12. Analytical chem question on sapling correct answer will get rated! Thanks A

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12. Analytical chem question on sapling

correct answer will get rated! Thanks

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 (tm), caffeine (tc), and aspartame (ta) were 40.0 s, 108.7 s, and 177.2 s, respectively.

A) What is the selectivity factor (relative retention), , for this column?

B) Calculate the hypothetical retention times for caffeine and aspartame if their retention factors were reduced by 27.0%. Assume the mobile phase retention time (tm = 40.0 s) remains constant.

t caffeine = ? seconds; t aspartame = ? seconds

C) Choose the statements that correctly describe the outcome if the partition coefficient for caffeine (Kc) were to decrease (Hint: there is more than one correct answer).

1 The retention time of caffeine would increase. 2 The retention factor of caffeine would increase. 3 Caffeine would spend a greater amount of time in the mobile phase. 4 The retention time and retention factor of caffeine would not change. 5 The retention factor of caffeine would decrease. 6 The retention time of caffeine would decrease.

HINTS= The selectivity factor, or relative retention, for the column is the ratio of the retention factors of caffeine and aspartame. The retention factor is the ratio of the time a solute spends in the stationary phase to the time it spends in the mobile phase.

= The partition coefficient (Kc) describes the equilibrium of caffeine molecules between the stationary and mobile phases.

Explanation / Answer

(A): Here methanol is the mobile phase

retention time for methanol, tm = 40.0 s

retention time for caffeine, tc = 108.7 s

retention time for aspartame, ta = 177.2 s,

Now retention factor of caffeine, kc = retention time of caeffine / retention time of methanol = 108.7 s / 40.0 s

= 2.72

retention factor of aspartame, ka = retention time of aspartame / retention time of methanol = 177.2 s / 40.0 s

= 4.43

selectivity factor (relative retention), = Retention factor of caffeine / retention factor of aspartame

= 2.72 / 4.43 = 0.614

(B) Now the retention factors are reduced by 27 % i.e the retention factor is 73 % of the original.

Hence for caeffine, the new retention factor is

(73/100) x 2.72 = 1.99 =  retention time of caeffine / retention time of methanol

=> 1.99 = tc / 40.0

=> tc = 1.99 x 40.0 = 79.6 s

Hence hypothetical retention time for caeffine is 79.6 s

For aspartame, the new retention factor is

(73/100) x 4.43 = 3.23 =  retention time of caeffine / retention time of methanol

=> 3.23 = ta / 40.0

=> ta = 3.23 x 40.0 = 129 s

Hence hypothetical retention time for caeffine is 129 s

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