Online MAE, UAE, ASE and SFE techniques coupled with LC are developed mainly for semisolid and solid samples. There are some drawbacks of using online injection as the coupling approach for these techniques. Apart from the sample loss problem, the application of this approach will be limited by the incompatibility between the mobile phase and the extractants such as nonpolar solvents and supercritical fluids. According to the column switching approach,
analytes extracted are firstly trapped by a solid-phase column and then eluted to an analytical column after the valve is switched to position 2, as shown in Fig. 4B (IV). This approach can be considered as the online coupling of extraction techniques (for solid matrices) with hyphenated SPE-LC systems. The SPE trapping process facilitates the injection of the entire amount of extracted analytes and the elimination of the sample matrix effects, which will improve the analytical sensitivity and selectivity significantly.