Matthias
Friebel
,
Eniko R.
Toth
,
Manuela A.
Fehr
and
Maria
Schönbächler
*
Institute of Geochemistry and Petrology, ETH Zürich, Clausiusstrasse 25, 8092 Zürich, Switzerland. E-mail: mariasc@ethz.ch
First published on 19th December 2019
This paper presents a new method for the separation of Sn and Cd from geological matrices followed by high-precision isotope analyses that include low abundance isotopes (<1.25%). The new technique is of specific interest for the detection of small mass-independent nucleosynthetic or cosmogenic isotope variations in meteorites and other planetary materials. We also report a new precise estimate for Sn isotope abundances. The method employs a combination of ion exchange and extraction chromatography together with multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). Tin is separated from the sample matrix using an anion-exchange resin, followed by removal of remaining matrix elements and organics using the TRU and Pre-filter resins, respectively. The matrix fraction from the TRU resin step is further purified to isolate Cd using a two-stage anion exchange procedure. Analyses of Sn and Cd standard solutions doped with interfering elements were employed to define thresholds for tolerable amounts of interference producing elements. Our data demonstrate that our new procedure produces purified Sn and Cd solutions with sufficiently low levels of contaminants for high precision Sn and Cd isotope analyses. Removal of U is important for Sn isotope data because of doubly charged U ions. The internally normalised Sn isotope data of the two standard solution (NIST SRM 3161a and SPEX CLSN2-2Y) are in excellent agreement with previous data. Based on repeated analysis of independently processed lake sediment aliquots, an external reproducibility (intermediate precision) (2SD) is achieved of ±110 ppm for 112Sn/120Sn, ±170 ppm for 114Sn/120Sn, ±160 ppm for 115Sn/120Sn, ±21 ppm for 117Sn/120Sn, ±13 ppm for 118Sn/120Sn, ±20 ppm for 119Sn/120Sn, ±22 ppm for 122Sn/120Sn and ±24 ppm for 124Sn/120Sn. Replicate Sn analyses of the carbonaceous chondrite Allende are in excellent agreement with those of the lake sediments. For Cd isotope analyses, the lake sediment yields an external reproducibility (2SD) of ±170 ppm for 106Cd/111Cd, ±200 ppm for 108Cd/111Cd, ±34 ppm for 110Cd/111Cd, ±18 ppm for 112Cd/111Cd, ±24 ppm for 113Cd/111Cd and ±15 ppm for 114Cd/111Cd.
Tin isotopes are of special interest for geochemical and cosmochemical studies due to various reasons. With a 50% condensation temperature of 704 K for a gas of solar composition at a total pressure of 10−4 bar, Sn is cosmochemically classified as a moderately volatile element.19,20 Moreover, the primitive mantle of the Earth is depleted in Sn by a factor of 33 ± 3 relative to the CI chondrites.21 Therefore, mass-dependent Sn isotope fractionation is a promising tool to better constrain the mechanism of volatile depletion in the solar system. Geochemically, Sn is moderately siderophile to chalcophile, and incompatible during silicate differentiation.21 The behaviour of Sn during differentiation depends on its oxidation state, coordination in the crystal lattice and melt composition.22,23 Based on this, Sn isotope fractionation likely occurs during igneous processes and published Sn isotope data support this conclusion.17,18
Similarly, Cd is another moderately volatile element with a 50% condensation temperature of 652 K20 that has been studied for its mass-dependent isotope composition to address the origin of volatile depletion in rocky planets (e.g.ref. 24–27). More recently, it has also been shown that Cd isotopes not only fractionate due to volatility related processes e.g. in chondrites,26 but also during magmatic differentiation, with the crust displaying a slightly heavier Cd isotope composition.28
In recent years, nucleosynthetic isotope variations have become a powerful tool in cosmochemistry. These are mass-independent isotope variations identified in meteorites and terrestrial planets. They are caused by the heterogeneous distribution of presolar dust that survived the formation of the solar system and kept the extreme isotopic compositions of their stellar sources (e.g. AGB-star or supernovae).29 The distinct nucleosynthetic compositions of planetary materials can be used to investigate important processes in the early solar system, such as mixing or the physical conditions (e.g. temperatures) that prevailed in the protoplanetary disk, and influenced the composition of solar system material (e.g.ref. 30 and 31). Tin is the element with the highest number of stable isotopes with ten stable isotopes, formed by the p-process (112Sn and 114Sn), s-process (116Sn), the r-process (124Sn), or mixtures thereof (115Sn, 117Sn, 118Sn, 119Sn, 120Sn and 122Sn), and therefore an ideal candidate to study nucleosynthetic isotope variations in the solar system. Cadmium provides an excellent companion with its eight stable isotopes also produced by a variety of nucleosynthetic processes: 106Cd and 108Cd by the p-process, 110Cd by the s-process, and 111Cd to 116Cd by a combination of the s- and the r-process in different proportions.32,33 Cadmium has a mainly chalcophile affinity as opposed to the more siderophile nature of Sn, indicating that Cd and Sn may reside in different carrier phases within meteorites. Therefore, the combined study of Sn and Cd may provide a powerful tool to assess the origin of the current lack of resolvable nucleosynthetic isotope variations of moderately volatile elements at the bulk meteorite scale. However, since these mass-independent nucleosynthetic variations are generally small, mostly less than 0.1 per mil (e.g. for Zr,30 Mo34 and Ru35), an improved analytical procedure for mass-independent Sn and Cd isotope analyses is required.
In addition to nucleosynthetic variations, meteorites and lunar samples can experience mass-independent modification of their isotopes through exposure to galactic cosmic rays in space. Such modifications are, for example, reported for Cd isotopes in lunar samples.26,36 This is important to note, because the investigation of the mass-dependent Sn and Cd isotope compositions in meteorites and lunar samples using the double-spike method also depends on their mass-independent isotope composition. This is because the double spike method assumes a constant natural isotope composition that is only altered by mass-dependent isotope fractionation. In order to obtain precise and accurate data using the double-spike approach, one must identify whether the Sn or Cd isotope composition was subject to mass-independent processes, i.e. has a different mass-independent composition and if so, the double spike calculation needs adaption (e.g.ref. 37).
