Their approach consisted of mildly focusing a laser beam into a solution containing particles that are counter–flowing coaxially with a capillary. When in equilibrium, there is a balance between the laser scattering force and the drag force from the fluid medium. A challenge in this configuration is to keep the laser beam aligned at the center of the of capillary. Since these pioneering experiments, optical chromatography has led to a range of different applications, for example: to visualize immunological reactions by flowing a sample of antigens past optically–trapped beads that are coated with antibodies. Selective binding changes the effective size of the particles an thus their retention distance. This allowed to determine antigen concentrations in the range from nanomolar to nanograms per milliliter. Source: 10.3389/fphy.2020.603641
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