Natural Variations in Relative Atomic Mass

The masses of isotopes can be measured with accuracies better than parts per billion (ppb), e.g., m40Ar = 39.9623831235 0.000000005 u. Unfortunately, determinations of abundance ratios are less accurate, causing errors of several parts per million (ppm) in relative atomic mass. The real limiting factor, however, comes from the variation of isotopic abundances from natural samples, e.g., in case of lead which is the final product of radioactive decay of uranium, the atomic weight varies by 500 ppm depending on the Pb/U ratios in the lead ore. [8] 

For organic mass spectrometry the case of carbon is of much greater importance. Carbon is ubiquitous in metabolic processes and the most prominent example of variations in the isotopic ratio is presented by the different pathways 

Petroleum, coal and natural gas yield very low ratios of 0.01068-0.01099 

Isotope ratio mass spectrometry (IR-MS) makes use of these facts to determine the origin or the age of a sample. For convenience, the minor changes in isotopic ratios are expressed using the delta notation stating the deviation of the isotopic ratio from a defined standard in parts per thousand (%c). [8,10] The delta value of carbon, for example, is calculated from 

The internationally accepted value for C/ Cstandm-d is 0.0112372 from a belemnite fossil from the Pee Dee formation in South Carolina (PDB standard). Naturally occurring C/ C ratios range from about 8 C = -50 to +5 %c. 

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