Low-blank chemistry for Zn stable isotope ratio determination using extraction chromatographic resin and double spike-multiple collector-ICP-MS

Abstract

A new low-blank Zn separation method employing an extraction chromatographic resin using Aliquat 336 (commercially available as TEVA resin) has been developed for Zn isotope composition determination by double spike-multiple collector-ICP-MS. The silicate sample solution containing 0.1 μg Zn in 0.5 mol L⁻¹ HBr containing 0.15% ascorbic acid (AA) is passed through a 1 mL column with 0.33 mL TEVA resin. While Zn is absorbed, major elements are eluted with 0.5 mol L⁻¹ HBr with AA. Afterwards, Zn was recovered with 2 mol L⁻¹ HNO₃. Recovery yield using silicate samples was 96.7 ± 6.8% (n = 10) and total blank was 0.05 ng (n = 6). The blank level is from 1/300 to 1/20 of previous studies, meaning 0.1 μg of Zn is sufficient for analysis. The ⁶⁷Zn–⁷⁰Zn double spike method was used in MC-ICP-MS. δ⁶⁶Zn of three USGS standard silicate reference materials (BHVO-1, AGV-1 and PCC-1) and seven GSJ silicate reference materials (JB-1, -2, -3, JA-1, -2, -3 and JP-1) were determined. Reproducibility of δ⁶⁶Zn of the total silicates was 0.19‰ (2SD). δ⁶⁶Zn of three carbonaceous chondrites (Orgueil, Murchison and Allende) were also determined and compared with those of references.

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Introduction

Zinc has five stable isotopes, ⁶⁴Zn, ⁶⁶Zn, ⁶⁷Zn, ⁶⁸Zn and ⁷⁰Zn. Since isotopic fractionation of Zn was observed, Zn stable isotopes have been used for studies of the solar system.^1–4

The most popular method for purification of Zn is anion exchange chromatography employing Zn(II) chloro-complex and anion exchange resin, AG MP-1,⁵ Chelex-100,⁶ or a combination.^7,8

In this method, Zn is absorbed by a high concentration of HCl (e.g., 7 mol L⁻¹), and recovered with diluted HNO₃ (e.g., 0.5 mol L⁻¹). The merit of this method is that simultaneous separation of Cu and Fe is available. However, the total eluent volume became 50 mL and extremely large even though 1.6 mL of the AG MP-1 resin is used.⁵ The currently described procedures in literature, however, suffer from a high Zn blank as low as 1–15 ng.^5,8 Thus an aim of this study was to develop a method with low Zn blank levels. A further aim of the method was to correct mass fractionation by employing the Zn double spike technique.

In order to reduce the blank, extraction chromatographic resin employing trioctylmethylammonium chloride (Aliquat 336), which is sold as TEVA resin from Eichrom Technologies, Inc., USA,^9–11 with eluent of 0.5 mol L⁻¹ HBr containing ascorbic acid (AA) was used.

In a double spike technique,^12–14 a double spike enriched in two isotopes with known isotope ratios is mixed with sample isotopes, and the isotope ratios of the mixtures are measured by multiple collector-inductively coupled plasma source-mass spectrometry (MC-ICP-MS). Then the stable isotope composition is calculated. The largest merit of the double spike technique is that mass discrimination occuring in chemical separation and/or mass spectrometry can be corrected.

In order to determine reproducibility of the method developed in this study, δ⁶⁶Zn of three US Geological Survey (USGS) silicate reference materials (BHVO-1, AGV-1 and PCC-1) and seven GSJ (Geological Survey of Japan) silicate reference materials (JB-1, -2, -3, JA-1, -2, -3 and JP-1) were determined. δ⁶⁶Zn in three carbonaceous chondrites (Orgueil, Murchison and Allende) was also determined and compared with those in the references.

