Determination of Bromine Stable Isotope Ratios from Saline Solutions

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Determination of bromine stable isotope ratios from saline solutions by “wet plasma” MC-ICPMS, including a comparison between high- and low-resolution modes, and three introduction systems Pascale Louvat, Magali Bonifacie, Thomas Giunta, Agnès Michel, and Max Laurence Coleman Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.6b00062 • Publication Date (Web): 22 Feb 2016 Downloaded from http://pubs.acs.org on February 22, 2016

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Analytical Chemistry

Determination of bromine stable isotope ratios from saline solutions by “wet plasma” MC-ICPMS including a comparison between high- and low-resolution modes, and three introduction systems Pascale Louvat1*, Magali Bonifacie1, 2, Thomas Giunta1, Agnès Michel1, Max Coleman3 1

Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Université Paris-Diderot, UMR CNRS 7154, 1 rue Jussieu, 75238 Paris Cedex, France. 2 Observatoire Volcanologique et Sismologique de Guadeloupe, Institut de Physique du Globe de Paris, Le Houëlmont, 97113 Gourbeyre Guadeloupe, France. 3 NASA Jet Propulsion Laboratory, California Institute of Technology. Pasadena, CA 91109 USA *corresponding author, Fax n°: + 33 1 83 95 77 06, E-mail: [email protected]

ACS Paragon Plus 1 Environment


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ABSTRACT: We describe a novel method for measuring stable bromine isotope compositions in saline solutions such as seawater, brines and formation waters. Bromine is extracted from the samples by ion exchange chromatography on anion exchange resin AG 1X4 with NH4NO3 and measured by MC-ICP-MS in wet plasma conditions. Sample introduction through a small spray chamber provided good sensitivity and stability of the Br signal compared to direct injection (d-DIHEN) and desolvation (APEX). NH4NO3 media allowed fast (3 years) reproducibility between ± 0.11 and ± 0.27‰ (2SD) was obtained for the four HBr solutions, the international standard reference material NIST SRM 977 (δ81BrSMOB = -0.65 ± 0.1‰, 1SD), and seawaters (synthetic and natural). The accuracy of the MC-ICP-MS method was validated by comparing the δ81Br obtained for these solutions with dual-inlet IRMS measurements on CH3Br gas. Finally, the method was successfully applied to 22 natural samples. KEYWORDS: bromine stable isotopes, MC-ICP-MS, IRMS, ion exchange chromatography, extraction, saline solutions


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INTRODUCTION Chlorine and bromine are both volatile (Cl2, Br2) and very soluble (Cl-, Br-) in aqueous fluids where they mainly behave conservatively. Hence their stable isotope ratios (37Cl/35Cl and 81Br/79Br, respectively) and Cl/Br ratios are potential tracers of the fluid sources or of the physical and chemical processes that fluids have undergone. However, one of the main difference between Cl and Br is their natural abundance in geological samples (ie., while Cl- is often the major dissolved anion, Br- is a trace element with concentration hundreds to thousands times lower than chlorine). This likely limited the studies investigating bromine stable isotope compositions (expressed as δ 81Br in per mil variations relative to SMOB, Standard Mean Ocean Bromide) in natural samples compared to chlorine stable isotope compositions (δ37Cl relative to SMOC, Standard Mean Ocean Chloride) that have been used in a variety of geological settings (including fluid circulation in the crust1, exchange of halogens between the surface and the interior of the Earth over geological timescales2 or volcanic fluids3). Since the pioneering work of Eggenkamp and Coleman4 who developed a method for precise and accurate measurements of δ81Br by dual-inlet Isotope Ratio Mass Spectrometry (DI-IRMS hereafter) on CH3Br gas, most of the studies using Br isotopes have focused on the evolution of formation waters and/or brines in sedimentary basins 5-8, but there are other applications, e.g. anthropogenic and natural emissions of organobromines to the stratosphere and their destructive impact on the ozone layer9-12. With the broadening use of halogens or chlorine stable isotopic compositions (δ 37Cl) in volcanology13,3, further applications of δ81Br in this field are also promising. Another noticeable difference between Cl and Br, that might discriminate the behavior of their isotopes, is that Br is more susceptible to oxidation than Cl, and is also more involved in bio-metabolisms than Cl, e.g. review in (14). Altogether the behavior of bromine, as a halogen, is very similar to chlorine and it can trace groundwater and magma sources as efficiently, but Br is also slightly different and combinations of Cl and Br isotope ratios might elucidate physico-chemical processes. The first method of δ81Br measurement4 was developed with DI-IRMS on CH3Br gas after Br oxidative extraction (AgBr precipitation followed by CH3Br gas formation and GC separation). Since then, a few laboratories have designed their own methods15-21 for organic or inorganic bromine species, and by IRMS or MC-ICP-MS (Table 1). So far, only three publications have reported MC-ICP-MS determination of δ81Br by MC-ICP-MS in wet plasma conditions, i.e. where aqueous solutions are measured directly, but none with application to natural samples. Here we present a method for the first time aimed principally at extracting Br from saline solutions (seawater and brines, but with other applications too) on an ion exchange column and to measure its stable isotopic composition (δ81Br) by MCICP-MS. It uses a novel off-line Br extraction method with an anion exchange resin, to both remove the matrix and separate Cl from Br. The difficulty of measuring δ81Br in a wet plasma is due to possible Br memory effects in acid solutions and the interference of Ar2H+ on

81

Br+. However high-pH solutions can efficiently remove these memory effects and give much

shorter washout times between samples. Both low- and high-resolution strategies were investigated to correct Ar 2H+ interference on 81Br+. As the Br first ionization potential is quite high, sensitivity of MC-ICP-MS for δ81Br measurements is low and three different introduction systems were tested in order to minimize the amount of Br used for δ81Br determinations, but still with the lowest Ar2H+ level as well as best accuracy and reproducibility. This method has been tested for seawater samples and four HBr solutions with a δ81Br variation range of 3‰, and validated by DI-IRMS δ81Br measurements. Also it was applied to a series of 22 natural samples (oil field brines).

EXPERIMENTAL SECTION Reagents and standards. Bromine extraction was done with anion exchange resin AG 1X4, 200-400 mesh, in OHform (BioRad). All mineral acids used in the study were Normapur quality and were distilled with a DST-1000 apparatus



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(Savillex, USA); H2O was from milliQ apparatus (Millipore, Thermo Fisher Scientific). NaOH was AnalR Normapur 30% (10 mol L-1). NH4NO3 solutions were prepared from HNO3 and Selectipur NH4OH. Standard HBr solutions at 100 ppb, 1 ppm and 10 ppm in NH4NO3 0.2 mol L-1 were prepared for the development of the MC-ICP-MS measurements. Sample Br solutions were also diluted to 10 ppm Br in NH 4NO3 0.2M (when enough Br available). Residual matrix constituents might shift the Br isotope ratio during MC-ICP-MS measurements; such matrix effects were addressed by comparing measurements with and without adding increasing amounts of Cl-, SO42-, NO3-, Na+, Ca2+ and Si(OH)4 (from HCl, H2SO4, HNO3, and AccuSPEC solutions). A solution of Cl-, Br-, SO42- and NO3- at 0.125 mol L-1 was prepared from SCP Science AccuSPEC 1000 ppm standard solutions for the first elution tests. A first in-house synthetic seawater solution (SSW1) was prepared from HCl, H 2SO4 and HBr acids and from NaHCO3, CaCO3, MgCO3, K2CO3 salts (Pro Analysis, Merck) with final concentrations of: 0.33 mol L-1 Cl-, 0.23 mol L-1 Na+, 26 mmol L-1 Mg2+, 20 mmol L-1 Ca2+, 5 mmol L-1 K+, 4.5 mmol L-1 SO42- and 34 ppm Br-. A second inhouse synthetic seawater solution (SSW2) was prepared with NaOH, HCl and NIST SRM 977 Br, with concentrations: Na+ and Cl- 0.56 mol L-1; Br- 65.6 ppm. A natural seawater sample MOMMARSAT (sampled in the Indian Ocean in January 2010 on the MD 175 cruise, latitude -32.45°S, longitude +84.01°E) was also measured. The international standard reference material NIST SRM 977 (NaBr salt) was used for testing accuracy of δ81Br measurements. For comparison between IRMS and MC-ICP-MS measurements, we prepared four HBr solutions with distinct 81Br/79Br ratios by distilling two 9 mol L-1 HBr solutions in a closed environment (Evapoclean, Analab, France) at 100°C. The first distillation was stopped after one hour when 10% of the initial volume was recovered: distillate (named HBr Dist2) and distillation residue (HBr Dist1) were collected. The second distillation was stopped after six hours, with the recovery of 80% of the initial HBr volume (distillate, HBr Dist4; distillation residue, HBr Dist3). (SI, Figure S1) Ion exchange chromatography. The novel ion exchange columns were designed from Kartell funnels (162/4, 4 mm inner diameter) and PTFE frits, and filled with 1mL AG 1X4 resin preliminarily rinsed and re-suspended with H2O. Elution rate (by gravity) was about 0.2 mL min-1 (for H2O, much slower for NaOH). The elution protocol was first developed with the Br-/Cl-/NO3-/SO42- equi-molar solution (0.125 mol L-1 each) with different eluents: ammonium citrate (0.01 to 0.05 mol L1

), ammonium nitrate (0.05 to 0.5 mol L-1) and NaOH (1 and 4 mol L-1). For each elution test, the eluent solution was

introduced mL per mL and collected under the column. Concentrations of Cl, Br, NO 3 and SO4 in the elution fractions were determined by HPLC (Dionex DX 120, IonPac AS9-HC column, ASRS-UltraII supression). Mass spectrometry MC-ICP-MS. Operating conditions for Neptune (Thermo Fisher Scientific) MC-ICP-MS at Institut de Physique du Globe de Paris are summarized in Table 2. Three different introduction system were tested: small 20 mL cyclonic spray chamber with 50 µL min-1 PFA nebulizer (ESI, USA), direct injection nebulizer d-DIHEN22 (Analab, France), APEX-HF desolvating system (ESI, USA) with PFA 50 µL min-1 nebulizer (ESI, USA). Measurements were tested both in high- and low-resolution modes. In high-resolution mode

81

Br was measured on the left

mass 81, while in low-resolution mode the instrument was tuned to reduce the

81

Br+ shoulder of (40Ar2H+ +

81

Br+) peak at

40

Ar2H+/81Br+ ratio and the blank signal on

mass 81 and 79 was subtracted. The instrumental mass bias over MC-ICP-MS measurements was corrected by samplestandard bracketing, and 81Br/79Br were expressed as δ81Br according to the following equation: δ81BrStd = {81/79Brsample / average(81/79BrStd1 ; 81/79BrStd2) - 1} x 103 where the standard was either an in-house HBr solution (δ81BrHBr) or the NIST SRM 977 (δ81Br977). In order to reduce the memory effect of Br in the introduction system, all Br solutions were prepared with NH4NO3 at 0.2 mol L-1. δ81Br measurements were generally triplicate, in order to estimate an external reproducibility over five δ81Br values for each sample.23


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Analytical Chemistry DI-IRMS. Bromine stable isotope compositions were measured on CH3Br gas with a dual-inlet Thermo Fisher Delta

