A device is disclosed for use in a chromatographic system wherein a component of a mixture is paritioned between a liquid phase and an immobile phase. The device is comprised of a chromatographic material. In the chromatographic system the component traverses at least a portion of the chromatographic material. The device generally has at least one longitudinal edge substantially corresponding to the direction of traverse of the component. The longitudinal edge includes means for controlling the shape of the front of the traversing component.