Anion Exchange The stationary phase is cationic, so anionic molecules are bound and eluted.
Cation Exchange The stationary phase is anionic, so cationic molecules are bound and eluted.
Buffer A This is the buffer that equilibrates the column before the sample is bound. Buffer A also re-equilibrates the column after elution or cleaning.
Buffer B This is the eluting buffer. It usually consists of Buffer A plus a salt such as NcCl or KCl.
Gradient Refers to eluting a column with a changing mobile phase. In ion exchange this means an increasing concentration of salt during the elution. Methods are usually developed with linear gradients, but at the process scale step gradients are desired if possible.
Isocratic Refers to running columns with an unchanging mobile phase.
Isoelectric Point (pI) The pH where a protein has a net charge of zero. Calculated or measured by iso-electric focusing. Proteins can bind to ion exchange columns at their pI because macromolecules can have "faces" that are either cationic or anionic.
Strong Ion Exchange Strong ion exchange columns are charged at all pH levels. The sulfonic acid group on Mustang S membrane is always anionic and the quaternary amine on Mustang Q membrane is always cationic.
Weak Ion Exchange Weak ion exchange columns are not charged at all pH levels. An example is weak cation exchange where the ligand on the stationary phase is a carboxyl group that is negatively charged only at ~pH 4 and above.