Question

A.) What factors cause the order of elution to change?

B.) If two unknown compounds are analyzed via thin-layer chromatography first using pure petroleum ether as the eluant and then 75:25 petroleum ether-ethyl acetate, how will the R_f values between plates change or can you not tell with the given information?

Answer 1

A) elution of components depends on polarity of mobile phase, strength of stationary phase and length of column etc

1)Polarity of mobile phase =

Elution order changes with polarity of mobile phase , as polarity increases , movement of components increases towards downtrend . So elution time of individuals components decreases substantially with rise in polarity .so if you want separation of components within stipulated time ,then you have to select appropriate mobile phase combination .

2)Strength of stationary phase =

If affinity of individuals components towards stationary phase is greater than polarity of mobile phase , then components separation require more time to gets elute . So strength of stationary phase depends on size of particles , as particles goes smaller then resolution require more time .

3)Length of column=

As column length goes shorter , resolution speed increase but efficiency of separation gets decline .So proper column length selection is paramount step in separation .

B) Here you can definitely predict , polarity effect based on polarity index of individuals solvent.

Here petroleum ether is comparatively non polar than ethyl-acetate. So initially you have chosen petroleum ether (100%) as mobile phase , hence TLC pattern containing 100% petroleum ether as mobile phase would shows high Rf value for  individuals components as compared with TLC having ether-ethyl -acetate (75:25) as mobile phase .