Its about chromatography?

Question

In the process of paper chromatography i know that in seperation the most polar will be in the lowest band on the paper, while the least polar will be in the uppermost band .

My question is, why is the most polar on the botttom and the least on the top? Has it to do anything with weight?

Thanks in advance. :]

Answer 1

You're wrong.

I do my paper chromatogtraphy on round disks of paper flat on a table.

You say that the most polar is down and least polar is up?  Wrong.  Which way is "down" on a flat table?

If I did it on the edge of a table so that the solvent flowed down over the edge, you'd be wrong again.  

If you're getting the idea it has nothing to do with gravity, you be right.

If I used special paper, I could have the least polar and most polar switch places.  I guess it has nothing to do with weight.

Most chromatography is based on the interactions between the media and the chemicals.  If you look at it like a bunch of people walking down a busy sidewalk, it might help.  Some people stop and chat.  They say hello to the guy selling hot dogs.  Pick up a paper to read and maybe sit on a park bench for a while.  These people are interacting with their environment.  A lot.  Other people ignor everything and walk straight to their destination.  They avoid eye contact.  Don't say good morning to anybody.  (Hot dogs in the morning?)  They get to work first.  Since they're not distracted, they are fastest.

Paper is polar (unless it is specially treated).  Polar molecules have an "affinity" for it and spend more time close to it, maybe partially absorbed on the surface of the fibers.  The solvent will slowly wash it off one fiber only to find it clinging to another.  Non-polar molecules want nothing to do with the fibers.  While they don't repel it, they just bounce right off, easily flowing with the solvent.  

So, with paper chrom.  polar molecules will "hang" onto the paper and travel slowly, non-polar will igonre the paper and be washed on, following where the solvent flows.  

What do you think would happen if you used a nonpolar plastic powder as a media instead of paper?  Just the opposite.

Answer 2

It's not based on a substance weight, it's on the compound's polarity.

If the type of paper you are using is polar, then the most polar compound will stay at the bottom because it has high affinity with the paper (they have the same polarity). Therefore, the least polar compound will be found on the top part.

OR

Just remember this,

If PAPER is POLAR

most polar = bottom

least polar = top

if PAPER is NONPOLAR

most polar = top

least polar = bottom

Answer 3

It actually has more to do with the kind of chromotography paper you're using than atomic weight.  In one of my Biology classes, we were working with paper that was highly polar, so more polar compounds stuck to the paper, moving less with the water than the non-polar solutes that were being pushed up the paper with no attraction to the paper or water.

Since both the water and paper are polar, but in my case the paper was more polar, it was like dragging the polar solutes up the paper which required more force than the capilary action that drove the water up the paper could generate.  In lay-man's terms, it would be equivalent to dragging a plow behind you when the ground is hard and dry.  Sure, you'll move some, but you would get as far as would could without the plow.