Question

Which of the following will increase the number of theoretical plates in a chromatographic separation? Select all that are appropriate.

	decrease column packing particle size
	increase column mobile phase flow rate
	increase column length
	change mobile phase to one with a greater analyte diffusion coefficent

Answer 1

The relation between the number of theoretical plates (N), column length (L) and height equivalent to a theoretical plate (H) is given by:

N = L/H…………………………..(1)

From the above equation, longer the column higher will be the number of theoretical plates. Thus, increasing column length will increase number of theoretical plates.

From van-Deemter equation:

H = A + (B/u) + Cu ……………………..(2)

Where, A is Eddy diffusion parameter, (B/u) is longitudinal diffusion and Cu is equilibrium time, C is mass transfer coefficient between mobile and stationary phase and mobile phase and u is linear velocity.

From the equation 1 number of theoretical plates will increase with decreasing H. From equation 2, Value of A should be smaller. The value of A is smaller for the column with smaller particle size. Thus decrease column packing particle size number of theoretical plates will increase.

There should be an optimal flow rate because flow rate affects the two factors B/u and Cu inversely.