Quantification of Stable Isotopic Tracers

Radiotracers of an element differ physically from its stable isotopes in the number of neutrons and, as they are unstable, can be quantified directly via monitoring of the decay process through detection of emitted particles and/or gamma rays. Stable isotopic tracers, in contrast, are composed of the same isotopes as the natural element. The two differ only in the relative abundances of their isotopes with tracers being commonly enriched in the isotope(s) that is/are least abundant in nature. This means that stable isotopic tracers can only be quantified indirectly via the changes in the isotopic abundances of the natural element introduced by tracer addition, which is measurable by mass spectrometric techniques. 

The isotopic abundance of the isotope of mass number X and all other isotopes can then be calculated from the measured isotope abundance ratios  

Combining Eqs. (16.1) and (16.2) yields an alternative equation, which is based entirely on isotope abundance ratios  

Conventional applications of IDMS in quantitative elemental analysis usually involve the use of a single isotopically enriched tracer/spike. This is different in metabolic studies. The use of two or more tracers of the same element in parallel permits the use of more refined study designs and methodologies, as discussed later. When two isotopic labels X and Y are used in parallel [9], the tracer to tracee ratio for label Y in the sample can be calculated using 

Alternatively, calculations can be based entirely on isotope abundance ratios with a, p, y, and i5 being then defined as 

---