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.