## 3 Results and Discussion
### Investigating the Modulation of Skin Volatile Profiles after Fragrance Application
For instance, a number of fragrance-derived compounds with lower vapour pressures remained present in all participants' skin volatile profiles 2 h after application (e.g., methyl dihydrojasmonate, galaxolide, linalool acetate, and \(\gamma\)-decalactone shown in Table 2). Isopropyl tetradecanoate (solvent) and 2-ethylhexyl 4-methoxycinnamate (UVB absorber) were also observed to remain after 2 h, while several other compounds exhibited longevity on the majority of participants including 5-cyclohexadecen-1-one, \(\beta\)-myrrene, and _cis_-linalool oxide. Many other fragrance components displayed varied substantivity between participants, such as phenylethyl alcohol, isosafrole, and nerol acetate after 1 h (Table 2). Inter-participant differences in the persistence of fragrance components were apparent. For instance, isosafrole and \(\beta\)-damascenone exhibited greater tenacity on F1 compared to other participants. Similarly, cyrene persisted on F2, F3, and M1, nerol acetate on F1, \(\alpha\)-ionone on F1, M1, and M3, and dihydro-\(\beta\)-ionone on F1 after 2 h. Differences in compound tenacity can be attributed to variations in the diffusion of the fragrance from skin which is known to vary from individual to individual depending on skin properties, air movement, and the concentration of fragrance on the skin [32]. Local tissue hydration levels were investigated in participants as a function of tissue dielectric constants (TDC) and all values were within the normal range for healthy skin [33] (Table S1). Water present on the surface of the skin can influence the retention and rate of evaporation of hydrophilic components such as _cis_-linalool oxide and \(\beta\)-linalool, which remained after 2 h in the majority of participants. In the presence of moisture, the more hydrophobic moieties may be driven off the skin as seen for several terpene hydrocarbons including citronellene, \(\alpha\)-pinene, \(\beta\)-pinene, and cyrene, while moderately hydrophobic compounds may have some affinity for the lipid phase in the epidermis and would be expected to exhibit greater longevity as observed for linalool acetate. Inter-participant variance was investigated for all endogenous and fragrance-derived volatiles and is shown in Figure 2. The highest concentrations of VOCs detected in participants' skin HS were present immediately after fragrance application at 0 h, where \(\beta\)-linalool, linalool acetate, limonene, and methyl dihydrojasmonate were among the most abundant across the participant group (Figure 2). These and several other prevalent volatiles, namely \(\alpha\)-citral, nerol acetate, and \(\alpha\)-ionone, exhibited the greatest variance between participants at 0 h. When all participants were clustered together, there was a significant reduction (\(p\leq 0.01\)) in the majority of endogenous VOCs after fragrance application (0 h) including octanal, nonanal, decanal, tetradecanal, 2,6-dimethyl-2,6-octadiene, 6-methyl-5-hepten-2-one, nonanoic acid, 9-hexadecenoic acid, and isopropyl palmitate. Tetradecanoic acid, pentadecanoic acid, squalane, and \(n\)-decanoic acid also underwent a significant reduction (\(p\leq 0.05\)) in participants' skin HS immediately after fragrance addition to skin. As fragrance evaporation proceeded the modulation of endogenous volatiles became less apparent, with a significant increase (\(p\leq 0.01\)) observed for 6-methyl-5-hepten-2-one, nonanal, decanal, nonanoic acid, and \(n\)-decanoic acid after 1 h. Conversely, a significant reduction (\(p\leq 0.01\)) occurred in numerous fragrance-derived compounds after 1 h, including citronellene, \(\beta\)-myrrene, 3-hexen-1-ol acetate, cyrene, limonene, \(\alpha\)-pinene, \(\gamma\)-terpinene, _cis_-linalool oxide, \(\beta\)-linalool, phenylethyl alcohol, _trans_-2-pinanol, linalool acetate, \(\alpha\)-citral, nerol acetate, \(\beta\)-damascenone, longifolene, \(\alpha\)-ionone, dihydro-\(\beta\)-ionone, and \(\alpha\)-calacorene.