How to forensically identify specific styling products used on hair

Current methods for analyzing hair fibers found at crime scenes don’t have the resolution to determine submicroscale physical properties or localized chemical compositions. So scientists combined atomic force microscopy (AFM) with time-of-flight secondary ion mass spectrometry (ToF-SIMS) to study longitudinally sectioned hair fibers. They report their method that identifies trace cosmetic evidence in the Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures.

Complexities of hair reduce the detection of secondary ions in ToF-SIMS, inducing imaging artifacts. However, these artifacts are eliminated when ion detection is delayed. Combining ion-detection-delayed ToF-SIMS with AFM produced a highly detailed picture of the hair fiber surface. Authors of the report produced precise images of a hair fiber’s internal structure, constructed from detection of longitudinally cut cross-sections of cryogenically prepared hair samples.

The researchers correlated the physical structure of cosmetic residues on hair fibers to the local chemical composition determined by ToF-SIMS. They used this correlation data to demonstrate how their method could aide in forensic investigations of human hairs. They conducted a blind test of hair fibers treated with gel, wax or spray cosmetics and used principal component analysis to identify and segment the images from ToF-SIMS and AFM measurements. This enabled reconstructing the chemical residues and successfully discriminating between styling products.

“ToF-SIMS with this preparation and approach is easy to use. The hardest part is the interpretation of your data, but if you have standards to compare residues to it is very easy,” said co-author Tanguy Terlier. Terlier and collaborator Yeonhee Lee are now collecting data from different cosmetic products to construct a forensic database, with tools for matching between the different spectra.

Source: “Investigation of human hair using ToF-SIMS: From structural analysis to the identification of cosmetic residues,” by Tanguy Terlier, Jihye Lee, and Yeonhee Lee, Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures (2018). The article can be accessed at https://doi.org/10.1116/1.5015928 .