On-line detection of human skin vapors

Abstract

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.

Fast Detection of Volatile Organic Compounds from Bacterial Cultures by Secondary Electrospray Ionization-Mass Spectrometry

“…Alternatively, it has been shown that electrospray plumes can efficiently ionize not only analytes from the condensed phase (7,8,10,28,29) but also gaseous compounds (9,18,32), with the latter approach termed "secondary electrospray ionizationmass spectrometry" (SESI-MS) (22,33). These previous studies demonstrate the possibility of real-time detection and identification of VOCs by MS, which can be applied to the characterization of biological samples (19)(20)(21) and could achieve limits of detection as low as parts per trillion (22).…”

Collision-Based Ionization: Bridging the Gap between Chemical Ionization and Aerosol Particle Diffusion Charging

“…TMA was specifically added to ESI solutions to mitigate the potential effects of vapor absorption by ESI-generated droplets during the evaporation process. Although the outlet of the copper tube was directed away from the ESI source and when the droplets are micrometers in size, the Stefan flow created by evaporation drives gaseous species away from the droplets (Huckaby and Ray 1989;Martínez-Lozano and de la Mora 2009), it is difficult to guarantee that no vapor uptake occurred. Substantial vapor uptake could influence ion evaporation kinetics, which, in turn, could alter the size and charge distributions of the generated cluster ions (GameroCastano and de la Mora 2000; Gamero-Castaño and Fernández de la Mora 2000; Hogan et al 2008;Hogan and de la Mora 2009).…”

Analysis of pattern recognition and dimensionality reduction techniques for odor biometrics

“…The new system, which is schematically depicted in Fig. 1, resembles that described in [37], but it uses clean gas to reduce background levels, and a corona instead of the electrospray that allows the temperature of the system to be increased so as to prevent vapor deposition and the subsequent memory effects. The MS (a Qstar XL from Sciex) provides a full spectrum (number of ions detected as a function of their mass to charge ratio) every 0.1 s. The combination of high speed and high sensitivity provided by the new system allowed the analysis of the samples to perform in real time (on-line).…”

“…Therefore, our attributes will be the different mass intervals corresponding to each peak which provides information about a compound of the body odor. Initially, only those mass intervals corresponding to fatty acids were identified based on our previous work [37]; however, these intervals showed to be insufficient to classify people accurately. Peak detection was refined by performing a visual inspection of all the spectrograms, giving as a result 752 common intervals for all the subjects.…”