Vapors released by the skin in the hand of one human subject are detected in real time by sampling them directly from the ambient gas surrounding the hand, ionizing them by secondary electrospray ionization (SESI, via contact with the charged cloud from an electrospray source), and analyzing them in a mass spectrometer with an atmospheric pressure source (API-MS). This gas-phase approach is complementary to alternative on-line surface ionization methods such as DESI and DART. A dominating peak of lactic acid and a complete series of saturated and singly unsaturated fatty acids (C 12 to C 18 ) are observed, in accordance with previous off-line studies by gas chromatography-mass spectrometry. Several other metabolites have been identified, including ketomonocarboxylic and hydroxymonocarboxylic acids. -5]. Until the recent development of desorption electrospray ionization (DESI) [6] and its variant extractive electrospray ionization (EESI) [7], such studies were dominated by gas chromatography-mass spectrometry (GC-MS) [3, 4, 8 -10], which requires sample collection and preconcentration, and is time consuming. Selected ion flow tubemass spectrometry (SIFT-MS) [11] has provided realtime information on skin emanations for relatively volatile species such as acetone [12]. Proton transfer reaction-mass spectrometry (PTR-MS) claims parts-pertrillion (ppt) sensitivities [13] and has permitted on-line monitoring of UV-induced lipid peroxidation products from human skin and its dependence on the fatty acid composition of the skin [14]. The rich new possibilities for on-line analysis of surfaces brought about by DESI and EESI have been used to continuously monitor the release of caffeine from the skin of a person before and after drinking coffee [15], or of DESI ions from intact bacteria [16]. The present study will continue these promising ES-based approaches relying on an ionization mechanism referred to as secondary ESI (SESI) [17][18][19][20][21], where gas is sampled from the ambient and mixed with the charged cloud of an electrospray of clean solvent. Polar volatile species are then ionized, either directly by the charged electrosprayed drops or by the small ions released after drop evaporation. Therefore, SESI ionizes vapor species and suspended particles [18] directly in the gas phase [22], being complementary with other surface ionization methods such as DESI or direct analysis in real time (DART) [23], which actively desorb charged species from surfaces.We shall see here that hexadecanoic and octadecanoic acids are released directly from the skin as vapors, without the need for an active desorption.We have recently used real-time SESI to sense explosive vapors [24] and breath volatiles [22,25], with lower detection limits approaching 0.2 ppt [24]. In negative ionization of breath [25] and urine [26], we accidentally observed a rich group of skin vapors, primarily organic acids. This interesting finding motivates the present study on negative SESI of skin vapors.