Question

1. (a) Would the TLC analysis still work for all unknown drugs if you changed the stationary phase from silica to a C-18 hydrocarbon chain? Why or why not?

(b) If it would still work for some drugs, how might your R_f values be different and why?

2. (a) Would the TLC analysis still work for all unknown drugs if you changed the mobile phase from diethyl ether:methanol (90:10) to acetonitrile:methanol (90:10)? Why or why not?

(b) If it would still work for some drugs, how might your R_f values be different and why?

Answer 1

1a) The stationary phase used in TLC is silica, SiO₂. SiO₂ is polar due to the polar Si-O linkages. Polar stationary phases can bind polar analytes; hence more polar drug molecules (containing –OH, -C=O, -COOH, -NH₂, etc) groups are bound tightly by the stationary phase. A non-polar solvent like hexane or a combination of less polar diethyl ether and methanol in 90:10 ratio is used to elute the analytes. Less polar analytes, being more soluble in the non-polar solvent are eluted first while more polar solvents are eluted later. Infact, to elute polar analytes, the percentage of the polar methanol has to be increased.

When the stationary phase is changed from SiO₂ to C-18 bonded SiO₂, the stationary phase is less polar. This is due to the hydrophobic nature of the C-18 alkyl chain. Therefore, polar analytes will not be retained on the stationary phase and will be eluted out as soon as a solvent is added. Thus, separation of analytes will not be attained. Since most drug molecules contain polar side chains, changing the stationary phase will adversely affect the TLC separation.

b) A non-polar stationary is best suited for a non-polar analyte. Thus, when the stationary phase in TLC is changed from SiO₂ to C-18, non-polar drug molecules will be retained tightly while polar drug molecules will be eluted fast. Non-polar drug molecules show low R_f values with SiO₂ since they are poorly retained. However, when the stationary phase is changed to C-18, the R_f values will increase because the non-polar molecules are strongly retained.