Particles are subjected to travelling wave field migration (TWFM) to migrate the particles over an array of microelectrodes. Altered particles are produced by treating original particles in such a way so as to alter their TWFM characteristics and the altered TWFM characteristics are employed for analysis and/or separation of the altered particles. The particles may be cells, bacteria, viruses, biomolecules or plastics microspheres. They may be altered by binding to a ligand such as a metal microparticle via a selective linking moiety such as an antibody/antigen or oligonucleic acid, or be physical or chemical treatments.