Reagents and methods

Reagents. Water and HF were purified as described elsewhere.¹⁵ The EL (electronic industry) grade HCl, HNO₃ and analytical grade L(+)-ascorbic acid (Kanto Chemical Co. Inc., Japan) were used without purification. The TAMAPURE-AA-100 grade HBr (Tama Pure Chemicals, Co. LTD.) was used and diluted to be 0.5 mol L⁻¹ HBr without purification. The 0.6 g ascorbic acid (AA) was dissolved with 1.8 mL water to be ∼30% (w/v). This AA solution was made in each column chemistry session. The 0.5 mol L⁻¹ HBr containing 0.15% (w/v) AA was made just before the column chemistry. Zinc standard solution (10 000 μg mL⁻¹, Specpure, Product no. 14404, Alfa Aesar, USA) was used in the mixing experiment. Multi-element standard solutions (Specpure, Product no. 42885 and 44270, Alfa Aesar, USA) were used in the column calibration. IRMM-3702 Zn standard solution (3.09 μg mL⁻¹, Lot no. 000069, Belgium) was diluted to be 0.1 μg mL⁻¹ Zn solution with 0.5 mol L⁻¹ HNO₃. This Zn solution was used as Zn isotopic standard solution for MC-ICP-MS. Atomic absorption spectrometry standard solutions of Zn and Te (1000 μg mL⁻¹, Kanto Chemical Co. Inc., Japan) were used in a mass interference test.

Extraction chromatographic resin. The TEVA extraction resin (100–150 μm, Eichrom Technologies, Inc., USA) and Amberchrom CG-71C resin (Rohm and Haas, Co., USA), which is an organic resin without any additional liquid phase, were used. The 1 mL polypropylene column (5 cm × 5 mm in diameter, Muromachi Chemical Inc., Japan) was employed. The TEVA resin and CG-71C were stored in water. The resins were used only once.

Zinc double spike. Zinc isotopes of ⁶⁷Zn and ⁷⁰Zn, with enrichments of 95 and 95%, were purchased from Oak Ridge National Laboratory (USA) and Euriso-top (France), respectively. Each spike was dissolved and diluted with HNO₃, then mixed and used as the Zn double spike.

A theory of a double spike is briefly explained here. As Zn has five stable isotopes, ⁶⁴Zn, ⁶⁶Zn, ⁶⁷Zn, ⁶⁸Zn and ⁷⁰Zn, there are a lot of combinations for the double spike. We chose a ⁶⁷Zn–⁷⁰Zn double spike, which is recommended in Rudge et al.¹³⁶⁴Zn was used as an denominator isotope. Each Zn ratio is written as follows using an exponential law:

α = [ln(R_c-66/64/R_meas-66/64)]/[ln(m₆₆/m₆₄)]/m₆₄ (1)

R_c-67/64/R_meas-67/64 = (m₆₇/m₆₄) ^ [αm₆₄] (2)

R_c-70/64/R_meas-70/64 = (m₇₀/m₆₄) ^ [αm₆₄] (3)

where m_i is a mass of ⁱZn; R_meas-i/64 and R_c-i/64 indicate measured and constant isotope ratios of ⁱZn/⁶⁴Zn. These equations should be true for any sample and standard under a given set of R_c-i/64 (i = 66, 67 and 70). In other words, when R_c-66/64 is fixed to one value, the other two R_c-i/64 (i = 67 and 70) are determined. In this study, R_c-i/64 (i = 66, 67 and 70) are 0.563973, 0.082166 and 0.012418, respectively, which are values of the standard, IRMM-3702.

In order to determine R_sample-i/64 (i = 66, 67 and 70) of the unknown sample with an unknown degree of fractionation, the double spike method has been developed.^12,13,16 To a sample the enriched ⁶⁷Zn and ⁷⁰Zn are added. The spike isotope ratios of R_spike-i/64 (i = 66, 67 and 70) are determined to hold eqn (1)–(3) in advance (the spike calibration method is related later). For the mixture, the following equation holds:

R_mix-i/64 = [b_i (R_sample-i/64) + (R_spike-i/64)_spike]/(1 + b_i) (i = 66, 67 and 70) (4)

where b_i is the mixing ratio of the sample and spike in moles for i. Then three isotope ratios of the mixture (R_meas-i/64) are measured. The mixtures (R_mix-i/64) satisfy the exponential laws of eqn (1)–(3). As there are six variables (R_sample-i/64, and b_i; i = 66, 67 and 70) and six equations (eqn (1)–(4)), there should be solutions for R_sample-66/64, R_sample-67/64, R_sample-70/64. Thus we can determine fractionation degree of the unknown sample, δ⁶⁶Zn (see eqn (5)). There are many practical approaches to solve these equations (e.g., Johnson and Beard¹⁶). Here we followed their calculation in which exponential approximation is used.

