CO2 sequestration studies use carbon isotope measurements to monitor the migration of CO2 in the subsurface. Measurements of aqueous CO2 and dissolved inorganic CO2 (i.e., bicarbonate) are widely applied. Hydrology studies coupled to CO2 sequestration studies also measure oxygen and hydrogen isotope values of groundwater as natural tracers. The δ18O value of quartz fractures can be used to determine the source of fluids or temperatures of quartz precipitation. Hydrocarbon exploration including shale gas reserves often rely on carbon isotope measurements to trace the origins of methane, ethane, and propane. Our laboratory offers a range of measurement capabilities that are suited to these applications.
Carbon isotope ratios are measured from shale gas samples that are collected in the field from either core samples or from in situ samplers. We use a modified Thermo Gas Bench with a combustion furnace that is coupled to a CombiPal autosampler. CO2 is separated from alkanes (methane, ethane and propane) using this system and a bulk ‘alkane’ carbon isotope ratio is measured. This method is optimized for small sample sizes; less than 100nmols of alkanes are required. We are developing a new method that will offer similar detection limits, the separation of the alkanes and simultaneous carbon isotope and concentration measurements. This new method will be available in September, 2012.
Laser Fluorination for Quartz Oxygen Isotope Ratios
We are equipped with a laser fluorination oxygen extraction line that is capable of measuring δ18O and δ17O values in silicates and oxides. This system utilizes a New Wave Research MIR10-30 laser that is coupled to a BrF5 vacuum extraction system. An aliquot (approximately 0.1 atmosphere) of purified BrF5 is expanded into the sample chamber. The laser is used to heat the minerals in the presence of BrF5, producing oxygen gas and solids and gaseous fluorides as reaction products. The oxygen gas is then purified using a series of liquid nitrogen and NaCl traps. Purified oxygen is quantitatively collected on 5Å molecular sieve. Oxygen isotope values are measured using a dual inlet bellows system in our Thermo MAT253 stable isotope ratio mass spectrometer. Up to 44 samples and standards can be loaded at one time. We require approximately 2.0mg of sample for a typical analysis. Precision is ±0.05‰.
δ18O and δ13C values are measured from solid carbonates with a Gas Bench coupled to a CombiPal autosampler and a heated sample block. Approximately 0.25-0.5mg of carbonate is weighed into 12ml Exetainer vials that are helium purged. 103% H3PO4 (s.g.=1.90) is added to each vial and equilibrated at 50°C to form CO2 . Vials are flushed with helium using the Combipal to sample CO2 multiple times with a 100microliter sample loop. δ18O and δ13C values are measured using a Thermo MAT253 isotope ratio mass spectrometer.
Dissolved Inorganic Carbonates (DIC)
The δ13C value of DIC can be measured in aqueous samples using the Gas Bench coupled to a CombiPal autosampler and a heated sample block. Depending on concentration, a volume of sample is added to helium purged 12ml Exetainer vials. 103% H3PO4 (s.g.=1.90) is added to each vial and equilibrated at 50°C to form CO2 . Vials are flushed with helium using the Combipal to sample CO2 multiple times with a 100microliter sample loop. δ18O and δ13C values are measured using a Thermo MAT253 isotope ratio mass spectrometer.
We use CO2 equilibration to measure δ18O values from water samples. Approximately 0.5ml of water is added to a 12ml Exetainer vial which is then sealed and purged with a mixture of Helium and CO2 and equilibrated at 25°C. After equilibration the Combipal autsampler flushes each vial with helium to sample CO2 multiple times with a 100microliter sample loop. δ18O are measured using a Thermo MAT253 isotope ratio mass spectrometer.
A TCEA (Thermal Conversion Elemental Analyzer) coupled to a Combipal autosampler is used to measure hydrogen isotope ratios (δD values) from samples of water. Less than 1microliter of water is injected into the TCEA that is packed with glassy carbon and heated to 1450°C. Water pyrolysis products: CO and H2 are separated using a gas chromatography column. δD values are measured using a Thermo MAT253 isotope ratio mass spectrometer.