The invention relates to a positioning means for a flow cell used for optical detection, comprising a base having at least one contact face, the contact face being provided to make contact with the end of the flow cell, wherein the base has at least one reference face, which is to be aligned and/or positioned relative to the flow channel of a flow cell, and wherein the base is designed to receive a connecting piece in such a way that the latter assumes a predefined attitude and alignment relative to the flow channel.