Carbon isotope standards Carolina

The amounts of the standard isotopic species and the tracer isotopic species are represented by X and X for the sample and the reference material. The reference substance is chosen arbitrarily, but is a substance that is homogeneous, available in reasonably large amounts, and measurable using standard analytical techniques for measuring isotopes (generally mass spectrometry). For instance, a sample of ocean water known as Standard Mean Ocean Water (SMOW) is used as a reference for and 0. Calcium carbonate from the Peedee sedimentary formation in North Carolina, USA (PDB) is used for C. More information about using carbon isotopes is presented in Chapter 11. 

The most widely used standard is a belemnite from the Peedee Formation in South Carolina (PDB) therefore, some ratios may be expressed as negative values. Most carbon isotope ratio correlations are made on the C15+ fraction of crude oil because it is less affected by degradation processes. Valid correlations using carbon isotopes can only be conducted on the same fractions of samples. 

PeeDee Belemnite (PDB) Oxygen and carbon isotope ratios are measured as relative deviations (5 0 per mil 5 C per mil) from a laboratory standard value. The standard normally employed for the analysis of carbonates is a PDB limestone (belemnite shell from the Cretaceous Peedee Formation of South Carolina). 

Standards The standard for carbon isotope abundance measurements is based on a Cretaceous belem-nite sample from the Peedee formation in South Carolina, USA. The original material is no longer available. It has been replaced by the convention that NBS 19, a carbonate material, has a value of -i-1.95 l versus PDB. This new scale is termed V-PDB (Vienna-PDB). The IAEA distributes a number of secondary standards including graphite (USGS24) with a d C value of - 15.994 V-PDB, oil (NBS-22) at -29.741 V-PDB, and calcium carbonate (NBS-18) with a value of - 5.014 V-PDB. 

It has an I80 / 160 ratio of 2005.20 ( 0.45) x 106 (Baertschi, 1976). In many studies involving carbonates, PDB is the standard relative to which values are reported. The PDB standard is based on the oxygen isotope ratio of a Cretaceous belemnite from the Peedee formation in South Carolina. Because it was composed of calcite, many limestones have isotopic ratios close to it. Values ("X") reported relative to the two standards are related by equation 3.16. 

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