The most important part of SPE is selecting the proper method to the compound of interest. The separation of the compound depends on its polarity. If the analyte is mid- to nonpolar than a reversed phase SPE would be the right method where the solid phase is nonpolar and the liquid phase is polar. The hydrophobic interaction will be between the analyte’s CH bonds and the functional group of the silica surface.
If the analyte on the other hand polar than the normal phase SPE should be used where the interaction will be between the polar functional groups of the sorbent and the functional group of the analyte.
The other sorbents are either ion exchange or adsorbents. In the ion exchange sorbents the electrostatic attractions of the charged groups of the surface of the sorbent and the charge group of the analyte. The adsorption SPE uses interactions of the analyte to the unmodified materials of the sorbent, which can either be hydrophilic or hydrophobic.