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Rapid sample pretreatment and accurate quantification of radiosulfur in chemically complex atmospheric samples Mang Lin, and Mark H. Thiemens Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b05079 • Publication Date (Web): 19 Jan 2018 Downloaded from http://pubs.acs.org on January 19, 2018
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Analytical Chemistry
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Rapid sample pretreatment and accurate quantification of radiosulfur in
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chemically complex atmospheric samples
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Mang Lin* and Mark H. Thiemens
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Department of Chemistry and Biochemistry, University of California San Diego, La Jolla,
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California 92093, USA
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* Email:
[email protected] or
[email protected] 1
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Abstract
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An ultra-low-level liquid scintillation counting (LSC) technique has been used in measuring
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radiosulfur (cosmogenic
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new way to examine various biogeochemical problems. A major limit of the technique is that
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complex chemical compositions in atmospheric samples may lead to color quenching of LSC
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cocktails, a serious problem prolonging the pretreatment time (>1 week) and hampering the
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accurate determination of
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important atmospheric chemical processes are examined, significant interferences arise and
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accurate analysis in small samples is not possible. In this study, we optimized the LSC method to
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minimize/eliminate color quenching in high-sensitivity
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performance of this new method was evaluated using control laboratory experiments and natural
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aerosol samples. Results show that the new method offers comparable accuracy as the traditional
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method for normal environmental samples (bias: < ±0.03 disintegrations per minute [DPM]) and
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significantly shortens the pretreatment time to less than 3 days. For samples that were heavily
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contaminated by color quenching agents, the accuracy of this new method is notably higher than
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the traditional method (maximum bias: -0.3 v.s. -1.5 DPM). With the growing use of radiosulfur
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in the field of Earth and planetary sciences, the accurate determination of
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reliable field-based constraint for modeling
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range of atmospheric, hydrological, and biogeochemical applications.
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S) in natural samples. The ideal half-life of
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S (~87 d) renders it a
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S. For application of the technique where many of the most
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S measurements. The analytical
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S would provide a
S production in the atmosphere and allow a wide
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Analytical Chemistry
Introduction
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Sulfur has been ubiquitous in the global terrestrial atmosphere since the earliest geological
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record of the primitive Earth and has played an important role in the evolution of life and the
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ability to track its origin and evolution.1 Interest in the modern atmospheric sulfur cycle
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predominantly stems from the key role of sulfate in affecting climate2 and public health.3 Our
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knowledge of the sulfate budget in the atmosphere is incomplete due to widely varying emission
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sources and complicated chemical transformations of sulfur compounds,4-8 hampering an
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accurate and precise quantification of its association with aerosol radiative forcing and mortality.
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Isotopic analysis of atmospheric sulfate samples has been utilized to provide additional
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constrains on these processes.9 Conventional isotopic studies have been focused on the most
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abundant rare stable isotope 34S and 18O for source apportionment.10 Measurements of other rare
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stable isotopes (33S,
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independent isotopic fractionation effects, which provide essential information on both emission
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sources and chemical formation pathways that cannot be quantified by conventional sulfate
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concentration or single isotope ratio measurements.9, 11
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S and
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O) in sulfates became important after the discovery of mass-
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The radiosulfur nuclide (35S) is naturally produced by bombardment of 40Ar in the atmosphere
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by high energy cosmic rays. Cosmogenic 35S is the only radioactive sulfur isotope with a half-life
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(~87 days) of ideal age to track atmospheric and hydrological processes. Unlike stable isotopes,
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radiosulfur in atmospheric sulfates have been rarely measured because of its low activity in the
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atmosphere and associated analytical difficulties.12-15 In 2010, an ultra-low-level liquid
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scintillation counting (LSC) technique was developed16 in which sulfate samples were prepared
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as aqueous sulfate solution for reducing the background activity and enhancing the 35S counting
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efficiency. Since then, this method has been used in the high-sensitivity determination of 35S in
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atmospheric samples for quantifing a wide range of atmospheric processes such as neutron
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leakage at the Fukushima nuclear plant, gas-to-particle (SO2-to-sulfate) conversion rate
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determination, and horizontal/vertical air mass transport (e.g., westerly jet stream, convection,
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stratospheric intrusion, foehn wind, and polar vortex).17-21 This LSC method (using aqueous
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sulfate solution as radiosulfur carrier) was recently extended to radiosulfur measurements in
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hydrological and cryogenic samples,22 which has broad implications on understanding the
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interaction between the atmosphere, hydrosphere, and cryopshere (e.g., snow and glacier
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melting) and serves as an important supplement to existing radiosulfur biogeochemical and
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hydrologic studies (e.g., microbial sulfate reduction in lake and marine sediments,23 age and
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source determination of meltwater runoff,24-26 groundwater,27-30, and surface water in
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watersheds31) using a different type of LSC technique (using solid BaSO4 as radiosulfur
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carrier).15, 32 These studies illustrate the wide range of biogeochemical applications to date. This
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paper presents a technique to allow for measurement to include environments where
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measurement is not presently feasible.
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Presently, standard techniques require a relatively long pretreatment process to purify
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environmental samples because of their chemically complex nature that hampers an accurate
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determination of
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nitric acids)33-34 in the atmospheric samples is a major problem. The photons produced from
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scintillation are absorbed or scattered by the LSC cocktail (sample-gel mixture) and therefore
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reduce the
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quenching, one must repeat purification procedures multiple times requiring at least one week for
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each sample. Occasionally, the prepared LSC cocktail remains influenced by color quenching
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after multiple purification steps, leading to an underestimated
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S. Color quenching resulting from impurities (e.g., organic compounds and
S counting efficiency and sensitivity. To prevent the possible influence of color
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S activity in the environmental
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Analytical Chemistry
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sample. Hydrogen peroxide (H2O2), a strong oxidant, was suggested as an effective bleaching
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agent for LSC cocktails in measuring high-radioactivity samples (>10,000 disintegrations per
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minute [DPM]),35-36 but it remains unclear if this agent can be used in the high-sensitivity
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measurements (
