## 3 Results and Discussion  
### Endogenous Skin Volatile Profiles  
Analysis of VOCs following thermal desorption from SPME fibres revealed a variety of classes of compounds emanating from the skin. There were 24 compounds identified in endogenous skin volatile profiles across all participants as shown in Table 1, where confirmation of compound identities was performed using retention index matching with a tolerance of \(\pm\)10 RI units. Compounds deemed to be contaminants (e.g., siloxanes from SPME fibres, column bleed) were excluded from the table. The predominant species were acids and aldehydes (Figure S1) including nonanal, decanal, hexadecanoic acid, and tetradecanoic acid. Many other major skin VOCs [11] were also present including octanal, undecanal, 6-methyl-5-hepten-2-one, and geranyl acetone. Squalane and 2,6-dimethyl-2,6-octadiene were the principal hydrocarbon species identified, and one alcohol (1-dodecanol) and ester (isopropyl palmitate) were also present. There were 14 VOCs common to the volatile profile of all participants comprising 6 aldehydes, 5 acids, 2 ketones, and 1 ester.  
The primary constituents (acids and aldehydes) are derived from the oxidative degradation of fatty acids on the skin. Sebaceous secretions can also impact many VOCs emanating from skin, including hydrocarbons (e.g., squalane) and ketones (e.g., 6-methyl-5-hepten-2-one and geranyl acetone that derive from the oxidative degradation of squalene) [28; 29; 30]. Esters such as isopropyl palmitate in skin VOCs are linked to cosmetic usage, and their persistence in the skin can vary [9]. Variations in individual volatile profiles were expected due to the influence of underlying biochemical processes, skin gland secretions, and skin microbiota on VOC emissions, so variation across intra- and inter-participant volatile samples was investigated to understand potential of the method to detect differences within the population. The repeatability of the majority of compounds was very good with intra-participant samples, suggesting that these VOCs could be useful for differentiating participants from one another. However, a small number of compounds showed marked variance in intra-participant samples making them less valuable for such categorisation of participants. Of the compounds that demonstrated good intra-participant repeatability, the inter-participant variance was substantial as shown in Figures S2-S9 for hexanal (Figure S2), octanal (Figure S3), nonanal (Figure S4), \(n\)-decanoic acid (Figure S5), tetradecanoic acid (Figure S6), \(n\)-hexadecanoic acid (Figure S7), isopropyl palmitate (Figure S8), and squalane (Figure S9). Several compounds showing marked variance in intra-participant samples (6-methyl-5-hepten-2-one; Figure S10 and geranyl acetone; Figure S11) derive from the oxidative degradation of squalene. Squalene is the most abundant oxidisable component of skin surface lipids, with concentrations in adult skin reaching up to 20% [31]. The observed variation for both ketones may be due to variations in the fast oxidative degradation of this abundant skin surface lipid, which can occur upon the action of both environmental oxidants and skin microbial residents to give rise to a wide spectrum of by-products [31]. In contrast, squalane is not subject to the same auto-oxidation that squalene is due to its complete saturation, which may account for the better intra-participant repeatability as seen in Figure S9.