A student researcher performed a chromatographic separation of caffeine and aspa
ID: 523029 • Letter: A
Question
A student researcher performed a chromatographic separation of caffeine and aspartame with methanol as the mobile phase using a C = 18 column. The retention times for methanol (t_m), caffeine (t_c), and aspartame (t_a) were 28.8 s, 94.6 s, and 164.5 s, respectively. A) What is the selectivity factor (relative retention), alpha, for this column? alpha = B) Calculate the hypothetical retention times for caffeine and aspartame if their retention factors were reduced by 25.0%. Assume the mobile phase retention time (t_m = 28.8 s) remains constant. t_caffeine = t_aspartame = Choose the statements that correctly describe the outcome if the partition coefficient for caffeine (K_c) were to decrease The retention factor of caffeine would increase. The retention time of caffeine would decrease. The retention time of caffeine would increase. The retention factor of caffeine would decrease. Caffeine would spend a greater amount of time in the mobile phase. The retention time and retention factor of caffeine would not change.Explanation / Answer
A.] The selectivity factor () is defined as the ability of the chromatographic column to differentiate between two constituents of the sample. is always greater than 1. Higher the value of , greater is the resolution between two chromatographic peaks.
= (ta – tm) / (tc – tm)
= (164.5 – 28.8) / (94.6 – 28.8)
= 135.7 / 65.8
= 2.062
B.] Retention factor (k), also known as the capacity factor, is the measure of the retention of a constituent in a sample.
k = (tr – t0) / t0
where, tr = retention time of an analyte
Let’s calculate the initial k using the retention time values given in the question.
kcaffeine = (94.6 – 28.8) / 28.8
kcaffeine = 65.8 / 28.8
kcaffeine = 2.285
kaspartame = (164.5 – 28.8) / 28.8
kaspartame = 135.7 / 28.8
kaspartame = 4.712
The retention factors reduce by 25 %. So, the new retention factors will be kcaffeine = 1.714 and kaspartame = 3.534.
Use these new retention factors to calculate the new retention times.
kcaffeine = (tcaffeine – 28.8) / 28.8
1.714 = (tcaffeine – 28.8) / 28.8
tcaffeine = 78.163 s
kaspartame = (taspartame – 28.8) / 28.8
3.534 = (taspartame – 28.8) / 28.8
taspartame = 130.579 s
C.] Partition coefficient K is defined as the moles of solute in the stationary phase to the moles of the solute in mobile phase.
K = Cs/Cm
The retention time is directly proportional to the partition coefficient. Bearing this information in mind, we can say that:
The retention time of caffeine would decrease.
The retention factor of caffeine would decrease.
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