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In order to investigate the incorporation behavior of drugs into hair in early stage (within 24 h) after intake, time-course changes in drug distribution in black hair were carefully analyzed after a single oral administration of methoxyphenamine (MOP), a non-regulated analog of methamphetamine. Single-hair specimens collected by plucking with the roots intact at appropriate intervals post-intake were each divided into 1-mm segments from the proximal end, and MOP in each segment was determined by a validated liquid chromatography-tandem mass spectrometry procedure. At 10 min after intake, MOP was not detected in any of the segments. MOP became detectable 30 min after intake in the hair bulb (0–1-mm segment from the proximal end) and 1 h after intake in the upper dermis zone (1–2-mm to 4–5-mm segments). The amount of MOP in the hair bulb increased rapidly over 3 h after intake and reached a maximum concentration of about 100–900 pg/1-mm single hair (11–95 ng/mg) around 3–10 h after intake, whereas that in the upper dermis zone increased at a more gradual pace over 24 h and reached a plateau at about 30–100 pg/1-mm hair (3–11 ng/mg). These differences can be attributed to their different incorporation mechanisms. Results from this study can further elucidate the drug incorporation mechanism, which is crucial for accurately interpreting results in hair analyses. Our findings also suggest that hair drugs analysis with special attention to the hair root can serve as a useful complementary approach to urine- and blood-based testing in the field of forensic toxicology.

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Various cathinone‐derived designer drugs (CATs) have recently appeared on the drug market. This study examined the mechanism for the generation of dehydrated ions for CATs during electrospray ionization collision‐induced dissociation (ESI‐CID). The generation mechanism of dehydrated ions is dependent on the amine classification in the cathinone skeleton, which is used in the identification of CATs. The two hydrogen atoms eliminated during the dehydration of cathinone (primary amine) and methcathinone (secondary amine) were determined, and the reaction mechanism was elucidated through the deuterium labeling experiments. The hydrogen atom bonded to the amine nitrogen was eliminated with the proton added during ESI, in both of the tested compounds. This provided evidence that CATs with tertiary amine structures (such as dimethylcathinone and α‐pyrrolidinophenones [α‐PPs]) do not undergo dehydration. However, it was shown that the two major tertiary amine metabolites (1‐OH and 2″‐oxo) of CATs generate dehydrated ions in ESI‐CID. The dehydration mechanisms of the metabolites of α‐pyrrolidinobutiophenone (α‐PBP) belongs to α‐PPs were also investigated. Stable‐isotope labeling showed the dehydration of the 1‐OH metabolite following a simple mechanism where the hydroxy group was eliminated together with the proton added during ESI. In contrast, the dehydration mechanism of the 2″‐oxo metabolite involved hydrogen atoms in three or more locations along with the carbonyl group oxygen, indicating that dehydration occurred via multiple mechanisms likely including the rearrangement reaction of hydrogen atoms. These findings presented herein indicate that the dehydrated ions in ESI‐CID can be used for the structural identification of CATs.