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 218.3 s with a baseline peak width, wc, of 13.6 s and the retention time for aspartame, ta, was 274.3 s with a baseline peak width, wa, of 21.3 s. The retention time for the unretained solvent methanol was 49.0 s. Calculate the average plate height in micrometers for this separation, given that it was performed on a 25.9-cm long column.

A student researcher performed a chromatographic separation of caffeine and aspartame. The retention time for caffeine, to, was found to be 218.3 s with a baseline peak width, wc, of 13.6 s and the retention time for aspartame, ta, was 274.3 s with a baseline peak width, wa, of 21.3 s. The retention time for the unretained solvent methanol was 49.0 s. Calculate the average plate height in micrometers for this separation, given that it was performed on a 25.9- cm long column. Incorrect. Number See the lower panel for more information on how to determine the plate height. H 8050 Hm Calculate the resolution, R, for this separation using the widths of the peaks. Number R 3.21 Calculate the resolution if the number of theoretical plates were to increase by a factor of 2 Number R, 4.54 2N

Explanation / Answer

The relationship between theoretical plate height H, the length of the column L, and the number of theoretical plates N can be expressed as:

H = L/N

or we can write this also:

L = H·N

Since we know H, we just need to find N.


The formula for finding the number of theoretical plates is:

N = 16(ta/w)²

therefore;

N = 16(274.3/21.3)²
= 191

Here L = 25.9-cm

So,

H = L/N

= 25.9 cm/ 191

= 0.1356 cm

= 1356 m

B) The resolution between the two elutes can be calculated using this formula:

R = t/w(avg)

t = 274.3 - 228.3
= 56.0 s

w(avg) = (21.3 + 13.6)/2
= 17.45 s

Then;

R = t/w(avg)

R = 56.0 s /17.45 s

= 3.21

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