Improvements on high-precision measurement of bromine isotope ratios by multicollector inductively coupled plasma mass spectrometry

Improvements on high-precision measurement of bromine isotope ratios by multicollector inductively coupled plasma mass spectrometry

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Short communication

Improvements on high-precision measurement of bromine isotope ratios by multicollector inductively coupled plasma mass spectrometry Hai-Zhen Wei a,n, Shao-Yong Jiang a,b,nn, Zhi-Yong Zhu a, Tao Yang a, Jing-Hong Yang a, Xiong Yan a, He-Pin Wu a, Tang-Li Yang a a b

State Key Laboratory for Mineral Deposits Research, Department of Earth Sciences, Nanjing University, Nanjing 210093, PR China State Key Laboratory of Geological Processes and Mineral Resources, Faculty of Earth Resources, China University of Geosciences, Wuhan 430074, PR China

art ic l e i nf o

a b s t r a c t

Article history: Received 16 February 2015 Received in revised form 22 April 2015 Accepted 25 April 2015

A new, feasible procedure for high-precision bromine isotope analysis using multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS) is described. With a combination of HR mass resolution mode and accurate optimization of the Zoom Optics parameters (Focus Quad:  1.30; Zoom Quad: 0.00), the challenging problem of the isobaric interferences (40Ar38ArH þ and 40Ar40ArH þ ) in the measurement of bromine isotopes (79Br þ , 81Br þ ) has been effectively solved. The external reproducibility of the measured 81Br/79Br ratios in the selected standard reference materials ranged from 70.03‰ to 70.14‰, which is superior to or equivalent to the best results from previous contributions. The effect of counter cations on the Br þ signal intensity and the instrumental-induced mass bias was evaluated as the loss of HBr aerosol in nebulizer and potential diffusive isotope fractionations. & 2015 Elsevier B.V. All rights reserved.

Keywords: Bromine isotope analysis Multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS) Isobaric interference Instrument-induced mass bias Counter ions effect

1. Introduction Halogen ions are major anions in geological fluids and natural waters, and are usually considered as conservative tracers, providing useful indications of fluid sources and interactions in fluid systems [1]. With rapid improvement of various measurement techniques, significant fractionation of chlorine isotopes (δ37Cl values ranging from  14‰ to þ16‰) in geological processes has been observed [2–10]. The applications of chlorine isotopic variation in conjunction with other geochemical indices (e.g. Cl/Br molar ratios) have led to advances in the fields of geochemistry and environmental chemistry, such as tracking original sources and evolution of groundwater [11], investigating chlorine isotopic fractionation between brine and evaporite in salt lakes [12–15], exploring magmatic and hydrothermal processes [5,16,17], as well Abbreviations: NTIMS, negative thermal ionization mass spectrometry; PTIMS, positive thermal ionization mass spectrometry; DI-IRMS, dual inlet-isotope ratio mass spectrometry; CF-IRMS, continuous flow-isotope ratio mass spectrometry; MC-ICP-MS, multicollector inductively coupled plasma mass spectrometry; GC, gas chromatograph; IC, ion chromatography n Corresponding author. nn Correspondence to: Department of Earth Science and Engineering, Nanjing University, 163 Xianlin Avenue, Nanjing 210023, Jiangsu, PR China. Tel.: þ 86 25 83596832; fax: þ86 25 83592393. E-mail addresses: [email protected] (H.-Z. Wei), [email protected], [email protected] (S.-Y. Jiang).

as tracing the source and the degradation of organic pollutants [18,19], etc. However, there has been little study of bromine isotopic geochemistry in various geological processes, despite relatively high abundance of bromine. This is largely due to the difficulty of separating Br from geological matrices containing large amounts of Cl. The isotopic abundance of bromine was determined first by Aston in 1920 [20]; subsequent analytical approaches have been improved continuously as summarized in Table 1. Noteworthy contributions by Gelman et al. include the achievement of improved external precision of 70.1‰ for measurements of δ81Br in both inorganic and organic compounds as a result of coupling Gas Chromatography (GC) to MC-ICP-MS [29,30,32], and Ion Chromatography (IC) to MC-ICP-MS [33]. Recent research on CH3Br þ -IRMS and Br þ -MC-ICP-MS demonstrates that bromine isotope variations in nature are not only much larger than the analytical errors [27,28,34,35], but also distinct from those of chlorine isotopes [36]. For instance, a survey of δ81Br and δ37Cl of dissolved halides in brines from the Canadian and Fennoscandian Shields indicated that water-rock interactions are likely to influence halogen isotopic composition, and assessed the usefulness of δ81Br and δ37Cl isotopes as tools for evaluating the evolution of groundwater in crystalline rocks [1]. So far, isobaric interference associated with sample introduction system has remained a challenge for high-precision Br isotope analysis by MC-ICP-MS, because excess hydrogen atoms in the

