During the sample pre-treatment process, except for the selection of SPE cartridge sizes, there are still several factors that should be carefully considered.
Specification of SPE Cartridge
The first important factor to select an SPE cartridge is the filler. The principle is to ensure that the filler of the SPE cartridge has sufficient capacity to retain the target compound 100% on the column. On the other hand, the filler shall not be too much. The more the amount of the filler, the greater the amount of eluent will be required, which could be adequate for the target compound to be effectively eluted.
The capacity of the bonded silica cartridge is generally from 1% to 5% and the ion exchange cartridge is about 0.5 to 1.5 meq/g. Since there are many interferents except for the target compound, it is recommended that the filler capacity is more than twice the target compound content.
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.