{HEAD ~ The applied field} comprises {TAIL ~ a first, external magnetic gradient} which moves magnetically responsive particles to a region of a collection vessel 
comprises
{ HEAD ~ The applied field } comprises { TAIL ~ a first , external magnetic gradient } which moves magnetically responsive particles to a region of a collection vessel
HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH