Question

The chromatogram shows the separation of components in an over-the-counter pain reliever. Calculate the number of theoretical plates, N, for the peaks of benzoic acid and aspirin. N benzoic acid: N = plates 2 4 6 8 10 12 14 16 aspirin: N = plates min Peak Calculate the resolution, R, between benzoic acid and aspirin. 2 3 Componenttr (min) acetaminophen 3.97 caffeine 6.15 benzoic acid L 9.73 aspirin 14.8 W1/2 (min) 0.12 0.21 0.35 0.73 R=

Answer 1

Column efficiency defined as the number of theoretical plates per column, and number of theoretical plates can be obtained from a chromatogram from the expression

N = 5.54 (t_R / w_0.5)² where t_R is the retention time of the analyte and w_0.5 the width of the peak at half height.

given

for benzoic acid

t_r1 = 9.73 min

w_0.5¹=0.35 min

N= 5.54t_r1/w_0.5¹)²

N=5.54(9.73/0.35)²

N= 5.54*27.8²

N = 4281 plates

for aspirin

t_r2 = 14.8 min

w_0.5²=0.73 min

N= 5.54t_r2/w_0.5²)²

N=5.54(14.8/0.73)²

N= 5.54*20.27²

^N =2276 plates

The resolution of a elute is a measure of how two elution peaks can be differentiated in a chromatographic separation. It is defined as the difference in retention times between the two peaks, divided by the combined widths of the eluted peaks

Resolution for benzoic acid and aspirin is given by

R=1.18*(t_r2-t_r1)/(w_0.5² +w_0.5¹)

R=1.18(14.8-9.73)(0.73+0.35)

R=1.18*5.07/1.08

R=5.54

Here compound benzoic acid elutes before aspirin