Experiment Samples were collected onto square aluminum sheets (3cm x 3cm) gently wiped across a human forehead for 2 minutes. This was repeated for a total of 3 sheets. The lipid compounds were then extracted with sonication from the aluminum sheets using a 3mL of a 50/50 volume solution of methanol/dichloromethane. Next, the extraction solution was centrifuged and the supernatant was concentrated to 200 μL. The resulting solution was analyzed using GCxGC–HRTOFMS and the conditions shown in table 1.