Abstract: An apparatus (1) is used for the preparation or extraction of samples, especially for analytical purposes, from a liquid or using a carrier liquid, where only very little liquid and substrate is available or is to be used and where the sample separation is to be carried out very quickly. For this, the apparatus (1) comprises, etched or machined into a plate-shaped body (8), an essentially planar flow bed (10) which has inlets and outlets arranged at opposing ends in the direction of flow, and is covered and sealed on the processing side of the plate-shaped body (8) by a further plate, so that the flow bed (10), which has an approximately groove-shaped flat cross-section, is sealed on all sides. There extend parallel to the flow bed (10), but also machined or etched into the plate-shaped body (8), electrode chambers (13), which are connected by way of electrodes (13b) to a current source (13a) in order to generate an electric field in the flow bed (10) transversely to the direction of flow.