In order to visualize the separation/preconcentration process of the Cr(VI) on the C18 microcolumns embedded in the transparent PDMS chip, a mixture of high concentration of Cr(VI) and Cr(III) standard solutions of 10 μL volumes is pumped simultaneously to the parallel columns to retain the Cr(VI) (Fig. 4a). Because the chromate has a relatively intensive yellow color, by taking a picture of the microchip, a densitogram of each microcolumn can be created by means of a densitometric software (e.g. CP Atlas 2.0, freeware thin layer chromatogram evaluation software) [29]. The densitograms, that is the plots of intensity (RGB values) of colors per pixels of the picture of the microcolumn, are shown in Fig. 4c.