Chromatographhy Definition

Vapor Phase Chromatography. This is accomplished by constructing or buying complicated and
expensive equipment. Although this method is very effective, it is superseded by the simple,
inexpensive and effective column chromatography.
Thin Layer Chromatography.
Thin layer chromatography is primarily a tool for small qualitive
analysis (deciding which solvents elute which substances, etc.). A microscopic amount of sample
is applied at one end of a small plate covered on one side with a thin absorbent coating. The
plate is then dipped into a shallow pool of solvent which rises on the coated layer, permitting
the compounds of the sample to move with the solvent to differing heights. The individual
components can then be detected as separate spots along the plate. Unfortunately this process can only be scaled up to do several grams at a time, again making column chromatography the
champion of chromatography. If, however, you wish to use thin layer, consult your local
library on methods. I chose not to go into depth on thin layer because it is so inferior to column style. solvent Column Chromatography.

The main idea here is to dissolve your mixture and put it on the adsorbent, at the top of the
column. Then you wash the mixture down the column using at least one eluent (solvent), perhaps more. The compounds of your mixture are carried along by the solvents and washed out of the column at different rates and collected into separate flasks. Why do you want to do this? Let us say you have a substance that needs to be purified, but it cannot be distilled because it decomposes at a low temperature, or you wish to extract one of many mixable liquid substances that have been mixed together, etc. A column chromatography can separate,
purify and extract. sand alumina, etc.
Now you may open the valve until there is a little over 1 cm of solvent above the top layer
of sand. If there are any cracks or air bubbles in the adsorbent, dump everything and start over.
Dissolve the mixture (your substance) in the same solvent you are going to put through the
column, keeping the amount as small as possible (this is called the analyate). You should be using
the least polar solvent that will dissolve your substance. Now you may add the analyate very
carefully; do not disturb the sand. Open the valve until the level of the column is the same as
it was before you added the analyate (1 cm above the sand). At no time let the solvent level
drop below the sand! Add the required eluent (solvent) to the column, not disturbing the sand.
Open the valve to slowly let the eluent run through the column until the first compound comes
out. Collect the different compounds in different flasks. At no time let the solvent drop below
the top of the sand! If necessary, stop the flow, add more eluent, and start the flow again.
Should the compounds be colored, you can watch them travel down the column and separate,
changing collection flasks as the colors change. If your compound is clear then you will have
to use one of the following steps:
1. Occasionally let one or two drops of eluent fall onto a microscope slide. Evaporate the
solvent and see if there are any properties of the compound that should be coming through,
such as crystal shapes, tastes, smells, viscosities if oil, etc.
2. Occasionally use several drops to spot, develop, and visualize a thin layer chromatography
plate. Although thin layer is very similar to column, you should read up on it as I do not
have time to go into the complete operation.
If you find the eluents are taking an excessive amount of time to wash down the compounds,
then switch to the next most polar solvent. If you had two compounds and one of them is already
collected, then go ahead and get some really polar solvent and get that last compound pronto.
List of solvents arranged in order of increasing polarity.