Plus XP mass-spectrometer at the Institut de Physique du Globe de Paris. Sample preparation and measurements were made following the method described in (4) and applied by (24), without the need of separation of bromide from chloride prior to AgBr precipitation (since analysed samples contained no chloride). Briefly, a solution containing 50 μmol of Br- is mixed with 4 mL of KNO3 (1 mol L-1). 2 mL of a citric acid-phosphate mixture (for buffering the solution at a pH of 2.2) and 1 mL of AgNO3 (0.2 mol L-1) are consecutively added to quantitatively precipitate AgBr. After filtration in the dark over a Whatman GF/F glass fiber filter, the precipitate is dried overnight at 80°C. The next day each filter is sealed together with 75

μL CH3I in an evacuated glass ampoule and stored at 80°C for two days. During this period CH 3I reacts with AgBr to form CH3Br, which is separated from excess CH3I by gas chromatography with two successive packed columns filled with Porapak Q 80-100 mesh. The purified CH3Br is then transferred to a glass tube and connected to the mass spectrometer configured to allow precise measurements on CH3Br (on m/z 96 and 94). The IPGP Mass Spectrometer was modified to make δ81Br measurements with 109 Ω resistors on both cups 6 and 7, with a typical intensity of 4V.

RESULTS AND DISCUSSION Br extraction from seawater and brines. The difficulty of extracting Br from natural sample solutions is linked to its low concentration relative to chlorine, with g/g Cl/Br ratios generally higher than 200 (300 in seawater, >5000 in some brines). The other major anions in natural samples are sulfates and nitrates. Here we tested four different eluents for Br extraction, according to their affinity for the resin relative to Br -: ammonium citrate,, ammonium nitrate (NH4NO3) and sodium hydroxide (NaOH) (SI, Figure S2). Finally, we combined elution with NaOH 4 mol L-1 and NH4NO3 0.2 mol L-1 to efficiently extract Br-. The volume of NaOH 4 mol L-1 was adjusted in order to eliminate SO42- and a maximum of Clwithout losing Br-, the resin was then rinsed with 2 mL H2O, 2 mL NH4NO3 0.2 mol L-1 were introduced to elute most of the remaining Cl-, and then Br- was recovered in 5 mL NH4NO3 0.2 mol L-1. This elution scheme was further tested for the synthetic seawater solution SSW1, with a Cl/Br ratio of 300 (Figure 1). Br - extraction yields were between 95 and 103% but the final Br fraction still contained more Cl- than Br- (Cl/Br ratio of 9). The final elution protocol (Table 3) was successfully applied to natural seawater, with extraction yields between 96 and 103 % (within 5 % HPLC analytical error), and final Cl/Br ratios from 10 to 50. A set of six columns was prepared and the reproducibility of the extraction protocol was tested both on synthetic and MOMARSAT seawaters. Extraction yield was always close to 100 ± 5% (within HPLC analytical error). Total procedure blanks for the elution protocol were between 0.04 and 0.1 µg Br. MC-ICP-MS measurements Choice of the introduction system. Because the Br eluant of the ion exchange chromatography method developed here is NH4NO3 0.2 mol L-1, this medium was also used for introduction into the MC-ICP-MS. It allowed fast washing between samples/standards compared to HNO3, classically used for ICP-MS analyses, in which we observed strong Br memory effects. In NH4NO3 media Br remains under the form Br-, while in HNO3 solutions Br2 may form and stick to the surfaces of the introduction system. We tested 3 introduction systems (d-DIHEN direct injection nebulizer, APEX-HF desolvator and a small cyclonic spray chamber) with their respective performances described below. With direct injection (d-DIHEN) the long nebulizer is plugged in place of the injector in the torch and the sample spray is directly injected to the plasma22,23. Tuning the MC-ICP-MS with d-DIHEN injection of a 1 ppm Br standard in NH 4NO3 0.2 mol L-1 was impossible, the signal being too instable, probably due to the injection of NH 4NO3 salt particles together with Br. The same solution diluted ten times in H2O (100 ppb Br in NH4NO3 0.02 mol L-1) gave a stable 0.6 V signal of 79

Br+, but the Ar2H+ interference on mass 81 could not be lowered to less than 0.1 V without a significant loss in sensitivity.

High-resolution (HR) analyses also were not possible with d-DIHEN as the peak shape was rounded, due to the large amount



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of water introduced into the plasma, wide distribution of droplet size and lower temperature within the ionization region. Low-resolution (LR) measurements of the in-house HBr and SRM 977 solutions at 1 ppm in NH 4NO3 0.02 mol L, despite high sensitivity (5 V ppm-1), were not reproducible (± 5‰, 2SD, n=5). With the APEX-HF desolvator, the spray is heated to 120°C in a teflon spray chamber and then cooled down to 2°C in Peltier cooled fingers, where most of the solvent condenses, leaving a desolvated “dry” spray that is injected in the plasma. A 1 ppm Br solution in NH 4NO3 0.2 mol L-1 gave 2 V intensity with very low Ar2H+ level (below 10 mV), but washing was very much longer (>20 minutes) than with d-DIHEN (2-3 min). After one hour of NH 4NO3 0.2 mol L-1 injection to the APEX, the Br signal decreased significantly and was finally lost: APEX was clogged with NH4NO3 salts. Introduction of NH4NO3 at lower concentration (ie., 0.02 mol L-1) improved the longevity of the Br signal but did not change the washing efficiency. The 20 mL cyclonic spray chamber was finally chosen as it allowed: introduction in NH 4NO3 0.2 mol L-1 (directly after Br extraction), high Br sensitivity (1 V ppm-1 in LR), stable

