Exactly, elements do not disappear unless you’re transmuting something. But what’s interesting about the mass spectrometry and NMR studies is that they can actually discriminate intact compounds.Right, Zinc and Phos will still show up on UOA because it has not 'disappeared'.
Its whether it remains in a form that is still useful or as useful as it was originally.
The time it takes to deplete depends on a number of factors. Note the references given above apply to very specific cases.