  We theoretically predict and experimentally demonstrate that several
different particle species can be separated from each other by means of a
ratchet device, consisting of periodically arranged triangular (ratchet) shaped
obstacles. We propose an explicit algorithm for suitably tailoring the
externally applied, time-dependent voltage protocol so that one or several,
arbitrarily selected particle species are forced to migrate oppositely to all
the remaining species. As an example we present numerical simulations for a
mixture of five species, labelled according to their increasing size, so that
species 2 and 4 simultaneously move in one direction and species 1, 3, and 5 in
the other. The selection of species to be separated from the others can be
changed at any time by simply adapting the voltage protocol. This general
theoretical concept to utilize one device for many different sorting tasks is
experimentally confirmed for a mixture of three colloidal particle species.
