For THC drug testing, the carboxy metabolite is historically the analyte used to determine cannabis usage. This compound has a long half-life and can be detected in urine or blood for several weeks in heavy consumers. This can pose a challenge when determining if a user is intoxicated at the time of testing. Today, labs are interested in the addition of the hydroxy metabolite, the intermediate between THC and the carboxy metabolite. The intermediate is short lived, but it is useful in the determination of chronic usage and when determining if a user is under the influence. Delta-8-THC is a common isomer of delta-9-THC that also demonstrates psychoactive effects and is of analytical interest to drug testing laboratories. The chromatographic separation of delta-9-THC and delta-8-THC and their respective metabolites are required due to their shared masses.
Several column chemistries were scouted, and a method was developed to separate delta-9-THC and delta-8-THC as well as their carboxy and hydroxy metabolites. Biphenyl, ARC-18, and FluoroPhenyl stationary phases were tested on a 100 x 2.1 mm column dimension using water and methanol as mobile phases, both modified with 0.1% formic acid.
The Biphenyl stationary phase did not display selectivity for the three pairs of isomers under the scouting conditions tested. The ARC-18 phase showed selectivity for both the delta-9/8-THC isomers and their carboxy metabolites, but not the hydroxy metabolites. When the strength of the solvent is reduced to attempt to resolve the hydroxy metabolites, the THC isomers become excessively retained, and the hydroxy metabolites are still not resolved. The FluoroPhenyl column showed great selectivity for the target analytes and resolved all three pairs of isomers. A method to fully resolve them was developed on a 100 x 3 mm column with great resolution and retention along with a 12-minute total cycle time.
