MC ICP-MS δ(34)S(VCDT) measurement of dissolved sulfate in environmental aqueous samples after matrix separation by means of an anion exchange membrane.

Ondrej Hanousek,Thomas Prohaska,Torsten W Berger

doi:10.1007/s00216-015-9053-z

Ondrej Hanousek, Thomas Prohaska + Show 1 more

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https://doi.org/10.1007/s00216-015-9053-z

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Abstract

Analysis of 34S/32S of sulfate in rainwater and soil solutions can be seen as a powerful tool for the study of the sulfur cycle. Therefore, it is considered as a useful means, e.g., for amelioration and calibration of ecological or biogeochemical models. Due to several analytical limitations, mainly caused by low sulfate concentration in rainwater, complex matrix of soil solutions, limited sample volume, and high number of samples in ecosystem studies, a straightforward analytical protocol is required to provide accurate S isotopic data on a large set of diverse samples. Therefore, sulfate separation by anion exchange membrane was combined with precise isotopic measurement by multicollector inductively coupled plasma mass spectrometry (MC ICP-MS). The separation method proved to be able to remove quantitatively sulfate from matrix cations (Ca, K, Na, or Li) which is a precondition in order to avoid a matrix-induced analytical bias in the mass spectrometer. Moreover, sulfate exchange on the resin is capable of preconcentrating sulfate from low concentrated solutions (to factor 3 in our protocol). No significant sulfur isotope fractionation was observed during separation and preconcentration. MC ICP-MS operated at edge mass resolution has enabled the direct 34S/32S analysis of sulfate eluted from the membrane, with an expanded uncertainty U (k = 2) down to 0.3 ‰ (a single measurement). The protocol was optimized and validated using different sulfate solutions and different matrix compositions. The optimized method was applied in a study on solute samples retrieved in a beech (Fagus sylvatica) forest in the Vienna Woods. Both rainwater (precipitation and tree throughfall) and soil solution δ34SVCDT ranged between 4 and 6 ‰, the ratio in soil solution being slightly lower. The lower ratio indicates that a considerable portion of the atmospherically deposited sulfate is cycled through the organic S pool before being released to the soil solution. Nearly the same trends and variations were observed in soil solution and rainwater δ34SVCDT values showing that sulfate adsorption/desorption are not important processes in the studied soil.Graphical abstractSulfate separation by means of an anion exchange resin on a plastic membraneElectronic supplementary materialThe online version of this article (doi:10.1007/s00216-015-9053-z) contains supplementary material, which is available to authorized users.

Highlights

Various processes led to the sulfur isotope (34S/32S) fractionation such as bacterial SO42− reduction, fractional crystallization, or evaporation of seawater [1, 2]
In order to test for sulfate preconcentration by the anion exchange membrane, we reduced the volume of the elution solution to 10 mL and to 5 mL 2 % (m/m) HNO3
Since the soil solutions represent a higher matrix content among the investigated sample types, the effect of the matrix on the S isotope ratio was tested based on concentrations in these samples

Summary

Introduction

Various processes led to the sulfur isotope (34S/32S) fractionation such as bacterial SO42− reduction, fractional crystallization, or evaporation of seawater [1, 2]. Sulfur acts as an essential nutrient for vegetation It is a constituent of amino acids, proteins, coenzymes, or sulfolipids of plants. Part of the sulfate can be taken up by plants and microbes and reduced to build organic sulfur compounds. Another part might be adsorbed on soil particles. To a generally small extent, weathering of sulfur-bearing minerals contributes to the sulfate flow, as well [10]. Some of these processes (immobilization/mineralization, weathering) are known to result in a change of the isotopic composition of dissolved sulfate [1]. The change of the isotopic composition can serve as basis for ecological/biogeochemical modelling, helps in fertilization planning, and allows for prediction of soil recovery from acid rain effects [11]. (Throughout this publication, the term rainwater summarizes terms precipitation (rainwater above a forest canopy) and tree throughfall, i.e., precipitation after the passage through the canopy.)

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