The Mechanism of Chromatographic Retention - Chromatographic Interactions > Ionic Interactions > Page 14

Dissociated ions can not be volatile and thus, ionic interactions are not employed as a retentive mechanism in GC. Ionic interactions, however, are the dominant retentive mechanism in ion exchange chromatography which is widely used in analytical chemistry. The stationary phase usually consists of a cross-linked polystyrene resin to which ionic materials have been chemically bonded. They are formed in the shape of tiny spheres that are packed into a column in the usual way. The mobile phase usually carries a buffer that is set at a pH that allows the solutes and the ion exchange resin to be ionized and thus the charged groups are available for mutual interaction. Both anion and cation exchange resins are available to separate the complementary ions of a solute molecule. In addition, both anion and cation exchange resins are available as both strong and weak ion exchangers. An example of a separation achieved by ionic interactions employing strong and weak ion exchangers is shown in figure 6.

Courtesy of the TosoHaas Corporation

 

1 Trypsinogen 2 Ribonuclease 3 a-Chymotrypsinogen
4 Cytochrome 5 Lysozyme

 

Properties of Two Strong and Weak Ion Exchange Stationary Phases

 

TSK gel SP-5PW CM-5PW
Matrix Hydrophilic Resin Hydrophilic Resin
Particle Size 10 mm 10 mm
Pore Size 1000 1000
Functional Group -CH2CH2CH2SO3- -CH2COO-
Counter Ion Na+ Na+
pH Range 2-12 2-12
pKa 2.3 4.2
Column Size 7.5 cm long 7.5mm I.D. 7.5 cm long 7.5mm I.D.

Figure 6. The Separation of a Mixture of Globular Proteins Employing Strong and Weak Ion Exchange Resins