Several DP systems have been discussed and evaluated with respect to dynamic evacuation under actual operating conditions of an electron microscope. As a result, a cascade DP system has been found to be the most reasonable for an electron microscope where the column is evacuated by the first DP backed by the second DP, and for the camera chamber by the second DP with a large volume foreline backed by a mechanical pump. A spectrum of the residual gases in an electron microscope with a cascade DP system indicates a very clean vacuum with hydrocarbon partial pressures of less than 3×10−7Pa.