The Principles of Isotope Ratio Monitoring

Isotope ratios, although tabulated with average values for all elements, are not constant. All phase transition processes, transport mechanisms and enzymatic or chemical reactions are dependent on the physical properties of the reaction partners, that is, most importantly the mass of the molecule involved in the process or chemical reaction, for their kinetic properties. 

The high precision quantitative data on isotope ratios obtained are not absolute quantitation data. Natural isotope ratios exhibit only small but meaningful variations that are measured with highest precision relative to a known standard. 

In most of the applications in stable isotope analysis, the differences in isotopic ratios between samples are of much more interest and significance than the absolute amount in a given sample. The isotope ratio is independent of the amount of material measured (and at the low end only determined in precision by the available ion statistics). The relative ratio measurement can be accomplished in the required and even higher precision, which is at least one order of magnitude higher than the determination of absolute values. 

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