Amino acids are zwitterionic, they have both positive and negative charges on the molecule.
Transgenomic offers a complete line of polymeric cation exchangers for separating amino acids based on their differences in positive charge. With cation exchange resins, the amino acids are bound to the resin by their attraction to the negatively charged ion exchange sites on the resin surface. After they are bound, they are selectively eluted with buffers. These buffers are comprised of molecules that also have positive charges. The positively charged buffers then compete with the amino acids for the negatively charged ion exchange sites.