The ideal cut of an aerosol separator should remove all particles larger than the cut diameter and allow particles less than the cut diameter to penetrate. However, it is impossible to obtain a true cut because generally, the aerosol separator shows a fractional efficiency curve with a slope, and these differences in efficiency lead to structural errors in aerosol separation [15,11,14]. This study adapts the generalized Lapple equation to describe the fractional efficiency of a cyclone and calculated the penetration efficiency through a PM2.5 collector. Sensitivity analysis for penetration error has been conducted between a real separation collector and an ideal collector. This study also derives an analytical approximated solution and shows that the approximated calculations agree very well with numerical results, especially for low geometric standard deviation and large slope parameter. Based on this measurement error calculation, the mass difference ratio of PM2.5 separator between different collectors with different slope parameter (beta) and geometric mean diameter was compared. Subsequently, this study shows that conventional separators have different separation (penetration) efficiency even if their cut diameter is the same depending on the slope parameter and particle size distribution, calculates the bias with polydispersed aerosol size distributions with different slope parameter and shows an easy way to estimate the penetration error using analytical approximated solutions. (c) 2012 Elsevier B.V. All rights reserved.