Generally, SPE trapping is relatively easy using the weak extraction solvents (e.g., water as extractant for nonpolar stationary phases). In this case, the extracts can be directly pumped through a filter and then a column for trapping. This direct trapping method has been used in all the online PHWE-SPE-LC (or SWE-SPE-LC) systems [40–47,110,148] and a few online UAE-SPE-LC systems [54]. However, in most cases, it is necessary to dilute the strong extraction solvent with the aqueous solvent prior to trapping.