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I just learned about the thin layer chromatography, which I think is basically the same as the paper chromatography.

I am wondering what is the function of the thin layer? I mean, what's special about thin layer, compared to normal chromatography paper? What mechanism/principle/property makes the thin layer special?

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    $\begingroup$ Thin layer chromatography uses a stationary phase which is alumina or silica. These phases are much more polar than paper. So they are more efficient than paper for detecting impurities in a given mixture. $\endgroup$
    – Maurice
    Nov 2, 2021 at 20:26
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    $\begingroup$ In addition, to the above, TLC compares well to column chromatography as for the stationary phase is the same or at least similar. Column chromatography is a purification method of outmost importance in organic chemistry. $\endgroup$
    – Alchimista
    Nov 2, 2021 at 20:51

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If you recall Tswett's original discovery of chromatography, he used a calcium carbonate column to separate the chlorophyll pigments (or different types of chlorophylls). Now calcium carbonate is a very fine powder and the permeation of liquid through a powder is very slow, especially if the column is long. Pressure drop across a column scales as 1/($d_p^2$), where $d_p^2$ is the particle diameter.

Paper chromatography was well known in the 1930s, it was relatively faster but you are limited by choice of stationary phases, just cellulose. In order to have "paper chromatography" with other stationary phases such as silica, alumina etc., a thin layer chromatography was invented. The main purpose was speed of separation for quality control of pharmaceutical products, as it is today!

So this "thin" in TLC is akin to a thin paper sheet, but made of different stationary phases. Note that if you use a thick layer, the capillary action will not be uniform, and the bands of analytes will be distorted badly.

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    $\begingroup$ I think you should add something on the line of my comment under the question. In fact, by TLC one can check the completion of a reaction as well as the purity of the crude, but very often also deduce what would be the best column for preparative purification purpose. $\endgroup$
    – Alchimista
    Nov 3, 2021 at 8:29

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