Optical chromatographic sample separation of hydrodynamically focused mixtures

Abstract

Optical chromatography involves loosely focusing a laser beam into a fluid flowing opposite to the direction of laser propagation. When microscopic particles in the flow path encounter this beam they are optically trapped along the beam and are pushed upstream by the radiation pressure from the laser focal point to rest at a position where the optical and fluid drag forces on the particle balance. Because optical and fluid forces are sensitive to differences in the physical and chemical properties of a particle, fine separations are possible. A laser beam which completely fills a fluid channel has been operated as an optically tunable filter for the separation of polymeric/colloidal and biological samples. We demonstrate here how this technique coupled with an advanced microfluidic platform can be used as both a coarse and fine method to fractionate particles in an injected sample. The microfluidic network allows for a monodisperse mixed particle sample of polystyrene and poly(methyl methacrylate) to be injected, hydrodynamically focused and completely separated. To test the limit of separation, a mixed polystyrene sample containing two particles varying in size by less than 0.2% was run in the system. The analysis of the resulting separation sets the framework for continued work to perform ultra-fine separations.