An apparatus for regulating the movement of a preselected ion across a membrane, which comprises:
means for generating an applied magnetic field parallel to a predetermined axis projecting through a space in which a membrane is positioned in the presence of a preselected ion, the applied magnetic field resulting in a magnetic flux density in the space of a known average value parallel to the predetermined axis;
means associated with the magnetic field generating means for fluctuating the magnetic flux density such that the known average value is a non-zero average value;
means for creating and maintaining a predetermined relationship between the frequency of the fluctuations and the non-zero average value of the magnetic flux density, wherein the frequency of the fluctuating magnetic flux density is directly related to the charge of the preselected ion, a preselected integer greater than 1, and the non-zero average value of the flux density parallel to the predetermined axis, and wherein the frequency is also inversely related to the mass of the preselected ion. Also corresponding apparatus for simultaneously regulating the movement of at least two different ions across the membrane. Methods for using both type of apparatus.