The effects of molecular weight and thermal decomposition on the sensitivity of a thermal desorption aerosol mass spectrometer Aerosol Science and Technology

In free molecular flow the slower speed of heavier molecules means that they spend more time in the ion source of a mass spectrometer. Hence the sensitivity of the thermal desorption mass spectrometers such as the Aerodyne Aerosol Mass Spectrometer (AMS) should include a term that scales as the square root of the molecular weight. Thermal decomposition on the vaporizer reduces the molecular weight prior to ionization and changes electron impact cross-sections. Thermal decomposition therefore has the potential to change the sensitivity, in some cases by more than a factor of three. Current AMS calibrations that rely upon an ammonium nitrate calibration and scaling for other components with a relative ionization efficiency may overestimate the concentration of large, thermally stable molecules and underestimate small or thermally unstable molecules. The overall sensitivity of the AMS to organics includes a partial cancellation of these effects. There is an incomplete understanding of the vaporization process, including that of ammonium nitrate. 2016 OAR (Oceanic and Atmospheric Research) ESRL (Earth System Research Laboratory) CSL (Chemical Sciences Laboratory) Submitted https://doi.org/10.1080/02786826.2015.1136403 Public Domain 1952