Relative migration sequence of organic compounds:
In normal phase adsorption chromatography, the stationary phase is more polar than the mobile one. Thus low-polar solutes, like aliphatic hydrocarbons, are only slightly adsorbed while the polar solutes are strongly adsorbed.
As a result, low-polar compounds (like aromatic hydrocarbons) will then migrate faster than polar compounds: their retardation factor will be higher.
Eluent choice: it is crucial because it allows for the fine adjustment of the differential migration rates of compounds. To this end, solvent mixtures can be used.