Question

1.How is the retention (capacity) factor manipulated in liquid chromatography? in gas chromatography?

2. How is the selectivity factor manipulated in liquid chromatography? in gas chromatography?

3. Briefly describe the general elution problem.

Answer 1

1) Retention factor is essentially the ratio of the number of moles of solute in the stationary phase to that in the mobile phase. It can be varied as follows:

Liquid chromatography:

- Changing the nature of the column i.e the stationary phase (colum length, material etc)

- Changing the mobile phase composition

- Using reagents which serve as complexing agents

Gas Chromatography

- Changing the nature of the stationary phase

- Changing temperature

2) Selectivity factor is the ratio of retention fcator of two species. It can be varied as:

LC:

By changing the mobile phase composition

By changing column packing

GC:

By changing column packing

3. The general elution problem in chromatography revolves around finding a set of optimal conditions which give a well resolved chromatogram i.e one in which the peaks corresponding to the different components present in a given mixture are well separated. This is usually done by making increamental adjustments as the separation is carried out. In LC the mobile phase composition can be adjusted while the temperature can be changed in GC to achieve this objective.