Magnetron sputter inert gas condensation was coupled with quadrupole mass spectrometry for the in situ characterization of a nanoparticle beam. The proposed method allows us to determine the size distribution and the mass flux of the nanoparticles. The measured quadrupole mass spectrometer grid current is converted into a nanoparticle flux and subsequently into a mass flux. Cu nanoparticles were deposited onto Si substrates using different filtering modes of the quadrupole mass spectrometer. Characterization was carried out using a combination of x-ray photoelectron spectroscopy and scanning electron microscopy. Quantitative analysis of the elemental composition of the Si surface revealed a Gaussian distribution of the deposited nanoparticles over the diameter of the rotating substrate holder with a good quantitative agreement with the predictions made from the in situ quantification method.