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There is a nearly ubiquitous problem in mass spectrometry publications. It is frequently the case that the isotopically labeled internal standard is not well-specified, morphine D3 for example. The problem is that this does not uniquely specify how the internal standard is labeled. For example, there are many different possible labeling schemes that result can all result in three deuteriums in the molecule. The same goes for most labeling methods (13C4, 15N2, etc.) In almost all cases there is more than one combination of labeling positions that can result in the same overall degree of labeling.