I ran all sorts of samples from 10 mg to 2 g on 3 seperate plates, none of them showed anything at all on the plates. It seems that there is more to it than simple concentration of miniscule samples.
What I did was finely chop up and soak the samples of cactus skin in the very minimal amount of solvent necessary, between <1-6 ml depending on the sample size. All the samples soaked overnight. I made it as concentrated as I could get it and still be able to suck it up into the micro-pipette.
The only thing that did show up on the plates was the peruvian torch extract I was using as a mescaline standard and that appeared fine on all the plates. This consisted of some saturated hcl salting water from a 250g extraction.
It really does seem that the best bet for testing is to extract or at least use a fairly large sample size.
Edit: Is it possible that mescaline and other cactus alks are not soluble in IPA?