9. (15 pts) Consider the following chromatogram showing separation of two compou

Question

9. (15 pts) Consider the following chromatogram showing separation of two compounds, A and B. The mobile phase was 0.1 M phosphate buffer modified with acetonitrile (70:30 v/v). Answer the questions that follow. Conponent A 600s Unvetained solue 100 Times a. Using the last eluting peak, calculate the number of theoretical plates b. Find the plate height if the column is 150 cm long c. Calculate the selectivity d. How would you modify the mobile phase composition so that the total elution time is less than 700 s?

Explanation / Answer

From the given chromatographic chart.

a. number of theoretical plates (N) = 5.54(t/w(1/2))^2

with,

t = 600 s

w(1/2) = 14.10 s

we get,

N = (600/14.10)^2 = 1811

b. Plate height (H) = L/N

with,

L = 150 cm

N = 1811

we get

H = 150/1811 = 0.083 cm

c. Using retention times for the two peaks and the unretained sample

Selectivity of two peaks = (600 - 60)/(360 - 60) = 1.8

d. To decrease the total elution time, we would increase the percentage of acetonitrile in the eluting solvent. This would push out the components faster from the column.

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