Column switching is a flexible online approach, which has been widely accepted in online solid-phase-based extraction techniques, such as hyphenated SPE-LC and in tube SPME-LC [133] systems. The typical configuration of these hyphenated systems is shown in Fig. 5. By the combination with autosampler, the sample loading, extraction, washing, elution and introduction can be controlled by LC software, which are completely automated. Nowadays, online
SPE-LC instruments have been commercially available. As shown in Fig. 5, there are two flushing modes of column switching to elute analytes from the trap column: straight-flush (A1 and B1) mode and back-flush (A2 and B2) mode. The back-flush mode is usually adopted, since it allows the analytes retained at the head of the trap column, especially the highly retained compounds, to be easily transferred into the LC column. In this mode, peak broadening can be efficiently minimized. There are two main extraction modes of online in-tube SPME-LC systems. One is the flow-through
extraction mode, in which sample solutions pass through a trap column (typically a capillary column) in one direction as can be seen from Fig. 6A [134,135]. The other is the draw/eject extraction mode [136,137], in which sample solutions are repeatedly drawn and ejected through the capillary column (Fig. 6B).