The Sample Switching Unit SYS 520 has been developed for the determination of fractional separation efficiencies of filters or filter media at two different sampling points, that is before and after the filter. This task can be realized with the sample switching unit SYS 520 with only one particle size analyzer economically and reproducible. With the switching unit an automated and safe exchange between the measurement of sampling lines I, II and a sampling line of a purge routine is possible (using the integrated HEPA filter).