In TLC, which is better; higher value of Rf or smaller value of Rf?

Answer 1

It really depends on what you are trying to do. To purify something quickly, a higher Rf will be best, although, this may not get rid of all impurities since the compound will fly off of the column. A lower Rf will give you more control on the purification.

If you are separating 2 compounds, you should concern yourself with the delta Rf (or the distance between the two spots). A larger delta Rf is desirable.

Answer 2

normally you choose the eluent so that the desidered compound has Rf=0.3 for a column.

for qualitative purity analysis of a coumpound personally I do 3 TLC: 1 with the desidered compound with Rf=0.5 (for maximum space over and under the main spot) , then Rf=0.9 (for analysis of more polar impurities) then Rf=0.1.