An instrument has been developed for real-time, quantitative chemical analys is of organic particles in laboratory environments. In this apparatus, which we call a Thermal Desorption Particle Beam Mass Spectrometer (TDPBMS), particles are sampled into a differentially-pumped vacuum chamber, focused into a narrow, low-divergence particle beam using aerodynamic lenses, and then transported into a high-vacuum region where they impact on a heated surface, evaporate, and the vapor is mass analyzed in a quadrupole mass spectrometer. The average composition of a continuous stream of particles is thus measured in real time, and size-dependent composition can be obtained by passing the incoming aerosol through a differential mobility analyzer. The TDPBMS can analyze multi component organic particles in the 0.02-0.5mu m size range for compound concentrations 0.1-1mu g m3 without particle matrix effects. By using careful calibration techniques that account for particle shape and transport efficiency, the particulate organic components can be quantified with an estimated uncertainty of 20%. The utility of TDPBMS for laboratory studies of aerosol chemistry is demonstrated by monitoring the tridecanoic acid concentration in secondary organic aerosol formed during a smog chamber reaction of 1-tetradecene and ozone.