Briefly, a suspension (~ 100 μL) of freshly ultrasonicated, methanolic C18 particles of 5 μm was manipulated through a small-bore tubing (0.25 mm ID) using a peristaltic pump. The tubing was connected to the port E (Fig. 1a,b), and the particles were washed with methanol (~ 10 μL/min) toward the frit at the end of the channels (ports S).