Determination of the δ34S of low-concentration sulfate in water; RSIL lab code 1949

Techniques and Methods 10-C8
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Abstract

The purpose of the Reston Stable Isotope Laboratory (RSIL) lab code 1949 is to determine the δ(34S/32S), abbreviated as δ34S, of dissolved sulfate having a concentration less than 20 milligrams per liter. Dissolved sulfate is collected on an anion-exchange resin in the field, eluted in the laboratory with 3 M KCl, and precipitated with BaCl2 at pH 3 to 4 as BaSO4. The precipitated BaSO4 is filtered and dried before introduction into an elemental analyzer (EA) Carlo Erba NC 2500. The EA is used to convert sulfur in a BaSO4 solid sample into SO2 gas, and the EA is connected to a continuous flow isotope-ratio mass spectrometer (CF-IRMS), which determines differences in the isotope-amount ratios of stable sulfur isotopes (34S/32S) of the product SO2 gas. The combustion is quantitative; no isotopic fractionation is involved. Samples are placed in a tin capsule and loaded into the Costech Zero Blank Autosampler of the EA. Under computer control, samples are dropped into a heated reaction tube that combines the oxidation and reduction reactions. The combustion takes place in a helium atmosphere containing an excess of oxygen gas at the oxidation zone at the top of the reaction tube. Combustion products are transported by a helium carrier through the reduction zone at the bottom of the reaction tube to remove excess oxygen and through a separate drying tube to remove any water. The gas-phase products, mainly CO2, N2, and SO2, are separated by a gas chromatograph. The gas is then introduced into the isotope-ratio mass spectrometer (IRMS) through a Finnigan MAT (now Thermo Scientific) ConFlo II interface, which is also used to inject SO2 reference gas and helium for sample dilution. The IRMS is a Thermo Scientific Delta V Plus CF-IRMS. It has a universal triple collector with two wide cups and a narrow cup in the middle. It is capable of measuring mass/charge (m/z) 64 and 66 simultaneously. The ion beams from SO2 are as follows: m/z 64 = SO2 = 32S16O16O; m/z 66 = SO2 = 34S16O16O primarily.
Publication type Report
Publication Subtype USGS Numbered Series
Title Determination of the δ34S of low-concentration sulfate in water; RSIL lab code 1949
Series title Techniques and Methods
Series number 10-C8
DOI 10.3133/tm10C8
Edition Version 1.0 - May 2006, Version 1.1 - 2007, Version 1.2 - September 2012
Year Published 2006
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Contributing office(s) Reston Stable Isotope Laboratory
Description viii, 35 p.
Larger Work Type Report
Larger Work Subtype USGS Numbered Series
Larger Work Title Chapter 8 of Book 10, Methods of the Reston Stable Isotope Laboratory, Section C, Stable Isotope-Ratio Methods
Online Only (Y/N) Y
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