A newly developed apparatus for simultaneous in situ monitoring of size, mass and number concentration of a flame synthesised particle aerosol is presented. The diagnostics methodology is relying on combination of particle mass spectrometry and quartz crystal microbalance techniques. The analysis protocol includes molecular beam sampling, sorting the charged particles constituent according to mass/charge ratio (m/ze) via electrostatic deflection, and detection of the neutral particles by monitoring the variation of the quartz crystal oscillation frequency upon exposure to the particle - laden molecular beam. From these measurements, the total mass concentration and probability density distribution of m/ze could be directly determined, allowing to deduce the particle number concentration in the aerosol, at least in relative units. The feasibility of the method was demonstrated on the example of iron oxide nanopartides (NPs) forming in CH4/O-2/N-2 dual stage flame doped with iron pentacarbonyl, Fe (CO)(5) as a precursor of iron oxide NPs. Sensitivities of 6 x 10(4) NP/s and 8 x 10(6) NP/s were achieved for charged and neutral particles, respectively. Possible implications of the obtained results on the elucidation of iron oxide NP formation mechanism are discussed. (C) 2013 The Combustion Institute. Published by Elsevier Inc. All rights reserved.