Sometimes I see people adjust the pH of the mobile phase in TLC. How does this help in separation?
The principles of separation in TLC are the same as in (HP)LC, and so the same types of mechanisms apply. You will need to adjust the pH for TLC separations in the same situations you would consider changing the pH in LC - to avoid adsorption, to minimize tailing, etc. It depends on your application - what type of compounds do you need to separate (will they be affected by the change in pH, are they ionic or polar?) and what stationary phase do you use (will it change charge/form with pH)?
Sometimes organic acids or bases are added in small amounts to the developing solvent (in TLC the term mobile phase is inappropriate). You normally adjust the pH if you get bad tailing of your bands, but pH can also be a strong tool to fine tune the retention of the compounds on the stationary phase. For example, carboxylic acids often have pKa in the range 3-5 and are ionized at neutral conditions. This can cause poor retention of these analytes on RP TLC plates, but adding an acid to the developing solvent renders the analytes neutral, therefore you should get better retention. Additionally, low pH also minimizes silanol interactions with your basic analytes. Silica is by nature slightly acidic and at higher pH values the silanol groups can interact with your ionized basic analytes via ion exchange interactions. To avoid this phenomenon, again, a small amount of acid is usually added to the developing solvent to suppres the silanol interactions. Basic additives to the developing solvent have an analogous function.
Hope this helps...
If your analytes have ionizable groups they may give annoying tailing. Using pH control may help getting better shaped spots. Not always strict control is necessary but...
take 10 beakers, add 1,2,3,5,8% HAc to your solvent in 5 beakers, and the same percentages of ammonium hydroxide to the other 5, and compare them all with your neutral solvent to get an optimization. this is a 1 hour experiment.
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