Chromatography experiment gone wrong? crushed grass..?

Question

I was thinking of giving this lesson to some kids from a practical worksheet from one of our textbooks. However none of the other staff have done this particular one. So anyway I tried it a few times and it does work BUT the questions on the worksheet dont seem to add up to what i see they are:

A. how many spots do you see and

B. What colour are the spots

you crush grass with mortar and pestle in propanone.Apply drops repeatedly to a single area on chromatography paper each time allowing to dry .till its a dark green spot.Dip the end of the paper into propanone and let it soak it up till top of paper. But what I see then is just a trail of green colour going up the paper, dark at bottom light at top. Theres no spots as such? Any ideas what I am supposed to see or are these spot questions wrong?

help!

Answer 1

could you maybe alter it and compare where the pigment stops in two or three different grasses.

Ive never heard of it using grass before. Stick to food colouring

Answer 2

Hmmm.  I have done this before in organic chemistry lab and it worked very well.  First of all, are you using the correct chromatography paper?  Liquid chromatography relies on a balance between the forces between the solvent, the pigment, and the chromatography medium (the paper).  If you use too polar of a solvent relative to the paper, it will just carry all the pigment with it to the top (that's how our experiment turned out in High school chemistry where the teacher used any random paper and solvent).  Not polar enough, and the dot won't get washed up quickly enough and will stay there.  If you use bad paper, then it might have too much of a mix of stuff in it and the colors might smear (I'm guessing here, but that's what it looked like when we did it on cheap coffee filters--the paper probably needs to be very homogeneous) So, maybe you need to try different paper or different solvent (acetone, methylene chloride, methanol, ethanol...).  How fast is the solvent wicking up the paper?  I recall it took quite a while, maybe 10 minutes to a half hour for the experiment to finish, but we definitely got separation of colors.  

You are trying to separate the different types of chlorophyll, carotenes,  and I think xanthophyll (?).   The color differences are fairly subtle between the chlorophylls so you might need to just give it more time or distance to travel on the chromatography paper.  I think we once did the experiment with spinach, which was easy to extract chlorophyll from.  I think we also did it with autumn leaves, which have anthocyanins, which are red, and separate out easily.

Oh, and I'm not sure food coloring would necessarily work especially if it is all one single chemical.  The point of the experiment is ultimately to understand the makeup of the colors and molecules in plants.  That's an important part of the scientific method to understand biology, and chromotography is one of the ways that scientists isolate biological chemicals.  You can let the paper dry and cut out the different colors, extract them back into solution, and then do spectroscopy on the individual chemicals.