All pros­e­cu­tions for the unlaw­ful pos­ses­sion of mar­i­juana requires as an essen­tial ele­ment of the crime for the gov­ern­ment to advance proof that the unknown sub­mit­ted for test­ing is in fact mar­i­juana (con­tains THC). Can the gov­ern­ment actu­ally do that based upon its typ­i­cal test­ing method?

In this series of posts we are going to exam­ine this seemly sim­ple question:

1. What is the goal and the pur­pose of test­ing of unknowns gen­er­ally? How do we best design a test for marijuana?
2. How is most mar­i­juana test­ing con­ducted in the United States?
3. What is micro­scopic mor­pho­log­i­cal exam­i­na­tion? Is it a “good” test?
4. What is the mod­i­fied Duquenois-Levine test? Is it a “good” test?
5. What is Thin Layer Chro­matog­ra­phy? Is it a “good” test?
6. Is the com­bi­na­tion of all three tests cre­ate a “good” test­ing scheme?
7. Is there a bet­ter way to test for marijuana?

Part 5: What is Thin Layer Chro­matog­ra­phy? Is it a “good” test?

Thin Layer Chro­matog­ra­phy test­ing

What is it?

Thin Layer Chro­matog­ra­phy (TLC) is a chro­mato­graphic tech­nique. It is a com­bi­na­tion of a col­ori­met­ric test and is mea­sured in terms of separation.

Mechan­i­cally how is it preformed?

TLC depends on the sta­tion­ary phase, often a glass plate coated with sil­ica (it must be prop­erly  des­ic­cated or dried) and the mobile phase which is com­prised of a sol­vent mix­ture made of toluene and diethy­lamine typ­i­cally. When placed on a prop­erly dried plate and using a prop­erly mixed and pre­pared sol­vent, the sol­vent (the mobile phase) will be wicked up by the sil­ica with cap­il­lary action and travel up the plate. A sam­ple from the unknown is selected. It is mashed up in some mechan­i­cal process and in some processes call for it to be dis­solved. It is placed in the des­ig­nated spot. A test is com­pared by per­form­ing the reac­tion of the plate of a known sam­ple from an adju­di­cated source. This com­par­i­son is made with the analyst’s eyes. There will be dif­fer­ent spots on the plate at dif­fer­ent posi­tions from the ori­gin with var­i­ous inten­si­ties of color. The height and the color change are visu­al­ized. The spots typ­i­cally need to be visu­al­ized with a chem­i­cal spray such as Fast Blue B Salt (50 mg in 20 ml of NaOH (0.1 N)) or par­tic­u­lar lighting.

How is the typ­i­cal crime lab­o­ra­tory ana­lyst trained to con­duct this?

Once again, the ana­lyst is not trained in the fun­da­men­tals of how or why this process works. If you handed them a pen and paper and ask them to dia­gram and explain with specifics the chro­mato­graphic process, they would likely be baf­fled. Also for­eign to them would be the specifics as to why the sol­vent used to elute up the plate has to be in a spe­cific ratio and not another. In essence, it is another sub­jec­tive test as it is based upon per­cep­tion of color by the ana­lyst and the per­cep­tion of this height devel­op­ing on the plate ver­sus the adju­di­cated known.

The ana­lysts are not aca­d­e­m­i­cally trained in the the­ory of this tech­nique of chro­matog­ra­phy, and are not taught about cross-reactivity and false pos­i­tives or other sources of errors.

Is this a ver­i­fi­able test?

Much like the mod­i­fied Duquenois-Levine test, TLC is poten­tially ver­i­fi­able. Dig­i­tal cam­eras exist. Pho­tographs can be taken. Heck, even video can be taken to show how it is con­ducted on a par­tic­u­lar sam­ple from the unknown and this com­par­i­son to the adju­di­cated known. How­ever, the mod­ern prac­tice is to not take a sin­gle pho­to­graph, to not take advan­tage of video tech­nol­ogy and pro­duce noth­ing ver­i­fi­able in court that the test was even con­ducted or that the analyst’s per­cep­tion of the change did hap­pen and was cor­rectly interpreted.

Again, there are no crime lab­o­ra­to­ries that I am aware of that use the ACE-V (Analy­sis, Com­par­i­son, Eval­u­a­tion, and Ver­i­fi­ca­tion) tech­nique that one would find in fin­ger­print iden­ti­fi­ca­tion with a dou­ble check in real time by a fel­low bench ana­lyst. In essence, it is checked one time, by one per­son with no dou­ble check by another, and noth­ing pro­duced that proves that the analy­sis was done or that the fea­tures that are reported as present were in fact objec­tively present.

Is there empir­i­cal valid­ity stud­ies that prove that this is a spe­cific and con­fir­ma­tory test?

Once again, this type of test­ing has not been proven to be a val­i­dated method to test specif­i­cally for THC. There are well-known and dis­cov­ered false pos­i­tives which includes cof­fee, basil and even tobacco products.

Once again, the same issues that are dis­cussed prove true with this test­ing. This TLC test is pre­formed on a totally dif­fer­ent sam­ple from the unknown that is not sub­jected to the micro­scopic mor­pho­log­i­cal exam­i­na­tion or the mod­i­fied Duquenois-Levine col­ori­met­ric test.