The adsorbed amounts of Cr(VI) were eluted into small hydrophobic plastic vessels (5 mm part length of the bottom of a 0.1-mL polypropylene micropipette tip, ~ 40 μL) positioned just at the ends of the chromatographic microcolumns (Fig. 1d). The plastic vessels could be easily and tightly inserted into the flexible ports S; no leakage outside around the vessel could be observed.