Carbonate - δ¹³C, δ¹⁸O

Overview

We use a Kiel III Carbonate Device coupled to a Finnigan Delta Plus isotope ratio mass spectrometer for a dual-inlet based δ¹³C and δ¹⁸O measurement of small carbonate samples. The Kiel III carbonate device is an automated system for the digestion of carbonate with 100% phosphoric acid (specific gravity 1.90 - 1.92) and purification of the carbon dioxide (CO₂) product. For traditional pure-carbonate samples, five drops of phosphoric acid are counted onto 20-120 µg of carbonate in an evacuated, leak tested, pyrex reaction vessel and allowed to react for ten minutes at 70 °C. Both CO₂ and H₂O are trapped in a small-volume cold finger held at -196 °C with liquid nitrogen. When the reaction is complete, the trap is warmed to -110 °C to keep H₂O frozen but allow CO₂ to expand for gas volume measurement. The CO₂ is then frozen into a second small-volume cold finger trap held at -196 °C with liquid nitrogen. Once the transfer is complete, the trap is warmed to 26 °C allowing the CO₂ to pass into a Finnigan Delta Plus isotope ratio mass spectrometer (IRMS) via a dedicated capillary for mass / charge 44, 45, 46 measurement alternating with that of a reference CO₂ working gas roughly calibrated to the VPDB scale held in a variable volume bellow. Eight comparisons of sample / reference are made to yield initial measured δ¹³C and δ¹⁸O values. Measured values of all materials are then corrected to the VPDB scale using a suite of internal reference materials that have been calibrated to and span a similar range of NBS19, NBS18, L-SVEC, and IAEA-603.

Standard Operating Procedures

• Microbalance
• Making Phosphoric Acid
• Carbonate Bulk Isotope Analysis

Suggested Reading

• Craig H (1957) Isotopic standards for carbon and oxygen and correction factors for mass-spectrometric analysis of carbon dioxide. Geochimica et Cosmochimica Acta 18, 849. doi: 10.1016/0016-7037(57)90024-8.
• McCrea JM. (1950) On the isotopic chemistry of carbonates and a paleotemperature scale. The Journal of Chemical Physics 18, 849. doi: 10.1063/1.1747785 .
• Santrock J, Studley SA, Hayes JM. (1985) Isotopic analyses based on the mass spectra of carbon dioxide. Analytical Chemistry, 57, 1444–1448. doi: 10.1021/ac00284a060.
• Tobin TS, Schauer AJ, Lewarch E. (2011) Alteration of micromilled carbonate δ¹⁸O during Kiel Device analysis. Rapid Communications in Mass Spectrometry. 25: 2149-2152. doi: 10.1002/rcm.5093.