For online SFE-LC systems, the trapping of analytes is performed by depressurizing CO2 via a restrictor in the solid-phase trap as shown in Fig. 7F (F1). Residual CO2 gas is sometimes eliminated directly by eluents [116,121], but usually, water is required to rinse this SFE trap before elution (Fig. 7F2) in order to replace any residual CO2 gas. Simultaneously, the analytes that exhibit low water solubility would continue to be absorbed on the trap column.