Two-tier online solid phase extraction for HPLC and LC-MS analysis

Integration of solid phase extraction (SPE) or column cleanup with liquid chromatographic (LC) analysis may take two approaches: 1) Direct coupling, in which a sample loaded in a cleanup column is completely transferred to the analytical column using a switching valve; 2) Indirect coupling, in which samples are treated like in the case of conventional offline cleanup. A portion of the collected fraction is then injected into the analytical column using a built-in autosampler in the SPE system or via the HPLC's autosampler.