40

Ar2H+ signal of about 50 mV, and short washout times (2-3

minutes, Figure 2). During tuning a 100 ppb Br solution, Br sensitivity was maximized and Ar 2H+ interference minimized, by finding the best set of sample gas, auxiliary gas and distance between torch and cones. Wash and blank solutions were NH4NO3 0.2 mol L-1, blank intensities were ≈10 mV for 79Br and ≈60 mV at mass 81. Low- versus high-resolution measurements. The sample–standard bracketing measurement sequence was: blank – standard – wash – blank – sample – wash – blank – standard – wash – blank etc. Each sample measurement is bracketed by those of the standard solution (either in-house HBr or NIST SRM 977), and each sample is measured three times in a row, to determine external reproducibility for five δ81Br values23. In High Resolution (HR) mode, 81Br was measured at the middle of the low-mass shoulder of the mass 81 peak (Figure 3a). By carefully tuning the lenses of the mass spectrometer this shoulder had a 100 ppm wide plateau. The center mass of the center cup was set at the exact middle of this plateau. Br sensitivity in HR mode was highly attenuated (0.15 V ppm-1) compared to the Low Resolution (LR) mode (1 V ppm-1). In LR mode 40Ar2H+ interference was corrected by blank subtraction: the average 79Br+ and (81Br++Ar2H+) intensities of the blanks measured just before and after the standard / sample solution were subtracted from the

79

Br+ and (81Br++Ar2H+) intensities

measured for the standard / sample solution. In order to minimize the blank and interference contributions to the Br isotope measurements, solutions containing 10 ppm Br were usually measured.

The internal 2SD reproducibility of δ81Br

measurement for the set of four distilled HBr solutions was comparable (± 0.04 to 0.57‰; SI, Table S1) in HR and LR modes. Among these 64 analyses, only 8 were measured in HR mode but 3 of them are outlier values (out of 5 outliers in total). Thus, even if the internal reproducibility found here on triplicate δ81Br analyses with HR and LR modes were comparable, HR mode tends to be less reproducible from one measurement session to the next, or even inaccurate, though no reason (e.g. lab temperature variation) is immediately apparent. Internal standard errors of uncorrected 81Br/79Br ratios were between 10 and 30 ppm for both LR and HR modes. But the internal error tended to worsen in HR mode because the central mass of the small plateau drifted over time, while remaining stable in LR mode. Because of this HR plateau drift, LR mode was preferred, finally. 81

MC-ICP-MS δ Br measurement performance. External reproducibility between ± 0.02 and 0.7‰ (2SD, SI, Table S1, S2 and S4) were typically obtained, on triplicate measurements of 9 standard reference materials and 22 samples. The 2SD long-term reproducibility of the measurements of the four HBr solutions obtained by evaporation (ie., Dist 1 to Dist 4) was between ± 0.22 and 0.27‰ (Figure 4a, Table S1), these samples were measured up to 21 times over 3 years. These four HBr solutions have δ81BrHBr between - 0.85 and 1.86‰. Repeated measurements of our in-house HBr solution relative to


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NIST SRM 977 gave an average δ81Br977 value of 0.62 ± 0.09‰ (2SD, n=20, Figure 4b, SI, Table S2). For synthetic and natural seawater samples, Br was first extracted by ion chromatography (according to the protocol in Table 3). As Cl separation from Br is not complete, Br and residual Cl concentrations were determined and the Br bracketing standard was doped with the same Cl concentration as in the sample Br fraction. δ81Br measurements of natural and synthetic seawaters (MOMMARSAT, SSW1 and SSW2) gave a 2SD long-term reproducibility for 81Br measurements of ± 0.11 to 0.17‰ (Figures 4c and 4d), similar to the performance for pure HBr and KBr salts, and confirmed that our extraction procedure is reproducible. Moreover, Br extraction does not fractionate Br isotopes. Indeed, SSW1 was prepared with inhouse HBr solution and the δ81Br977 value for SSW1 is identical to that for HBr; SSW2 was prepared with NIST SRM 977 and the δ81Br977 value for SSW2 is 0.03‰ (Figure 4c). Our average δ81Br977 value for MOMMARSAT seawater (0.64 ± 0.11‰, 2SD, n=18, Figure 4d), as well as the external reproducibility we found here, are identical to previously published values15,18 (0.64 ± 0.06‰, 1SD, n=12; and 0.65 ± 0.06‰, 1SD, n=5; respectively), which validates our method. Concentration and matrix effects. We tested how close Br and NH4NO3 concentrations of the sample and bracketing standard have to be during δ81Br analyses, by bracketing Br solutions at 7.5 and 12.5 ppm with a Br solution at 10 ppm, and NH4NO3 solutions at 0.15 and 0.25 mol L-1 with a solution at 0.2 mol L-1 (all at 10 ppm Br). No significant change was observed with a 25% variation in Br or NH 4NO3 concentrations (SI, Table S3). We also measured HBr solutions with concentrations between 0.1 and 10 ppm, in order to assess the lower limit in concentration our method was able to reach (Figure 5; SI, Table S3). We only found analytically resolvable deviating δ81BrHBr values for the test with the 0.1 ppm Br solution (-0.33‰ instead of 0‰), with large error bars (a few ‰). This likely results from the much higher impact of blank subtraction when Br concentration is smaller: the sample to blank signal ratio at mass 81 varies from 4 to 400 when Br concentration varies from 0.1 to 10 ppm. For the 0.5 ppm Br solution, even if the average δ81Br value remained correct (δ81BrHBr = 0.01‰), the uncertainty was much higher (± 0.39‰, 2SD) than for the 1, 5 and 10 ppm solutions (± 0.06, 0.08 and 0.10‰, respectively). Thus we consider that our method is best for Br concentrations between 1 and 10 ppm. As the volume required for triplicate measurements is 1 mL, the Br amount needed for a precise measurement with our method is between 1 and 10 µg Br. However, as our extraction protocol collects Br in 5 mL NH 4NO3, the amount needed for triplicate measurements of a natural sample is 5 to 50 µg Br. For lower concentrations the extraction procedure could easily be modified proportionately. Finally, matrix effects were also tested by measuring Br isotopes in Br solutions doped with various proportions of Cl -, SO42-, NO3-, Na+, Ca2+ and Si(OH)4, relative to the pure Br bracketing solution. All solutions contained 10 ppm Br-. These doping species were chosen because they are major constituents of natural waters. Because the ratio of Cl and Br contents in natural waters is roughly between 200 and 10 000 (e.g. halite samples), part of Cl- remains in the Br- fraction after Br extraction. Since Na+ is introduced as NaOH in the extraction protocol, it could also be present in the Br fraction after extraction. The presence of either Cl -, SO42- or Si(OH)4, at a concentration of up to 500 ppm did not noticeably affect δ81Br measurements (Figure 5c, e and f; SI, Table S3). The influence of Ca 2+ is a little bit greater since the δ81Br value was shifted by + 0.2 and + 0.4 ‰ with 100 and 500 ppm Ca, respectively (Figure 5d). The largest matrix effect is caused by sodium, with up to - 2.6‰ δ81Br deviation at 500 ppm Na+, and still -0.3 ‰ at 100 ppm Na+ (Figure 5b). Na concentrations in the Br fractions extracted from a series of 22 formation waters were all below 10 ppm (and Na/Br between 0.01 and 7), except for one sample (with 235 ppm, probably because of a remaining droplet of NaOH solution) but for which the Na/Br is 9. Observed Na matrix effects thus did not affect sample δ81Br measurements. Most importantly, these matrix effect tests showed that imperfect separation of Cl from Br is not really a problem for δ81Br measurements in natural saline samples, for Cl/Br ratios apparently as high as 500 (Figure 5e).