Available analytical methods for Sn or Cd isotope analyses (e.g.ref. 14 and 27) were not designed to obtain such mass-independent isotope data at sufficiently high precision to address these issues. Therefore, we developed a new analytical procedure to obtain high precision Sn and Cd isotope data from the same sample aliquot. The method includes a three-stage chromatographic procedure to efficiently separate Sn from complex rock matrices, followed by anion-exchange chromatography to purify Cd. The resulting Sn and Cd sample solutions are analysed on a Nu Plasma II MC-ICP-MS, thereby determining all Sn and Cd isotopes at high precision. Spectral interferences and matrix effects from traces of elements remaining after sample purification can hamper data quality during MC-ICP-MS analyses. This is in particular an issue when improving the analytical precision, because effects, which were unproblematic at lower precision, can be resolved at high precision. For this reason, we performed extensive test with Cd and Sn standard solutions doped with critical elements. The accuracy and precision of the new procedure was also verified using two geological samples processed through the chromatographic procedure: a lake sediment and the carbonaceous chondrite Allende. The new technique is powerful because it can be applied for the search of nucleosynthetic or cosmogenic Sn and Cd isotope variations in extraterrestrial materials. Since extraterrestrial materials are in general rare and therefore analyses are sample limited, the combined Sn and Cd isotope analyses on the same sample aliquot constitute a significant advantage. Here, the method is also used for a new accurate estimate of Sn isotope abundances.
Eluent | Volume (ml) | Step |
---|---|---|
(1) Column: 2 ml BioRad AG1-X8 anion-exchange resin (200–400 mesh) | ||
1 M HNO3 | 10 | Resin cleaning |
1 M HCl | 10 | Resin conditioning |
1 M HCl | 10 | Sample loading |
1 M HCl | 20 | Matrix elution |
6 M HCl | 10 | Matrix elution |
1 M HNO3 | 2 | Matrix elution |
1 M HNO3 | 5 | Sn, Cd, Zn |
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(2) Column: 40 μl Eichrom Pre-filter resin (100–150 μm) + 120 μl Eichrom TRU resin (100–150 μm) | ||
1 M HNO3 | 5 | Resin cleaning |
0.1 M HCl–0.3 M HF | 5 | Resin cleaning (U) |
1 M HCl | 5 | Resin conditioning |
1 M HCl | 1 | Sample loading |
1 M HCl | 5 | Cd, Zn |
1 M HNO3 | 3.5 | Sn |
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(3) Column: 160 μl Eichrom Pre-filter resin (100–150 μm) – organics removal | ||
1 M HCl | 5 | Resin cleaning |
0.1 M HCl–0.3 M HF | 5 | Resin cleaning (U) |
1 M HNO3 | 5 | Resin conditioning |
1 M HNO3–0.01 M HF (sample solution) | 3.5 | Sn |
1 M HNO3 | 1 | Sn |
The Eichrom Pre-filter resin (40 μl) used in the second stage was not sufficient to remove all organic compounds (octyl(phenyl)-N,N-diisobutylcarbamoylmethylphosphine oxide (CMPO) and tributyl phosphate (TBP)) of the TRU resin. For further organic removal, an additional column using solely Pre-filter resin was introduced (Table 1). The eluted Sn fraction of the second column was directly loaded on a Teflon column with Eichrom Pre-filter (160 μl). The resin was pre-cleaned and conditioned with 5 ml 1 M HCl, 5 ml 0.1 M HCl–0.3 M HF and 5 ml 1 M HNO3. Tin was directly collected upon loading and in an additional 1 ml 1 M HNO3.
Eluent | Volume (ml) | Step |
---|---|---|
a The Cd-containing fraction of the (2) column from the Sn separation procedure (Table 1) was dried down and dissolved in 3 M HCl as sample load. | ||
2 ml BioRad AG 1-X8 anion-exchange resin (100–200 mesh) | ||
2 M HNO3 | 10 | Resin cleaning |
H2O | 1 | Rinse |
0.5 M HCl | 1 | Conditioning (conversion to Cl− form) |
6 M HCl | 20 | Conditioning |
0.5 M HCl | 11 | Conditioning |
3 M HCl | 10 | Sample loadinga |
3 M HCl | 1 | Matrix elution |
0.5 M HCl | 30 | Matrix elution |
1 M HCl | 10 | Matrix elution |
2 M HCl | 10 | Matrix elution |
0.5 M HNO3–0.1 M HBr | 1 | Elute remaining HCl |
0.5 M HNO3–0.1 M HBr | 20 | Zn |
2 M HNO3 | 2.5 | Matrix elution |
2 M HNO3 | 4 | Cd |
2 M HNO3 | 4 | Cd (post fraction) |
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160 μl BioRad AG 1-X8 anion-exchange resin (100–200 mesh) | ||
2 M HNO3 | 2 | Resin cleaning |
H2O | 0.1 | Rinse |
0.5 M HCl | 1 | Conditioning (conversion to Cl− form) |
3 M HCl | 1 | Sample loading |
0.5 M HCl | 0.2 | Matrix elution |
0.5 M HNO3–0.1 M HBr | 0.1 | Elute remaining HCl |
0.5 M HNO3–0.1 M HBr | 1.6 | Zn |
2 M HNO3 | 0.25 | Matrix elution |
2 M HNO3 | 0.4 | Cd |
2 M HNO3 | 0.4 | Cd (post fraction) |
The second separation step is a downscaled version of the first (by a factor of ∼10) using ∼160 μl anion-exchange resin (AG 1-X8, 100–200 mesh) prepared in Teflon columns (Table 2). It was specifically designed to remove remaining Zn required for samples with a 66Zn/111Cd ratio above ∼0.01. This ratio was calculated based on mass-scans performed on the Nu Plasma II MC-ICP-MS that was used for Cd isotope analysis. This ratio is the threshold determined for obtaining accurate 106Cd and 108Cd isotope data (see below) onto which Zn argides can interfere. The sample was prepared prior to loading as before, but with 0.5 ml 6 M HCl and 0.5 ml MQ water. The Cd fraction was collected in 0.4 ml 2 M HNO3. The post-Cd elution contained on average <5% of the total eluted Cd. However, on one occasion, a column processed standard contained ∼7% of the total Cd in this post-cut and was thus recombined with the main Cd fraction.
