Acute and chronic health effects have been associated with diesel particulate matter (DPM). Since both ultrafine particles and metals have been implicated in this correlation, we are conducting investigations to characterize the metal content of diesel nanoparticles. For this study, DPM was generated by a 1.51 engine and ferrocene was added to the fuel to raise the level of metal in the system. The exhaust particles were analyzed in real time using a recently developed single-particle mass spectrometer (SPMS) that has the capability of ablating each particle down to its elemental constituents, thereby yielding the relative mass of elements in each particle. Particle-size calibration of the instrument was achieved by correlating the SPMS signal intensity with measured DPM size. Using this approach, we present size- and composition-re solved elemental species distributions for both the nuclei mode and ultrafine portion of the accumulation mode of DPM. Results show that when the fuel is doped with ferrocene, iron-rich nanoparticles are formed and their number and size increase with level of doping. Larger iron-bearing particles are also formed, but it is observed that the metal to carbon ratios increase for smaller particle sizes. Hydrogen to carbon ratios were measured as a function of particle size, which allowed us to determine the relative amounts of organic carbon and elemental carbon in the particles and showed that the hydrogen to carbon ratios increase for smaller sized particles. The combined results are used to discuss the effects of metal doping level and engine load on particle nucleation and mechanisms of DPM formation. (C) 2005 Elsevier Ltd. All rights reserved.