Abstract: The invention relates to a process for producing ferrofluid-forming magnetic particles substituted by DMSA (FFSH) that can be covalently coupled, directly or through a difunctional reactant, to an effector, which process comprises combining DMSA with ferrofluid-forming magnetic particles to form FFSS particles having disulfide bonds, and reducing the disulfide bonds of the FFSS at basic pH to form FFSH. The invention includes a composition comprising such FFSH magnetic particles. The invention also relates to a method in which FFSH magnetic particles are incubated with an effector comprising at least one functional group which forms an S--S, C--S, C--C, or C--N bond with DMSA, so that a covalent effector-FFSH complex is formed. The invention further includes a method for separating and/or distinguishing between molecules or cells having a ligand that forms an affinity complex with an effector-FFSH particle of the invention, and those molecules or cells that do not have such a ligand.