An ultrahigh-pressure dual on-line solid phase extraction/capillary reverse-phase liquid chromatography (DO-SPE/cRPLC) system is provided. The system comprises: a sample loading valve into which a first solvent and a sample to be analyzed are loaded; a first column valve in flow communication with a first solid-phase extraction column and a first reverse-phase liquid chromatography column; a second column valve in flow communication with a second solid-phase extraction column and a second reverse-phase liquid chromatography column; a column-switching valve for determining whether the sample is transferred to either the first column valve or the second column valve; a solvent selection valve in flow communication with the first and second column valves to supply the first solvent or a mixed solvent of the first solvent and a second solvent to the first and second column valves; a second solvent loading valve, in flow communication with a solvent mixer, into which the first and second solvents are loaded; a supply pump for loading the second solvent into the second solvent loading valve; and a supply pump for loading the first solvent into the sample loading valve, the second solvent loading valve and the solvent selection valve. The system requires minimal time (i.e. dead time) for column equilibration between successive experiments to shorten the total time required for the experiments by a factor of about two. In addition, the system enables rapid sample injection, on-line sample desalting and sample enrichment. Furthermore, the system is highly reproducible in terms of liquid chromatography (LC) retention time and can be operated at a pressure as high as 10,000 psi.