Solid - δ34S
We use a Eurovector Elemental Analyzer, Conflo III, Thermo MAT253 for a continuous flow based measurement of our solid organic material δ34S. Samples are flash combusted at 1000 °C with excess oxygen in a Eurovector Elemental Analyzer (EA) equipped with a Costech zero-blank autosampler. The combustion column is packed with alternating layers of quartz chips and quartz wool, a layer of reduced copper, and another layer of quartz chips as in Fry et al. 2002. A helium carrier set to 80 mL / min moves combustion products through a magnesium perchlorate desiccant column and an 850 °C quartz chip column to ensure complete exchange of all SO2 oxygens with quartz oxygens. This treatment completely removes any oxygen isotope variability and allows one to attribute variation in mass 64 to variation in S-34 isotopes. Sample gases are then carried through a 0.8 m gas chromatography column to separate SO2 from N2 and CO2. The EA effluent flows into a ThermoFinnigan Conflo III where the stream is split to reduce the flow rate from 80 mL / min to 2 mL / min. The reduced flow rate then flows through an open split with excess helium via fused silica capillary tubing. A pressure differential between the ThermoFinnigan MAT253 isotope ratio mass spectrometer and the Conflo III along fused silica capillary tubing allows helium and sample gas to move into the electron bombardment source for subsequent ionization, acceleration, focusing, separation, and measurement of masses 64 and 66 for δ34S. Internal laboratory reference materials are interspersed with samples for a two point calibration which allows for conversion to the δ34S VCDT scale. We use a suite of internal reference materials calibrated against IAEA-S-1, NBS127, and IAEA-S-3 to place samples on the VCDT scale.
The IsoDat software provided δ34S values relative to the working reference gas cylinder (along with all other selected data) are imported into a matlab script for data reduction. A linear regression is completed for δ34S using the measured δ values and the accepted values of our internal reference materials. All data are corrected to the VCDT scale using this linear equation. Precision and accuracy are estimated for each run using a third internal reference material that is treated as an unknown. Reference materials are chosen to bracket the sample range in δ34S values.
Standard Operating Procedures
- Fry B, Silva SR, Kendall C, Anderson RK. (2002) Oxygen isotope corrections for online δ34S analysis. Rapid Communications in Mass Spectrometry 16, 854-858. doi: 10.1002/rcm.651.