###### Abstract  
Volatile organic compounds (VOCs) emanating from human skin contribute to an individual's body odour. Understanding the modulation of human odour by a fragrance is of significant importance to the cosmetic sector in the design, development and evaluation of new products. The present research describes an in vivo approach for passive headspace sampling of skin volatile emissions in human participants. A wearable headspace solid-phase microextraction (HS-SPME) method has been employed to investigate baseline endogenous skin volatiles and the subsequent modulation of skin volatile profiles after application of a fragrance to skin. Coupled with gas chromatography-mass spectrometry (GC-MS) this method enables characterisation of scent profiles and fragrance longevity in vivo. A total of 51 compounds were identified in participants' skin, including 19 endogenous and 32 fragrance-derived compounds. The temporal variation in volatile profiles at different times after fragrance application was investigated. Fragrance diffusion from skin varied between participants resulting in diversified scent profiles over time. This non-invasive approach could be employed during cosmetic product development for in vivo evaluation of fragrance profiles and for assessment of the retention of fragrance components in skin to reduce reliance on expert panels during product development. v 2018 5 6  
diversity of microbial flora at the skin surface varies across the body, and this is partially reflected in the regional diversity of skin volatiles. For instance, volatile profiles from feet are often dominated by short chain fatty acids, while axillary volatiles primarily contain C6-11 carboxylic acids and alkanes, and hand and forearm volatiles tend to exhibit a greater distribution of aldehydes and ketones [8; 11; 12; 13; 14]. Furthermore, the chemical profile of an individual's skin VOCs can vary with pathophysiological status [15], and it is thought that some VOCs may derive from the potential passage of compounds from blood vessels [16]. Diet can affect gland secretions and consequently VOC profiles [17; 18], and age-related metabolic activity has also been linked to changes in odour profiles [8]. Environmental factors and personal habits also have a significant influence on an individual's VOC profile, including level of physical activity, personal hygiene, and cosmetic or fragrance usage [6]. The use of fragrances to modulate body odour dates back to ancient times and has experienced remarkable growth throughout the centuries. In 2016, the global fragrance market was estimated to be worth US840 billion [19]. Understanding the modulation of skin volatile profiles by a fragrance is of significance to the cosmetic sector in the design, development, and evaluation of new products [2; 20]. Efficacy testing is most often reliant on subjective sensory evaluation employing panel testers or on the self-evaluation of volunteer users. The use of panels is time-consuming and costly. Analytical characterisation techniques such as gas chromatography (GC) and gas chromatography-olfactometry (GC-O) can provide a more objective measurement of odour which is important during product development and efficacy testing, but these techniques are typically applied to in-vial analyses. Cosmetic testing is most informative when carried out in human participants, and there is a growing interest in the development of non-invasive analytical tools in this area [3; 21; 22]. Our group have recently reported a non-invasive approach for in vivo profiling of skin VOCs that employs HS-SPME in a wearable format with subsequent gas chromatography-mass spectrometry (GC-MS) analysis [23]. The present research employs this approach for identification of the major skin VOCs present in healthy human participants and investigation of the modulating effect fragrance application has on skin volatile profiles.