“…[95] Recently it has been suggested that another possibility to identify external contamination is the detection of a specific marker, i.e. [96] To date the only techniques that are able to reach the cut-off of 0.2 pg mg -1 recommended by SoHT for THC-COOH determination are GC-MS/MS, using electron impact (EI) ionization mode [97,98] or negative ion chemical ionization (GC-NCI-MS/MS), that allows a further increase of the sensitivity [90][91][92] and GC/GC-MS. [99] LC-MS/MS has been used in phytocannabinoids hair analysis just in the last few years, [100][101][102] THC-COOH was included only in one study, [103] but the LOQ obtained is significantly higher than the cut-off values and the method can be applied only for chronic use studies. [96] To date the only techniques that are able to reach the cut-off of 0.2 pg mg -1 recommended by SoHT for THC-COOH determination are GC-MS/MS, using electron impact (EI) ionization mode [97,98] or negative ion chemical ionization (GC-NCI-MS/MS), that allows a further increase of the sensitivity [90][91][92] and GC/GC-MS. [99] LC-MS/MS has been used in phytocannabinoids hair analysis just in the last few years, [100][101][102] THC-COOH was included only in one study, [103] but the LOQ obtained is significantly higher than the cut-off values and the method can be applied only for chronic use studies.…”