Absolute Mag™ Hydroxyl Magnetic Particles, Dextran, 50 nm (# WHM-C150) are ferrofluids consisting of an aqueous dispersion of magnetic iron oxides. The beads are covered with hydrophilic polymers which protect them against aggregation by foreign ions. Terminal functional groups such as ion-exchange groups or reactive groups for covalent immobilization can be used for binding to biomolecules. Ferrofluids can further be used for MRI-diagnostics and magnetic drug targeting applications. Ferrofluids with particle sizes of 50 nm have a single domain core and therefore not separable by an external magnetic field. These substances are real magnetic fluids, and the magnetic isolation is only possible with a High Gradient Magnetic Separator. This is expensive and time consuming; therefore we recommend using magnetic nanoparticles with a minimum size of 118 nm.