A rapid and accurate liquid chromatographic method for hemp compliance testing

A liquid chromatography ultraviolet (LC-UV) method, which used methanol as the organic solvent for both extraction and separation, was developed for hemp compliance testing. While published methods occasionally used methanol for separation, all of them used gradient elution which would cause baseline drifts and a subsequent high limit of quantification (LOQ), and only eight or less cannabinoids were included. Our method was characterized by a baseline separation of Δ⁹-tetrahydrocannabinol (Δ⁹-THC) and Δ⁹-tetrahydrocannabinolic acid (Δ⁹-THCA) among nineteen cannabinoids in ten minutes and a calibration between 0.04 to 50 µg/mL, making it the most effective method validated so far. However, a reproducible separation of Δ⁹-THCA could not be achieved with the common formate buffer system, presumably due to its adsorption onto the C18 column which could be prevented by either using acetonitrile in the organic solvent that was widely undertaken by published methods or adding 0.1 % (v/v) trifluoroacetic acid in the aqueous solvent that was achieved only in this study. While similar problems were not reported by published methods, it was speculated that the problem could be common for all C18 columns, more or less, as the stationary phase in this study was di-isobutyl-n-octadecyl-silane without any polar imbedded moieties. Additionally, while quantification of Δ⁹-THC at 230 nm achieved good specificity, quantification of Δ⁹-THCA required 269 nm for acceptable specificity, which was verified by electrospray ionization time-of-flight mass spectrometry (ESI/TOFMS), indicating no false quantification and identification of Δ⁹-THC and Δ⁹-THCA, despite the observation of ten matrix peaks in the LC-UV chromatograms and identification of three untargeted/unknown cannabinoids by ESI/TOFMS in ten samples.