A simple and reliable anion-exchange resin method for sulfate extraction and purification suitable for multiple O- and S-isotope measurements

Procedures from samples to sulfur isotopic data: A review

RATIONALE

Sulfur isotopes have been widely used to solve some key scientific questions, especially in the last two decades with advanced instruments and analytical schemes. Different sulfur speciation and multiple isotopes analyzed in laboratories worldwide and in situ microanalysis have also been reported in many articles. However, methods of sampling to measurements are multifarious, and occasionally some inaccuracies are present in published papers. Vague methods may mislead newcomers to the field, puzzle readers, or lead to incorrect data-based correlations.

METHODS

We have reviewed multiple methods on sulfur isotopic analyses from the perspectives of sampling, laboratory work, and instrumental analysis in order to help reduce operational inhomogeneity and ensure the fidelity of sulfur isotopic data. We do not deem our proposed solutions as the ultimate standard methods but as a lead-in to the overall introduction and summary of the current methods used.

RESULTS

It has been shown that external contamination and transformation of different sulfur species should be avoided during the sampling, pretreatment, storage, and chemical treatment processes. Conversion rates and sulfur isotopic fractionations during sulfur extraction, purification, and conversion processes must be verified by researchers using standard or known samples. The unification of absence of isotopic fractionation is needed during all steps, and long-term monitoring of standard samples is recommended.

CONCLUSION

This review compiles more details on different methods in sampling, laboratory operation, and measurement of sulfur isotopes, which is beneficial for researchers' better practice in laboratories. Microanalyses and molecular studies are the frontier techniques that compare the bulk sample with the elemental analysis/continuous flow-gas source stable isotope ratio mass spectrometry method, but the latter is widely used. The development of sulfur isotopic measurements will lead to the innovation in scientific issues with sulfur proxies.

Geological evidence of extensive N-fixation by volcanic lightning during very large explosive eruptions

Most of the nitrogen (N) accessible for life is trapped in dinitrogen (N2), the most stable atmospheric molecule. In order to be metabolized by living organisms, N2 has to be converted into biologically assimilable forms, so-called fixed N. Nowadays, nearly all the N-fixation is achieved through biological and anthropogenic processes. However, in early prebiotic environments of the Earth, N-fixation must have occurred via natural abiotic processes. One of the most invoked processes is electrical discharges, including from thunderstorms and lightning associated with volcanic eruptions. Despite the frequent occurrence of volcanic lightning during explosive eruptions and convincing laboratory experimentation, no evidence of substantial N-fixation has been found in any geological archive. Here, we report on the discovery of a significant amount of nitrate in volcanic deposits from Neogene caldera-forming eruptions, which are well correlated with the concentrations of species directly emitted by volcanoes (sulfur, chlorine). The multi-isotopic composition (δ18O, Δ17O) of the nitrates reveals that they originate from the atmospheric oxidation of nitrogen oxides formed by volcanic lightning. According to these first geological volcanic nitrate archive, we estimate that, on average, about 60 Tg of N can be fixed during a large explosive event. Our findings hint at a unique role potentially played by subaerial explosive eruptions in supplying essential ingredients for the emergence of life on Earth.

Sample pretreatment effects on isotopic compositions of oxygen and sulfur in BaSO4 derived from atmospheric sulfate

RATIONALE

Preparation of BaSO4 from samples of atmospheric rain, snow, and aerosols has been used for δ18 O and δ34 S analyses. In the present study, we investigated the effect of various sample pretreatments on δ18 O and δ34 S values determined from a Na2 SO4 reagent solution and samples of atmospheric precipitation to improve assay time and cost efficiency.

METHODS

BaSO4 was prepared from a Na2 SO4 solution by (a) evaporative concentration, (b) evaporation to dryness, (c) evaporation to dryness after adding HCl, and (d) evaporation to concentration after adding HCl, followed by cooling and then precipitation using a BaCl2 solution. To analyze the atmospheric precipitation samples for δ18 O, BaSO4 prepared from the samples was treated with diethylenetriaminepentaacetic acid (DTPA) and SO4 2- and separated chromatographically. The values of δ18 O and δ34 S were measured using a continuous-flow elemental analyzer coupled to an isotope-ratio mass spectrometer.

RESULTS

The δ34 S values in BaSO4 precipitated from Na2 SO4 solution determined by methods (a)-(c) were consistent within precisions of ±0.5‰. The δ18 O values of methods (a) and (b) were consistent within ±0.2‰, whereas the δ18 O values of methods (c) and (d) increased with increasing HCl concentrations. Similar results were obtained from samples of atmospheric precipitations. The δ18 O values from DTPA-treated BaSO4 were consistent with those obtained by chromatographic separation within ±0.5‰.

CONCLUSIONS

We found no significant differences in the effects of various pretreatments (acidification, heating, concentration, and drying) on δ18 O and δ34 S values in sulfate from samples of atmospheric precipitation and aerosols extracted as BaSO4 if HCl was not added to the sample before heating and BaSO4 was treated with DTPA for the δ18 O analysis.

