An apparatus for loading a sample into a particle-optical instrument that includes a slider bearing having a base plate in contact with the vacuum chamber at one side, said base plate showing a first through-hole in contact with the vacuum chamber, and a second plate, one side of the second plate in contact with the base plate, said second plate also showing a through-hole, where the faces of the base plate and the second plate facing each other are sufficiently smooth to form a non-elastomeric vacuum seal and where the second plate is a flexible plate and the face of the flexible plate opposite to the base plate is equipped to seal against a cup equipped to hold a sample. Preferably the through-hole in the base plate shows a rim facing the flexible plate with a controlled curvature, the curvature of the rim formed such that the vacuum seal between the base plate and the flexible plate forms on a pre-defined contour and that the Hertzian contact pressure is smaller than a pre-defined maximum contact pressure, the pre-determined maximum contact pressure chosen to minimize particle generation. By forming the second plate as a flexible plate the pressure with which the base plate and the second plate are pressed together is better reproducible than when both plates are rigid. By forming the rim with a controlled radius, particle generation is minimized.