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Estimation of Intrinsic Magnetic Properties of Single Particles by Particle Tracking Velocimetry

Characterization of paramagnetic particles is of utmost importance in biotechnology, cell separations, and medical applications. The particle size uniformity and colloidal stability of paramagnetic particles have been the focus of most producers, but magnetization is never specified and is typically mentioned only in terms of percent iron oxide content. Bulk measurements of magnetic susceptibility and saturation magnetization, using a superconducting quantum interference device (SQUID) or a vibrating sample magnetometer (VSM), provide an average value at best and do not account for the distributed nature of these variables or for particles with zero or very low susceptibility and magnetization. By means of particle tracking velocimetry in dark-field illumination, we measured multiple characteristics of several thousand individual micrometer-sized particles per sample. The Hyperflux velocimeter is utilized to provide quantitative video analysis of cells and particles using a high-definition camera to capture the images of the particle trajectories in an isodynamic field. Image analysis software converts the image data to the parameters of interest based on velocimetry calculations. The intrinsic magnetic properties of the magnetic beads estimated by both the velocimeter and VSM are in good agreement. Estimation of the magnetophoretic mobility distributions and intrinsic magnetic properties of magnetic microparticles using dark-field particle tracking allows economical and time-efficient magnetic evaluation of a broad range of magnetic particle sizes.