Assessment of Particulate Matter (PM) emissions have to be accurately performed in order to have a better understanding on their environmental and human health effect. Cascade impactors have been one of the very first way to size fractionate aerosol particles. A lot of studies (environmental, biological, chemical,...) need substantial amounts of PM size fractions from cascade impactors during long time collections in order to meet the experimental needs. The following study examines the effect of long term collection on the Electrical Low Pressure Impactor (ELPI), using its impaction ability to perform on-line measurement. The same diluted aerosol from miscanthus combustion in a domestic scale boiler is sampled with two different ELPIs, the only difference between the two ELPIs being that one of them has undergone long term collection (120, 178 and 234 min). Significant differences of size distribution measurement have been recorded; therefore a comprehensive study of jet-to-impaction surface distance (S) changes has been carried out using reflection microscopy. The material collected below a jet nozzle onto an impaction plate which was especially studied exhibits a dome shape (shown by 3D reconstruction of z-stacks reflection microscopy volume). More than 1700 impaction dome heights have been recorded in order to suggest corrections for the changes of the jet-to-impaction surface distance during long term collection and subsequently take them in account into equivalent long-term collection diameters. (C) 2015 Elsevier Ltd. All rights reserved.