I have written before how in criminal drug prosecutions, the government analyst should be cross-examined and their case file and other information should be requested to prepare a defense case. A colleague sent me a great example of this effort paying off.
A chemist at the DEA Mid-Atlantic Laboratory wrote in a report that three samples suspected to be marijuana turned purple in a Duquenois-Levine color test. The problem is that the picture of the test (not included in the report but in the analyst’s case-file) showed only one sample that might conceivably indicate the sample was marijuana (and even that one is weak).
Is there any reason for the analyst to photograph the color test against a green background other than to obscure the real colors obtained? Compare the picture above against an example of what the D-L color test should look like (copied from Justin McShane‘s site for purposes of illustration and comment):
The truth is that color tests like these are not specific for marijuana, meaning that the test may show a color even if the sample is not marijuana – there’s a huge list of plants and chemicals that could cause a purple or purple-ish color. But if there is no color at all, the chemists tell us in their testimony that this would indicate that the sample is not marijuana. So by that token, the lack of color in the test would be evidence that the sample is not marijuana.
You can only get this evidence by requesting the information from the government and cross-examining their expert.