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Table 1 Evolution of analytical procedures for bromine isotopic compositions. References

Date

MS types

Detected ions

External precision (2r)

Blewett [21] Williams et al. [22] Cameron et al. [23] Catanzaro et al. [24] Willey et al. [25] Xiao et al. [26] Eggenkamp et al. [27] Shouakar-Stash et al. [28] Gelman et al. [29] Gelman et al. [30] Yang et al. [31] Gelman et al. [32] Gelman et al. [33]

1936 1946 1956 1964 1978 1993 2000 2005 2010 2011 2011 2013 2014

Dempster-type MS Nier-type MS NTIMS NTIMS DI-IRMS PTIMS DI-IRMS CF-IRMS GC-MC-ICP-MS MC-ICP-MS Gas Bench II-IRMS GC-MC-ICP-MS IC/MC-ICP-MS

Br þ , Br2 þ , Br2 þ , Br  Br þ , Br2 þ , Br2 þ Br  Br  CH3Br þ Cs2Br þ CH3Br þ CH3Br þ Br þ Br þ CH3Br þ Br þ Br þ

725‰ 74‰ 74‰ 71.8‰

plasma lead to formation of the ions 40Ar38ArH þ and 40Ar40ArH þ , interfering with 79Br and 81Br at m/z values of 79 and 81 respectively [30]. Moreover, it has been demonstrated that the introduction of bromine solution via a desolvation nebulizer, normally used to reduce the formation of polyatomic ions, results in significant signal loss due to the escape of volatile bromine species through the desolvation membrane [30]. Gelman and Halicz have dealt with the problem by using in-line oxidation of bromide to bromine and its subsequent introduction into the MC-ICP-MS in a stream helium carrier gas. In their method, instrumental mass bias was corrected by using an external spike of Sr (standard solution NIST SRM 987) from an Aridus desolvation nebulizer [29,30]. These modifications have extended the range of geochemical applications of bromine isotopes to abiotic and biotic environmental processes. An attempt to correct instrumental mass bias by using 80Se as an internal standard proved to be unsuccessful, because of serious isobaric interference at m/z ¼81 from the polyatomic ion 80Se1H þ where 80Se makes up 49.80% in naturallyoccurring Se [37]. In this preliminary work, we report an effective and feasible approach, in an attempt to solve the inherent isobaric interferences and the instrumental mass bias effect associated within the bromine isotope analysis by MC-ICP-MS. We provide a detailed account of the instrumental setup required to distinguish isobaric interference from the signals of the two bromine isotopes, and the influence of counter ions on Br þ signal sensitivity and instrumental-induced mass bias. In addition, we assess the analytical reproducibility of this approach. The new technique opens the possibility of carrying out a systematic isotopic survey of δ81Br in geological processes, in order to understand processes of ore genesis in deposits of rare and dispersed mineral commodities in hydrothermal systems and saline environments in western China.

70.18‰ 70.12‰ 70.06‰ 70.1‰ 70.1‰ 70.1‰ 70.1‰ 70.1‰

Note

4–32 μg Br 2000–8000 μg Br 80 μg Br 0.0016 μg organic Br 4000–10,000 μg Br Brominated phenol 0.05 μg inorganic Br

KBr-A (Aldrich-Sigma, SP) and KBr-B (Aldrich-Sigma, SP) were prepared in purified water for parallel analysis of bromine isotopes. 2.2. Apparatus and instrumental parameters A Neptune Plus (Thermo Fisher Finnigan, Germany) MC-ICP-MS with an ESI PFA (50 mL/min) nebulizer in a quartz cyclonic spray chamber was used for bromine isotope analysis. The bromine content in both sample solution and NIST 977 solution was kept close to 2500 ng mL  1 (i.e. 2.5 ppm) to obtain  0.5 V of 79Br þ signal with the conventional Ni-skimmer cones. The mass bias is corrected by a sample-standard bracketing procedure (SSB, Eqs. (1) and (2)). Memory effect was clearly observed in the washing procedure by MC-ICP-MS, and it could be reduced to 1.5 mV of 79 Br þ with continuous washing with 1% HNO3 and Milli-Q H2O for ca. 20 min.

