An insert for a sample chamber of a multi-well plate, which delimits the sample chamber upwardly, so as to form a measurement space, when the insert is in the state in which it is fitted into the sample chamber. The insert has at least two portions with outer contours differing from one another, one of the portions being a sealing portion with a continuous sealing surface, which sealing portion is adapted to a corresponding, in particular hollow-cylindrical sealing portion, with a likewise continuous sealing surface, of the sample chamber, in such a way that the insert sealing portion and the sample chamber sealing portion can be connected to one another releasably and in a liquid-tight manner by means of a press fit.