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Stable Isotope Methods

Compound Specific Isotope Analyses (CSIA) for δ13C and δD

The lab is equipped with a Thermo Trace GC Ultra interfaced with Thermo Delta V Plus through a Thermo GC-C-III combustion unit for GC-C-irms ofδ13C and δD. The GC has one split/splitless and one PTV inlet, with a Triplus autosampler. For GC-C-irms analyses, the compounds of interest (C16 to C40n-alkanes or fatty acids) are extracted from environmental samples (soils, plant material, etc.) in our Organic Geochemistry Laboratory, where they are first screened for purity and abundance with a Thermo Trace GC Ultra/DSQ IIGC-MS system. Mixtures of n-alkanes A6 and B3 (from Arndt Schimmelmann, Dept. of Geological Sciences, Indiana University) are measured for calibrating the GC-C-irms δ13C and δD results.

Solid Phase Micro Extraction (SPME) GC-C-irms for δ13C

Using the novel technique of Solid Phase Micro Extraction coupled with Gas Chromatography isotope ratio mass-spectrometry (SPME-GC-C-irms), it has been possible to analyze carbon isotopes in selected organic compounds from natural samples at extremely low concentrations. Volatile organic compounds within the head space of the sampling vial are adsorbed onto a polymer fiber, and are eventually released inside the GC’s PTV inlet at an elevated temperature. The eluted compounds are then oxidized and δ13C analyzed by the mass-spectrometer.

δ13C and δ18O in carbonates

Powdered carbonates (~ 300 to 400 mg of calcite or aragonite) are reacted with orthophosphoric acid in Helium flushed Exetainer vials, and the headspace gas is analyzed for δ13C and δ18O using Thermo Gasbench II with CTC PAL autosampler. The Exetainer vials loaded with samples are flushed with helium in a specially designed glass manifold which allows for complete evacuation of multiple vials prior to their flushing. In-house CaCOstandard is used as the primary calibration standard, which has been calibrated against NBS 18 and NBS 19.

δ13C in dissolved inorganic carbon

Between 1 and 5 mL of water samples are needed for analysis of δ13C in DIC. Water samples are stored refrigerated prior to analysis in airtight vials with crimped rubber stoppers. Samples are drawn from the vials with a 10 mL gas-tight syringe and injected into Helium flushed Exetainer vials containing 8-10 drops of orthophosphoric acid. The Exetainer vials with acidified DIC are then agitated with a vortex and allowed to equilibrate at room temperature (25 °C) for 2 hours and the headspace gas is analyzed for δ13C. The δ13C results are calibrated against an in-house sodium bicarbonate (NaHCO3) standard.

δ13C and δ15N in organic matter

Powdered and homogenous soil or plant material (5 to 20 mg) is packed in tin capsules and combusted in the presence of oxygen at 980 °C using the Costech ECS4010 Elemental Analyzer (with chromium oxide/silvered cobalt oxide oxidation column and reduced copper wire reduction column at 650° C). The gases (CO2 and N2) are resolved by a 4 m long stainless steel GC column (at 70 °C), dried with magnesium perchlorate and measured for δ13C and δ15N by Thermo Delta V Plus IRMS after appropriate dilution by Conflo IV (0% for N2 and 58 or 83% for CO2). Concentrations of C and N (C% and N%) are obtained both from Costech’s Thermal Conductivity Detector (TCD) and from the IRMS chromatograms. Commonly used rimary standards are Acetanilide #3, Urea #2 and EDTA #2 (from Arndt Schimmelmann, Dept. of Geological Sciences, Indiana University), IAEA-600 Caffeine, IAEA-CH-3 Cellulose, and IAEA-CH-6 Sucrose.

δ34S in sulfides and sulfates

For measurements of δ34S in rocks, sulfur is first extracted from the samples in our Sedimentary Geochemistry Laboratory either in the form of silver sulfide (Ag2S) or barium sulfate (BaSO4). About 0.8 mg of Ag2S or BaSO4 samples are packed in tin capsules with V2O5 additive and combusted in presence of oxygen at 980 °C using Costech ECS4010 Elemental Analyzer (with tungstic oxide/copper oxidation column). SO2 is resolved using a 2 m long Teflon GC column (at 110 °C), dried with magnesium perchlorate, and δ34S of SO2 are analyzed with Thermo Delta V Plus IRMS. Primary standards used are IAEA S1, S2 and S3 (for samples as Ag2S) and IAEA SO5, NBS-127 and an in-house Barite standard (for samples as BaSO4).

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