Question

Explain how polarity P' is related to retention of the sample analytes on the silica column for both the normal-phase and reverse-phase procedures.

Answer 1

In normal-phase chromatography:

Normal phase chromatography, an adsorptive mechanism, is used for the analysis of solutes readily soluble in organic solvents, based on their polar differences such as amines, acids, metal complexes, etc.

the stationary phase is polar and the mobile phase is nonpolar. Typical stationary phases for normal-phase chromatography are silica. There are also bonded normal phase material. They have organic moieties with cyano and amino functional groups.

In normal-phase chromatography

, the least polar compounds elute first and the most polar compounds elute last. The mobile phase consists of a non-polar solvent such as hexane or heptane mixed with a slightly more polar solvent such as isopropanol, ethyl acetate or chloroform.

Retention increases as the amount of non-polar solvent in the mobile phase increases.

In normal-phase chromatography, the stationary phase is more polar than the mobile phase. There are a number of stationary phases available for normal-phase chromatography.  Silica is the most common of the non-bonded phases and can provide very high selectivity for many applications,

REVERSE PHASE PROCEDURE:

Any inert non-polar substance that achieves sufficient packing can be used for reversed-phase chromatography. The most popular column is an octadecyl carbon chain (C18)-bonded silica (USP classification L1) with 297 columns commercially available.^[5] This is followed by C8-bonded silica (L7 - 166 columns), pure silica (L3 - 88 columns), cyano-bonded silica (L10 - 73 columns) and phenyl-bonded silica (L11 - 72 columns),

while cyano columns can be used in a reversed-phase mode depending on analyte and mobile phase conditions. It should be noted at this point that not all C18 columns have identical retention properties. Surface functionalization of silica can be performed in a monomeric or a polymeric reaction with different short-chain organosilanes used in a second step to cover remaining silanol groups (end-capping).

While the overall retention mechanism remains the same, subtle differences in the surface chemistries of different stationary phases will lead to changes in selectivity.