is seen that there is good agreement among the values obtained by independent filament Analytical Chemistry, Vol. 81, No. 17, September 1, 2009

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Table 6. Results of 11B/10B Isotope Ratios in Two Synthetic Mixtures by N-TIMS and MC-ICPMS MS technique used N-TIMS mixture no.

set A

set B

MC-ICPMS

(N-TIMS) average/ (MC-ICPMS)

mixture-1 mixture-2

9.3027 4.0874

9.2937 4.0863

9.2901 4.0788

1.0009 1.0020

Table 7. Results of δ11B (‰) in Different Samples by N-TIMS δ11B (‰) sample

code

coral

JCp-1

coral

KTP-1

filament mean δ11B uncertainty expected δ11B loading (‰) (%) (‰) 1 2 1 2

23.4 22.0 25.2 25.1

0.1 0.1 0.1 0.1

δ11B ) 24.4 δ11B ) 24.7

loadings, and also the values agree well with the expected results for these materials, which demonstrates the validity of the present approach. It was of interest to monitor the changes in the 18O/16O and 187 Re/185Re ratios with time calculated using eqs 4 and 5. Figure 3 shows the results obtained from a set of 50 blocks. It is seen that though some changes in both the isotope ratios occur over an extended period of about 1 h, the changes are negligibly small for a time duration of 10-15 min required for acquiring a set of data consisting of 5-10 blocks. Figure 4 displays the variation in the 18O/16O isotope ratio observed during the different sets of measurements using ReO4 species. Here again, it is noted that a variation of about 0.5% occurs in the 18O/16O isotope ratios during the different sets of measurements. Though these changes will give some uncertainty on the final 11B/10B isotope ratios, these can be controlled by having a well-defined protocol for mass spectrometric analysis. It may also be mentioned that during different mass spectrometric analyses, it was observed

Figure 3. The results of the 18O/16O (187Re16O318O/187Re16O4) and 187 Re/185Re (187Re16O4/185Re16O4) plot obtained from a set of 50 blocks using the ReO4 species. 7426

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Figure 4. The variation of 18O/16O (187Re16O318O/187Re16O4) observed during the different sets of measurements using the ReO4 species.

Figure 5. An empirical correlation between the R values calculated from 187Re16O4/185Re16O418O and 11B16O18O/10B16O18O using the exponential law for fractionation. A good correlation is observed which shows that the fractionation behavior of Re and B from the two different species containing oxygen can be correlated. Note there are two results (shaded circles) that significantly deviated from the average line.

that the filament temperature required to obtain a similar intensity of the peak at mass 252 was about 100 °C higher in set B as compared to that for set A (Tables 2 and 3). This was attributed to depletion of the oxygen amount during the first set of 5 blocks. Figure 5 shows an empirical correlation between the R values calculated from 187Re16O4/185Re16O418O and 11B16O18O/ 10 16 18 B O O using the exponential law for fractionation. A good correlation is observed which shows that the fractionation behavior of Re and B from the two different species containing oxygen can be correlated. The strength of the present studies lies in the fact that there is no need to make any assumption for the 18O/16O ratio in the sample as it can be determined in situ from each filament loading. Finally, from the present data, it is not possible to arrive at any definite conclusions about the source of oxygen, i.e.,

either from the oxidized rhenium filament or the sample loaded on the filament or both. These studies would require extensive experiments using 18O labeled compounds and will be performed in the future. As a matter of fact, differences between the 18O/16O isotope ratio in the oxidized filament and that present in boron containing compound loaded on the filament could be one of the factors leading to a systematic difference of 0.4% in the 11B/10B isotope ratio determined in NIST-SRM951 using the present approach. We are confident that future studies will be able to provide insights into these factors. CONCLUSIONS A simple and robust analytical approach based on internal normalization is described for the precise and accurate isotopic analysis of B using N-TIMS and employing BO2- ion. The first step of this approach is based on determining experimentally the 18O/16O isotope ratio from each filament loading by monitoring the 187Re/185Re ratio using ReO4- ions. The mass fractionation correction is applied to the observed O isotope ratio based on that calculated from the observed 187Re/185Re ratio. In the second step, when analyzing B as the BO2- ion, the predetermined and fractionation corrected O isotope ratio from the same filament is used for internal normalization and mass bias correction to the observed B isotope ratio. The validity of this approach has been demonstrated by analyzing the NIST-SRM-951 B isotopic certified reference, synthetic mixtures, one international coral reference sample, and one in-house coral standard. Precision values of 0.6‰ (n ) 6) have been demonstrated using this methodology for the NIST-SRM951 B standard. We believe that this would facilitate routing the isotopic analysis of B using N-TIMS in corals and foraminifera samples, which contain only a few nanogram amounts of B. However, the blank for B must be strictly controlled in the

entire sample preparation and mass spectrometric filament loading procedure. There is no need to monitor different parameters like sample size and pH of the solution, loading temperature, filament heating temperatures, time of data acquisition, etc. for the mass spectrometric analysis procedure. We are confident that this new development to obtain fractionation free precise and accurate results for the 11B/10B isotope ratio, using nanogram amounts of the sample, will be useful in different areas including paleo-oceanography, geochemistry, and cosmo-chemistry.35-37 In particular, the technique will be valuable to study individual foraminifera in sediments as well as to measure radiogenic 10B from the decay of 10Be in samples with less extreme Be/B amount ratios.37 ACKNOWLEDGMENT The authors are thankful to NCKU, Tainan, for adequate funding to set up the clean room laboratory and instrumentation facilities. It is with great pleasure that authors acknowledge the excellent cooperation provided by Ms. Tsai-Luen Yu (Helen) and Dr. Kuo-Fang Huang (Denner) during the present work. S.K.A. thanks the authorities at BARC, Mumbai, for allowing him to accept his appointment as a Visiting Professor at EDSRC, NCKU, Tainan, for a period of 3 months. Constructive comments provided by the three reviewers and the handling editor improved the paper significantly and are greatly appreciated. Received for review June 4, 2009. Accepted July 17, 2009. AC901224M (35) Kasemann, S. A.; Schmidt, D. N.; Bijma, J.; Foster, G. L. Chem. Geol. 2009, 260, 138–147. (36) Williams, L. B.; Hervig, R. C. Appl. Geochem. 2004, 19, 1625–1636. (37) McKeegan, K. D.; Chaussidon, N.; Robert, F. Science 2000, 289, 1334– 1337.

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