Abstract In forensic toxicology, high-performance liquid chromatography tandem mass spectrometry (LC–MS-MS) is increasingly used for the fast and sensitive measurement of a wide range of drugs. For our routine casework, a LC atmospheric pressure chemical ionization MS-MS method for the quantification of Δ9-tetrahydrocannabinol (Δ9-THC), cannabinol (CBN) and cannabidiol (CBD) in hair was established and fully validated. Separation was achieved using a Kinetex® C18 column (100 mm × 2.1 mm, 100 Å, 1.7 μm, Phenomenex) at a flow rate of 0.5 mL/min. Measurements were performed on a QTRAP 5500 mass spectrometer (Sciex, Darmstadt, Germany). Unexpected signals were observed in authentic THC-positive hair samples. First, a signal with a slightly shifted retention time of THC whose origin could be assigned to the isomer Δ8-THC was detected. Second, additional peaks exhibiting the same fragments as CBN and Δ9-THC but eluting at different retention times were detected. Spiking experiments and enhanced product ion scans pointed to the origin of these additional signals as result of in-source decarboxylation of Δ9-tetrahydrocannabinolic acid A (Δ9-THCA-A) into Δ9-THC and further partial oxidation of Δ9-THC into CBN, respectively. Positive findings of Δ9-THCA-A in hair have been shown to derive from external contamination; therefore, the herein described artifacts may be used as indirect markers for external contamination.