111Cd | 112Sn | 113Cd | 114Sn | 115Sn | 116Sn | 117Sn | 118Sn | 119Sn | 120Sn | 122Sn | 124Sn | 125Te | 126Te | 129Xe | |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Collector configuration | |||||||||||||||
Line 1 | L3 | L2 | L1 | Ax | H1 | H2 | H3 | H4 | H5 | H6 | H7 | H8 | |||
Line 2 | L4 | L3 | L2 | L1 | H1 | H3 | H4 | H5 | H7 | ||||||
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Isotope abundances of Sn and isobaric interferences | |||||||||||||||
Cd | 12.8 | 24.1 | 12.2 | 28.7 | 7.5 | ||||||||||
Sn | 0.97 | 0.66 | 0.34 | 14.5 | 7.7 | 24.2 | 8.6 | 32.6 | 4.6 | 5.8 | |||||
In | 4.3 | 95.7 | |||||||||||||
Te | 0.10 | 2.6 | 4.8 | 7.1 | 19.0 | ||||||||||
Xe | 0.09 | 0.09 | 26.4 | ||||||||||||
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Major molecular interferences | |||||||||||||||
M1H | 110Pd (11.7) | ||||||||||||||
110Cd (12.5) | 111Cd (12.8) | 112Cd (24.1) | 113Cd (12.2) | 114Cd (28.7) | |||||||||||
115In (95.7) | |||||||||||||||
121Sb (57.2) | 123Sb (42.8) | ||||||||||||||
124Sn (5.8) | |||||||||||||||
128Te (31.7) | |||||||||||||||
M14N | 97Mo (9.5) | 98Mo (24.1) | 100Mo (9.6) | ||||||||||||
99Ru (12.7) | 100Ru (12.6) | 101Ru (17.0) | 102Ru (31.5) | 104Ru (18.6) | |||||||||||
103Rh (100) | |||||||||||||||
104Pd (11.1) | 105Pd (22.3) | 106Pd (27.3) | 108Pd (26.4) | 110Pd (11.7) | |||||||||||
110Cd (12.5) | 111Cd (12.8) | 112Cd (24.0) | |||||||||||||
115In (95.7) | |||||||||||||||
M16O | 95Mo (15.9) | 96Mo (16.6) | 97Mo (9.5) | 98Mo (24.1) | 100Mo (9.6) | ||||||||||
99Ru (12.7) | 100Ru (12.6) | 101Ru (17.0) | 102Ru (31.5) | 104Ru (18.6) | |||||||||||
103Rh (100) | |||||||||||||||
104Pd (11.1) | 106Pd (27.3) | 108Pd (26.4) | 110Pd (11.7) | ||||||||||||
110Cd (12.5) | 113Cd (12.2) | ||||||||||||||
109Ag (48.1) | |||||||||||||||
M40Ar | 71Ga (39.7) | ||||||||||||||
72Ge (27.6) | 73Ge (7.7) | 74Ge (35.8) | |||||||||||||
76Se (9.3) | 77Se (7.6) | 78Se (23.7) | 80Se (49.4) | 82Se (8.7) | |||||||||||
75As (100) | |||||||||||||||
79Br (50.5) | |||||||||||||||
40Ar–40Ar–40Ar (99.6) | |||||||||||||||
82Kr (11.5) | 84Kr (56.8) | 86Kr (17.2) | |||||||||||||
85Rb (71.9) | |||||||||||||||
86Sr (9.8) | |||||||||||||||
89Y (100) | |||||||||||||||
M++ | 232Th (100) | 238U (99.3) |
Isobaric interferences (Table 3) were corrected using 111Cd, 125Te and 129Xe signals as monitors as follows. First a Cd correction was performed on 116Sn by subtracting 116Cd (using 116Cd/111Cd = 0.563754 (ref. 6)) without mass bias correction. For the mass bias correction, the corrected 116Sn signal was then used to determine the fractionation factor (β) of the exponential law:
Rtrue = Rmeas × (m1/m2)β | (1.1) |
εxSn = ((xSn/120Snsample/xSn/120SnNIST 3161a) − 1) × 10![]() | (1.2) |
Solutions for yield and blank determination were analysed on an Element XR. The blanks from digestion and those from the chemical separation procedure were dried down and taken up in 0.5 M HNO3–0.005 M HF for analysis. To estimate the yields of the separation procedure, small aliquots (5% of sample material) of the Sn fraction collected in the first and third stage were compared to a small aliquot (1% of sample material) taken before the chromatographic procedure. These aliquots were also analysed in a 0.5 M HNO3–0.005 M HF media.
After baseline and background correction, the Cd isotope data were internally normalised to a 116Cd/111Cd ratio of 0.578505 42 with the exponential law. This ratio was chosen to ascertain a large spread, which improves precision, but also to minimize the effects of interferences from neighbouring elements (i.e. Pd) and to avoid isotopes affected by neutron capture (113Cd, 114Cd) during galactic cosmic ray irradiation in space.43
Interferences from Pd, In and Sn were corrected for using a similar iterative approach as described for the Sn data reduction (see Section 3.2). Results are reported in the epsilon notation relative to 111Cd:
εxCd = ((xCd/111Cdsample/xCd/111CdAlfa Aesar) − 1) × 10![]() | (1.3) |
The total Sn procedural blank ranged typically between 1.4 and 2.2 ng for samples of 0.5–1 g. The Sn blanks of the reagents differed, concentrated HCl (twice Teflon distilled) contained 16–30 pg ml−1, while concentrated HNO3 (twice Teflon distilled) and SQ water contained less than 2 pg ml−1 Sn. The total procedural blank was therefore strongly dependent on the digestion procedure. While the complete separation procedure usually yielded a blank of 0.4 ng Sn, the digestion of the sample alone resulted in blank values from 0.27 ng per beaker in a Parr bomb digestion, up to 2 ng blank for the digestion of 0.5–1 g sample material in 60 ml vials on the hotplate. Generally, the Sn content of the analysed lake sediment and basalt aliquots was above 500 ng resulting in a blank contribution of <1%, which is therefore negligible.
The Cd blanks after the Sn column chemistry in the Cd–Zn cut were ≤14 pg, and including the subsequent Cd ion exchange chemistry ≤87 pg, on average. The sample digestion and the complete separation procedure together yielded blanks of <90 pg Cd. Considering that the Cd contents of all lake sediments and column processed standards were ∼150 ng or higher, this results in a negligible blank contribution of <0.1%.