⎛ 81Br ⎞ ⎛ 81Br ⎞ ⎛ M 81 ⎞ β ⎜ 79 ⎟ = ⎜ 79 ⎟ × ⎜ 79 ⎟ ⎝ Br ⎠corrected ⎝ Br ⎠measured ⎝ M ⎠

β = ln

( 81Br/ 79Br)NIST 977certified 81

79

( Br/ Br)NIST 977measured

/ ln

(1)

M 81 M79

(2)

Where β is a correction factor per unit of atomic mass, M81 and M79 are the atomic masses of 81Br and 79Br isotopes, M81 ¼ 80.916291(3)u, and M79 ¼78.9183376(20)u [39]. All reproducibilities described in this work are determined from replicated measurements (n Z5, 2 S.D., 95% confidence limits). The detailed descriptions of instrumental parameters are given in Table S1.

3. Results and discussion 2. Experimental section 2.1. Materials and reagents

3.1. Distinguishability of isobaric interference from bromine isotopes with high resolution mode

Milli-Q water (Resistivity, 18.2 MΩ cm) was used throughout the experiments. Concentrated HNO3 was purified twice by subboiling distillation. The standard bromine isotopic reference material, NIST 977 (in NaBr-Form), was purchased from the National Institute of Standards and Technology, USA. The certified absolute abundance ratio (79Br/81Br) of 1.02784 7 0.00105 (95% confidence limit) is provided by Catanzaro et al. [24]. A solution of NIST 977 in HBr-Form was prepared by a two-step ion-exchange procedure using strongly acidic cation resin (Dowex 50W  8, H-Form, USA, 200–400 mesh), as described in our previous study [38]. Solutions of CsBr (Aldrich-Sigma, 99.999%, CAS 7787-69-1),

On the basis of the distribution of relevant isotopes of bromine and its neighbor elements, the Faraday cups from low mass to high mass are assigned individually as following: 76Se-L3, 77Se-L2, 78SeL1, 79Br-Central, 80Se-H1, 81Br-H2, 82Se-H3, and 83Kr-H4. The cup configuration was optimized until all peaks were well focused as shown in Fig. 1. According to the atomic masses of 79Br (78.9183376(20)), 81Br (80.916291(3)), 38Ar (37.9627322(5)), 40Ar (39.96238123(2)), and 1H (1.0078250321(4)) [39], the exact mass to charge ratios (i.e. m/z) of Br þ and relevant isobaric interference ions are derived below (Table 2). Resolution in mass spectrometry is defined as R¼M/ΔM, where R is the resolution, M is the mass of

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Signal intensity (V)

the second peak, and ΔM is the mass difference between two ion species. A larger resolution indicates a better separation of peaks [40]. The resolutions between 79Br þ and 40Ar38ArH þ and between 81 Br and 40Ar40ArH þ are 5261 and 4964 respectively. As the mass resolution with the High Resolution (HR) mode is  8000 for the Neptune Plus MC-ICP-MS in our laboratory, our instrument is capable of distinguishing the isobaric ions from the two isotopes of bromine. As shown in Fig. 2, the ion peaks of 81Br þ (m/z¼80.916) and 40Ar40ArH þ (m/z ¼80.933) could be distinguished with the HR mode, and the two fine plateaus for 81Br þ ion (Plateau A) and 40 Ar40ArH þ ion (Plateau B) were achieved with fine optimizing of the Zoom Optics parameters. By assigning a mass of 78.867 to the Central cup, it was possible to focus the narrow plateau of 81Br þ ion (m/z ¼80.916, Plateau A in Fig. 2) into cup H2 while avoiding 1.2 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1

L3- Se L2- Se L1- Se C-78.867( Br) H1- Se H2- Br H3- Se H4- Kr

0.006 0.004 0.002

-0.002 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74

Scanning time (s) Fig. 1. Cup configuration for bromine isotope analysis, where the ions 79Br and 81Br are detected using the Central and H2 cups. The strongest signal in the H1 cup is caused by the polyatomic ion 40Ar40Ar þ (m/z ¼80).

Table 2 Exact mass to charge ratios (m/z) of

79

Br and

81

Mass to charge ratio at 79 Ions m/z Resolution

Signal intensity (V)

