Chromatography is a chemical
technique for separating mixtures of coloured chemicals. This technique is important in
biology as well as chemistry; it is also used by forensic scientists.
We all know that green plants are green because they
contain chlorophyll; we know that chlorophyll is green. Hang on; why are plants different
shades of green if they all contain the same green pigment? Well, perhaps it is not quite
as simple as you were told by your biology teacher. In fact there are several different
kinds of chlorophyll and some other photosynthetic pigments in green plants. So exactly
what colour they are depends upon which types of chlorophyll and other pigments their
leaves contain.
What has all this to do with chromatography?
What is chromatography and what can we do with it?
Well, for a start we could separate the different
pigments in leaves and find out a bit more about how photosynthesis works. Leaves contain
a mixture of two or more of the following pigments: chlorophyll a, chlorophyll b,
chlorophyll c, xanthophyll, carotene, phaeophytin and other pigments. The leaves of
different plants contain different pigments, hence their different colours. We could also
use chromatography to separate the different pigment in writing ink: a forensic scientist
might do this to find out if all the writing on a cheque was written with the same pen.
We can even use chromatography to separate mixtures
of colourless chemicals: it is possible to use the technique to separate the amino-acids
produced when a protein is digested. Once we have separated the mixture it is necessary to
stain the amino-acids using a coloured chemical. Ninhydrin is used; unfortunately this is
not a very nice chemical. Smoke cigarettes if you must but don't use carcinogenic
chemicals without taking the proper precautions.
Here is an experiment to try for yourself at home
(mum won't mind):
Get a square of blotting paper (you could use
paper towel).
Draw a line parallel to one edge about 2cms onto
the paper.
Put a series of small crosses in pencil along the
line.
Make a circular mark over one cross with one of
your felt-tip pens (water soluble).
Do the same to the other crosses with different
colours.
Now hang the piece of blotting paper in a dish of
water so that it is just touching the water.
You should see the water soak up into the blotting
paper. As the water rises up the blotting paper it will carry the ink from the felt-tip
pens with it. When the water gets near to the top you can take it out of the water and
leave it to dry. You should see that the ink marks have separated out into their different
colours. Maybe some of you felt-tip pens contain inks made of only one coloured chemical,
but most of them will contain mixtures.
This experiment will not work with non-washable
felt-tip pens unless you use alcohol or acetone instead of water. You could use some of
your parents' gin or vodka if there is any and they will let you have it, otherwise you
will have to do the experiment at school.Don't be surprised if you cannot get the
experiment to work with water and the chlorophyll in leaves, again you will have to use an
organic solent.
The solvent rises up the chromatography paper
(blotting paper) by capillarity. When the solvent reaches the "spot" it
dissolves the mixture of coloured chemicals. There is now a solution; this is a mixture of
solutes dissolved in the solvent. The molecules of these different chemicals are all
different sizes. The simple explanation is that the smallest solute molecules travel
almost as quickly as the solvent molecules and so get carried to the top of the
chromatogram. The largest solute molecules travel very slowly and stay near the bottom. So
some of the coloured chemical travel further than others. If you are doing this with
amino-acids you will not see anything happen until the end of the experiment when you
stain it with ninhydrin.
Here are some more terms for you to look up in your
textbook or library:
Thin Layer Chromatography (TLC). My
wife uses TLC to mean "tender loving care".
Electrophoresis (this is not
getting rid of the hairs on your legs with electric currents).
