Mid-infrared in vivo depth-profiling of topical chemicals on skin

Thermal emission decay-Fourier transform infrared (TED-FTIR) spectroscopy is a non-contact and non-destructive analytical technique and was used in this study to detect the presence of external chemicals on human skin in vivo. The detection was possible due to the ability of the TED-FTIR technique to acquire the mid-infrared spectrum of the outmost layers (less than 10 microm) of Stratum Corneum (SC) and the ability to identify the absorption bands of the chemical.

As an illustration of such measurements, propylene glycol (PG) was applied on human stratum corneum and depth-resolved TED-FTIR spectra of the SC were measured to quantify the concentration of PG in deeper layers of SC.

The mid-infrared spectrum of the surface 0.7 microm layer of skin had 50% contribution from SC and 50% from PG. At 3 h after application, the contribution of PG at the surface decreased to 7% as PG molecules diffused deeper into the skin and were lost at the surface. At a depth of 6 microm, the maximum concentration was 20% after 25 min after PG application.

This work shows the feasibility of the TED-FTIR technique to detect the presence of chemicals on human SC in vivo and without contact, and for a wide range of other applications, such as detection of toxic chemicals used as warfare (vesicant agents like sulphur mustard and organophosphate nerve agents), pesticides, and other toxins on fruit and vegetable skins, water, or even other contaminated surfaces.