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Inter-calibration with DI-IRMS measurements on a 2.5‰ range of  Br. Over the course of this study, our in81

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house reference solutions were prepared and measured several times by IRMS for their δ81Br composition: seawater MOMMARSAT shows δ81BrSMOB of 0.01 ± 0.05‰ (1SD, n=5; which is typical of the values routinely found in IPGP for 81Br analyses by DI-IRMS measurements on CH3Br gas samples24) and HBr solution shows δ81BrSMOB of - 0.16 ± 0.05‰ (1SD, n=3). The four distilled HBr solutions with distinct δ81Br values were also measured by IRMS. The very good agreement between MC-ICP-MS and IRMS δ81BrHBr values for these four solutions (Figure 6 and Table S1) validates the accuracy of our MC-ICP-MS method for 81Br measurement. Comparison with other methods. Overall the method developed here allows reproducible and accurate Br isotope ratio measurement by MC-ICP-MS in wet plasma conditions for samples containing 1 to 50 µg Br. The average long-term reproducibility obtained over a 3 year period for δ81Br measurements of pure Br solutions and seawaters is between ± 0.1 and 0.25‰ (2SD), competitive with the best reproducibility reported by previous methods 4,15,17-21. Compared to the previous methods for inorganic Br isotope ratio analysis4,15,17-21, the minimum amount of Br needed here is much lower (Table 1), with the exception of Zakon et al.21 who coupled an IC with a MC-ICP-MS and measured transient signals. More generally, the lowest Br amounts are reported for methods measuring transient signals, mostly from organic brominated compounds 10,16,19. Use of a sample loop to inject the samples (flow injection analysis) could significantly reduce the Br amounts needed, in the near future. Among the published methods, four17,19-21 concerned MC-ICP-MS Br isotope analysis with liquid sample injection. Wei et al.20 described high-resolution measurements with external reproducibility equivalent to ours, but very long washout times (20 min) and lower sensitivity (0.2V/ppm for

79

Br against 1V/ppm here). The NH4NO3 introduction media

used here (or NH3 as in [21], or solution buffering to pH 8 with NaHCO3/Na2CO3 as in [19]) clearly eliminates the memory issue. Our comparison between high- and low- resolution modes clearly shows that low-resolution measurements are much more stable than HR, once the torch settings have been properly tuned; and this is because the 81Br plateau on the low mass shoulder of the mass 81 peak is very narrow and can easily drift with time. This issue has however not been reported 20,21. Ion exchange extraction of Br was proposed recently 21, but consisted of removing the sample matrix with a cation exchange resin, with the drawback of not separating Br from the other anions in the sample. That method was however successfully tested on seawater21. Zakon et al.19 describe a very elegant measurement of δ37Cl, δ34S and δ81Br (however not on the same sample injection) by coupling Ion Chromatography (IC) to MC-ICP-MS, which enables them to measure as low as 50 ng of Br with a precision of 0.2 ‰ (2SD), by transient signal acquisition. However, their method was only tested on NaBr and CaBr2 salts, and on a synthetic seawater solution, but not on natural samples (not even seawater) and, as for (20), applicability to natural samples is not demonstrated. Their artificial seawater solution contained 0.01% Br and 1.9% Cl and, as the injection loop used in their study has a volume of 50 µL, if seawater is injected undiluted, this correspond to 5 µg Br, comparable to the lowest amount needed for our method. The big difference is thus transient signal measurements against stable continuous signal measurements here. The IC – MC-ICP-MS method19 is very attractive and could be optimized for natural samples with low Br contents, but also needs a dedicated IC. However, our off-line method is valuable for samples with very high salinity and low Br content that cannot be directly injected on-line to the IC, e.g. brines and formation waters. Our present off-line Br extraction method, is best for brines and formation waters, but can also be adapted to other types of water samples. Application to natural samples. Bromine isotope values for a series of 22 formation waters from different wells in the Paris Basin and North Sea oil fields were measured as a test of the method. Samples had Br concentrations between 70 and 600 ppm, with a relatively narrow range of Cl/Br of 100 to 300. 30 µL to 2 mL of the sample solutions were introduced onto the columns in order to have a minimum Br amount of 5 µg, ideally 50 µg. Br was extracted according to the protocol in Table 3. Cl and Br concentrations in the Br fractions were measured and resulted in Cl/Br ratios between 3 and 60. In some