The spike calibration method is simple. First, only the double spike with higher concentration is measured. Thus, one set of spike solution, R_spike-i/64 (i = 66, 67 and 70) are obtained. In this set, when one of R_spike-j/64, for example R_spike-66/64, is determined, the other two R_spike-j/64 (j = 67 and 70) are determined. Secondly, lots of spike-standard (the IRMM-3702 solution) mixtures are made, then the mixture isotope ratios are measured. More than ten mixtures are made and measured. Finally, by changing the spike isotope ratio of R_spike-66/64, the best-fit spike isotope ratio is searched to minimize the differences of IRMM-3702 between the measured and ideal values. Thus the Zn double spike was calibrated to be ⁶⁶Zn/⁶⁴Zn = 1.8425, ⁶⁷Zn/⁶⁴Zn = 88.293 and ⁷⁰Zn/⁶⁴Zn = 88.877 by this method, however, this is off the ideal value of the double spike.

Ideal values of the double spike were 42.65 and 57.35% for ⁶⁷Zn and ⁷⁰Zn, respectively.¹³ Such pure spike cannot be obtained and ⁶⁴Zn and ⁶⁶Zn are contained, therefore, the spike values in this study were 1.8425, 88.293, 88.877 for ⁶⁶Zn/⁶⁴Zn, ⁶⁷Zn/⁶⁴Zn and ⁷⁰Zn/⁶⁴Zn, respectively. The spike: sample mole ratio is required to be 37 : 63,¹³ therefore, 0.05 μg spike was added with 0.1 μg of sample. In order to achieve this ratio, a Zn amount of the sample was determined before isotope analysis.¹⁶

Silicate reference materials and digestion. Three USGS (US Geological Survey) silicate reference materials, BHVO-1 (basalt), AGV-1 (andesite) and PCC-1 (peridotite), and seven GSJ (Geological Survey of Japan) silicate reference materials, JB-1, -2, -3 (basalts), JA-1, -2, -3 (andesites) and JP-1 (peridotite) were used. These samples were decomposed with HF–HClO₄, dried with HClO₄, and dissolved with 0.5 mol L⁻¹ HNO₃. Details of the sample digestion method are found in Makishima and Nakamura.¹⁷ In order to determine reproducibility, the sample digestions were repeated for each sample. The sample solution containing 0.1 μg of Zn was taken and added with the appropriate amount of the double spike. Then the solution was dried with 0.4 mL of 8 mol L⁻¹ HBr at 80 °C to change the sample into bromide form, and the sample was loaded on the TEVA column, washed and conditioned as described below.

Carbonaceous chondrite powders of Orgueil (CI1), Murchison (CM2) and Allende (CV3) were digested in a similar manner to the silicate reference materials.

Chromatographic separation of Zn. The new single stage column chemistry was developed for purifying Zn using an extraction resin, TEVA in this study. The 0.33 mL of TEVA resin was packed in a 1 mL polypropylene column (5 cm × 5 mm in diameter, Muromachi Chemical Inc., Japan) above 0.67 mL of CG-71. CG-71 controls the elution speed and removes organic materials extracted from the TEVA resin. The TEVA resin is one of anion exchange resins, but the largest merit of this resin is resistance to oxidation.¹⁰

The column chemistry procedure developed in this study is summarized in Table 1. All experiments were done in a clean room.¹⁷ The column was pre-washed with 6.4 mL of 2 mol L⁻¹ HNO₃ and 1.6 mL of water, followed by 3.2 mL of 0.5 mol L⁻¹ HBr. The resin bed was then conditioned with 0.5 mol L⁻¹ HBr containing 0.15% (w/v) AA. The dried sample was dissolved with 0.1 mL of 0.5 mol L⁻¹ HBr containing 0.15% (w/v) AA and loaded on the resin bed. The major elements (Na, Mg, Al, P, Ca, Cr, Mn and Fe, etc.) were eluted by addition of 3.2 mL of 0.5 mol L⁻¹ HBr containing 0.15% (w/v) AA. Subsequently, the resin bed was washed with 0.8 mL of 0.5 mol L⁻¹ HBr, and Zn was collected by addition of 0.8 mL of water and 5.6 mL of 2 mol L⁻¹ HNO₃. The purpose of 0.8 mL of 0.5 mol L⁻¹ HBr and water before Zn collection are to prevent AA from forming complex organic materials and to prevent HBr from generating gaseous Br₂ or NO_x by reaction with HBr and HNO₃, respectively.

