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Abstract: The discovery of mass-independent isotope effects observed in Archean rocks, certain classes of meteorites, and atmospheric aerosols has had profound implications to our understanding of ancient and present atmospheric sulfur chemistry. We present a new technique that takes advantage of continuous He flow isotope-ratio-monitoring gas chromatography–mass spectrometry to achieve precise analysis of all four stable sulfur isotopes (32S, 33S, 34S, and 36S) at nanomole level samples. The technique involves fluorination of sulfide (silver sulfide or pyrite), and separation of product gas by gas chromatography and the removal of mass-131 interference by a liquid-nitrogen ethanol slush at −110 °C. This technique works with an optimum sample size of 100 to 200 nmol with precision for Δ33S and Δ36S at 0.1 and 0.5‰ (2σ). Samples, as small as tens of nanomole, can be analyzed using this new method. One of the major sources of error in irm-GCMS is found to be tailing of the major ion beam (32SF5 +) onto minor beams (33SF5 + and 36SF5 +), which results in contraction of the measured δ 33S and δ 36S scales. This effect is corrected by measuring a series of reference sulfide samples with mass-dependent sulfur isotope compositions. This methodology increases the spatial resolution of the laser ablation in situ analysis and considerably reduces the analysis time as compared with conventional dual inlet methods. [Copyright &y& Elsevier]

Abstract: This paper describes a comparison of ultraviolet photolysis experiments undertaken with SO2 (oxygen with isotopes at natural abundance levels) and S18O2 (18O-substituted oxygen). Experiments were conducted in a closed photocell using a deuterium lamp (principally 190–235nm) under pressure regimes (5–25Torr) that produced optically thick conditions for 32SO2 and variable optical depths for other isotopologues. The experiments, which were designed to examine the effects of intramolecular isotopic substitution of oxygen atoms on the S-MIF produced during UV photolysis of SO2, reveal generally reduced sulfur fractionation for 18O-rich SO2 as compared to 16O-rich SO2. Model shielding calculations were undertaken using spectra that were shifted due to changes in rotational and vibrational energy levels. The model calculations suggest that processes in addition to rotational and vibrational shifts in absorption spectra play a role in the experimentally produced isotope effects. Such additional processes may include differences in primary photoexcitation arising from smaller peak-to-valley amplitudes for fine structure of 18O-rich SO2 absorption spectra or an isotopically selective process associated with transitions between excited states. [Copyright &y& Elsevier]