Question

this question from HPLC lab :

b. What are ways, other than changing the mobile phase compositions, to change the total retention or selectivity of the system towards the compounds? other than changing the mobile phase compositions (Temperature, sorbent, pH, etc...?)

Answer 1

Other than changing the mobile phase composition, the important way to change the retention of compounds is to change the stationary phase (sorbent).

Chromatography is based on the principle that a substance placed in contact with two immiscible phases, one moving and one stationary, will equilibrate between them based on the interactions.

A reproducible fraction will partition into each phase, depending on the relative affinity of the substance for each phase.

A substance which has affinity for the moving or mobile phase will be moved rapidly through the system.

A material which has a stronger affinity for the stationary phase  will spend more time immobilized in that phase, and will take a longer time to pass through the system. Therefore, it will be separated from the first substance.

Chromatography is a separation technique in which a sample is equilibrated between a mobile and a stationary phase.

so, if mobile phase is kept same then two columns with different stationary phases show different retention capacity since the materials used have different interactions with the compounds (mixture).