Table 1 Chromatographic procedure for separation of Zn using TEVA resin column

Washing
	2 mol L^−1 HNO3	6.4 mL
	Water	1.6 mL
	0.5 mol L^−1 HBr	3.2 mL
Conditioning
	0.5 mol L^−1 HBr with 0.15% ascorbic acid	3.2 mL
Loading the sample
	0.5 mol L^−1 HBr with 0.15% ascorbic acid	0.1 mL
Removing major elements
	0.5 mol L^−1 HBr with 0.15% ascorbic acid	3.2 mL
	0.5 mol L^−1 HBr	0.8 mL
Collecting Zn
	Water	0.8 mL
	2 mol L^−1 HNO3	5.6 mL

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Then the Zn fraction was dried on a hot plate at 120 °C in a clean fume hood with the addition of 1 drop of HClO₄ to decompose organic materials and small resin particles. To remove HClO₄ completely from the sample, the dried sample was further heated at 195 °C for 6 hours, and then dissolved with 1 mL of 0.5 M HNO₃.

Measurement of Zn by MC-ICP-MS. MC-ICP-MS, NEPTUNE (ThermoElectron, Germany) at the Pheasant Memorial Laboratory (PML) was used in low resolution mode. Details of the MC-ICP-MS operating conditions and the cup configuration are shown in Table 2. 0.5 mol L⁻¹ HNO₃ was used as carrier, washing and sample solutions. The desolvator, ARIDUS II (CETAC, USA) and the jet cone with additional rotary pump¹⁸ were used. The signal of ⁶⁶Zn⁺ for 0.1 μg mL⁻¹ Zn solution was 4 × 10⁻¹¹ A. When the jet cone and the additional rotary pump are not used, the signal intensity of ⁶⁶Zn⁺ was ∼1.2 × 10⁻¹¹ A, therefore, the jet cone increased the signal intensity by ∼3 times. Gains of amplifiers of each Faraday cup were calibrated before each session. 0.5 mol L⁻¹ HNO₃ was measured before each sample measurement as background, and subtracted from the following sample analysis (on-top zeros). As ⁶²Ni signal was ⁶⁴Ni/⁶⁴Zn = 10⁻⁷ to 10⁻⁸ levels and small, ⁶²Ni was measured by an amplifier with a 1 TΩ resistor,¹⁸ and ⁶⁴Ni/⁶²Ni = 0.25 was used without considering mass discrimination. Zinc isotope ratios were obtained by double spike calculation,¹⁶ and expressed in δ-notation (a permill difference from that of the Zn standard, IRMM-3702):

δ⁶⁶Zn = [(⁶⁶Zn/⁶⁴Zn)_sample/(⁶⁶Zn/⁶⁴Zn)_IRMM-3702 − 1] × 1000 (5)

Table 2 MC-ICP-MS operating conditions

a The amplifier with a resistor of 1 TΩ.
1. Sample introduction and ICP conditions
Nebulizer					Micro-flow PFA nebulizer, PFA-50 (ESI, USA), self-aspiration (flow rate: ∼50 μL min^−1)
Desolvator					Aridus II
Ar gas flow rate					8–9 L min^−1
Plasma power					1.2 kW (27.12 MHz)
Torch					Quartz glass torch with a sapphire injector
Plasma Ar gas flow rate					15 L min^−1
Auxiliary Ar gas flow rate					0.80 L min^−1
Nebulizer Ar gas flow rate					0.90 L min^−1
2. Interface
Sampling cone					Jet cone, made of Ni, with additional rotary pump
Skimmer cone					X-skimmer, made of Ni
3. Data acquisition conditions
Resolution					∼500
Washing time					480 s after measurement
Uptake time					90 s
Background data integration					4 s for 1 scan
					20 scans in one run
					On-top zeroes
Sample data integration					4 s for 1 scan
					20 scans in one run
4.Cup configuration
	L4	L3	L2	L1	C	H1	H2	H3	H4
	^62Ni^a	^63Cu	^64Zn	^65Cu	^66Zn	^67Zn	^68Zn	^70Zn

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One run consists of 20 scans, and the average and standard error (1SE) were obtained. One measurement takes almost 15 min including washing time.