0.010

Ar40ArH þ (m/z ¼80.933, Plateau B). By comparison, no separate peaks for 79Br þ and 40Ar38ArH þ are observed on the scanning curve for the Central cup (the black line in Fig. 2), owing to the low isotopic abundance of 38Ar in nature (i.e. 38Ar: 0.0632%, 40Ar: 99.6003%) [41]. On the other hand, as the mass to charge ratio of 40 Ar38ArH þ is larger than that of 79Br þ , the setting mass number of 78.867 at the middle position of Plateau A in the Central cup also avoid any overlapping 79Br þ from the 40Ar38ArH þ ion. In order to assess the validity of this procedure, signals from the two Faraday cups were monitored during the washing procedure (Fig. 3). As in the case of memory effects observed in boron isotope analysis by MC-ICP-MS, the bromine signal is difficult to remove in a short time period. Our numerous trials verified that pure water (Milli-Q H2O) is the best choice of cleaning solvent, and can remove the remaining Br þ signal in 20 min. As shown in Fig. 3, all ion signals decreased rapidly with increasing washing time. It is worth noting that the interference signal of 40Ar40ArH þ ion became more visible as the signal of Br þ decreased, as indicated in the two amplified signal plots inserted in Fig. 3. After the 79Br þ signal had been washed to blank level, the signal of 40Ar40ArH þ remained at  0.06 mV, which is equivalent to  0.5 V of 40Ar40Ar þ (m/z¼80) signal verified from the H1 cup. The combination of HR mass resolution mode and the fine optimizing of Zoom Optics parameters avoid the inherent isobaric interference on the two isotopes of bromine successfully, which ensures accurate analysis of bromine isotopic compositions with an effective and feasible procedure. 40

3.2. Influence of counter ions on bromine isotopic ratios and signal sensitivity

0.000

0.012

3

79

Br 78.9183376(20) 5261

40

38

Ar ArH 78.93294

Br and relevant isobaric ions. Mass to charge ratio at 81

þ

81 Br 80.916291(3) 4964

40 Ar40ArH þ 80.93259

In this preliminary work, 2.5 ppm solutions of NIST SRM 977 in both NaBr-Form and HBr-Form were prepared for a parallel study. As shown in Fig. 4, very stable 81Br/79Br ratios could be maintained for NIST 977 NaBr solution during different analytical sessions. This indicates that the instrument-induced mass bias for bromine isotopes is relatively stable because of medium mass of bromine compared to the dynamic mass bias observed for the lighter element boron [42]. The signal intensity of 79Br þ produced from 2.5 ppm of NaBr solution is as high as 0.4 V compared to that of  0.08 V obtained from 2.5 ppm of HBr solution, which would indicate a significant loss of HBr from aerosol in the nebulizer because of the high volatility of HBr molecules. Similar experimental phenomena were observed in δ37Cl and δ81Br analysis by IC/MC-ICP-MS [32]. Gelman et al. attributed the loss of Cl and Br

Central Cup (m/z 79) H2 Cup (m/z 81)

Mass resolution mode: High Resolution Zoom Optics: Focus Quad: -1.30 Zoom Quad: 0.00

0.008 0.006

Plateau B: Ar Ar H (m/z=80.933)

0.004 0.002

Plateau A: Br (m/z=80.916)

0.000 296

298

300

302

304

306

308

310

312

314

316

Scanning time (s) Fig. 2. Scanning curves of ion signals detected from the Central cup and H2 cup. The shaded rectangles (Plateau A and Plateau B) correspond to the red scanning curve of the H2 cup. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 3. Variation of ion signals from the Central cup and H2 cup with increased washing time. Insets show two amplified plots of signal intensities recorded after different washing times.

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4

a

Table 3 Determined bromine isotopic ratios in inter-laboratory standards by MC-ICP-MS.

1.023

0.70 0.65

1.022

0.60 0.55

Intensity of Br (V)

0.45 1.020

0.40 0.35

1.019

0.30

81

SE (single run)b

δ81Br (‰)

1r

NIST 977

0.97297 0.97292 0.97288 0.97277 0.97285 0.97274 0.97285

0.00002 0.00002 0.00003 0.00003 0.00003 0.00003 0.00009 (2s, n¼6)

 0.06

0.09

0.97405 0.97406 0.97403 0.97406 0.97399 0.97405

0.00004 0.00003 0.00004 0.00004 0.00004 0.00003 (2s, n¼5)

1.16

0.03

0.97476 0.97450 0.97425 0.97465 0.97447 0.97446 0.97459

0.00006 0.00007 0.00007 0.00007 0.00007 0.00006 0.00013 (n¼ 6)

1.74

0.14

0.97449 0.97482 0.97467 0.97454 0.97453 0.97461

0.00005 0.00006 0.00005 0.00006 0.00008 0.00014 (n¼ 6)

1.75

0.14

Br/ Br ratios

1.021

0.50

Standards no.

