A method of isomerizing Δ9-tetrahydrocannabinol (“Δ9-THC”) to Δ10-tetrahydrocannabinol (“Δ10-THC”). The method includes the steps of: extracting Δ9-THC from cannabis biomass, which optionally contains one or more of the components found in fire retardant such as PHOS-CHEK®; dewaxing of crude extracts by winterization; pH-adjusting extracts by washing the extracts in heptane solution with aqueous solutions of: citric acid, sodium bicarbonate, and brine; isomerizing Δ9-THC to Δ10-THC by exposure to suitable conditions and in the presence of a catalyst based on the components of fire retardant; vacuum distillation of Δ10-THC at a predetermined temperature range and vacuum level; collecting the distillate and redistilling it up to three times to acquire distillate containing less than 60% Δ10-THC; and purification of the MO-THC to a purity of 99% or greater by crystallization from n-pentane solution.