An assembly suitable for use in a laboratory instrument is set forth. The assembly comprises a plurality of guide rods that are disposed generally parallel with one another and a plurality of carriage assemblies connected for movement along the plurality of guide rods. A sub-carriage assembly is disposed on at least one of the carriage assemblies and is connected to the carriage assembly for movement in a direction transverse to the plurality of guide rods. A plurality of carriage drive mechanisms are employed to move the plurality of carriage assemblies independently along said plurality of guide rods and a sub-carriage drive mechanism is provided to move the sub-carriage assembly in a direction transverse to said plurality of guide rods.