On the heels of the discussion launched by Daniel, can anyone explain (or provide a good reference) why there is a mass-dependent linearity effect in the first place? I was quite surprised to learn a few months back that it is not isolated to IRMS: I sat in on a seminar about analyzing isotope-labeled compounds by other MS techniques (such as ion trap) and very clearly, the enrichment of a compound increased linearly with the amount injected. The presenter had no explanation for it and just tossed it as the quirks of mass spec, but it's curious that this seems to be a universal trait.
Any thoughts?