Generally, formic acid improve the quality of the bands of components in HPTLC. How it works? What is the chemistry behind it?
That's a very broad statement, and not true for all compounds or TLC. By "quality", I am assuming you mean band fronting or tailing.
Some ionizable compounds can convert between an ionized, polar form and a non-polar form. Assuming silica TLC, the polar form retains more on the plate than the non-polar form. Formic acid shifts the equilibration so that the compound mostly stays in one form. For acidic compounds, the compound would tend to remain non-polar, and would elute earlier on silica. Basic compounds would tend to be ionized, and elute later on silica. Neutral compounds would show little effect. This also holds for reverse phase TLC as well, except non-polar compounds elute later.
There are many chemical explanations, depending on the components you want to separate, the TLC plate choice and the composition of the elution solution.
If for example you are working with componenents that are weak acids and a non-polar TLC plate, than adding formic acid (and hence working in an acid environment around pH 2-3) reduces the dissociation of you sample components. Making them more non-polar and thus improving the separation.
because of charged atoms in silica gel, the formic acid occupied that free zone of charge and potentiate a compound to run freely.
if you will not and formic acid or any , the charge atoms will interact with -OH group of compound and makes it a obstacle to run it out and lead fronting or tailing.
moreover, this interaction will depend upon number of -OH group in a compound,
means if your compound having more polarity, will interact more and if non polar , less interaction.
so this is the only reason to add formic acid or any, depends on nature of dissociation of acid.
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