9 Adsorption Chromatography: Normal-Phase Chromatography 9.1 WHAT IS ADSORPTION? The silica structure is saturated with silanol groups at the ends. The silanol groups represent the active sites in the stationary phase. They form a weak type of ‘bond’ with any molecule in the vicinity when any of the following interactions are present: Dipole-induced dipole; Dipole-dipole; Hydrogen bonding π-complex bonding

9.2 THE ELUOTROPIC SERIES

If, for example, a solvent strength of ε0=0 If, for example, a solvent strength of ε0=0.4 is need, this can be obtained by the following binary eluents:

9.3 SELECTIVITY PROPERTIES OF THE MOBILE PHASE Eluent selectivity is the ability of different mobile phases to change the separation factor α of two or more compounds present in the sample. Adsorption chromatography selectivity has two different aspects, localization and basicity. Localization is a measure the ability of the solvent molecules to interact with the adsorbent which is used as stationary phase.

The upper half of the table from fluoroalkane to dichloroethane represents nonlocalizing solvents with the exception of diethyl ether. This solvent as well as the lower half of the table from triethylamine to water are the localizing eluents. Therefore the two solvents dichloromethane (nonlocalizing) and diethyl ether (localizing) differ in this important selectivity property although they have almost identical strength.

Basicity is one of the axes of the solvent selectivity triangle of Figure 5.1. The most basic of the common HPLC solvents are the ethers .

9.4 CHOICE AND OPTIMIZATION OF THE MOBILE PHASE The best mobile phase strength for a specific separation problem can be determined by thin-layer chromatography (TLC). By this TLC test the correct strength (ε0) can be found but the selectivity may need optimization as individual peaks may remain only partly resolved. In order to obtain selectivity changes it is necessary to choose solvents which differ in their localization and basicity. In many cases the mobile phase consists of two solvents, A and B. The usual A solvent is hexane (or heptane, ε0=0) which has no strength, localization or basicity. For B it is best to use either a nonlocalizing(e.g. dichloromethane) a nonbasic localizing(e.g. acetonitrile or ethyl acetate), , or a basic localizing solvent(e.g. tert. Butylmethyl ether).

Deactivators Water is the most important deactivator(moderator) Deactivators Water is the most important deactivator(moderator). It is highly polar and present in all solvents, albeit sometimes only in trace amounts. The alcohols methanol, ethanol or isopropanol are also possible deactivators. Gradient elution is possible in adsorption chromatography if the polarity difference of the solvents is not too large.

It can be more difficult, thus less recommended, than in reversed-phase chromatography. The solvents could demix the column, i.e. the more polar one could be adsorbed; Their modifier content may differ; Re-equilibration times may be long; It is advantageous not to start the gradient at 0% B but with a certain (perhaps very low) amount of the stronger solvent.

9.5 APPLICATRIONS Silica is an excellent stationary phase for the separation of isomers. Figure 9.6 shows a separation of diastereomers. Figure 9.8 shows the separation of very polar compounds on silica.

The separation of acidic or basic products may cause difficulties in adsorption chromatography. Reversed-phase or ion-exchange chromatography would be more suitable but solubility problems may arise. One answer is to use buffered silica, as shown in Figure 9.9. The silica is treated ether as bulk material or in the column with a buffer solution of a suitable pH (acidic buffer of acidic samples, basic buffer for bases).