Analysis of hair styling wax residues on human hair: A forensic investigation with ATR-FTIR spectroscopy and chemometric interpretation

A comparative analysis of hair evidence recovered from the crime scene and the hair of suspects or victims is common in the forensic scenario. However, its evidentiary value increases with cosmetic applications like gel, wax, spray, or other hair treatments. Therefore, after extracting these products from hair strands, they can link the victim and accused to the crime scene and with each other, serving as corroborative evidence. Hair styling wax is one of the daily-wear hair cosmetic products used to manage hairstyles to provide stiffness to the hair strands and offer a high-setting performance. In this work, 19 different brands of hair styling waxes were analyzed using ATR-FTIR spectroscopy and chemometrics (principal component analysis and linear discriminant analysis). The results showed 96.49 % PCA-LDA training and 94.73 % validation accuracies. A blind test was also performed to test the built model's reliability, which showed 100 % accuracy. Furthermore, a wearing effect study was conducted using human hair samples with hair styling wax where the wax residues were extracted with acetone which showed 100 % matching when visually comparing the spectra of pristine wax samples and wax residues from hair samples. PCA-LDA prediction of the spectra obtained from wax residues from hair samples was also done which showed 87.5 % accuracy. This research presents a rapid and non-destructive method of analyzing hair styling wax products, which may act as corroborative evidence to help criminal investigating agencies.