Results and discussion

TEVA column chemistry

In the early stage of this study, 2 mol L⁻¹ HCl, 0.05, 0.25 and 0.5 mol L⁻¹ HBr mixed with 0.15% ascorbic acid (AA) were tested as eluent. Elution curves of Fe, Cu and Zn in these tests are shown in Fig. 1(a)–(d), respectively. The diluted multi-element standard solutions of Specpure no. 42885 and 44270 were used as the sample solution.

Fig. 1 Elution curves of Fe, Cu and Zn obtained by (a) 2 mol L⁻¹ HCl with 0.15% ascorbic acid (AA), (b) 0.05 mol L⁻¹ HBr with AA, (c) 0.25 mol L⁻¹ HBr with AA and (d) 0.5 mol L⁻¹ HBr with AA. The horizontal axis indicates the eluent volume (mL). The first eluent includes that of sample loading (0.1–0.2 mL). The vertical axis shows the yield (%) of each element relative to the loaded amount of the sample.

Fig. 1(a) shows the elution curves of Fe, Cu and Zn when 2 mol L⁻¹ HCl with AA was used. Fe, Cu and Zn adsorb on the TEVA resin forming chloro-complex but do not elute. This elution condition cannot be used because there is no separation of Fe from Zn. In the next step, the sample solution and eluents were changed into HBr. Using 0.05 mol L⁻¹ HBr, Fe elutes together with Zn, but Cu does not appear (Fig. 1(b)), resulting in no separation between Fe and Zn. In a case of 0.25 mol L⁻¹ HBr with AA, Fe and Zn can be separated, however, Cu is not separated from Zn (Fig. 1(c)). Thus, this condition also does not fit to the Zn purification. However, when 0.5 mol L⁻¹ HBr with AA is applied, Cu as well as Fe elute, while Zn is still on the resin. Therefore, Zn can be purified from Cu and Fe. Likewise, alkaline, alkaline earth and rare earth elements are separated. Luck et al.³ used AG 1 × 4 and HBr, but, the details of the method are not described in their publication.

Performance of TEVA resin chemistry

Alkaline, alkaline earth and rare earth elements of >99% are removed in the first 1.6 mL of 0.5 mol L⁻¹ HBr with AA. V, Cr, Mn, Fe, Co, Ni, Ga, Ge, Zr, La, Ce and Hf show similar behavior to Fig. 1(d). Coexistent elements in the Zn fraction are ∼20% of Nb, Ta and W, and >90% of Cd, In, Sn, Tl, Pb and Bi, which are obtained by using the mixture of multi-element standard solutions of Specpure no. 42885 and 44270. Total amounts of Nb, Ta and W in the sample solution should be lower than these values, because coprecipitation with Ti oxides is expected to occur in natural silicate analysis.¹⁹ Generally, abundances of these coexistent elements are low compared to Zn, thus, our method suits most silicate samples.

The recovery yield of Zn in the various actual silicate composition containing 0.1 μg Zn was 96.7 ± 6.8% (n = 10). Small loss in the column chemistry should cause mass fractionation in column chemistry,²⁰ however, as the double spike method is employed in this study, the fractionation in the column chemistry should be corrected.

The flow rate of the eluent was 0.3 mL min⁻¹, therefore, the column chemistry can be finished within two hours including the resin wash and column conditioning procedure. This is another advantage of the TEVA resin column chemistry developed in this study compared to the conventional methods, for example, the flow rate of the eluent using 1.6 mL of AG MP-1 is ∼0.1 mL min⁻¹, and total elution volume is 50 mL.⁵ Thus it takes ∼8 hours for the Zn column purification. If the washing time of the resin is included, total separation time is much longer.