Sample | Sn (ppb)f | Cd/Sn | Te/Sn | N | ε 112Sn | ε 114Sn | ε 115Sn | ε 117Sn | ε 118Sn | ε 119Sn | ε 122Sn | ε 124Sn | |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
a Indicates the number of analysed standards within one analytical session, instead of the number (N) of analytical sessions for standard solutions or individual measurements for the other samples. b Denotes ε115Sn measured relative to SPEX CLSN2-2Y and corrected for difference between SPEX CLSN2-2Y and NIST SRM 3161a. c Measured with Aridus II on a Nu Plasma II. d Uncertainty refers to 95% confidence interval t0.95,n−1 × SD/√n). e For the lake sediment, two aliquots were digested. “bomb” refers to the Parr bomb digestion, the second aliquot was digested on the hotplate. NIST: NIST SRM 3161a; SPEX: SPEX CLSN2-2Y; Lake Zürich Ix: x refers to separate column chemistry. f Sn (ppb) concentration refers to measurement solution. | |||||||||||||
Sn standard solutions | |||||||||||||
NIST SRM 3161a | 200 | 1.4 × 10−5 | 3.6 × 10−5 | 20 | 0.0 ± 0.9 | 0.0 ± 1.3 | 0.0 ± 1.2 | 0.00 ± 0.16 | 0.00 ± 0.11 | 0.00 ± 0.17 | 0.00 ± 0.21 | 0.00 ± 0.31 | |
100 | 11 | 0.0 ± 1.4 | 0.0 ± 2.3 | 0.0 ± 2.0 | 0.00 ± 0.22 | 0.00 ± 0.13 | 0.00 ± 0.21 | 0.00 ± 0.25 | 0.00 ± 0.39 | ||||
40 | 29a | 0.0 ± 2.6 | 0.2 ± 4.9 | 0.1 ± 5.4 | 0.00 ± 0.20 | 0.00 ± 0.21 | 0.01 ± 0.39 | 0.00 ± 0.35 | 0.01 ± 0.40 | ||||
25 | 25a | 0.0 ± 5.0 | 0 ± 11 | 0.1 ± 6.7 | 0.01 ± 0.50 | 0.01 ± 0.26 | 0.02 ± 0.71 | 0.00 ± 0.91 | 0.00 ± 0.75 | ||||
9 | 19a | −1 ± 11 | 0 ± 23 | 1 ± 21 | −0.1 ± 1.4 | 0.04 ± 0.61 | 0.1 ± 1.9 | 0.1 ± 1.9 | 0.2 ± 1.7 | ||||
5.5 | 21a | 0 ± 28 | 1 ± 49 | 0 ± 42 | 0.0 ± 1.6 | −0.02 ± 0.91 | 0.0 ± 1.6 | 0.0 ± 2.6 | 0.0 ± 2.5 | ||||
SPEX CLSN2-2Y | 100 | 2.4 × 10−5 | 6.2 × 10−5 | 20a | 0.0 ± 1.2 | 0.1 ± 1.8 | 7.3 ± 2.2 | −0.05 ± 0.18 | 0.01 ± 0.11 | −0.03 ± 0.19 | −0.05 ± 0.27 | −0.13 ± 0.44 | |
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Column processed Sn standard solutions | |||||||||||||
NIST + Cd | 200 | 1.6 × 10−5 | 3.3 × 10−5 | 1 | −0.4 ± 1.0 | −0.3 ± 1.7 | −0.6 ± 1.1 | 0.18 ± 0.18 | 0.01 ± 0.13 | 0.11 ± 0.18 | −0.14 ± 0.16 | −0.16 ± 0.27 | |
NIST + Cd | 100 | 2.8 × 10−5 | 2.6 × 10−5 | 1 | 0.6 ± 1.7 | 1.3 ± 2.3 | 0.1b ± 1.7 | 0.02 ± 0.23 | 0.10 ± 0.20 | 0.04 ± 0.15 | −0.03 ± 0.14 | −0.17 ± 0.27 | |
SPEX | 100 | 8.6 × 10−6 | 2.8 × 10−5 | 1 | −0.6 ± 1.6 | −1.6 ± 2.1 | −1.1b ± 1.5 | −0.09 ± 0.25 | −0.04 ± 0.10 | −0.15 ± 0.10 | −0.18 ± 0.27 | −0.48 ± 0.36 | |
SPEX Ag1x8 | 100 | 7.3 × 10−6 | 1.8 × 10−5 | 1 | −0.1 ± 1.6 | −0.1 ± 2.1 | −0.5b ± 1.5 | 0.05 ± 0.25 | 0.08 ± 0.10 | 0.18 ± 0.10 | 0.00 ± 0.27 | 0.15 ± 0.36 | |
SPEX TRU | 100 | 1.3 × 10−5 | 1.2 × 10−4 | 1 | −1.2 ± 1.6 | −1.0 ± 2.1 | −1.0b ± 1.5 | 0.08 ± 0.25 | −0.03 ± 0.10 | 0.10 ± 0.10 | −0.06 ± 0.27 | −0.09 ± 0.36 | |
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Allende (CV3) | |||||||||||||
Allende bomb 1a | 200 | 4.3 × 10−5 | 3.6 × 10−5 | 1 | 0.4 ± 1.0 | 1.9 ± 1.7 | −2.8 ± 1.1 | 0.05 ± 0.18 | 0.04 ± 0.13 | 0.10 ± 0.18 | −0.07 ± 0.16 | −0.13 ± 0.27 | |
Allende bomb 1a | 200 | 4.4 × 10−5 | 6.8 × 10−5 | 1 | 0.27 ± 0.83 | 0.9 ± 1.1 | −2.6 ± 1.1 | 0.16 ± 0.23 | 0.10 ± 0.12 | 0.04 ± 0.21 | 0.02 ± 0.24 | −0.06 ± 0.41 | |
Allende bomb 1b | 200 | 1.2 × 10−4 | 3.3 × 10−5 | 1 | −0.11 ± 0.92 | 1.9 ± 1.4 | −0.09 ± 0.94 | 0.08 ± 0.22 | 0.02 ± 0.11 | 0.09 ± 0.14 | 0.17 ± 0.24 | 0.03 ± 0.24 | |
Allende bomb 1b | 200 | 2.3 × 10−5 | 5.1 × 10−5 | 1 | 0.33 ± 0.57 | −0.13 ± 0.82 | −2.38 ± 0.81 | 0.20 ± 0.12 | −0.03 ± 0.10 | 0.00 ± 0.15 | −0.23 ± 0.19 | −0.21 ± 0.23 | |
With additional Pre-filter clean up | |||||||||||||
