A novel aerosol ion trap mass spectrometer combining pulsed IR laser desorption with electron impact (EI) ionization for single particle studies is described. The strengths of this instrument include a two-step desorption and ionization process to minimize matrix effects; electron impact ionization, a universal and well-characterized ionization technique; vaporization and ionization inside the ion trap to improve sensitivity; and an ion trap mass spectrometer for MS n experiments. The instrument has been used for mass spectral identification of laboratory generated pure aerosols in the 600nm–1.1μm geometric diameter range of a variety of aromatic and aliphatic compounds, as well as for tandem mass spectrometry studies (up to MS 3 ) of single caffeine particles. We investigate the effect of various operational parameters on the mass spectrum and fragmentation patterns. The single particle detection limit of the instrument was found to be a 325nm geometric diameter particle (8.7×10 7 molecules or 22fg) for 2,4-dihydroxybenzoic acid. Lower single particle detection limits are predicted to be attainable by modifying the EI pulse. The use of laser desorption-electron impact (LD-EI) in an ion trap is a promising technique for determining the size and chemical composition of single aerosol particles in real time.