Compound-Specific Sulfur Isotope Analysis of Petroleum Gases

Abstract

We describe a simple, sensitive, and robust method for sulfur isotope ratio (S/S) analysis of ppm-level organic sulfur compounds (OSCs) in the presence of percent-level HS. The method uses a gas chromatograph (GC) coupled with a multicollector inductively coupled plasma mass spectrometer (MC-ICPMS). The GC, equipped with a gas inlet and a valve that transfers the HS to a thermal conductivity detector (TCD), enables a precise heart cut and prevents the saturation of the MC-ICPMS. The sensitivity and accuracy of the method are better than 0.3‰ for OSCs at a concentration of 25 pmol or 1.4 ppm, and better than 0.5‰ for concentrations ≥0.7 ppm of OSCs. An order of magnitude increase in sensitivity, with no effect on accuracy, can be achieved if the loop volume (0.5 mL) is changed to 5 mL. High concentrations of methane (95% v/v) and/or HS (20% v/v) had no effect (within 0.5‰) on the precision and accuracy of the gas sample containing 2 ppm of OSCs after heart cut. The applicability and robustness of this method are demonstrated on a gas sample (10% v/v HS) that was produced by pyrolysis of sulfur-rich kerogen. The results show good precision and reveal sulfur isotope variability between individual OSCs that may represent key processes during formation and degradation of OSCs.

Sulfur isotopes ratio of atmospheric carbonyl sulfide constrains its sources

“…The method we used was a combination and modification of two existing methods. One is the Tenax resin pre-concentration of COS from air 15,19 and the second is the δ 34 S analysis of gases using GC/MC-ICPMS 18 . To ensure that our method preserves the original δ 34 S value of the measured COS, we have measured the following COS standards using two introduction methods, direct injection and pre-concentration.…”

“…The GC was equipped with a split/splitless injector for direct injection of volatile samples and a heated (70 °C) six-way valve gas inlet system (Valco Instrument Co, TX, USA) for the introduction of gaseous compounds with a computer-controlled actuator. The GC column (60 m * 0.320 mm, GS-GASPRO, Agilent Technologies) is able to separate cleanly between SO 2 or CS 2 and COS. A transfer line, heated to 200 °C, connected the GC to the plasma source 18 .…”

“…Table 1 presents the operational conditions of the GC-MC-ICPMS system. Data processing procedure was as described in detail elsewhere 16,18 .…”

“…The δ 34 S values of Mix 1 were calibrated against in-house liquid standards DMS (−3.0 ± 0.1‰), CS 2 (17.2 ± 0.1‰) and Thiophene (9.6 ± 0.2‰) (“Mix 5”) that were pre-calibrated against international standards (using elemental analyzer isotope ratio 16,18 . These standards, diluted in toluene to form ~81 pmol μL −1 , were injected directly in to the GC injector (1 µl, split 5, ~16 pmol on column) as detailed in Said-Ahmad et al .…”

“…A more tractable analytical approach for the analysis of trace atmospheric sulfur compounds is the coupling of a gas chromatograph (GC) with a multicollector inductively coupled plasma mass spectrometer (MC-ICPMS) 16 . This method enables the measurements of δ 34 S values in individual volatile and gas compounds and requires a sample size of the pmol level or about 10 4 -fold lower than that of a regular IRMS 16–18 . The current sensitivity of the GC/MC-ICPMS required only 1–2 L of air for reliable δ 34 S analysis of atmospheric COS.…”