(a) Why does mobile phase strength increase as solvent

becomes less polar in reversed-phase chromatography, whereas

mobile phase strength increases as solvent becomes more polar in

normal-phase chromatography?

(b) What kind of gradient is used in supercritical fluid chromatography?

Separation on normal-phase chromatography implies that the stationary phase is polar and the mobile phase nonpolar. The separation depends onthe interactions of a polar analyte with the polar stationary phase.

In the reverse-phase chromatography separation, the stationary phase is non-polar and the mobile phase is polar. The separation is based on thehydrophobic property of the analyte. Thus if a solvent is less polar, it will separate easily on the stationary phase in the column making the mobilephase strength increase.

Supercritical fluid chromatography (SFC) is a hybrid technique of gas and liquid chromatography.

The importance of SFC is that it allows the separation and determination of groups of compounds that cannot normally be determined by either gas or liquid chromatography. It is a method in which the mobile phase is a gas at temperature and pressure above critical values. It uses a gradient of increasing pressure to give increasing eluent strength.