Experimental and ab initio investigation of the products of reaction from Δ9-tetrahydrocannabinol (Δ9-THC) and the fast blue BB spot reagent in presumptive drug tests for cannabinoids

•First report on the colorimetric reaction product between Fast Blue BB and THC.•Confirmation of product by HNMR and exact mass spectrometry.•Support for coupling of FBBB to the para position of Δ9-THC.•Sub-microgram detection limits for the presence of THC are reported.•Differentiation between CBD and THC are reported.