Characterization of unknown events observed by the third generation JPL electronic nose using sensor response models

The Third Generation Electronic Nose (ENose) from the Jet Propulsion Laboratory operated continuously and autonomously on the International Space Station (ISS) for more than 6 months during 2008-2009. During this time, the ENose monitored air quality in the US lab and recorded anomalous events. The data gathered and analyzed showed that in addition to detecting several analytes on the target list (ethanol, methanol, Freon 218 and formaldehyde), many events caused by chemical species outside the target list were observed and these were classified as "unknown." We report an investigation to identify the chemical nature of the analyte(s) producing "unknown" events. The identification is achieved by using sensor response models based on molecular principles developed using Quantitative Structure-Activity Relationships (QSAR) and First Principles Molecular Dynamics approaches. Molecular descriptors calculated from the models include dipole, hydrogen bond characteristics, molar refractivity, molar volume, and solubility parameters. Calculated molecular descriptors were then compared with descriptors of analytes in the Spacecraft Maximum Allowable Concentration (SMAC) database. The results indicate that a possible identification of the unknown response is sulfur hexafluoride (SF₆).