This study proposes using a magnetically stabilized fluidized bed (MSFB) to separate zwitterionic molecules (i.e., molecules containing negatively and positively charged moieties) by virtue of differing isoelectric values (i.e., the pH at which the molecule is not charged). The fluidized bed consists of a tube in which negatively charged resin particles are added from the top of the tube, and the fluidizing liquid is distributed from the bottom. Removal of the particles from the bottom, and liquid from the top allows for continuous operation. A pH gradient is maintained in the bed by inserting the liquid feed at a high pH and the solid particles at a low pH. A mixture containing dissolved solutes (one of which is targeted for separation) is fed into the liquid stream somewhere near the bottom, and to one side of the column. As the solute moves up the bed by virtue of the liquid motion, the local pH that the solute is exposed to decreases. The reduction in the local pH causes the solutes to eventually become positively charged (at the positions in the bed just above the positions where the local pH is equal to the isoelectric values of the solutes). Once positively charged, the solute binds electrostatically to the negatively charged resin, and is carried with the resin particles down to the bottom of the column. As it descends down the column, the local pH decreases, and this causes the solute to reverse its charge, remove itself from the resin particle, and become entrained in the upflowing liquid. The solute thus becomes trapped in a narrow zone at a level in the column where the local pH is equal to the solute's isoelectric point. Ideally, the solutes in the mixture will separate out into zones occupying different levels in the column. By withdrawing fluid and solid from these zones, liquid rich in one solute can be obtained, thereby effecting the separation. The proposed research will develop a working prototype, and scope the effectiveness of the bed to prepare separations of biological molecules.
