Development of a Tunable Laser Spectroscopic Method for Determining Multiple Sulfur Isotope Composition of Nanomoles of SO2

Multiple sulfur isotope (³²S, ³³S, ³⁴S, ³⁶S) analyses of geological material provide important constraints on the sulfur cycles on Earth [1] and other planetary bodies, e.g., Mars [2]. However, most current multiple sulfur isotope measurements are performed on magnetic sector isotope ratio mass spectrometers (IRMS) and thus require relatively large sample size (usually about several micromoles of sulfur, except the MC-ICPMS and SIMS methods) and time-consuming sample preparation procedures. More importantly, these IRMS methods demand relatively sophisticated instrumentation, and are not ideal for field measurements or flight missions. In contrast, laser spectroscopic methods provide opportunities for significantly reducing the sample size requirement and enabling real-time monitoring in the field, and have been proven to be of great importance in the isotopic measurements of many molecules in nature, e.g. CO₂, H₂O, N₂O, CH₄. Based on a prototype built for measuring δ³⁴S of SO₂ [3], we're developing a new tunable laser spectrometer (TLS) for simultaneously determining the δ³⁴S and Δ³³S of nanomoles of pure SO₂. We have identified a new spectral window (<1 cm^-1 wide) suitable for measuring ³²SO₂, ³³SO₂, ³⁴SO₂ simultaneously. Ongoing work focuses on increasing the optical path length of the analysis cell and determining the optimal analytical conditions, with the goal of achieving ≤0.5‰ precision in both δ³⁴S and Δ³³S over 30 seconds of analysis duration of ~20 nmol of pure SO₂. Progress of these developments and comparison with conventional IRMS methods will be presented at the meeting. As a case study, we will also present preliminary TLS results from laboratory low pressure SO₂ UV photolysis experiments where δ³⁴S and Δ³³S of the residual SO₂ are expected to decrease as the photolysis proceeds [4]. Future developments of this method will involve the coupling of a sample introduction system to enable multiple sulfur isotope analysis of samples other than pure SO₂ [3]. [1] Johnston (2011) Earth Sci. Rev. 106, 161-183; [2] Farquhar et al. (2000) Nature. 404, 50-52; [3] Christensen et al. (2007) Anal. Chem. 79, 9261-9268; [4] Guo et al. (2010) Geochim. Cosmochim. Acta. 74, A366.