we usually use gradient illusion but for isocratic, I want to know the percentage calculation of organic solvent
Nalla, for what purpose? Any previous info- use that to formulate, or try based on the polarity (calculate hydrophilic/lipophilic character through software-based prediction, and check for viability with the solvent polarity. Rate theory of chromatography, if you dealing with the separations, might help you!
Normally, the easiest and most practical way of choosing solvent system for isocratic elution is to make couple TLC plates (the same adsorbent which you have in column), and the mixture which cause good separation should be used. Usually, we try to achieve retention factor (Rf) of 0.3. If you have mixture of two solvents (for example ethyl acetate and hexane) you can try at start 10% ETAC/hexane and 50% ETAC/hexane. If you use normal phase chromatography (polar adsorbent), the more ETAC you add, the higher Rf will be. So, if Rf in both eluents is too high, you need to use more percentage of hexane, if too low in both you need to use less percentage of ETAC. If in 50% it is too high, and in 10% too low, you need to find something between 10% and 50%. If you can't achieve good separation you need to change solvent system.
Here you have more voluminous explanation:
https://www.reachdevices.com/SetUpColumn.html
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