Page 9 of 21

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cases sample sizes were much less than optimum but were included nevertheless even if poorer precision might result to see if the results conformed with the trends of the others in each set. For each sample a standard was prepared with Cl and Br (inhouse HBr) concentrations matching those of the sample. All solutions were in NH 4NO3 0.2 mol L-1. Sample-standard bracketing measurements were always in triplicate. For three samples the Br extractions were duplicated, and for eleven samples the entire measurement was duplicated. This series of formation waters from three different sampling sites had δ81BrHBr values varying between + 0.08 and +1.86‰ relative to our in-house HBr standard (δ81BrSMOB = -0.16 ± 0.05 ‰, 1SD, n=3 by IRMS), with external 2SD reproducibility between ±0.01 and 0.7‰ (SI, Table S4). The poorest reproducibility was for samples with lowest Br concentration available (down to 0.5 ppm Br in the measured solution). The samples analyzed in the first application of this new method were archived oilfield produced waters, for which a large amount of other information had already been accrued. They comprise different sets of samples and should not be considered all together. The initial aim was to see if there was a general anti-correlation of chlorine and bromine stable isotope compositions, as had been observed by Eggenkamp and Coleman4. In fact, none did. Three different phenomena have been observed so far: confirmation of the presence of two different brine compositions in the same oilfield (Figure 7a); mixtures of formation water mixed with varying extents of the injected seawater; and the influence of an older more saline formation water on the compositions of waters in overlying much younger formations (Figure 7b). In all cases the bromine isotope analyses confirm or enhance previous data. These results will be published in other articles for specialist journals, but show the potential value of this analytical approach as part of the armory of tools to explore these important scientific and economic problems. CONCLUSIONS We have developed a method for the determination of δ81Br in saline solutions by wet plasma MC-ICP-MS after Br extraction on an anion exchange resin. Different instrument settings were tested (three different sample introduction systems, as well as low- and high- resolution measurements). The best conditions (in terms of external reproducibility and accuracy of δ81Br measurements) were obtained by sample introduction through a small spray chamber in NH 4NO3 media and in lowresolution mode after careful tuning of the mass spectrometer in order to reduce the level of 40Ar2H+ spectral interference on 81

Br+. The method gives accurate results, checked with SRM 977, the international reference salt from NIST, and comparison

of the method with DI-IRMS measurements on CH3Br gas samples covering a wide, > 2.5‰, variation range in their δ81Br values. It gives reproducible measurements (± 0.1 to 0.25 ‰, 2SD) for simple Br solutions, as well as for KBr salt, natural and synthetic seawaters. Our method is competitive compared to others also using MC-ICP-MS, both in terms of Br amounts needed and reproducibility of δ81Br measurements. It is also an interesting alternative to the DI-IRMS method for samples containing low Br concentrations, as it requires 10 to 100 times less Br. The ion exchange extraction of Br can be adapted for other types of natural waters, and this method will allow broadening of the currently limited knowledge of Br isotope behavior in various geological settings. ASSOCIATED CONTENT Supporting Information Two figures (S1: Preparation of the in-house reference HBr solutions; and S2: Elution tests) and four tables (Br isotope measurements) are reported as supporting information: This material is available free of charge via the Internet at http://pubs.acs.org. ACKNOWLEDGMENT PL and MB were funded by PTeV (Programme Transverse en Volcanologie), multidisciplinary program PARI of IPGP / region Ile de France (SESAME Grant no. 12015908), and CNRS INSU ALEAS program. MC was partly funded by both the



