Discuss the principles on which methods of amino acid separation are based.

ANSWER: All the methods used for the separation and analysis of amino acids take advantage of the relative differences in the physical and chemical characteristics of amino acids, particularly ionization behavior and solubility characteristics. Separations of amino acids are usually based on partition properties (the tendency to associate with one solvent or phase over another) and separations based on electrical charge. In all of the partition methods, the molecules of interest are allowed (or forced) to flow through a medium consisting of two phasessolid-liquid, liquid-liquid, or gas-liquid. The molecules distribute themselves between the two phases depending on their particular properties and their consequent preference for associating with one or the other phase. Methods important for amino acid separations include ion exchange chromatography, gas chromatography (GC), and high-performance liquid chromatography (HPLC).