Question

The following data has been calculated from the chromatogram. The column is 100 cm long and the unretained marker takes 2 minutes to be detected. Calculate the resolution for the peaks (A-B, B-C). How long must the column be in order to have baseline resolution? Assume the plate height, flow rate, selectivity and retention factor (capacity factor) will remain constant. If you use a shortcut equation in calculating the resolution, justify.

Compound	Plate Number	Retention Factor
A	2450	6.4
B	2500	7.8
C	2850	8.8

Answer 1

Formula,

Resolution (Rs) = {[sq.rt.(N)[(alpha - 1)/alpha][k/(1 + k)]}/4

with,

N = plate number

alpha = ratio of retention factors

k = retention factor for higher peak

For A-B,

N = (2450 + 2500)/2 = 2475

alpha = 7.8/6.4 = 1.22

k = 7.8

Resolution (Rs) = {[sq.rt.(2475)[(1.22 - 1)/1.22][7.8/(1 + 7.8)]}/4

                          = 2.0

For B-C

N = (2500 + 2850)/2 = 2675

alpha = 8.8/7.8 = 1.13

k = 8.8

Resolution (Rs) = {[sq.rt.(2675)[(1.13 - 1)/1.13][8.8/(1 + 8.8)]}/4

                          = 1.3

With plate height (H) a constant, the column length (L) is directy proportional to the plate number (N)

L = H x N

H = constant

So the length of column would be dependent upon the plate number in both the cases for the baseline resolution.