The present invention relates to means for covering one or more sample(s) that are suitable to avoid or minimize evaporation and/or condensation of any vaporizable substance that may be present in the sample(s) or reaction mixture(s), in particular evaporation of substance at the fringes of a vessel or an array of vessels or condensation of said substance on the lid of a reaction vessel or a plate/block containing the sample(s) and/or the means for covering. This is achieved by providing a device comprising, among others, a means for positioning a covering means and a means for guiding at least one lid of the covering means.