Stable Isotope Facility

University of Wyoming

Gas Bench II Analyses (GC-IRMS)

δ¹³C and δ¹⁸O Carbonates

δ¹³C  and δ¹⁸O ratios in carbonate minerals can be determined simultaneously by the analysis of CO₂ generated by the reaction of carbonate with 100% phosphoric acid.  At SIF, headspace vials are used to process carbonate samples. A known mass of sample is placed in the headspace vial, dried overnight and then flushed with helium. A quantity of 100% phosphoric acid is then added. Depending on the carbonate material, the reaction is allowed to proceed for 24-48 hours.  Calcite and aragonite require 24 hours, while dolomite and apatite require 48 hours.  All of the carbon ends up in the headspace of the vial in the form of CO₂ gas, so that the δ¹³C of the CO₂ gas is equivalent to that of the carbonate.  Oxygen is partitioned both into water and CO₂ so that a calculation is required to get the isotopic composition of the carbonate from the CO₂ gas. The headspace CO₂ is analyzed using the GasBench attached to an isotope ratio mass spectrometer (GC-IRMS).
Typical sample size:    100-200 μg (calcite)

δ¹³C and δ¹⁸O of Atmospheric CO₂

At SIF, the measurement of δ¹⁸O and δ¹³C ratios of CO₂ in atmospheric air employs a GasBench headspace autosampler attached to an IRMS.  Once atmospheric air is introduced into a headspace vial, the vial is simply placed in the GasBench for automated sampling.  No further processing is required. (GC-IRMS)
Typical sample size:    not applicable

δ¹³C and δ¹⁸O of Breath CO₂

In breath, the carbon isotope value (δ¹³C) provides information on the diet of the animal whereas the oxygen isotope value of breath CO₂ reflects the δ¹⁸O value of body water.  For humans, this closely matches drinking water.  Animals that obtain a large portion of metabolic water from food sources (vegetation, etc.) obviously would have isotope ratios of body water that reflect this input. At SIF, the measurement of δ¹⁸O and δ¹³C isotope ratios of CO₂ in breath employs a GasBench headspace autosampler attached to an IRMS.  Breath samples are collected in headspace vials and simply placed in the GasBench for automated sampling.  No further processing is required (GC-IRMS).
Typical sample size:    not applicable

δ¹³C of Dissolved Inorganic Carbon (DIC)

When carbon is dissolved in water, it occurs in three different forms: CO₂, HCO₃, and CO₃.  These species are commonly referred to as dissolved inorganic carbon (DIC).  To determine the δ¹³C ratio of this carbon, it is necessary to quantitatively remove all the carbon from the water sample.  At SIF, the procedure used is to add an aliquot of 100% phosphoric acid (H₃PO₄) to a headspace vial, which is then flushed with helium.  The sample water is injected into the vial.  The acid protonates the DIC, releasing CO₂ into the vial headspace. The CO₂ is then analyzed using the GasBench attached to an isotope ratio mass spectrometer (GC-IRMS).
Typical sample size:    1-5 ml

¹⁸O of Water  (CO₂/H₂O Equilibration)

The δ¹⁸O value of water can be easily determined by equilibrating the water with CO₂. When CO₂ dissolves in water, oxygen atoms are exchanged through a mechanism that involves the hydration of dissolved CO₂ to form carbonic acid (H₂CO₃).  By using an excess of water compared to the amount of CO₂ present, the above hydration process will allow the ^CO₂ to take on the value of the δ18O ratio in the water (after correcting for a known liquid-gas phase equilibrium fractionation).  At SIF, the headspace vial is flushed helium gas followed by an injection of the water sample. The vials are then allowed to equilibrate for 24 hours.  The Finnigan GasBench is then used to automatically sample the CO2 in the headspace of the vials before the sample is analyzed in the IRMS. (GC-IRMS)                                     Typical sample size:    1-5 ml