Inorganic Solids Analysis Techniques
Carbon and oxygen isotopes of carbonates
Direct Analysis of Carbonates on the GasBench
Pure carbonate mineral species can be analysed directly in continuous flow on the Thermo Finnigan GasBench coupled to a DeltaPlus XP IRMS.
Requirements for material sent for analysis:
An Exetainer vial is filled with about 10mL of specially prepared anhydrous phosphoric acid (method adapted from: Coplen et al., 1983), capped, and evacuated on the Pyrex vacuum line for at least 1 hour. About 0.5-0.7mg of sample or standard is measured into each Exetainer. A calibrated internal calcite or dolomite standard is also loaded with each batch to be run as an unknown.
Once the samples and standards have been weighed, the vials are loaded without caps into an extra GasBench rack turned on its side (i.e. Exetainers are kept almost horizontal). A 1mL disposable syringe is flushed with helium. The helium is injected into the evacuated Exetainer of acid, which is then kept in a cup of hot water to reduce the viscosity. The 1mL syringe is used to carefully drop 0.1mL of acid just inside the threaded top of each Exetainer. Vials are kept horizontal while being recapped (after every 8). Once all the vials have acid added and caps on, each column of 8 vials is flushed and filled with helium off-line for 6-8 minutes.
Prepared vials are then tipped upright and immediately placed in the heated block of the GasBench (either 25.0°C or 50.0°C), and left to react for 24 hours. Regular analysis of CO2 headspace follows.
The analytical precision is +/- 0.1 permil.
Off-line Extraction Techniques (calcite-dolomite mix):
Carbonate mineral species are reacted under vacuum at either 25.0°C or 50.0°C with anhydrous phosphoric acid to release CO2 (McCrae, 1950).
Sample material must be very fine sand size for selective extraction; powdered material will not work. Powdered material is ok for single mineral extraction.
Depending on the purity of the sample, up to 400mg of powdered mineral or rock is loaded into a Pyrex vessel with side-arm. An amount of 1-5mL of specially prepared anhydrous phosphoric acid (method adapted from: Coplen et al., 1983) is dispensed into the side-arm of the vessel. A cap with a valve is added so that the vessel can be attached to a Pyrex vacuum line for evacuation. The vessel and acid are left to pump for at least 1 hour; the acid is heated briefly with a heat gun to fascillitate the de-gassing of the acid. The vessels are then closed and placed in a controlled-temperature water bath to equilibrate for 15 minutes minimum. The acid is then tipped onto the sample and left to react for the appropriate amount of time (Al-Aasm, ??).
After the reaction, the vessels are re-attached to the 5-port vacuum line. Liquid nitrogen and -80°C ethanol baths are used to trap and clean water from the CO2 then transfer the gas to a breakseal which is sealed with a torch.
Selective acid extraction technique (Al-Aasm, I.S. et al., 1990) is used to obtain CO2 from mixtures of calcite and dolomite.