Pre-treatment Solvent Selection
For a reverse-phase cartridge or an ion exchange cartridge, the methanol or a polar organic solvent can usually be used as a pretreatment solution. Water or a buffer solution could be used as an equilibrium solution. For the pre-treatment of the normal-phase column, an elution solvent or a non-polar organic solvent would be a better choice.
Washing Solution Selection
When the target compound is a natural one, containing a hydrophobic non-polar filler (C4, C8, C18, for example), the cartridge is suggested to be washed with water or a buffer used for column equilibration. If the analyte has strong adsorption on the SPE cartridge, you need to enhance the washing effect with a water-soluble organic solvent added.
Hope this guidance can help you a lot!