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Invited Professor program of Université Paris Diderot and the Jet Propulsion Laboratory (JPL), California Institute of Technology, under contract with the National Aeronautics and Space Administration (NASA). Assya and Abdellah Krim are thanked for their help in calibrating ion exchange chromatography. This is IPGP contribution number 3712. REFERENCES (1) Bonifacie, M., Monnin, C., Jendrzejewski, N., Agrinier, P., Javoy, M. Earth Planet. Sci. Let. 2007, 260, 10-22. (2) Bonifacie, M., Jendrzejewski, N., Agrinier, P., Humler, E., Coleman, M., Javoy, M. Science 2008, 319, 1518 - 1520. (3) Li, L.; Bonifacie, M.; Aubaud, C.; Crispi, O.; Dessert, C.; Agrinier, P. Earth Planet. Sci. Lett. 2015, 413, 101-110. (4) Eggenkamp H. G. M.; Coleman M. L. Chem. Geol. 2000, 167, 393-402. (5) Chen, L; Ma, T; Du, Y; et al. J. Geochem. Explor. 2014, 145, 250-259. (6) Bagheri, R.; Nadri, A.; Raeisi, E.; et al. Chem Geol. 2014, 384, 62-75. (7) Stotler, R. L.; Frape, S. K.; Shouakar-Stash, O. Chem Geol. 2010, 274, 38-55. (8) Shouakar-Stash, O.; Alexeev, S. V.; Frape, S. K.; et al. App. Geochem. 2007, 22, 589-605. (9) Frische, M.; Garofalo, K.; Hansteen, T. H.; Borchers, R.; Harnisch, J. Environ. Sci. Pollut. Res. 2006, 13, 406-413. (10) Sylva S. P.; Ball L.; Nelson R. K.; Reddy C. M. Rapid Commun. Mass Spectrom. 2007, 21, 3301-3305. (11) Holmstrand, H.; Unger, M.; Carrizo, D.; Andersson, P.; Gustafsson, Ö. Rapid Commun. Mass Spectrom. 2010, 24, 2135–2142. (12) Horst, A.; Holmstrand, H.; Andersson, P.; Andersson, A.; Carrizo, D.; Thornton, B. F.; Gustafsson, Ö. Rapid Commun. Mass Spectrom. 2011, 25, 2425–2432. (13) Aiuppa, A., Baker, D. R., & Webster, J. D. Chem. Geol. 2009, 263, 1-18. (14) Eggenkamp, H. G. M. Geochemistry of Stable Chlorine and Bromine Isotopes, Springer, 2014. (15) Shouakar-Stash, O.; Frape, S.K.; Drimmie, R.J. Anal. Chem. 2005, 13, 4027-4033. (16) Gelman, F.; Halicz, L. Int. J. Mass Spectrom. 2010, 289, 167-169. (17) Gelman, F.; Halicz, L. Int. J. Mass Spectrom. 2011, 307, 211-213. (18) Du Y.; Ma T.; Yang J.; Liu L.; Shan H.; Cai H.; Liu C.; Chen L. Int. J. Mass Spectrom. 2013, 338 ,50–56. (19) Zakon, Y.; Halicz, L.; Gelman, F. Anal. Chem. 2014, 86, 6495-6500. (20) Wei H.-Z., Jiang S.-Y., Zhu Z.-Y., Yang T., Yang J.-H., Yan X., Wu H.-P., Yang T.-L. Talanta 2015, 143, 302-306. (21) De Gois J.S., Vallelonga P., Spolaor A., Devulder V., Borges D.L.G., Vanhaecke F. Anal. Bioanal. Chem. 2015, 35, 7588. (22) Louvat, P.; Bouchez, J.; Paris, G. Geostand. Geoanalyt. Res. 2011, 35, 75-88. (23) Louvat, P.; Moureau, J.; Paris, G.; Bouchez, J.; Noireaux, J.; Gaillardet, J. J. Analyt. Atom. Septrom. 2014, 29, 16981707. (24) Eggenkamp, H.; Bonifacie M.; Ader, M.; Agrinier P. , in revision for Chem. Geol. 2016.


Page 11 of 21

FIGURES

NaOH 4 mol L-1

16

H2 O

0.05

NH4NO3 0.2 mol L -1

14

0.04

12

8

Cl Br 0.03 NO3 SO4

6

0.02

10

4 0.01

2 0

Br concentratio n (ppm)

18 Cl, SO4 & NO3 concentration (ppm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

2

4

6

8 10 volume (mL)

12

14

16

0.00

Figure 1. Br extraction on AG1X4 anion exchange resin with 2 mL of synthetic seawater (SSW1).

ACS Paragon Plus11 Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 21

Figure 2. Typical Br washout in NH4NO3 0.2 mol L-1, after a 10 ppm Br solution. At time 0 the autosampler probe went to the wash solution.


Page 13 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. High and low-resolution peak shape and signal intensity (with the cyclonic spray chamber)



a/ distil l en d HBr so lutio s vs in-house HBr solutio 2.5

HBr Dist3: d81BrHBr = 1.86 ± 0.25 ‰ 2.0

d81BrHBr (‰)

1.5

1.0

HBr Dist1: d81BrHBr = 0.18 ± 0.22 ‰

0.5

in-house HBr: d81BrHBr = 0 ‰

0.0

HBr Dist4: d81BrHBr = -0.50 ± 0.22 ‰ -0.5

-1.0

-1.5 0

HBr Dist2: d81BrHBr = -0.85 ± 0.27 ‰ 2

4

6

8

10

12

14

16

18

20

22

d81Br977 (‰)

b/ in-house n HBr solutio v s NIST SRM 977 1.0

d81Br977 = 0.62 ± 0.09 ‰ n

0.5

0.0 0

2

4

6

8

10

12

14

16

18

20

22

c/ syntheticsea waters vs NIST SRM 977 SSW1: d81Br977 = 0.62 ± 0.17 ‰

d81Br977 (‰)

1.0

0.5

SSW2: d81Br977 = 0.03 ± 0.13 ‰

0.0

-0.5 0

2

4

6

8

10

12

14

16

18

20

22

d/ natural seawater MOMMARSAT vs NIST SRM 977 d81Br977 (‰)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 21

1.0

d81Br977 = 0.64 ± 0.11 ‰

0.5

0.0 0

2

4

6

8

10

12

14

16

18

20

22

Analysis number

Figure 4. Long-term reproducibility of δ81Br measurements for (a) four in-house reference HBr solutions obtained by evaporation (vs inhouse initial HBr), (b) in-house HBr (vs NIST SRM 977), (c) synthetic seawater solutions prepared with in-house HBr (SSW1) and with NIST SRM 977 (SWW2) (vs NIST SRM 977), (d) natural seawater MOMMARSAT (vs NIST SRM 977). Stars are for IRMS measurements.


