In Vivo and In~Vitro Evaluation of PTeCA (1H-Pyrrole-2,3,4,5-Tetracarboxylic Acid) in Hair Matrix as a Marker for Oxidative Cosmetic Treatment

Alteration of the hair matrix by cosmetic products presents a challenge for forensic hair analysis. Oxidative treatments lead to analyte depletion and false-negative results. Currently, the degradation product of eumelanin, 1H-pyrrole-2,3,5-tricarboxylic acid (PTCA) is being investigated as a marker for oxidative hair treatment; however, it requires the definition of the cut-off value. Recently, it has been shown that 1H-pyrrole-2,3,4,5-tetracarboxylic acid (PTeCA) also increased significantly after in vitro oxidative hair treatments. Here, our previously published LC-MS/MS method for hair PTCA has been fully validated for the simultaneous quantification of PTeCA (range 0.01–2.5 ng/mg; lower limit of quantification [LLOQ] 0.003 ng/mg). The method was applied to 3378 self-reported treated (T) and untreated (UT) hair samples (3–6 cm proximal). In addition, the in vitro formation of PTeCA was assessed in 225 UT hair samples by different professional cosmetic treatments with and without oxidative agents. In the UT group (N = 1144), PTCA was determined in about 40% of the samples with a median PTCA of 0.04 ng/mg (range 0.01–14.9 ng/mg) and PTeCA was >LLOQ in < 2% of the samples (N = 53). In the T group (N = 425), PTCA was determined in 84% of the samples with a median of 0.75 ng/mg (range 0.01–58.1 ng/mg); while the median PTeCA was 0.40 ng/mg (N = 243; range 0.02–31.2 ng/mg). Moreover, the in vitro cosmetic treatment confirmed the PTeCA formation only in oxidative conditions. Finally, a PTCA cut-off value was proposed using PTeCA as the gold standard. Our data suggest that PTeCA could be a reliable marker for detecting oxidative cosmetic treatments in the hair matrix.