  An optical method for precise purification of chemical elements is introduced
in this paper. The materials are supposed to be in the states of gaseous beams,
which are coherently coupled to an external traveling light during
purification. Before decoherence occurs, atoms periodically move in the light
with different speeds that depends on masses and optical transition wave
lengths of these atoms. The speed gradient leads to deflections of different
atoms in different directions. The model is described by Schr\"{o}dinger
equations with analytical results. This method could be used for some hardly
separable atoms and isotopes depending on the condition of atom coherent time.
The present work opens a platform for applications of cold atom technology in
the purification of atoms and molecules.