The total blank including sample digestion was 0.05 ng (n = 6), which was determined by a calibration curve method. This blank was determined by measuring solution obtained after all sample digestion and column chemistry procedures. As 0.1 μg of Zn is separated, and isotope composition between the sample and blank is assumed <±10‰, this blank levels affect <±0.01‰ to the sample isotope composition, which can be neglected. This blank level is 1/300 of Marechal et al. (15 ng)⁵ or 1/20 of Archer and Vance (1 ng).⁸ In other words, 300 or 20 times Zn are required in the conventional methods to make blank effects negligible.^5,8 Such low blank is one of advantages of this new column chemistry, and 0.1 μg of Zn is a sufficient amount to determine Zn isotope composition.

Isobaric interferences

In Fig. 2, interferences on Zn are summarized.²¹ Argides such as ArMg⁺, ArAl⁺ and ArSi⁺ should not exist, because Mg, Al and Si are removed in the TEVA column chemistry. Barium, La and Ce form interfering ions of ¹³⁸Ba²⁺, ¹³⁸La²⁺ and ¹⁴⁰Ce²⁺, however, they are also removed by the TEVA column, therefore, there is no need to consider them. Ge has an isobaric isotope with Zn, ⁷⁰Ge. In our sample digestion method using HF, Ge is removed as gaseous GeF₄.^17,22 Tellurium and Xe, form interfering doubly charged ions, ¹²⁸Te²⁺ and ¹³²Xe²⁺ in the plasma. Tellurium could remain in the Zn fraction. Even if 100% of Te remains in the Zn fraction, its effect should be <0.03‰ and negligible, because Te/Zn of general silicates is <1000,²² Te²⁺/Te⁺ is <0.1% and sensitivity of Te/Zn = ∼0.4. The double charge forming ratio and sensitivity ratio between Te and Zn were determined by the measurement condition in Table 2.

Fig. 2 Typical background levels of Neptune with a Jet cone, an X-skimmer cone, and a large rotary pump. The horizontal axis indicates mass (m) over charge (z) plotted in linear scale. The vertical axis shows the background levels in A in log scale. The background levels were obtained by NEPTUNE introducing 0.5 mol L⁻¹ HNO₃ through the desolvator at low (LR), middle (MR) and high (HR) mass resolutions (M/ΔM = ∼500, ∼3000 and ∼7500), respectively. The error bars indicate 1SE (standard error) of 20 measurements. The possible major interferences in the Zn measurement are shown at the top of the figure.

In order to estimate the effects of ⁴⁰Ar¹²C¹²C⁺, ¹³²Xe²⁺, ⁴⁰Ar¹²C¹⁶O⁺, ⁴⁰Ar¹⁴N¹⁴N⁺, and ⁴⁰Ar¹⁴N¹⁶O⁺ on m/z = 64, 66, 67, 68 and 70, respectively, backgrounds were measured at low (LR; M/ΔM = ∼500), middle (MR; M/ΔM = ∼3000) and high (HR; M/ΔM = ∼7500) resolutions (Fig. 2). As the resolution increases, the transmission of the mass spectrometer decreases so the background level decreases. At the highest resolution, the pattern of peaks becomes almost flat, indicating peculiar molecular ions are eliminated. However, at the low resolution, backgrounds show a zigzag pattern with higher backgrounds at m/z = 64, 66 and 68. As only 0.5 mol L⁻¹ HNO₃ through the desolvator is introduced into the plasma, these higher backgrounds are considered to be caused by ⁴⁰Ar¹²C¹²C⁺, ¹³²Xe²⁺, ⁴⁰Ar¹⁴N¹⁴N⁺ and ⁴⁰Ar¹²C¹⁶O⁺. The intensities of these backgrounds are 3–5 × 10⁻¹⁴ A. After the double spike addition, the smallest peak is ⁶⁶Zn⁺ of ∼6 × 10⁻¹¹ A, therefore, the largest effect is on the background of ⁶⁶Zn⁺ and 0.5‰. The standard and sample solutions are basically the same Zn solution, it can be assumed that the intensity of ¹³²Xe²⁺ should not change larger than 10%. If so, the accuracy of ⁶⁶Zn⁺ is <0.05‰.

Performance of double spike-MC-ICP-MS

The average and standard deviation of double spike-MC-ICP-MS for the Zn standard of IRMM-3702 were δ⁶⁶Zn = −0.00 ± 0.04 (2SD; n = 58) in this study. This error of 0.04 is considered to be the minimum error in the double spike technique including errors in the spike isotope ratio estimation.

If the silicate sample is already digested and diluted with HNO₃, the total analysis time is 3 days. The first day is for the column preparation and column separation; the second day is for drying HClO₄ and final dissolution into 0.5 mol L⁻¹ HNO₃; and the third day is for measurement by double spike-MC-ICP-MS.

Mixing experiment of double spike-MC-ICP-MS

It is difficult to evaluate the accuracy for unconventional stable isotope systems such as Zn, because there are a few standard solutions for Zn²³, which are difficult to obtain. One of the available standard solutions was IRMM-3702, which is already used for sample-standard bracketing measurements and also for calculating the isotope ratio as δ-notation. If there are two standards, evaluation of analytical methods or data becomes far easier. In this study, in order to estimate the accuracy, the mixed Zn solutions are made by mixing of two samples, and δ⁶⁶Zn was compared with the calculated one. Such mixing experiments were done in Tl isotope determination to evaluate the accuracy of the method.²⁴ The two samples were chosen to have different isotope and matrix compositions, therefore, matrix effects on isotopes were also examined in this test.

For the two samples, the JB-2-digested solution (sample #1; 0.33 μg mL⁻¹) of δ⁶⁶Zn = −0.13 ± 0.08 (2SD; n = 5) and the diluted Alfa Aesar Specpure Zn standard solution (sample #2; 0.10 μg mL⁻¹) of δ⁶⁶Zn = −10.01 ± 0.24 (2SD; n = 29) were used. δ⁶⁶Zn of sample #2 was determined by a bracketing method, because both sample #2 and IRMM-3702 were the same concentration Zn solutions with similar high purity. Sample #1 and sample #2 solutions were taken to be 0.1 μg with three different compositions. Then the mixed solutions were added with the Zn double spike, 0.4 mL 8 mol L⁻¹ HBr and dried, re-dissolved, and passed through the TEVA column. Then δ⁶⁶Zn of the mixtures was determined by the double spike-MC-ICP-MS. The calculated δ⁶⁶Zn was obtained from the mixing ratio and δ⁶⁶Zn of the samples #1 and #2. The error of the calculated δ⁶⁶Zn of the mixture was obtained from errors of concentration of ∼5% of the two solutions and pipetting error of ∼1%.

Measurement results of the δ⁶⁶Zn values of the three mixtures are shown in Table 3. The calculated value is also shown in Table 3. The results are also plotted in Fig. 3. The observed isotope fractionation is consistent with the calculated δ⁶⁶Zn within error. Although the three samples have different isotope ratios and major element compositions, the observed δ⁶⁶Zn in mixture is consistent with the calculated δ⁶⁶Zn. Thus, it is concluded that the double spike-MC-ICP-MS of this study gives the analytical results independent of concentration of major element concentrations and δ⁶⁶Zn, resulting in high reliability and accuracy.

Table 3 Calculated and measured results of mixing experiments

	Calculated	2SD (‰)	Measured	2SD (‰)	n
	δ ^66Zn		δ ^66Zn
Mixture #1	−1.67	−0.10	−1.54	0.14	4
Mixture #2	−3.45	−0.20	−3.73	0.18	7
Mixture #3	−6.94	−0.40	−7.27	0.10	4

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Fig. 3 Analytical results of the mixing experiments. The samples #1 and #2 are mixed, and their observed and calculated values of δ⁶⁶Zn are plotted. The diagonal line shows slope 1. The dotted lines indicate errors of isotope ratios combined by the samples #1 and #2 (0.26‰, 2SD). Error bars are 2SD.

Reproducibility of Zn isotope ratios in silicate reference materials

Reproducibility is obtained by 2SD of the same sample starting from different sample digestion. In Table 4, the δ⁶⁶Zn values with 2SD for the USGS standard reference materials, BHVO-1, AGV-1 and PCC-1 are shown. δ⁶⁶Zn of GSJ reference materials, JB-1, 2, 3, JA-1, 2, 3 and JP-1 and reproducibility (2SD) are also in Table 4. There are no δ⁶⁶Zn values to compare to those of this study, so future extensive studies are required. The reproducibility of all measurements of the silicate samples in Table 4 was 0.08–0.32‰ (the average was 0.19‰ in 2SD), so this average (0.19‰) can be considered as the reproducibility in actual silicate sample analysis of this study.

