A new principle of the magnetophoretic velocity modulation mass analysis of microparticles, which can determine simultaneously the mass and magnetic susceptibility of a single microparticle, has been proposed, and the measurement system was constructed by applying a magnetophoretic force on a falling microparticle through a magnetic field gradient in an atmosphere. A polystyrene microparticle as a test particle adsorbed on a glass plate was selectively knocked off by a pulsed Nd:YAG laser impact into a narrow gap of pole pieces of permanent magnets having a magnetic field gradient with a maximum intensity of 850 T2 m(-1). The falling particle was irradiated by a He-Ne laser, and the scattered light was detected through a slit array mask as a function of time. A bundle of spiked signals of scattered light intensity was analyzed to obtain velocities, which gave acceleration and deceleration of the falling particle. On the basis of the equation of motion under the magnetic field gradient, the mass and magnetic susceptibility of the test particle were reasonably determined.

Optically detected magnetophoretic acceleration mass analysis of an individual micro-particle in an atmosphere has been remarkably improved in sensitivity by using a reflective microscope objective, by which forward scattered light from a particle could be effectively collected. From the light-scattering simulation, the detection limit for the radius of a micro-particle was estimated to be smaller than 0.4 μm, and about 60 times intensity enhancement was observed for a polystyrene particle with a radius of 2.8 μm. For both paramagnetic and diamagnetic micro-particles, the mass magnetic susceptibility and the relaxation time could be determined without knowing any parameters of the particles. From the relaxation time, the mass of a particle was obtained if the radius or the density of the particle was known. For a test sample silica particles were used to adsorb paramagnetic dysprosium(III), the surface concentration of dysprosium(III) on a single particle could be successfully determined by use of this method.