High precision analysis of all four stable isotopes of sulfur (S-32, S-33, S-34 and S-36) at nanomole levels using a laser fluorination isotope-ratio-monitoring gas chromatography-mass spectrometry

摘要

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 (S-32, S-33, S-34, and S-36) 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 degrees C. This technique works with an optimum sample size of 100 to 200 nmol with precision for Delta(33)S and Delta(36)S at 0.1 and 0.5 parts per thousand (2 sigma). 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 ((SF5+)-S-32) onto minor beams ((SF5+)-S-33 and (SF5+)-S-36), which results in contraction of the measured delta(33)S and delta(36)S 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. (c) 2005 Elsevier B.V. All rights reserved.