Due to the non-polar nature and the absence of an ionizable group on the cannabinoids, the ionization efficiency in electrospray is low and leads to poor limits of detection (LOD). The reaction of chloride dabsyl with the phenolic OH group of cannabinoids results in a product containing a tertiary amine, which is easily protonated in positive electrospray mode and can significantly improve the cannabinoids LOD. A rapid, selective and sensitive LC/MS-MS method was developed for quantitative determination of Δ(9)-tetrahydrocannabinol (THC), 11-hydroxy-Δ(9)-tetrahydrocannabinol (11-OH-THC), 11-nor-9-carboxy-Δ(9)-tetrahydrocannabinol (THC-COOH), cannabinol (CBN) and cannabidiol (CBD), in micro volume blood samples following dabsyl derivatization to enhance signal intensity. The method comprised protein precipitation followed by derivatization with dabsyl chloride and subsequent analysis using liquid chromatography-tandem mass spectrometry (LC/MS-MS). Chromatographic separation was achieved using a 150 mm × 2.1 mm C18 analytical column maintained at 65°C and eluted with a gradient of water and acetonitrile, both containing 0.2% formic acid. The run time was 8 min. The assay was successfully validated using the approach based on the accuracy profile. Lower limits of quantification (LOQ) were 0.25 ng/mL for THC and THC-COOH, 0.30 ng/mL for 11-OH-THC, 0.40 ng/mL for CBN and 0.80 ng/mL for CBD. A comparative study of cannabinoids in blood and plasma, as determined by the developed LC/MS-MS method or the in-house GC/MS-MS technique, demonstrated an excellent correlation between the two methods. Dabsylation was also tested on-line with a spiral of peek tubing placed in the LC/MS-MS column heater at 65°C before the analytical column. The results obtained with on-line dabsyl derivatization were similar to those observed off-line.
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