U.S. Geological Survey Open-File Report 01-257
By Kinga M. Revesz, Jurate M. Landwehr, and Jerry Keybl
Calcium carbonate or calcite (CaCO3) can be analyzed for the stable isotopes of carbon and oxygen to determine the ratios of the rare to the more common isotopes, respectively. The classical method used to obtain carbon and oxygen isotope ratios in calcite (McCrea, 1950) is labor intensive and requires relatively large sample sizes (20 mg). Samples are loaded in a y-shaped vessel with 100 percent phosphoric acid (2 ml) in one branch and the carbonate sample in the other. The vessel is evacuated on a vacuum line, and placed in a constant temperature bath (25 ± 0.1ºC). When the temperature stabilizes, each vessel is oriented in such a way as to have the acid flow and react with the carbonate sample as:
The reaction of acid with calcite produces solid calcium hydrogen phosphate, liquid water, and two gases, water vapor and carbon dioxide. The water is removed manually by cryogenic separation. First, both gases are frozen in liquid nitrogen. The frozen gases then are exposed to a dry ice slush, where the carbon dioxide sublimates and the water stays frozen. After two iterations of melting and then freezing of the gases, it is possible to remove the water vapor from the carbon dioxide. At this point, the carbon dioxide can be analyzed with a dual inlet isotope ratio mass spectrometer (DIIRMS).
A mass spectrometer is used to determine the ratio (R) of the heavy (rare) isotope to the light (abundant) isotope in a sample. The value or proportional difference from a standard is used almost exclusively in the Earth sciences for reporting stable isotope abundances and variations. Carbon and oxygen isotope data are reported as differences in parts per thousand (per mill or ‰) from their respective reference materials. Thevalue is defined as
where Rx=(C13/C12)x or (O18/O16)x for the sample X, and Rstd is the corresponding stable isotope ratio in the reference standard (Friedman and O'Neill, 1977). Thevalues for the carbon isotope or 13C reported relative to Vienna Peedee belemnite [VPDB], are defined as 13CNBS19/VPDB = +1.95 per mill (Hut, 1987). Thevalues for the oxygen isotope or 18O are reported relative to Vienna Standard Mean Ocean Water [VSMOW] on a scale normalized to 18O of Standard Light Antarctic Precipitation [SLAP] = -55.5 per mill (Coplen, 1988).
The classical method (McCrea, 1950) requires a relatively large sample (20 mg) and each sample must be prepared by hand. The classical method was streamlined to make use of robotic technologies and a more sensitive continuous flow isotope ratio mass spectrometer (CFIRMS) that is presently available (Finnigan Delta XL). This new method uses only 400 µg of calcite or 2 percent of the sample required in the classical method. Also, only 10 percent of the acid previously required is used. To be viable, this new method should provide results that are similar in accuracy and precision to those of the classical method. The purpose of this report is to describe the development and application of this new method.
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