IDCalc was developed to provide a simple way to view the expected isotope distribution for biological molecules measured by mass spectrometry. The program also includes the the ability to consider enriched stable isotopes.
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Kubinyi, Analytica Chimica Acta, 247 (1991) 107-119.
The C, H, O, and N isotope abundances are specific to biological molecules
(Dwight E. Matthews personal communication)