Question

2. (5) How is column chromatography related to TLC? Gas chromatography? Be sure to discuss mobile phase, stationary phase, sample size, sample type in your answer. 3. (2) How is Rf in TLC related to elution order in column chromatography? What do you expect to come out first and last?

Answer 1

2. Chromatography is an analytical method to seperate different components of a mixture. There are two important terms in chromatography - stationary phase and mobile phase. Stationary phase is generally a solid (sometimes can be a liquid also) is held stationary while mobile phase is a liquid or gas that moves through the stationary phase and carry the components of the mixture along with it.

In column chromatography, the stationary phase used is silica or alumina and the mobile phase is the solution of the mixture with a suitable solvent. The mobile phase is eluted along the stationary phase and the components are seperated according to their polarity. The stationary phase being polar will bind strongly with the polar component of the mixture and hence it is eluted later, So the non polar components are eluted first followed by polar components.

In thin layer chromatography, the stationary and mobile phase is same as column chromatography. the only difference is in the order of elution. Since in TLC the movemet is against the gravity unlike in column chromatography which is with the gravity. Thus in TLC, the non polar solvent are eluted first followed by the increase in polarity of components.

In gas chromatography, the stationary phase is a liquid and the mobile phase is a carrier gas that consists of inert gas like helium or unreactive gas like nitrogen. Its principleis similar to that of column chromatography. The sample is injected inside the column and the rate of adsorption determines the seperation of various components.

3. R_f (retention factor) is the ratio of distance travelled by a component by the distance travelled by the solid. The order is opposite in column chromatography and TLC. Since in TLC the movemet is against the gravity unlike in column chromatography which is with the gravity. In column chromatography, the stationary phase being polar will bind strongly with the polar component of the mixture and hence it is eluted later, So the non polar components are eluted first followed by polar components. But in TLC since the mottion is opposite, so,the non polar components will show spots above in the TLC plate followed by the polar components which will shpw spots below in the TLC plate.