Ion Chromatography – Different Classes of Ion exchange Interactive Groups

Different Classes of Ion exchange Interactive Groups.

There are a number of different ionic groups that can be attached to the stationary phase matrix for ion exchange purposes. Probably the most common cation groups are the sulphonate group(-SO₃^–), the phosphonate group (-PO₃^-2) and the carboxyl group (COO^–).The sulphonate group is classed a a strong cation exchanger and the phosphonate group and carboxyl groups are considered. as weak cation exchangers. In a similar manner the most common anion exchange groups are probably are quaternary amines (-N(R)₃⁺) the tertiary amines (-N(R)₃)and the secondary amine (-NR). In a similar manner to the cation exchangers, the quaternary amine group is classed as a strong anion exchanger and the tertiary and secondary amine groups are considered. as weak anion exchangers.

The classification of a strong or weak ion exchanger depends on the effect of pH on its ion exchange capacity or, in other words, the degree of dissociation of the ion exchanger situated on the stationary phase. This is fairly obvious as the the ion exchanger can only be effective when it is in its ionized form.

If the ion exchanger remains fully ionized over a wide pH range (i.e. a cation exchanger remains ionized over a pH range from 2.0 to more than 13.0) then it is a strong cation ion exchanger (e.g. a sulphonate). If, however, as with the cation exchanger such as a carboxylate, the material is only fully ionized over a limited pH range (e.g over .pH values ranging from about 8.0 to 14.0) and only starts to be ionized between a pH of 4.0 to 5.0 then the material is said to be a weak cation exchanger.

The Physical Characteristics of an Ion Exchangers

Ion exchange media can be prepared from a polystyrene resin cross linked with divinyl benzene, polymethacrylate resins, silica gel bonded with organic moieties, cross-linked polydextranes and cellulose. The resins are usually in the form of beads a few micron in diameter, and the silica ion exchangers are prepared from silica gel particles 5 to 10 micron in diameter. In aqueous media the resins often swell and, thus, can deform under the high pressures necessary in high performance liquid chromatography (HPLC) that can result in the mobile phase flow being impeded.

The situation can be improved in a number of ways; the degree of cross linking can be increased, the pore size of the resin surface can be increased (macroporous resins) and the resin can be coated onto the surface of glass beads or silica particles. Some of the different types of resin beads are show in figure 6.

Figure 6. Different Types of Ion Exchange Resin Beads

The types of resin beads containing pores of different size are called microporous and macroporous beads. The beads having a glass or silica core are called pellicular beads. The pellicular media are mechanically strong and can very easily cope with the high pressures necessary in HPLC. However, as the amount of actual ion exchange material on the packing is very small, the columns have very low chromatographic capacity and, although may produce very fast separations, columns packed with pellicular materials are easily overloaded.