Gravimetric Measurements Gravimetric measurements rely on measuring the in crease in mass of some substrate following its exposure to an ambient airstream at a known flow rate. If the sam pling efficiency of the substrate is known, the average particle concentration in the air may then be calculated The simplest such approach is to use a filter paper in series with the airflow, leading to the trapping of particles by impaction, diffusion, electrostatic attraction and interception among other mechanisms. Filter papers are commonly formed from paper or glass fiber, although numerous other substrates exist. The efficiency with which particles are retained varies with their size for example, while larger particles are efficiently trapped by impaction of an airstream, and smaller particles by dif fusion, intermediate sized particles may not be so readily retained; consequently, the relationship between the retained particle mass and the ambient concentration may not be straightforward.