clarification needed

[rrm1987]

why concentration of buffer is very crucial in Mobile phase?

[Johnny Rod]

The dissolved ionics species give the liquid its ionic strength. More means more so more polar. Also if you put too much buffer in then mix it (offline or in the pumps) with too much organic solvent it could precipitate out. Too little buffer obviously could fail to maintain the pH.

[lmh]

... and the way I've heard it is this: too little buffer means the pH may change when the sample is acidic/basic, and if the pH changes so that it's close to the pKa of any analyte, there will be the following problem: the solvent very close to the edge of the column may be at a slightly different temperature to that right in the middle of the column, because as solvent passes down the column it's doing mechanical work, heating the column, and only the outside of the column is being maintained at constant temperature by the column oven. Ionisation and pH effects depend on temperature, so if the analyte is close to its pKa, it may be differently ionised at the outside and inside of the column, if the temperature dependence of its acid-base behaviour is different to that of the buffer. If it's differently ionised, it moves at a different speed, so you get peak broadening.

Perhaps someone else with better chemistry can correct me, or explain more clearly?

[Johnny Rod]

That could wlel be the case. Close to the pKa all manner of bad things can happen as you have too close a ratio of ionised and molecular species. Keep it simple!

[Klaus I.]

why concentration of buffer is very crucial in Mobile phase?

Maybe you should google with this term "How to glue wings on a pig".

[Vlad Orlovsky]

statements above are true for reversed-phase chromatography, buffer pH and buffer concentrationfor mixed-mode are much more critical and provides a great tool to change selectivity: