Sample is injected (using a separate injector) onto an analytical column. Early eluting components (front cut) pass through a trapping loop and are detected at port 3. The valve is then switched, and the center (or heart cut), which was retained in the trapping loop, is injected onto the second column to the detector (at port 4).
Late eluting components (end cut) are trapped on the first column. When the valve is switched again, the end cut passes through the trapping loop to the first detector, completing the analysis.