With SPE, the analyte in the sample matrix is first loaded onto the SPE sorbent with one solvent and later eluted off of it with another. Because the SPE technique must be quantitative to be of use, these loading and elution steps must be complete. If not all of the adsorbed analyte is washed off of the sorbent, for example, then the method recovery will be lower. One factor that can contribute to incomplete loading or elution is the speed at which the solvent is introduced. If the solvent flows through the sorbent too quickly, the analyte may not have sufficient time to completely load or elute from the sorbent due to the kinetics of the interaction.
Hence, when using an SPE procedure, be sure to select a flow rate of the loading/ wash/ elution solvents such that the physicochemical processed occurring in the cartridge have a chance to occur completely. Don’t forget that SPE sorbents typically have larger particle sizes than are used in analytical HPLC columns, so the mass transfer is slower!