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1.1.1 DETERMINATION OF THE CONDITIONS BY THIN LAYER CHROMATOGRAPHY
Before doing the purification by flash chromatography, you should first determine the experimental conditions by using thin layer chromatography. With TLC you will know how many products are in your crude reaction and you will be able to determine the relative polarity of each one. So let have a brief overview of the thin layer chromatography technique. TLC is a very simple and useful chromatographic method. With this technique you can optimize your experimental conditions without using a lot of your compounds and it is fast. This is good if you are using the same stationary phase for the TLC and the flash chromatography. Using both SiliCycle TLC plates and SiliCycle UltraPure Flash Cartridges (or silica gel) will give you reproducibility of your experimental conditions. The mechanism of TLC is based on the affinity of the compound for the stationary phase and the mobile phase. Every product will not have the same strength with which it binds to the silica gel (or the stationary phase) so the retention time will be different for each product. The principle of the TLC is very easy to understand. You spot your crude reaction mixture at the bottom of the TLC plate and you elute the plate into a beaker in which you previously put a small quantity of solvent (called eluent). The spot where you put your sample on the TLC should not be mixed with the solvent. You let the solvent elute up to around 1 cm from the top of the TLC plate. When the elution is finished, you should be able to reveal the different products on the plate. The easiest way to reveal your product is with an UV lamp. But this is possible only for unsaturated products. So that is why dyes are often used to be able to see where the products on the TLC plate are.
After revelation of the TLC, you can interpret the results. The better way to know if the experimental conditions that you used will be good enough to do the separation is to use the retention factor (Rf) of every product. The Rf refers to the distance that a product eluted compared to the solvent front. The determination of the Rf parameter is based on a mathematical equation that uses the distances traveled by the compounds and the solvent.
The optimal condition to do a flash chromatography is to use a solvent that will provide an Rf parameter of the product to purified of around 0.25 and all the other components as far as possible from the desired product (at least a difference of Rf of 0.2 unit). To reach that goal and have a good separation, sometime you can’t only use one solvent. That is why you should play a little bit with the mobile phase. There is known possible techniques to determine the mobile phases: