High-resolution visualization of cosmetic active compounds in hair using nanoscale secondary ion mass spectrometry

A wide range of small molecules are used in our daily hair products to improve the appearance of hair and to protect it from damage from the environment. In order to better design formulations of these products, an understanding of the partitioning and distributions of these small molecules in hair is critical. In this study, we used preferential extraction methods to measure the partitioning of active compounds commonly found in hair cosmetic products on the hair surface and inside hair, and investigated the use of stable isotope labelling, cryo-sample preparation and nanoscale secondary ion mass spectrometry (NanoSIMS) for high-resolution visualization of distributions of these compounds. With this approach, we quantified partitioning and directly visualized distributions at high-resolution of four molecules (e.g. resorcinol, salicylic acid, pentadecyl alcohol and pentadecanoic acid) in hair. This has not been achieved before and revealed distributions of high lipophilicity active compounds in the lipid-rich and hydrophobic cell membrane complex network in hair, while low lipophilicity ones distributed dispersedly.