Page 15 of 21

1

a

0.3

d81BrHBr (‰)

d81BrHBr (‰)

0.5

0.1 0.0 -0.1 -0.3 0.1

0.5

1

5

Br concentration (ppm)

c

0.6 0.4 0.2 0.0

-0.2

0

20

SO4/Br

40

0.8

0.4 0.2 0.0

-0.2

60

e

0.6

0

200

Cl/Br

400

-1 -2 0

20

Na/Br

40

0.8

d81BrHBr (‰)

d81BrHBr (‰)

0.8

b

0

-3

10

0.4 0.2 0.0 0

20

Ca/Br

40

0.8

600

60

d

0.6

-0.2

d81BrHBr (‰)

-0.5

d81BrHBr (‰)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60

f

0.6 0.4 0.2 0.0

-0.2

0

20

SiO2/Br

40

60

Figure 5. Concentration and matrix effects on δ81Br measurements. a: concentration effect: in-house HBr solutions between 0.1 and 10 ppm, bracketed by themselves. b to f: matrix effects: in-house 10 ppm HBr solutions were doped with increasing amounts of either Na+, SO42-, Ca2+, Cl- or Si(OH)4, and bracketed by the “pure” 10 ppm HBr solution.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 6. Comparison of δ81Br measurements for four in-house reference HBr solutions by this method and by IRMS.


Page 16 of 21

Page 17 of 21

a/

2.5

d81BrSMOB (‰)

2.0 11

1.5

12

1.0 y = 0.39x + 1.61 (R 2 = 0.64)

0.5 5

0.0

3

15

4

2

-0.5 -1.0 -5

-4

-3

-2

-1

0

d37ClSMOC (‰)

b/ 2.0 1.5

d81BrSMOB (‰)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


19 y = -2.10 -5 x + 2.37 (R 2 = 0.87)

1.0 0.5

20 22

0.0

21

-0.5 0

40000

80000

120000

Cl (ppm)

Figure 7. Application of the method to natural samples of formation waters from wells in the Paris Basin and North Sea oil fields (with sample numbers labels). a/ δ81Br vs δ37Cl shows mixtures of two different oilfield brines from various wells in the same field. Very small samples from the set, despite having greater uncertainties, plot within the trend. b/ δ81Br vs Cl concentration shows mixtures of two oilfield brines: more concentrated Triassic and more dilute Jurasssic brines.



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Page 18 of 21

TABLES

Table 1. Comparison of methods for Br stable isotope ratio measurements Study Eggenkamp & Coleman Shouakar-Stash et al.15 Sylva et al.

10

Holmstrand et al.11 Horst et al.

12

Gelman & Halicz

16

Gelman & Halicz17 Du et al.18 Zakon et al.19

4

Instrument

Br form

Br extraction

On-line

Min. Br (µg)

± 2SD, ‰

Signal

DI-IRMS

inorganic

oxidation/AgBr/CH3Br

N

1600

0.2

continuous

CF-IRMS

inorganic


N

80

0.12

continuous

GC/MC-ICP-MS

organic

GC

Y

0.024

0.6

transient

GC/MC-ICP-MS

organic

GC

Y

1.6 to 3.2

1 to 4

transient

GC/MC-ICP-MS

organic

GC

Y

3.2

0.8

transient

GC/MC-ICP-MS

organic

GC

Y

0.08

0.2 to 0.4

transient

oxidation

Y

20

0.2

continuous

inorganic


N/Y

2000

0.12

transient

IC/MC-ICP-MS inorganic

IC- Dionex AS11-HC

Y

0.048

CF/MC-ICP-MS inorganic GasBench/CFIRMS

Wei et al.20

MC-ICP-MS

inorganic

none

De Gois et al.21

MC-ICP-MS

inorganic

IC- Dowex-50WX8

N

This work

MC-ICP-MS

inorganic

IC - AG1X4 resin

N


0.2

transient

0.1-0.2

continuous

4

0.2

continuous

1 to 5

0.1-0.25

continuous

Page 19 of 21

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Table 2. MC-ICP-MS and DI-IRMS operating parameters MC-ICP-MS Instrument Cones Intro. systems Nebulizer RF Power Ar Cool Gas Ar Auxiliary Gas Ar Sample Gas Cup configuration Resolution Integration time Uptake Time Wash Time Sensitivity IRMS Instrument Introduction mode High Voltage Emission Electron energy Reference gas Cup configuration Integration time

Neptune, Thermo Fisher Scientific Ni: Jet sampler and regular skimmer • 20 mL spray chamber (preferred) • or direct injection nebulizer (d-DIHEN) • or APEX HF (ESI) PFA microconcentric 50 µL min-1 (ESI) 1200W 15 L min-1 1.0 to 1.2 L min-1 1.0 L min-1 81 Br in C, 79Br in L2, 1011 ohms resistors • LR: m/∆m=400 (preferred) • or HR: m/∆m=8000 15 cycles of 8s 1 min 3 min 1 V ppm-1, on mass 79Br (LR)

DeltaPlus XP, Thermo Fisher Scientific Dual Inlet 2.54 kV 1.5 Amp 123 eV CH3Br gas (from in-house KBr salt) 81 Br (m/z 96): cup 6, 109 Ω resistor 79 Br (m/z 94): cup 7, 109 Ω resistor 10 cycles of 8s each



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table 3. Br extraction protocol (ion exchange chromatogaphy 1mL resin AG1X4 200-400 mesh, OH- form Wash: 10 mL NH4NO3 0.5 mol L-1 5 mL H2O 20 mL NaOH 4 mol L-1 2x 5 mL H2O Sample 1 to 2 mL at pH 9 introduction: Rinse/matrix 2x 1 mL H2O 1 elution: 5 mL NaOH 4 mol L-11 2x 1 mL H2O 1 2 mL NH4NO3 0.2 mol L1

Br collection:

5 mL NH4NO3 0.2 mol L1


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For TOC only


Determination of Bromine Stable Isotope Ratios from Saline Solutions

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