Ion exchange chromatography separates molecules based on differences in the net charge of the molecules. In cation exchange, the analyte is positively charged (a cation), while the chromatographic material is negatively charged (an anion). Ion exchange is usually used for protein purification, but it may be used for purification of oligonucleotides, peptides, or other charged molecules. For interaction to occur, the protein of interest must have a charge opposite to that of the functional group of the sorbent particle. For example, immunoglobulins, which generally have a net positive charge, will bind well to cation exchangers because they possess negatively-charged functional groups. Because this interaction is ionic, binding must take place under low ionic strength conditions. Elution is achieved by increasing the ionic strength of the mobile phase to reduce ionic attractions, or by changing the pH of the mobile phase to alter the ionization state of the protein. TSKgel columns are highly efficient for sample purification with excellent recovery. Anion- and cation-exchangers are available on porous polymer-based and silica-based particles. Proteins and nucleic acids can be analyzed faster on a TSKgel non-porous resin column, although STAT and NPR columns have lower capacity than large pore-size-polymer particles. TSKgel BioAssist S columns are constructed from PEEK (polyether ether ketone).