Allende bomb 2a | 200 | 5.8 × 10−5 | 4.0 × 10−5 | 1 | −0.17 ± 0.84 | 0.90 ± 1.15 | −2.5 ± 1.4 | 0.07 ± 0.16 | 0.10 ± 0.12 | 0.12 ± 0.16 | −0.02 ± 0.20 | 0.00 ± 0.38 | |
Allende bomb 2b | 200 | 4.5 × 10−5 | 4.5 × 10−5 | 1 | −0.26 ± 0.84 | 0.64 ± 0.93 | −2.20 ± 0.92 | 0.21 ± 0.12 | 0.10 ± 0.08 | 0.17 ± 0.23 | 0.09 ± 0.12 | −0.14 ± 0.25 | |
Allende bomb 2a + 2bc | 200 | 4.9 × 10−5 | 3.0 × 10−5 | 1 | −0.9 ± 1.1 | −0.22 ± 1.62 | −0.3b ± 1.7 | 0.26 ± 0.18 | 0.03 ± 0.10 | 0.07 ± 0.21 | −0.04 ± 0.25 | −0.16 ± 0.37 | |
Allende bomb 1 mean | 200 | 4 | 0.23 ± 0.46 | 1.1 ± 2.0 | −2.0 ± 2.5 | 0.12 ± 0.14 | 0.03 ± 0.11 | 0.06 ± 0.09 | −0.03 ± 0.34 | −0.09 ± 0.21 | |||
4d | 0.23 ± 0.36 | 1.1 ± 1.6 | −2.0 ± 2.0 | 0.12 ± 0.11 | 0.03 ± 0.09 | 0.06 ± 0.08 | −0.03 ± 0.27 | −0.09 ± 0.16 | |||||
Allende bomb 2 mean | 200 | 3 | −0.44 ± 0.85 | 0.4 ± 1.3 | −1.7 ± 2.4 | 0.18 ± 0.19 | 0.07 ± 0.11 | 0.12 ± 0.17 | 0.01 ± 0.21 | −0.10 ± 0.31 | |||
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Basalts | |||||||||||||
BHVO-2 1a | 100 | 1.2 × 10−5 | 3.0 × 10−5 | 1 | −1.1 ± 1.7 | −2.4 ± 2.3 | 0.3b ± 1.7 | −0.08 ± 0.23 | −0.17 ± 0.20 | 0.13 ± 0.15 | 0.20 ± 0.14 | 0.06 ± 0.27 | |
BHVO-2 1b | 100 | 2.3 × 10−5 | 2.7 × 10−5 | 1 | 0.4 ± 1.7 | 1.9 ± 2.3 | −0.2b ± 1.7 | 0.08 ± 0.23 | −0.03 ± 0.20 | −0.01 ± 0.15 | 0.10 ± 0.14 | 0.08 ± 0.27 | |
BHVO-2 2 | 100 | 1.4 × 10−5 | 3.2 × 10−5 | 1 | −0.3 ± 1.7 | −1.0 ± 2.3 | 0.3b ± 1.7 | −0.06 ± 0.23 | −0.14 ± 0.20 | 0.03 ± 0.15 | 0.07 ± 0.14 | 0.16 ± 0.27 | |
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Lake sediment | |||||||||||||
Lake Zürich Ia | 200 | 3.5 × 10−5 | 3.6 × 10−5 | 1 | 1.1 ± 1.0 | 1.3 ± 1.7 | −1.5 ± 1.1 | 0.13 ± 0.18 | 0.03 ± 0.13 | −0.04 ± 0.18 | −0.16 ± 0.16 | −0.17 ± 0.27 | |
Lake Zürich Ia | 200 | 3.3 × 10−5 | 2.8 × 10−5 | 1 | −0.45 ± 0.92 | 0.6 ± 1.4 | −1.86 ± 0.94 | −0.10 ± 0.22 | −0.09 ± 0.11 | 0.20 ± 0.14 | 0.24 ± 0.24 | −0.07 ± 0.24 | |
Lake Zürich Ib | 200 | 1.5 × 10−5 | 3.2 × 10−5 | 1 | 0.1 ± 1.0 | 0.0 ± 1.7 | −1.7 ± 1.1 | 0.09 ± 0.18 | 0.04 ± 0.13 | 0.02 ± 0.18 | 0.00 ± 0.16 | −0.05 ± 0.27 | |
Lake Zürich Ib | 200 | 1.3 × 10−5 | 3.0 × 10−5 | 1 | −0.31 ± 0.92 | −1.1 ± 1.4 | −1.96 ± 0.94 | −0.10 ± 0.22 | −0.08 ± 0.11 | 0.06 ± 0.14 | −0.06 ± 0.24 | 0.05 ± 0.24 | |
Lake Zürich Ib | 200 | 1.5 × 10−5 | 4.0 × 10−5 | 1 | 0.0 ± 1.2 | 0.4 ± 2.0 | −2.0 ± 1.4 | 0.13 ± 0.19 | 0.08 ± 0.12 | −0.16 ± 0.11 | −0.08 ± 0.21 | −0.14 ± 0.29 | |
Lake Zürich Ic | 200 | 1.8 × 10−5 | 2.8 × 10−5 | 1 | 0.09 ± 0.92 | 0.8 ± 1.4 | −1.64 ± 0.94 | 0.05 ± 0.22 | 0.15 ± 0.11 | 0.01 ± 0.14 | −0.16 ± 0.24 | −0.33 ± 0.24 | |
Lake Zürich Ic | 200 | 2.2 × 10−5 | 2.8 × 10−5 | 1 | −0.68 ± 0.92 | −0.5 ± 1.4 | −1.86 ± 0.94 | −0.18 ± 0.22 | −0.02 ± 0.11 | 0.01 ± 0.14 | −0.17 ± 0.24 | −0.21 ± 0.24 | |
Lake Zürich Ic | 200 | 8.8 × 10−6 | 4.0 × 10−5 | 1 | 0.3 ± 1.2 | −0.7 ± 2.0 | −2.7 ± 1.4 | 0.04 ± 0.19 | 0.03 ± 0.12 | −0.06 ± 0.11 | −0.02 ± 0.21 | −0.10 ± 0.29 | |
With additional Pre-filter clean up | |||||||||||||
Lake Zürich Id | 200 | 8.0 × 10−5 | 3.4 × 10−5 | 1 | −0.25 ± 0.94 | −0.5 ± 1.4 | −0.9 ± 1.1 | −0.10 ± 0.12 | 0.03 ± 0.11 | −0.11 ± 0.17 | −0.07 ± 0.22 | −0.16 ± 0.38 | |
Lake Zürich Ie | 200 | 5.1 × 10−5 | 3.3 × 10−5 | 1 | −0.8 ± 1.1 | −0.3 ± 1.6 | 0.0 ± 1.7 | −0.10 ± 0.18 | 0.02 ± 0.10 | −0.10 ± 0.21 | −0.03 ± 0.25 | −0.10 ± 0.37 | |
Lake Zürich I bomb a | 200 | 1.1 × 10−4 | 3.7 × 10−5 | 1 | 0.14 ± 0.84 | 1.4 ± 1.2 | −0.5 ± 1.4 | −0.13 ± 0.16 | −0.01 ± 0.12 | −0.03 ± 0.16 | −0.01 ± 0.20 | −0.03 ± 0.38 | |
Lake Zürich I bomb b | 200 | 2.9 × 10−5 | 3.3 × 10−5 | 1 | 0.24 ± 0.94 | 0.2 ± 1.4 | −0.6 ± 1.1 | 0.00 ± 0.12 | 0.11 ± 0.11 | −0.21 ± 0.17 | −0.14 ± 0.22 | −0.19 ± 0.38 | |
