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Maybe this belongs in rec.bicycles.racing.chemistry.organic.synthesis.

From what I've been reading, the isotope ratio of synthetic
testosterone is only somewhat different than that of natural
testosterone. Does anyone know how much the difference is? And better
yet, why is there a difference? I am assuming that all carbon in the
biosphere has the same C12/C13 ratio, and that the difference in masses
is unlikely to produce any kinetic differences during the biosynthetic
reactions, in stark contrast to H1/H2 isotopes.

Is it that a starting material is taken from the soy source and then
modified with petroleum derived reagents (which have no C13 since they
have been in the ground for millenia)?

The difference is small (~3 parts per thousand PDB), but readily
measurable if you've got good technique. It arise from the fact that
synthetic testosterone is produced from plant sterols, which are lower
in 13C than animal hormones/tissues/etc. due to isotopic
discrimination.

Andy Coggan

This isotope test are very difficult. The manufacturer of the testing
equiptment says "quite regularly there are errors."

Exactly my point from yesterday.
I don't see how isotopic tests of C12/C13 could hold any value with
WADA or UCI.

The amounts being tested are phenomonally miniscule.

You need hard evidence and so far there is NONE.

-Ken


http://online.wsj.com/public/article... main_tff_top

Someone should question the UCI use of the equiptment for IRMS. The
article concludes: "The apparent sensitivity of the testosterone test's
numbers to alcohol consumption, and the announcement of partial test
results without full disclosure by the cycling union, has created a
milieu for cyclists that is "almost Kafka-esque," Dr. Davis said. "The
phrase often bandied about is 'chemical McCarthyism'.""