SPE cartridge can be divided into three classes on the markets: silica-based cartridges, polymer-based cartridges, and adsorption-based cartridges. And here we talk about the three types of silica-based cartridges.
The first type is the C18 cartridges. This kind of cartridges bond the octadecyl to the silica gel, that can make use of the non-polar interaction to the solid phase extraction. Because of highly–sealed which inhibit the interaction of the cation interchange caused by silanol group, they decreased adsorption of alkaline compounds. This kind of cartridges is the best choice when using non-polar interaction to extract substance in the experiments.
The second one is the C8 cartridges. Compared with C18 cartridge, this kind of cartridges works on when the force of non-polar interaction is weak. It bonds the octyl group to the silica gel.
The last one is NH2 cartridges. NH2 cartridge is SPE cartridge with silica gel bonded aminopropyl. The main interaction is the interaction of anion exchange and the polar interaction. The secondary interaction is non-polar interaction. They are usually used in removing the anionic compounds such as organic acids and sugars in herbicide residues.