Question

3. If the TLC plate development had stopped before the solvent front reached the top of the TLC plate (i.e. maybe it reached the middle of the TLC plate when it was stopped, instead):

a) Would the Rf values have been affected? If so, how? If not, why not? Give a numerical example to support your answer.

b) Would the size of the spots have been affected? If so, how? If not, why not? (Hint: think about the diffusion of the spot on the TLC plate as it spends more time on the stationary phase. For example, what happens over time if you put a small drop of food coloring in water?)

4. What are the highest and lowest possible Rf values on a TLC plate?

5. Leaks from underground tanks spread gasoline and additives into the water supply chromatographically. The water carries them away as it moves through the soil. Among gasoline and the additives MTBE and ethanol, which would spread fastest? Which would spread slowest? Explain your answer in terms of intermolecular forces.

CH3(CH2)6CH3-Octane (one component of gasoline)

CH3CH2OH-Ethanol

CH3OC(CH3)3- MTBE

6. When a TLC separation is performed, it is typically done in sealed and a liner is sometimes used to help keep the air inside the TLC chamber saturated with the mobile phase solvent. If a TLC chamber is not saturated with the solvent in the air, how do you think the speed of development would have been affected? (Hint: how volatile is methylene chloride? Do you think it would say on the TLC plate if it was open to the air?)

7. If the distances had been measured in inches instead of mm:

a) Would the Rf values have been affected?

b) If so, how? If not, why not? Give a numerical example to support your answer.

8. If the solvent was allowed to reach the top of the TLC plate and not stpped until well after this happened:

a) Would the Rf values have been affected?

b) If so, how? If not, why not? (Hint: would the solvent keep traveling up the TLC plate? How does that affect the distance traveled of the compounds being separated?)

9. Does Rf have units? If so, what? If not, why not?

10. a) Ethyl acetate rises slower on a silica plate than methylene chloride. Explain why in terms of polarity.

b) How fast would water rise compared to ethyl acetate and methylene chloride.

11. A spot and its solvent front rise 1.3 cm and 6.4 cm, respectively.

a) What is the Rf value of the spot?

b) If the solvent front had risen 7.9 cm, how far would the spot have risen?

c) If the spot had risen 6.0 cm, how far would the solvent front have risen?

Answer 1

3. If the TLC plate development had stopped before the solvent front reached the top of the TLC plate (i.e. maybe it reached the middle of the TLC plate when it was stopped, instead):

a) Would the Rf values have been affected? If so, how? If not, why not? Give a numerical example to support your answer.

The solvent will have the uniform affinity for the stationary phase to move on it. But the the spot have different affinity for both the stationary phase and mobile phase. the resolution of peak or spot is governed by by no of theorical plates. The more no of theoritical plates more the resolution is and the spot separated with each other on the plate.

So here the development is stopped in between. then it will definately impact on the Rf value because it not attained the desired no of theoretical plates. it depends on case to case but most often the separation of two spots occur when they have different affinity for both the mobile phase and stationary phase. i have attached a photo for the explanation.  

  b) Would the size of the spots have been affected? If so, how? If not, why not? (Hint: think about the diffusion of the spot on the TLC plate as it spends more time on the stationary phase. For example, what happens over time if you put a small drop of food coloring in water?)

Answer: yes definately if will affected it depends on the affinity of the molecules for Stationary phase. initially the spot is sperical and when it goes up on the TLC its shape changes becuse if some impurities are there which is similar to the analyte but have different affinity for Stationay phase it will affect the shape of the spot. Tailing in the spot is also a example.

4. What are the highest and lowest possible Rf values on a TLC plate?

Answer: Highest will be 1 when the spot has no affinity for stationary phase and Lowest when it has no affinity for mobile phase. In both the cases we have to check the proper selection of Mobile and Stationary Phase.

5. Leaks from underground tanks spread gasoline and additives into the water supply chromatographically. The water carries them away as it moves through the soil. Among gasoline and the additives MTBE and ethanol, which would spread fastest? Which would spread slowest? Explain your answer in terms of intermolecular forces.

CH3(CH2)6CH3-Octane (one component of gasoline)

CH3CH2OH-Ethanol

CH3OC(CH3)3- MTBE

Answer: Like dissolves like. it depends on the affinity of these chemicals towards the soils. As the Octane and MTBE are not soluble in water while ethanol is soluble in water it shows how these chemicals have affinity for water. Gasoline and MTBE are spread fast as compared to ethanol.

6. When a TLC separation is performed, it is typically done in sealed and a liner is sometimes used to help keep the air inside the TLC chamber saturated with the mobile phase solvent. If a TLC chamber is not saturated with the solvent in the air, how do you think the speed of development would have been affected? (Hint: how volatile is methylene chloride? Do you think it would say on the TLC plate if it was open to the air?)

Answer: The TLC chamber should be saturated with vapor of mobile phase. An unsaturated chamber causes solvent to be evaporated from the TLC plate. if you are using a Solvent mixture as mobile phase in which One Solvent may be evaporated faster than the other can lead to a poorly shaped solvent front, and can give irreproducible results. A filter paper sheet placed in the chamber with the solvents helps to saturate the chamber.

7. If the distances had been measured in inches instead of mm:

a) Would the Rf values have been affected?

Answer: it will not much impact as such.

b) If so, how? If not, why not? Give a numerical example to support your answer.

if Solute front is 30mm and solvent front is 60mm. Then Rf= 30mm/60mm = 0.50

when it will be in inches then Rf = 1.1811/2.3622=0.50

8. If the solvent was allowed to reach the top of the TLC plate and not stpped until well after this happened:

a) Would the Rf values have been affected?

Answer: Yes it will impact on the Rf Value.

b) If so, how? If not, why not? (Hint: would the solvent keep traveling up the TLC plate? How does that affect the distance traveled of the compounds being separated?)

Answer: the binding between the Spot and stattionary phase is reversible not irreversible. if solvent keep on travelling with the spot it will be slowly washed off from the plate.

9. Does Rf have units? If so, what? If not, why not?

Rf is unitless. for e.g. if Solute front is 30mm and solvent front is 60mm. Then Rf= 30mm/60mm = 0.50.

both mm is cancelled.

10. a) Ethyl acetate rises slower on a silica plate than methylene chloride. Explain why in terms of polarity.

Answer: like dissolve like.

If depends on the affinity of solvent towards the stationary phase. if a solvent has more affinity for stationary phase it will move slowly and vice versa if a solvent have less affinity for stationary phase it will move faster. based on that a Eluotropic series of solvent has been made.

b) How fast would water rise compared to ethyl acetate and methylene chloride.

Answer: Same as Above

11. A spot and its solvent front rise 1.3 cm and 6.4 cm, respectively.

a) What is the Rf value of the spot?

Answer: 0.2

b) If the solvent front had risen 7.9 cm, how far would the spot have risen?

c) If the spot had risen 6.0 cm, how far would the solvent front have risen?