Question

In van deemter equation, why does increasing the flow rate reduce the contribution of longitudinal diffusion to total plate height?

a. increased flow rate causes the solute to migrate through the packing material at a greater rate, resulting in less interaction, and a faster, more efficient separation.
b. increasing the flow rate will decreases the time the solute resides in the column, resulting in less diffusional broadening.
c. the equilibrium rates of exchange between the solute and the stationary phase are decreased at increased flow rates.
d. it is due to a reduction in the mean free path of a solute molecule as it moves through a packed column (eddy diffusion).
e. increasing the flow rate increase the retention factor (or capacity factor)

Answer 1

Increasing the flow rate in chromatography reduces the contribution of longitudinal diffusion to the total plate height. This is because the solute spends less time in the column, resulting in less diffusional broadening. Option b. increasing the flow rate will decreases the time the solute resides in the column, resulting in less diffusional broadening is the correct option

The correct answer to the question is option (b): increasing the flow rate will decrease the time the solute resides in the column, resulting in less diffusional broadening.

The Van Deemter equation describes the factors contributing to the total plate height in chromatography. One of these factors is longitudinal diffusion, which refers to the spreading of solute molecules as they move through the column due to random motion.

When the flow rate is increased, the solute spends less time in the column. This reduced residence time means that there is less opportunity for longitudinal diffusion to occur, leading to less diffusional broadening. In other words, the solute molecules have less time to spread out and mix with the surrounding stationary phase.

By decreasing the time the solute resides in the column, the contribution of longitudinal diffusion to the total plate height is reduced. This can result in a faster and more efficient separation, as less spreading of the solute occurs during its transit through the column.