Optimizing sulfate pyrolysis triple oxygen isotope analysis for samples from desert environments

RATIONALE

The triple oxygen isotope composition of sulfate may reveal the formation pathway and depositional sources and may indicate slow biologic cycling in the environment. Pyrolysis mass spectrometry is better suited for large sample workloads during environmental profiling but sufficient precision and a thorough verification of accuracy are required for comparison with higher precision laser fluorination data.

METHODS

Quantitative sulfate extraction from soil samples at neutral pH, purification, conversation into Ag-sulfate, and pyrolysis mass spectrometry were modified for high sample throughput. Samples were analyzed after pyrolysis in quartz cups and gold capsules in a modified EuroVector model 3000 elemental analyzer. Sample O₂ was measured in continuous He-flow after purification by cryo-trapping and chromatography on a Thermo Finnigan MAT253 isotope ratio mass spectrometer. A protocol for routine quality control and data normalization ensures long-term accuracy of the pyrolysis method.

RESULTS

The 1σ SD external reproducibility is 0.12‰ for Δ¹⁷ O_SO4 values on 30 μmol samples. Careful normalization for a daily analytical session accounts for changing pyrolysis conditions over the course of multiple sessions. The precision and accuracy obtained with quartz cups are comparable with those obtained with gold capsules. Pyrolysis and fluorination data for in-house standards from four laboratories and from an Atacama Desert gypsum-soil profile are identical and demonstrate the accuracy of our simplified method.

CONCLUSIONS

Pyrolysis of sulfate in quartz cups and a modified simple elemental analyzer setup allows for accurate, precise, fast, cost-efficient, and non-hazardous mass spectrometric analysis. Exchangeability of data from pyrolysis and laser fluorination methods was demonstrated by repeat analysis of standards and natural samples despite high contents of interfering, easily soluble nitrates and chlorides.

Multi O- and S-isotopes as tracers of black crusts formation under volcanic and non-volcanic atmospheric conditions in Sicily (Italy)

The deterioration of monument or building stone materials is mostly due to the growth of black crusts that cause blackening and disaggregation of the exposed surface. This study reports on new oxygen (δ¹⁷O, δ¹⁸O and Δ¹⁷O) and sulphur (δ³³S, δ³⁴S, δ³⁶S, Δ³³S and Δ³⁶S) isotopic analyses of black crust sulphates formed on building stones in Sicily (Southern Italy). The measurements are used to identify the possible influence of volcanic emissions on black crust formation. Black crusts were mostly sampled on carbonate stone substrate in different locations subject to various sulphur emission sources (marine, anthropogenic and volcanic). Unlike atmospheric sulphate aerosols that mostly exhibit Δ³³S > 0‰, here most of the analysed black crust sulphates show negative Δ³³S. This confirms that black crust sulphates do not result from deposition of sulphate aerosols or of rainwater but mostly from the oxidation of dry deposited SO₂ onto the stone substrate. The δ³⁴S and δ¹⁸O values indicate that most of black crust sulphate originates from anthropogenic activities. Δ¹⁷O values are found to be related to the sampling location. The largest ¹⁷O-anomalies (up to ~4‰) are measured in black crust from areas highly influenced by volcanic emissions, which demonstrates the strong involvement of ozone in the formation of black crusts in volcanically influenced environments.

An evaluation on the extraction capability of anion exchange membranes for high-precision sulfur isotope measurement by multiple-collector inductively coupled plasma mass spectrometry

Anion exchange membranes (AEMs) are adept at extracting sulfate for sulfur isotope analyses by multiple-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) from natural samples typically with low sulfate concentrations.

A simple and reliable method reducing sulfate to sulfide for multiple sulfur isotope analysis

RATIONALE

Precise analysis of four sulfur isotopes of sulfate in geological and environmental samples provides the means to extract unique information in wide geological contexts. Reduction of sulfate to sulfide is the first step to access such information. The conventional reduction method suffers from a cumbersome distillation system, long reaction time and large volume of the reducing solution. We present a new and simple method enabling the process of multiple samples at one time with a much reduced volume of reducing solution.

METHODS

One mL of reducing solution made of HI and NaH₂ PO₂ was added to a septum glass tube with dry sulfate. The tube was heated at 124°C and the produced H₂ S was purged with inert gas (He or N₂ ) through gas-washing tubes and then collected by NaOH solution. The collected H₂ S was converted into Ag₂ S by adding AgNO₃ solution and the co-precipitated Ag₂ O was removed by adding a few drops of concentrated HNO₃ .

RESULTS

Within 2-3 h, a 100% yield was observed for samples with 0.2-2.5 μmol Na₂ SO₄ . The reduction rate was much slower for BaSO₄ and a complete reduction was not observed. International sulfur reference materials, NBS-127, SO-5 and SO-6, were processed with this method, and the measured against accepted δ³⁴ S values yielded a linear regression line which had a slope of 0.99 ± 0.01 and a R² value of 0.998.

CONCLUSIONS

The new methodology is easy to handle and allows us to process multiple samples at a time. It has also demonstrated good reproducibility in terms of H₂ S yield and for further isotope analysis. It is thus a good alternative to the conventional manual method, especially when processing samples with limited amount of sulfate available.