Question

When ACE, ASP and CAF were analyzed by TLC using 2:9:9 mixture of t-butyl alcohol, 4-methyl-2-pentanone and ethyl acetate respectively the following Rf values were obtained: ACE (0.51), ASP (0.15) and CAF (0.35). These results are not the same ones obtained when using 2% acetic acid and 98% ethyl acetate as the mobile phase? Explain why were the results different for our case (TLC shown) particularly for caffeine.

I know that the 2%/98% mobile phase is mostly nonpolar, so asprin has a higher Rf (because its the most nonpolar) and then the second is acetaminophen longest, and caffeine (being the most polar) travels the shortest distance. But, I don't understand the first part of the question. Thank you for your help!

Answer 1