A student researcher performed a chromatographic separation of caffeine and aspa

Question

A student researcher performed a chromatographic separation of caffeine and aspartame. The retention time for caffeine, tc, was found to be 208.7 s with a baseline peak width, wc, of 13.0 s and the retention time for aspartame, ta, was 258.2 s with a baseline peak width, wa, of 22.5 s. The retention time for the unretained solvent methanol was 47.1 s.

A) Calculate the average plate height in micrometers for this separation, given that it was performed on a 29.4-cm long column.

B) Calculate the resolution, R, for this separation using the widths of the peaks.

C) Calculate the resolution if the number of theoretical plates were to increase by a factor of 2.5.

Explanation / Answer

A) Number of theoretical plates (N) = 16(tr/w)

tr for caffeine = 208.7 - 47.1 = 161.6 s

w = 13 s

N = 16 x 161.6/13 = 199

Plate height = L/N = 29.4/199 = 0.15

tr for aspartame = 258.2 - 47.1 = 211.1 s

w = 22.5 s

N = 16 x 211.1/22.5 = 150

Plate height = 29.4/150 = 0.196

Average plate height = 0.173 cm = 1730 mm

B) Resolution = 2(tr2 - tr1)/(w1+w2)

= 2 x (211.1 - 161.6)/(13 + 22.5)

= 2.8

C) If N = 2.5 times of 174 = 435

with 174 being the average number of plates

tr2 = 16 x sq.rt.(497.5/2) = 252.35 s

tr1 = 16 x sq.rt.(375/2) = 219.1 s

Resolution = 2 x (252.35 - 219.1)/(13 + 22.5) = 1.87

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