in the november scientific american, there is a good article about the isitope test that is used to show if an athlete is using synthetic compounds.

it explains that all the food we eat (since it was all recently other living organisms) has a pretty similar and narow ratio of carbon 12 to carbon 13. once your body digests that food and starts turning it into bilogical compounds (fats, cells, hormones, whatever) they will ALL have the same ratio.

syntheticly produced hormones have different ratios (less carbon 13 IIRC) than organically produced hormones because the chemistry and devices used to make them "sort out" the carbon 13 in ways that your body doesnt.

so in the test they look at the carbon 12/13 ratio of several compounds in your urine. if one (in floyd's case testosterone) has a different ratio than all the others (like cholesteral and such that no one would dope for), its a smoking gun that that compound was synthetically derived. it seems pretty air tight.

the expert they interveiwed did give one possible out for landis. he said that his only chance would be to argue that the synthetic cortisone he injected was some how metabilized and turned into a disproportionate amout of testosterone compared to the other compounds look at in the test, which could make floyd's natural testosterone look synthetic. but he didnt seem to put much faith in that possiblity.

ill try to find a link, but it was an intersting read, give it a look if you see it on the news stand.