Table 4 δ ⁶⁶Zn of silicate reference materials

Sample	δ ^66Zn	2SD	n
	Average	(‰)
BHVO-1	−0.42	0.26	5
AGV-1	−1.90	0.30	3
PCC-1	−0.09	0.10	5
JB-1	−0.04	0.32	3
JB-2	−0.13	0.08	5
JB-3	−0.29	0.20	3
JA-1	−0.03	0.08	3
JA-2	−0.33	0.14	4
JA-3	−0.11	0.18	3
JP-1	−0.15	0.20	4

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Comparison of δ⁶⁶Zn values of chondrites with those in other laboratories

The δ⁶⁶Zn values of carbonaceous chondrites, Orgueil, Murchison and Allende are shown in Table 5. It is difficult to compare our data with those of other laboratories, because each laboratory uses different standards. For example, Moynier et al.² and Luck et al.³ use Zn from Johnson Matthey Company (JMC) 400882B, called “Lyon solution” as the standard. Cloquet et al.²³ reported that δ⁶⁶Zn of Lyon solution is −0.32 ± 0.16 (2SD) compared to IRMM-3702. This correction factor makes δ⁶⁶Zn of carbonaceous meteorite data of Moynier et al.² and Luck et al.³ possible to be compared with our data. The error of correction factor of 0.16 (ref. 23) is considered as 2SD error of ref. 2 and 3. Ghidan and Loss⁴ use IRMM-3702, therefore, their data can be directly used here.

Table 5 δ ⁶⁶Zn of carbonaceous chondrites

Sample	δ ^66Zn	2SD	n	References
	Average			(Error is 2SD)
Orgueil	0.59	0.26	3	0.14 ± 0.16^3, 0.20 ± 0.16^3, 0.60 ± 0.30^4
Murchison	0.16	0.04	4	0.12 ± 0.16^2, 0.01 ± 0.16^3, 0.13 ± 0.16^3
Allende	−0.06	0.12	3	0.06 ± 0.16^3, −0.32 ± 0.16^3, 0.76 ± 0.32^4

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In Table 5, δ⁶⁶Zn of recalculated reference values are shown. In Fig. 4, the data of this study are compared with those of the reference values. From Fig. 4, δ⁶⁶Zn of the carbonaceous chondrites measured in this study seem to be consistent with the reported values, indicating that our method can yield accurate results. However, variations of Allende in references are too large. This possibly indicates heterogeneity of the Allende meteorite.

Fig. 4 δ ⁶⁶Zn of carbonaceous chondrites in this study vs. δ⁶⁶Zn of those in ref. 2–4 The error bars are 2SD. A diagonal line indicates slope = 1. The solid circle, square and triangle indicate δ⁶⁶Zn of Orgueil, Murchison and Allende, respectively.

Conclusions

A new low-blank column chemistry for separating Zn has been developed employing HBr-ascorbic acid and TEVA resin. Recovery yield using silicate samples was 96.7 ± 6.8% (n = 10) and blank was 0.05 ng (n = 6). The blank level is from 1/300 to 1/20 of previous studies, meaning 0.1 μg of Zn is sufficient for analysis. Therefore, 0.1 μg of Zn was used in this study. We found that the total separation time of Zn including resin washing time can be reduced to < two hours.

The δ⁶⁶Zn value was determined using double spike MC-ICP-MS. The IRMM-3702 was used as the standard. The δ⁶⁶Zn values of 0.04‰ (2SD; n = 58) was obtained as reproducibility of the standard solution by the double spike-MC-ICP-MS.

In order to show the applicability of this method, δ⁶⁶Zn of three USGS and seven GSJ standard silicate materials were determined. The reproducibility of δ⁶⁶Zn was estimated from the average of total silicate samples to be 0.19‰ (2SD). δ⁶⁶Zn of carbonaceous chondrites of Orgueil, Murchison and Allende are measured and compared with the results of previous studies to evaluate the accuracy of double spike MC-ICP-MS.

Acknowledgements

The authors thank Kayo Tanaka for performing the column chemistry; T. Moriguti, C. Sakaguchi and all members of PML for maintaining our clean laboratory. The authors also thank the two anonymous journal referees for improving the manuscript.

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