Lake Zürich I bomb b | 200 | 2.4 × 10−5 | 2.8 × 10−5 | 1 | −0.6 ± 1.1 | 0.5 ± 1.6 | −0.9 ± 1.7 | −0.12 ± 0.18 | −0.01 ± 0.10 | 0.03 ± 0.21 | 0.06 ± 0.25 | 0.10 ± 0.37 | |
Lake Zürich I bomb c | 200 | 8.1 × 10−5 | 3.7 × 10−5 | 1 | 0.38 ± 0.78 | 1.20 ± 0.97 | −0.6 ± 1.0 | −0.08 ± 0.14 | −0.04 ± 0.10 | −0.18 ± 0.16 | −0.13 ± 0.18 | −0.20 ± 0.21 | |
Lake Zürich I bomb c | 200 | 8.1 × 10−5 | 2.2 × 10−5 | 1 | −0.59 ± 0.73 | −0.56 ± 0.97 | −0.3 ± 1.2 | −0.01 ± 0.12 | −0.07 ± 0.11 | −0.02 ± 0.14 | 0.11 ± 0.20 | 0.03 ± 0.27 | |
Lake Zürich I bomb c | 200 | 8.5 × 10−5 | 1.5 × 10−5 | 1 | −0.80 ± 0.91 | −0.8 ± 1.3 | −0.4 ± 1.4 | 0.06 ± 0.18 | −0.04 ± 0.13 | −0.22 ± 0.16 | −0.22 ± 0.22 | −0.33 ± 0.35 | |
Lake Zürich I bomb c | 200 | 8.4 × 10−5 | 1.5 × 10−5 | 1 | 0.38 ± 0.91 | 1.1 ± 1.3 | 0.1 ± 1.4 | 0.07 ± 0.18 | 0.07 ± 0.13 | −0.10 ± 0.16 | 0.01 ± 0.22 | −0.12 ± 0.35 | |
Lake Zürich I bomb cc | 200 | 8.4 × 10−5 | 2.7 × 10−5 | 1 | −1.2 ± 1.1 | 0.4 ± 1.6 | −1.9b ± 1.7 | −0.19 ± 0.18 | −0.04 ± 0.10 | −0.07 ± 0.21 | −0.07 ± 0.25 | 0.00 ± 0.37 | |
Lake Zürich I bomb c | 200 | 9.4 × 10−5 | 2.7 × 10−5 | 1 | 0.29 ± 0.91 | 1.5 ± 1.3 | −0.7 ± 1.4 | −0.16 ± 0.18 | −0.01 ± 0.13 | 0.00 ± 0.16 | 0.09 ± 0.22 | −0.04 ± 0.35 | |
Lake Zürich I bomb c | 200 | 8.1 × 10−5 | 2.7 × 10−5 | 1 | 0.2 ± 1.1 | 1.5 ± 1.2 | −0.9 ± 1.0 | −0.07 ± 0.12 | −0.07 ± 0.07 | −0.09 ± 0.11 | −0.05 ± 0.19 | 0.06 ± 0.25 | |
Measured with Aridus II on NU3 | |||||||||||||
Lake Zürich Ib | 100 | 2.8 × 10−5 | 3.9 × 10−5 | 1 | 0.1 ± 1.3 | 0.7 ± 2.7 | −1.0 ± 2.0 | −0.13 ± 0.14 | 0.04 ± 0.16 | −0.05 ± 0.21 | −0.15 ± 0.22 | −0.31 ± 0.33 | |
Lake Zürich Id | 100 | 2.3 × 10−5 | 2.3 × 10−5 | 1 | 0.0 ± 1.2 | 0.3 ± 2.9 | −2.1 ± 1.7 | −0.07 ± 0.13 | −0.07 ± 0.09 | −0.18 ± 0.22 | −0.01 ± 0.25 | 0.07 ± 0.53 | |
Lake Zürich Ie | 100 | 6.5 × 10−5 | 2.2 × 10−5 | 1 | −0.2 ± 1.2 | 0.4 ± 2.3 | 0.3 ± 2.7 | 0.01 ± 0.27 | 0.00 ± 0.18 | −0.14 ± 0.27 | 0.09 ± 0.31 | −0.18 ± 0.57 | |
Lake Zürich I bomb b | 100 | 3.2 × 10−5 | 1.8 × 10−5 | 1 | 0.0 ± 1.2 | 0.4 ± 2.9 | −0.8 ± 1.7 | −0.02 ± 0.13 | −0.06 ± 0.09 | −0.33 ± 0.22 | 0.15 ± 0.25 | 0.00 ± 0.53 | |
Lake Zürich I bomb c | 40 | 8.0 × 10−5 | 1.3 × 10−5 | 1 | −3.3 ± 2.6 | −3.9 ± 4.9 | 3.2 ± 5.4 | −0.13 ± 0.20 | 0.12 ± 0.21 | −0.17 ± 0.39 | 0.15 ± 0.34 | −0.14 ± 0.40 | |
Lake Zürich I bomb c | 9 | 1.0 × 10−4 | −5.7 × 10−5 | 1 | −1 ± 11 | −10 ± 23 | −7 ± 21 | 0.8 ± 1.4 | 0.16 ± 0.61 | −0.8 ± 1.9 | −1.1 ± 1.9 | −0.3 ± 1.7 | |
Lake Zürich I bomb c | 5.5 | −3.8 × 10−5 | 1.9 × 10−4 | 1 | 0 ± 28 | 25 ± 49 | 3 ± 42 | −0.4 ± 1.6 | 0.13 ± 0.91 | −0.5 ± 1.6 | −0.7 ± 2.6 | −1.4 ± 2.5 | |
Lake Zürich I mean | 200 | 20 | −0.1 ± 1.1 | 0.3 ± 1.7 | −1.1 ± 1.6 | −0.04 ± 0.21 | 0.00 ± 0.13 | −0.05 ± 0.20 | −0.04 ± 0.22 | −0.10 ± 0.24 | |||
100 | 4 | 0.0 ± 0.2 | 0.4 ± 0.3 | −0.9 ± 2.0 | −0.05 ± 0.12 | −0.02 ± 0.10 | −0.18 ± 0.23 | 0.02 ± 0.27 | −0.11 ± 0.35 | ||||
40 | 1 | −3.3 ± 2.6 | −3.9 ± 4.9 | 3.2 ± 5.4 | −0.13 ± 0.20 | 0.12 ± 0.21 | −0.17 ± 0.39 | 0.15 ± 0.34 | −0.14 ± 0.40 | ||||
9 | 1 | −1 ± 11 | −10 ± 23 | −7 ± 21 | 0.8 ± 1.4 | 0.16 ± 0.61 | −0.8 ± 1.9 | −1.1 ± 1.9 | −0.3 ± 1.7 | ||||
5.5 | 1 | 0 ± 28 | 25 ± 49 | 3 ± 42 | −0.4 ± 1.6 | 0.13 ± 0.91 | −0.5 ± 1.6 | −0.7 ± 2.6 | −1.4 ± 2.5 |
