Evaluation of PTeCA (1-H-pyrrole-2,3,4,5-tetracarboxylic acid) in in vivo and in vitro hair matrix as a marker of oxidative cosmetic treatment

Background Alteration of the hair matrix by cosmetic products presents a challenge for forensic hair analysis. In particular, oxidative treatments can lead to analyte depletion and, thus, false-negative results. Currently, the degradation products of eumelanin, 1H-pyrrole-2,3,5-tricarboxylic acid (PTCA) and its isomer isoPTCA, are being investigated as markers for oxidative hair treatment; however, they require the definition of threshold values. Recently, the 1H-pyrrole-2,3,4,5-tetracarboxylic acid (PTeCA) concentration was significantly increased also after in vitro oxidative hair treatments with low percentages of H2O2 (1.9%)1. Methods Our previously published high-performace liquid chromatography tandem mass spectrometry (LC-MS/MS) method for PTCA analysis2 (LOQ 0.01 ng/mg) in hair matrix has also been fully validated for the simultaneous quantification of PTeCA (linearity range 0.01-2.5 ng/mg; LOQ 0.003 ng/mg) in real hair samples. Then, the method was applied to N=3378 self-reported treated (T; N= 503) and untreated (UT; N=2875) hair samples (<6 cm proximal) collected from the Laboratory of Forensic Toxicology of the University of Milan. In addition, the in vitro formation of PTeCA was assessed in N=240 untreated hair samples by N=9 different professional cosmetic treatments with and without oxidative agents (H2O2). Results In the UT group, N=1144 (40%) samples showed a median PTCA concentration of 0.04 ng/mg (range 0.01-14.9 ng/mg), among which N=53 (<2%) samples presented PTeCA concentration >LOQ (median 0.30 ng/mg; range 0.01-4.8 ng/mg). In the T group, only N=247 released color (49%) into the extracting solution. PTCA was determined in N=425 (84%) samples with a median concentration of 0.75 ng/mg (range 0.01-58.1 ng/mg); while PTeCA median concentration was 0.40 ng/mg (N=243; range 0.02-31.2 ng/mg), according to median PTeCA concentration measured in the N=53 samples with PTeCA>LOQ belonging to the UT group. Finally, the in vitro cosmetic treatment confirmed the PTeCA formation only in oxidative conditions. Conclusion Our data suggest that PTeCA could be a reliable marker to detect oxidative cosmetic treatments in the hair matrix