# Determination and Profiling of Human Skin Odors Using Hair Samples
## 1 Introduction  
Human odors have been investigated for a range of purposes, such as for human pheromone identification or the investigation of volatile biomarkers in breath, which can be used as a non-invasive and low-cost diagnostic tool for many diseases [1; 2]. Many studies have focused on human skin odors and their relation to the attraction of insects harboring human pathogens, aiming to understand the relationship between vectors and hosts and aid in the development of alternative methods for vector-borne disease control [1; 3; 4; 5]. There is no gold standard method for human skin odor determination; many techniques can be applied to collect, extract, transfer, anddetect human skin odors [6]. However, solid-phase microextraction (SPME) in headspace (HS) mode coupled to gas chromatography (GC) and mass spectrometry (MS), hereafter referred to as HS-SPME/GC-MS, has been successfully used for the confinement, extraction, separation, anddetection of volatile organic compounds (VOCs) that constitute such odors [7; 8; 9]. The first step that must be performed in HS-SPME/GC-MS analysis is sample collection, which provides the matrix containing the VOCs to be extracted by HS-SPME. This matrix can be any one of a diverse set of materials, such as gauze, cotton, or glass beads, that can come into contact with the skin surface for a while and become impregnated by any VOCs responsible for skin odor. These can then be extracted by thermal volatilization or solvent extraction, followed by fiber exposure in an enclosed SPME system [1]. Other sampling methods can be used, such as the use of solvents directly on the skin or even scraping to obtain skin tissue samples and, as with other odor sampling methods, the collection of fresh sweat [9; 10]. A less invasive method than the last method is the timed exposure of an SPME fiber directly onto the skin surface for VOC collection [1; 11]. Although each one of the aforementioned skin odor sampling methods was applied successfully in previous studies, all of them have some significant limitations. For example, the matrices can be biologically sterile (except the skin itself) without being chemically selective, allowing for the detection of other compounds as well as those that are truly responsible for skin odor [11]. The direct use of fibers has the advantage of not requiring any steps prior to GC-MS analysis (the HS-SPME step can be skipped), as the fibers that are usually used in the HS-SPME step can be directly inserted into this system after VOC sampling [12]. Nevertheless, such a sampling method probably limits the number of sampled subjects at the same time, as it would require a fiber for each individual, which would both raise the sampling costs per individual and demand a long period of time to sample all subjects. In the event that there are a limited number of fibers, itwould be necessary to clean the fiber between odor collections, requiring a laboratory setting with a GC instrument, which would be impractical to have in many field areas, including rural zones. The use of skin tissue can be considered an invasive method because there is some risk of accidental cuts. The use of solvents can also be invasive, as it can evoke an allergic reaction in the individual. Furthermore, all of these sampling methods often require a long time to be applied, as they involve several steps and can demand volunteers to spend a long period of time collecting samples of their own skin odor (e.g., with Petri dishes, glass beads, gauze, or cotton, which frequently require solvent extraction as the next step), or expose areas from their body for the application of organic solvent directly onto skin (which can cause allergic reactions or skin irritation) and, income cases, the volunteers may need to perform exercises in order to allow for sweat sampling [1; 9; 13; 14; 15]. These limitations are certainly inconvenient for studies with large populations, especially for field-scale studies.