Policy on isotopes for molecular entities (e.g., MRI contrast agents)

[Peyman-N]

I am creating molecular entities that are used as MRI contrast agents.
What is our policy concerning isotopes? Should isotopically enriched forms (e.g., Gd-157, Xe-129) be represented as separate molecular entities, or do we only model the general element/compound?
@lzehl, your thoughts on this.

[apdavison]

My suggestion would be to represent individual isotopes as MolecularEntities, and mixtures of isotopes as ChemicalMixtures

[Peyman-N]

This is an interesting approach as it eliminates the guesswork and would be consistent. There is also precedent for this; Interlex has taken the same approach and has registered all the istopes separately.
However, I see only two problems: one is that some atoms have numerous isotopes, and the other is that it would make some simple tasks complex for no reason.
If we go this route, we should also do a cleanup.
Anyhow, this seems the most consistent way of modeling it, so maybe we should do the extra work.