Effects and Stable Isotope Fractionation

These procedures have had two principal applications analysis of degradation and transformation in natural populations, and the determination of degradation pathways. Illustrative examples are 

Elements such as C, N, O, S, and Cl that are components of many organic compounds exist naturally as mixtures of stable isotopes. The ratios of these in a compound reflect the different rates of reaction at isotopically labeled positions, and therefore reflect the fractionation—biotic or abiotic—by which it was synthesized or to which the compound has been subjected. Techniques have been developed whereby the ratios C/ C (5 C), (5 N), (5 0), 

Results have been expressed in a number of ways. In the Raleigh model that has been extensively used, the fractionation factor a is given by R/Rq = when the fraction of remaining substrate is/and where R is the isotopic composition of the substrate during degradation and Rq is the initial value. The enrichment factor e where e = 1000 (a - 1) has also been used. There are certain conditions that must be fulfilled for the Raleigh model to be applicable  

The concentrations of the heavy isotope must be low, which is true for experiments using natural levels of abundance, or the reaction must be near the initial starting point. 

The model is strictly applicable only when applied to a single-step reaction, and not to a sequence of reactions. 

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