Q.4.(a) Give difference stationary phases used in different modes of HPLC.
Ans.4. (a)Types of Stationary Phases used in HPLC: Various stationary phases available for HPLC are Polystyrene/Divinylbenzene- Based Resins In ion chromatography, the support material is polystyrene/ divinylbenzene (PS/DVB) based resin that is relatively stable with respect to pH.
1. The copolymerization of PS with DVB is used to give the resin mechanical stability
2. The amount of DVB in the resin is denoted as “percent crosslinking”. The percentage of cross linking is directly related to the extent to which PS/DVB resin shrinks or swells in an aqueous media or in the presence of organic solvents.
3. If the resin shrinks a loss in column efficiency occurs as a dead volume occurs at the beginning of the column.
4. Swelling of the resin leads to higher column backpressures. The optimum degree of cross linking is said to be 2-5%
(A). Anion –Exchange Resin. These resins used by Dionex are composed of a surface sulphonated PS/DVB core (10-25 µm) and a totally porous latex particle (0.1 µm), which is completely animated. Electrostatic and van der Waals interactions are used to agglomerate the latex particles onto the core particles. It is the latex particles that carry the actual ion exchange function. Advantages of this stationary phase include mechanical stability of the resin due to the inner core, which also ensures moderate backpressures. Rapid exchange processes and thus high efficiencies occur due to the small, totally porous, latex particles. Surface sulphonation greatly reduces swelling and shrinking of the material.
(B). Cation-Exchange Resins. The stationary phase of a cation exchange column is based on inert, surface sulphonated, cross linked polystyrene. The exchange process for a cation M+, occurs as follows:
~SO3H+ + M+A-
~SO3M+ + H+A-
Silica based Resins Silica based resins are one of the most important classes of ion-exchangers used in chromatography. There are two main groups of silica based materials namely polymer coated and functionalized silica materials. Polymer coated materials consist of silica particles which are coated with a layer of polymer such as polystyrene, silicone or fluorocarbon and then derivitised to introduce functional groups. The advantages of polymer coated materials are that diffusion in the thin layer of the polymer occurs more rapidly than it would in totally polymeric particles. Functionalized silica materials comprise a functional group, which is chemically bonded directly to a silica particle. A disadvantage of silica based resin is that they can only be operated over a limited pH range.
Chelating Resins Chelating resins, which are able to separate metal ions, are made up of a suitable ligand immobilized onto a stationary phase. Many chelating resins have been synthesized using styrene-divinyl benzene polymers or silica as the support material. The ligands are chemically bound to the stationary phase by an appropriate reaction. Broad peaks are characteristic of slow formation and dissociation rates.
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