NASA - National Aeronautics and Space Administration

A metastable atom bombardment (MAB) ionization source has been coupled to an existing thermal desorption aerosol mass spectrometer. The design allows real-time alternation between MAB and electron ionization (EI). A jet of metastable species produced in a DC discharge is directed at the ionization volume of the mass spectrometer, where Penning ionization is thought to be the dominant mechanism. Performance is characterized in experiments with oleic acid particles. By changing discharge gases between N2 , Kr, and Ar, the excited state energy of the metastable species can be adjusted in the range 8.5–11.7 eV. For vaporization at 180 ◦ C, all gases yield significantly less fragmentation than EI, which could improve results of factor analysis. Fragmentation increases with vaporization temperature, but generated fragments have higher average mass than those produced by EI. Analyte signal levels are 0.1% and 0.006% of equivalent analysis with EI when using Ar* and Kr* , respectively. These sensitivities are not practical for ambient studies, but are sufficient for source measurements, as demonstrated with direct measurements of biomass burning emissions. The measured Ar* flux of 3.6 × 1013 sr−1 s−1 is ∼30 times lower than the best literature values for similar metastable beam sources, suggesting that sensitivity can be increased by source design improvements.