Because of the high abundances of 120Sn, 122Sn and 124Sn, the Te interference correction can tolerate larger amounts of Te compared to Cd or In. Without Te correction on the denominator isotope 120Sn, Te/Sn ratios of up to 3 × 10−3 can be tolerated (Fig. 1d–f). If an additional Te interference correction on 120Sn is applied, a Te/Sn ratio of up to 1.2 × 10−2 still yields accurate Sn isotope results (Fig. 1g–i). The Te/Sn ratios of the analysed samples after the chemical separation procedure were generally below 8.0 × 10−5. The Te interferences were monitored using 125Te and 126Te and the results after interference correction with each isotope were compared. Our data show that interference correction using 125Te results in a slightly better reproducibility, because 126Te needs an additional correction for 126Xe and this increases the uncertainty of the correction (Table S2†). A proportionally higher background was observed for 125Te compared to 126Te, and this elevated 125Te background correlated with the signal intensity of Sn. This increased background signal was likely caused by 124Sn-hydrides. Therefore, the correction with 125Te results in slightly lower 122Sn/120Sn and 124Sn/120Sn ratios when compared to the 126Te corrected data (Table S2†). However, this difference is smaller than the uncertainties of the analyses.
Uranium (238U) can interfere on 119Sn as double-charged ions. Tests show that the production rate of U++ is ca. 4% and that U/Sn ratios of up to 5 × 10−5 leave the results unaffected (Fig. 1c). This demonstrates that relatively small blank amounts of U can already affect the data and thus U requires a clean separation from the Sn fraction. The U content of samples after our chemical separation procedure were generally below this threshold.
The limits for adequate and reliable isobaric interference corrections for Cd isotope analyses were also assessed. To this end, the 200 ppb Alfa Aesar Cd standard solution was doped with different amounts of Sn (0.050–0.200 ppb), In (0.005–0.200 ppb) and Pd (0.005–0.075 ppb) (Fig. 2 and ESI Fig. S2†). The tolerance levels were defined as the relative signal ratios of the isotopes used for interference correction (118Sn, 115In and 105Pd) against 111Cd. This allows a direct comparison with samples scanned for purity before analysis. Standards with a 105Pd/111Cd ratio of up to ∼5.5 × 10−4 (0.075 ppb Pd, Pd/Cd = 3.8 × 10−4) and a 115In/111Cd ratio of up to ∼1.2 × 10−2 (0.200 ppb In, In/Cd = 1.0 × 10−3) can still be accurately corrected for (Fig. S2a–c† and 2c). For Sn, the tolerance threshold was determined at 118Sn/111Cd of ∼2.4 × 10−3 (0.150 ppb Sn, Sn/Cd = 7.5 × 10−4) (Fig. 2a and b). Column processed standards and samples in general yielded ratios below these limits (Table S3†).
For Cd analyses, major molecular interferences originate from Zn, Zr and Mo, that in particular influence the two least abundant Cd isotopes, 106Cd and 108Cd.27 Standard solutions doped with up to 0.400 ppb Mo (Mo/Cd = 2 × 10−3, 95Mo/111Cd ∼ 1.3 × 10−3 from mass-scans) and 0.030 ppb Zr (Zr/Cd = 1.5 × 10−4, 90Zr/111Cd ∼ 3.0 × 10−4) yield accurate Cd isotope data and thus these levels can be tolerated in the final sample solutions (ESI Fig. S2d–f†). For Zn, a tolerance threshold on 66Zn/111Cd of ∼1 × 10−2 was estimated (∼1 ppb Zn, Zn/Cd ∼ 5 × 10−3) (Fig. 2d–f). These limits were in general achieved for Cd–Zn standards and samples passed through both Cd ion exchange columns (Table S3†). However, the Zn blank of sample solutions often increased following dry-down and redissolution to levels above this tolerance threshold, as can be observed for samples where repeat analyses were possible (Cd Std 1 and Lake Zürich If, Table S3†). This issue was noted early during establishing our method and was combatted by adopting a more thorough cleaning regime of the pipette tips and vials used, and by working with PE gloves instead of vinyl. This ensured that Zn levels remained low. This is reflected by the last two measurements of Cd Std 1 that already show a much more constant Zn/Cd ratio than before, and also from the low Zn/Cd ratio of later processed standards and samples.
