d is retained on a stationary r was a measure of how long the 1. Retention time
ID: 541770 • Letter: D
Question
d is retained on a stationary r was a measure of how long the 1. Retention time (tR) is a measure of how long a compoun phase in column in column chromatography. In TLC, R compound was retained on the stationary p Rf? (2pts) hase. What is the relationship between tR and 2. What would be the order of elution if one separated the three compounds from base/base extraction (3-nitrotoluic acid, 2-nphthol and 1,4-dimethoxybenzene) on column chromatography using silica as stationary phase and ethyl acetate/hexanes as mobile phase? (6 pts) Explain. What would be the effect of changing the mobile phase in question #3 to ethyl acetate/triethyl amine (N(CH3CH3)3)? (4 pts) Explain 3. 4. In TLC lab when you separated spinach pigments', most sections found 45:55 acetone:hexanes mobile phase composition to give the best results. In column chromatography, the best mobile phase turned out to be 40:60 or 35:65 acetone:hexanes. What is the reason that the exact mobile phase composition from TLC does not give the best separation in column chromatography? Explain (4 pts) Why is it important to keep the solvent level above the stationary phase? What would happen if one lets the solvent get much lower than the silica statio 5. nary phase? (4 pts)Explanation / Answer
1. Retention Time (tR)
Retention time is the time it takes a solute to travel through the column. The retention time is assigned to the corresponding solute peak. The retention time is a measure of the amount of time a solute spends in a column. It is the sum of the time spent in the stationary phase and the mobile phase.
Retention Factor (k)
k = (tR - tM) / tM = tR' / tM ......... Equation 1, where tR = retention time, tM = retention time of unretained compound and tR' = adjusted retention time
The retention factor (k) is another measure of retention. It is the ratio of the amount of time a solute spends in the stationary and mobile phases (carrier gas). It is calculated using Equation 1. The retention factor was previously called the partition factor or capacity factor. Since all solutes spend the same amount of time in the mobile phase, the retention factor is a measure of retention by the stationary phase. It is a relative measurement and is linear. For example, a solute with a k = 6 is twice as retained by the stationary phase (but not the column) as a solute with a k = 3. The retention factor does not provide absolute retention information, but it provides relative retention information. An unretained compound has k = 0.
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