Stable Isotope Facility

Sulfur (³⁴S) Analysis of Solid Samples

The SIF provides ³⁴S isotope analysis of organic solid materials, such as plant and animal tissues, using an elemental analyzer and pre-concentration unit interfaced with a continuous-flow isotope-ratio mass spectrometer.

Analysis
Stable isotope ratios of ³⁴S in solid samples are measured using a SerCon elemental analyzer and custom cryofocusing system interfaced to a SerCon 20-22 IRMS (Sercon Ltd., Cheshire, UK). Samples mixed with an oxidant (e.g., niobium pentoxide, Nb₂O₅) are combusted at 1020 °C in a reactor packed with copper wires and quartz chips. Immediately following combustion, water is removed by a selectively-permeable nafion dryer, sample gases pass through a reactor filled with quartz chips at 850 °C, and SO₂ and CO₂ are separated by a GC column (HayeSep D, 55 °C, 75 mL/min). Sample gases then enter a cryo-focusing loop through a six-port rotary valve (Valco, Houston TX), programmed to trap SO₂ after the CO₂ peak has passed to waste. The entire SO₂ content is frozen, held, and released to a second HayeSep D microcolumn (to improve peak shape) before flowing into the IRMS. For samples containing greater than 40 μg S, the cryofocusing unit is bypassed, and SO₂ passes from the GC column directly to the IRMS.

A pure reference gas (SO₂) is used to calculate provisional delta values of the sample peak. Final δ³⁴S values are obtained after adjusting the provisional values such that correct δ³⁴S values for laboratory standards are obtained. During analysis, samples are interspersed with several replicates of at least three different laboratory standards to monitor and correct for variation in drift and linearity. Laboratory standards are calibrated directly against IAEA S-1, S-2, and S-3. The long-term reproducibility of this method is +/- 0.2 per mil.

Please be aware that EA-IRMS measurement of ³⁴S from samples that contain significant amounts of both organic and inorganic sulfur, such as soil or sediments, will represent a mixture of both sulfur components. Inorganic or organic sulfur must be removed prior to the analysis of such samples, if the measurement of an individual sulfur pool is desired.