Notations in irm-GC-MS

The small abundance differences of stable isotopes are best represented by the delta notation (Eq. 2.29) in which the stable isotope abundance is expressed relative to an isotope standard, measured in the same run for reference  

Other systems in use for expressing isotope ratios are ppm (part per million, relative value), at.% (atom percent, absolute value) and as the most common terminology in biomedical tracer studies APE (atom percent excess, relative value). The choice is dependent primarily on the specific field of application, for example, with high degrees of enrichments being very much different from natural abundances. Also, typical in this field are historical traditions in the use of different notation systems. Values of at.% and 5-notation can be converted as follows  

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