Chapter 25: QCE (page 706)

Two peaks emerge from a reversed-phase chromatography column as sketched in the illustration.

According to Equation 23-33, resolution is given by
Resolution $= \frac{\sqrt{N}}{4} \frac{(α - 1)}{α} (\frac{k_{2}}{1 + k_{2}})$
where $N$ is plate number, $α$ is relative retention (Equation23-20), and $k_{2}$ is the retention factor for the more retained component (Equation 23-16).
(a) If you decrease the amount of organic solvent in the mobile phase, you will increase retention. Sketch the chromatogram if retention factors increase but $N$ and $α$ are constant.
(b) If you change the solvent type or the stationary phase, you will change the relative retention. Sketch the chromatogram if $α$ increases but $N$ and $k_{1}$ are constant.
(c) If you decrease particle size or increase column length, you can increase the plate number. Sketch the chromatogram if $N$ increases by (i) decreasing particle size and (ii) increasing column length. Assume $α$ and $k_{2}$ are constant.

Short Answer

Expert verified

a)The sketch of chromatogram if retention factors increase but $N$ and $α$ are constant:

b) The sketch of chromatogram if $α$ increases but $N$ and $k_{1}$ are constant:

c) The sketch of chromatogram if $N$ increases by (i) decreasing particle size and (ii) increasing column length.

Step by step solution

Definition of retention factor.

The retention factor (Rf) is used in thin-layer chromatography to compare and identify substances. The distance travelled by the compound divided by the distance travelled by the solvent front gives the Rf value of a compound.

Step 2: The sketch of chromatogram if retention factors increase but and are constant.

When the quantity of organic solvent in the mobile phase is reduced, the retention is automatically enhanced. As a result, if $N$ and $α$ are held constant, the chromatogram will look like this