Answer :
Hey there!
The reason why a pencil must be used to draw in a paper chromatography is because its meant to split up the mixture of compounds into its sub parts.
Hope this helped you
Tobey
Because ink in a pen would be seperated into it's different components on the chromatography paper. Graphite pencils just... don't dissolve like that, they aren't soluble like ink. Graphite would not seperate on the paper, whilst ink would.