The invention is a device for processing materials comprising a mixing chamber having a valve for removal of the material from the mixing chamber and a piston which fits within the mixing chamber in a manner such that the piston can be moved to remove substantially all material from the mixing chamber via the valve; at least one, preferably at least two, retractable mixing element(s) wherein each of such elements is movable in and out of the mixing chamber through a port wherein the element and port are configured such that during mixing and when the element is withdrawn from the mixing chamber substantially no material is removed from the mixing chamber via the port. The invention is also a method of using such a device and a system comprising use of such devices in an automated or partially automated array.