The invention relates to an improved O-ring seal for use as a high vacuum seal. A limitation of standard O-ring seals is the permeation of water through the O-ring. Especially for instruments in which parts are kept at a cryogenic temperature, such as a cryogenic electron microscope, the presence of water in the vacuum is a problem, as this results in ice growth on the cryogenic parts. As a solution often a double O-ring seal is used, or a metal seal. Both of these solutions have severe draw-backs.The invention proposes to place the O-ring in a channel in which dry gas, such as dry nitrogen, is blown. In this way no water can permeate through the O-ring, resulting in a reduced base pressure and highly reduced ice growth.