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We experimentally investigate a particle detection scheme that makes use of optical gradient forces. The path of nanoscale particles carried in a microfluidic channel is perturbed by a strongly focused laser beam. The perturbation is interferometrically recorded with a quadrant detector and the temporal asymmetry of the detector signal yields information about the force exerted on the particles by the laser focus. Large particles experience strong forces, thus rendering a highly asymmetric signal whereas small particles pass the laser focus almost unperturbed thus rendering a symmetric signal. We analyze the influence of experimental parameters such as laser power and fluid speed on the precision of the method and discuss the influence of Brownian motion. © 2003 American Institute of Physics.