Ratio | ETHa (200 ppb) | ETHa (100 ppb) | Lee and Halliday (1995) | Rosman et al. (1984) | Devillers et al. (1983)c | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
NIST 3161a | SPEX1 | Johnson Matthey AAS standard solution | Johnson Matthey Sn oxide and metalb | VENTRON metallic wire, Alfa Products | |||||||||||
Mean | 2SD | 2SD (ppm) | Mean | 2SD | 2SD (ppm) | Mean | 2SD | 2SD (ppm) | Mean | 2SD | 2SD (ppm) | Mean | 2SD | 2SD (ppm) | |
a External reproducibility (n sessions). b “Specpure” tin oxide (JMC 530 laboratory S8346) and tin metal (JM 540 laboratory S2807). c Absolute ratios determined using a 116Sn–122Sn double spike. | |||||||||||||||
112Sn/120Sn | 0.029823 | 0.000004 | 149 | 0.029826 | 0.000008 | 266 | 0.029812 | 0.000004 | 134 | 0.029860 | 0.000050 | 1674 | 0.029840 | 0.000100 | 3351 |
114Sn/120Sn | 0.020190 | 0.000004 | 206 | 0.020194 | 0.000007 | 346 | 0.020195 | 0.000014 | 693 | 0.020220 | 0.000050 | 2473 | 0.020000 | 0.000100 | 5000 |
115Sn/120Sn | 0.010361 | 0.000002 | 193 | 0.010369 | 0.000002 | 228 | 0.010366 | 0.000007 | 675 | 0.010390 | 0.000040 | 3850 | 0.011000 | 0.000100 | 9091 |
116Sn/120Sn | 0.446000 | 0.446000 | 0.446000 | 0.446000 | 0.446000 | 0.001100 | 2466 | ||||||||
117Sn/120Sn | 0.235320 | 0.000028 | 119 | 0.235302 | 0.000023 | 100 | 0.235313 | 0.000048 | 204 | 0.235380 | 0.000080 | 340 | 0.235500 | 0.000700 | 2972 |
118Sn/120Sn | 0.742945 | 0.000029 | 39 | 0.742929 | 0.000024 | 32 | 0.742935 | 0.000076 | 102 | 0.742950 | 0.000200 | 269 | 0.743200 | 0.001100 | 1480 |
119Sn/120Sn | 0.263479 | 0.000032 | 121 | 0.263447 | 0.000027 | 103 | 0.263430 | 0.000046 | 175 | 0.263450 | 0.000130 | 493 | 0.263400 | 0.000400 | 1519 |
122Sn/120Sn | 0.142095 | 0.000011 | 80 | 0.142081 | 0.000007 | 48 | 0.142086 | 0.000013 | 91 | 0.142110 | 0.000070 | 493 | 0.142010 | 0.000280 | 1972 |
124Sn/120Sn | 0.177583 | 0.000035 | 197 | 0.177549 | 0.000027 | 152 | 0.177588 | 0.000052 | 293 | 0.177530 | 0.000100 | 563 | 0.177600 | 0.000550 | 3097 |
n | 17 | 7 |
The associated uncertainties on the ratios are generally more precise than in previous studies. Nevertheless, the ratios 117Sn/120Sn, 118Sn/120Sn and 119Sn/120Sn are affected by drifting (long-term and during a session) (Fig. 3). Therefore, for sample analyses, the measurements were bracketed with NIST SRM 3161a to correct for these drifts. The reason for the long-term drifts is unclear. It could indicate that the Faraday cups do not behave completely linear due to differences in cup efficiencies, however, the relatively prominent drifts on the odd/even isotope ratios 117Sn/120Sn and 119Sn/120Sn may also suggest nuclear field shift effect48 or the magnetic isotope effect49 as a potential source (Fig. 3). The typical reproducibility (2SD) of the NIST SRM 3161 200 ppb solution (drift corrected) within a single session was 0.9 for ε112Sn, 1.3 for ε114Sn, 1.2 for ε115Sn, 0.16 for ε117Sn, 0.11 for ε118Sn, 0.17 for ε119Sn, 0.21 for ε122Sn and 0.31 for ε124Sn.
Based on the long-term average for a 200 ppb NIST SRM 3161a Sn solution (Table 5), new Sn isotope abundances were calculated. The new Sn isotope abundances are more precise and in good agreement with the previous recommendation of IUPAC 2001 (ref. 50) (Table 6).
ETHb | 2SD | Böhlke (2005)c | |
---|---|---|---|
a The 2SD uncertainties in parenthesis refer to last digits. b Abundances and associated uncertainties were calculated based on data for NIST SRM 3161a 200 ppb (Table 5). c Representative Sn isotope composition from Böhlke.50 | |||
112 | 0.0097220 | (15) | 0.0097(1) |
114 | 0.0065822 | (14) | 0.0066(1) |
115 | 0.0033775 | (7) | 0.0034(1) |
116 | 0.1453838 | (45) | 0.1454(9) |
117 | 0.0767065 | (94) | 0.0768(7) |
118 | 0.242175 | (12) | 0.2422(9) |
119 | 0.085883 | (11) | 0.0859(4) |
120 | 0.325973 | (23) | 0.3258(9) |
122 | 0.0463168 | (40) | 0.0463(3) |
124 | 0.057880 | (12) | 0.0579(5) |
![]() | ||
Fig. 5 The Sn isotope composition for repeated analyses of two independently digested aliquots of Allende. Symbols and uncertainties are the same as in Fig. 4. |
To verify the method and to check for analytical artifacts associated with organics released by the TRU Spec resin, aliquots of lake sediments were analysed using different introduction systems (Aridus II versus DSN). The isotopic composition of the lake sediment Lake Zürich I measured with two different desolvating systems, DSN 100 and Aridus II, are in good agreement (Table 4). In addition, aliquots of samples prepared by using the two-stage chromatographic separation procedure only were measured and compared to samples with the additional Pre-filter stage. Similar results are obtained when comparing samples before and after additional treating with Pre-filter resin (Table 4 and Fig. 4). The exception are the lake sediments with additional Pre-filter treatment, which show a tendency to higher ε115Sn values and therefore are closer to the values obtained for the NIST SRM 3161a Sn standard solution. The reason behind this is unclear, but may indicate that these samples pick up a small additional In blank during Pre-filter treatment. Alternatively, organics may be partly responsible for the observed negative shift in ε115Sn. However, considering the analytical uncertainty, all isotopic ratios measured with or without the third stage column (Pre-filter resin) are identical.
For Cd isotope analyses, the purified Cd fractions contain Pd, Sn and In below their determined tolerance limits and Zn is efficiently separated to enable accurate 106Cd and 108Cd isotope data. Inference tests demonstrate that notably high Zn levels (66Zn/111Cd > 1 × 10−2) yield inaccurate 106Cd and 108Cd data due to interfering Zn-argides formed in the plasma. Such levels can be easily generated as blank contribution during the analytical procedure and therefore need careful monitoring, if the low abundance Cd isotopes are targeted.
Our new analytical procedure allows for the simultaneous detection of all ten Sn isotopes and the correction of direct isobaric interferences. Similarly, it also includes the simultaneous measurement of all eight Cd isotopes including Pd, In and Sn isotopes for isobaric interference correction. The new data obtained for the NIST SRM 3161a Sn standard is in excellent agreement with those previously determined.2,7,47 Based on our data, more precise absolute abundances of natural Sn were determined. The precisions for Sn and Cd isotope data achieved using our new procedure are sufficient to identify potential small nucleosynthetic or cosmogenic effects in meteorites. Our findings also illustrate the challenges related to extending an analytical method to low abundance isotopes or improving the measurement precision through increased counting statistics and improved instrumentation available. It requires careful attention to potential interferences. Molecular interferences (e.g., argides or oxides) as well as double charged ions formed in the plasma, or contamination of single standard solutions, previously unproblematic, can hamper the data quality when moving to higher precision.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9ja00289h |
This journal is © The Royal Society of Chemistry 2020 |