Ave

0.25 0.20

Br/ Br mean=1.02184 (0.00011), n=6 Correction Factor = -1.963

0.15 0.10

1.018

CsBr

1.017

0.05 0.00

1.016 0 10 20 30 40 50 60 70 80 90 100 110120130 140 150160170180

Cycles

Ave

b

KBr-A

1.026

0.14

1.025 1.024

0.10

1.023 1.022

0.08

1.021

0.06

1.020

0.04

Br/ Br mean=1.02400 (0.00030), n=5 Correction Factor = -2.047

0.02

Br/ Br ratios

Intensity of Br (V)

0.12

KBr-B

1.018

1.016

0

Ave

1.019

1.017

0.00

Br/79Br correcteda

10 20 30 40 50 60 70 80 90 100 110 120 130 140 150

Cycles Fig. 4. Variations in signal intensity and measured 81Br/79Br ratios recorded from different analytical runs (30 cycles in each run). (a) 2.5 ppm NIST 977 solution in NaBr-Form. (b) 2.5 ppm NIST 977 solution in HBr-Form. Gray columns represent the signal intensity of 79Br þ with reference to the left y-axis, and point symbols are 81 Br/79Br ratios with reference to the right y-axis.

signal to evaporation of HCl and HBr produced by interaction of chloride and bromide anions with cation exchange resin in the IC [32]. As a result, we obtained a better external precision of 70.11‰ for δ81Br measured on NaBr solution than for HBr solution, for which the external precision was 70.29‰. The correction factors for instrument-induced mass bias in aqueous NaBr and HBr media are calculated with Eq. (2). The obvious difference in the correction factors (i.e. βNaBr ¼ 1.963, βHBr ¼  2.047) most likely reflects possible diffusive isotopic fractionation that the two isotopes of Br have different diffusion coefficients in different aqueous media, as demonstrated by Eggenkamp and Coleman [36]. Other solution parameters, such as counter cations, bromide concentration, pH etc., might influence bromine isotope measurement by MC-ICP-MS. Further investigation of the influence of solution chemistry on bromine isotope analysis will be carried out in our near future, in order to gain a better understanding of isotopic fractionation of bromine ions within the sample introduction system. 3.3. Bromine isotopic compositions in internal laboratory standard reference materials In order to check the precision and reproducibility of this new procedure in our study, four internal laboratory standard materials were subjected to measurement. As shown in Table 3, the external reproducibility of δ81Br values in 2.5 ppm solutions ranged from

Ave a 81

79

Br/ Br corrected values obtained from mass bias correction using SSB procedure (Eqs. (1) and (2)). b SE is the absolute standard error from one single measurement (60 cycles).

70.03‰ to 70.14‰, which is clearly superior or equivalent to the best results from previous contributions listed in Table 1. The excellent consistency of δ81Br values obtained in the standard KBrA and KBr-B purchased from the same chemical company with different batches. In addition, further investigations related to precise analysis of bromine isotopes in various geological materials are being undertaken in our laboratory, including the establishment of an effective procedure to separate trace bromine from saline matrix, the optimization of high-precision measurement by TIMS and MC-ICP-MS techniques etc. A systematic survey of δ81Br will be carried out in order to understand the processes of ore genesis of rare and dispersed mineral deposits, both hydrothermal and associated with salt accumulations, in western China.

4. Conclusions Isobaric interference from the ions 40Ar38ArH þ and 40Ar40ArH þ induced in the sample introduction system remains a great challenge for high-precision bromine isotope analysis by MC-ICP-MS. In this study, we have shown that a combination of HR mass resolution mode and fine optimizing of Zoom Optics parameters (Focus Quad: 1.30; Zoom Quad: 0.00) successfully avoids the isobaric interference with the two isotopes of bromine, ensuring precise analysis of bromine isotopic compositions with an effective and feasible procedure. The influence of counter ions on Br þ signal sensitivity and the instrumental-induced mass bias appears to be significant, reflecting the differences in diffusion and isotopic fractionation behavior of bromide ions in different aqueous media. Further investigation into the effects of solution chemistry (such as, counter cations, bromine concentration, pH etc.) on bromine

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isotope analysis is in process. The analytical precision and reproducibility of this new procedure have been assessed from replicated measurements on four selected internal-laboratory standards, which yielded an external reproducibility of 70.1‰ on the measured 81Br/79Br ratio. The improved procedure will facilitate the study of bromine isotopes and the geochemical cycle of bromine in the earth and environmental sciences.

Acknowledgments We are grateful to Prof. J. H. Wang, Prof. C. J. Eastoe and the three reviewers for their scientific and editorial comments. This research was supported by the National 973 Project (2012CB416706), the National Natural Science Foundation of China (Nos. 41422302 and 41473042) and the Fundamental Research Funds for the Central Universities (No. 20620140380).

Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at doi:10.1016/j.talanta.2015.04.073.

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Please cite this article as: H.-Z. Wei, et al., Talanta (2015), http://dx.doi.org/10.1016/j.talanta.2015.04.073i