​Fentanyl (hydrochloride) (CRM) and all fentanyl analogs (provided as neat materials) used in the study are Cayman products. Single component solutions for each fentanyl analog were prepared by dissolving 1 mg of neat material in 1 ml of HPLC-grade methanol (EMD Millipore). Twenty multicomponent mixtures were prepared by transferring 200 μl aliquots of each single component solution with a 200 μl aliquot of internal standard into a vial and dried under nitrogen. The multicomponent mixtures were reconstituted with 200 μl of methanol for a final nominal concentration of 1.0 mg/ml per component. The multicomponent solutions were transferred to autosampler vials with insert prior to injection. The 20 multicomponent mixtures were used in the GC separation study.