Establishing the 13C Isotope Content in Natural Samples

The natural cycle of carbon involves compounds of the atmosphere, hydrosphere, lithosphere, and biosphere. A certain difference in the 13C isotope content exists between the samples, depending on their origin. To estimate the deviation from the average value of 13C isotope contents 8(%o) scale is used. The deviation may be calculated by Equation 5.14  

Indexes o and s define the ratio of the carbon isotopes in the sample and in the standard. A lithospheric carbonate material was accepted as standard. The closest to this zero point value belongs to standard sample NBS-19 (1.95%c). There are some other standard samples NBS-22 oil (—29.74%c), NBS-18 calcium carbonate (— 5.01%c). Usually 813C values for plants are in the range ( 15%o) to (— 30%c), and for oil (— 20%c) to ( 36%c). Atmospheric methane has the lowest content of 13C. Its 813C value is approximately —47%o. 

Isotopic mass spectrometry is used to establish 813C values. The sample is burned to C02 and the intensities of the ion peaks of m/z 44, 45, and 46 are measured. Then correction to eliminate the influence of 170 isotope is achieved. If using sector magnetic instrument and three detectors (for each mass) the standard deviation of the results will be better than 0.001%. The high accuracy of the measurements allows valuable results to be obtained. The ancient Europeans and Americans may be distinguished by the analysis of their remnants. The reason involves the fact that wheat constituted the basic food ration in Europe, while com played the same role in America. The difference in the isotopic composition of these plants forms several units of 813C scale [34], 

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