The particles of an aerosol that is being sucked into a point sink move more slowly than the air as the orifice is approached; this is on account of their inertia. Their weight causes them to deviate from the radial streamlines of air flow at a distance from the orifice. If the circumstances are such that either of these effects operates alone, that is when either the inertia of the particles or their rate of fall due to gravity is negligible, the sample of the aerosol drawn into the orifice will have the correct concentration. Should both factors act together, however, the concentration of the sample will be low. This paper contains an analysis of the problem and a few numerical results which indicate practical steps which can be taken to avoid sampling errors.