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.

Sample Submission

Your samples should be ground to a fine powder and you should provide percent carbonate values if they are not > 90 % carbonate. We require 20 to 120 µg of carbonate for a single analysis. Ideally you will have ~200 µg of carbonate at your disposal for this analysis. Analysis costs are here. Typically the only cost associated with this method is Carbonate - δ13C and δ18O, however, samples may require Simple sample grinding or Sample Pulverizing. You should also know the species of carbonate in your sample and make certain to provide this information to anyone handling your sample. Additional analyses may be required for samples of unknown percent carbonate.

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.