A microfluidic device with ion-exchange preconcentration column and photometric detection with Schlieren effect correction

This work describes the construction and evaluation of a microflow analyzer containing a Dowex® 1X8 packed column and LED-based photometric detection with Schlieren effect correction. Flow structures, including a reservoir for the preconcentration column with dimensions 25.0 × 2.0 × 1.0 mm (l × w × h), were manufactured in a urethane-acrylate resin, and the ion-exchange particles (50-100 mesh) were manually packed before sealing the device. Photometric measurements were performed using an integrated flow cell with a 5.0 mm optical path by using optical fibers to guide the radiation from a 3.0 W RGB LED to the microfluidic device and from the microdevice to a photodiode (OPT-101). It was demonstrated that the Schlieren effect is appropriately corrected by subtracting the signals for two diodes of the RGB LED and, by setting the intensity of light sources before measurements, the discordant response to the effect at different wavelengths is compensated. The preconcentration column was easily and successfully integrated into the microfluidic device without any clogging or leaking of the working solutions, and swelling of the packing particles caused no damage to the sealed structures because of the elastomeric nature of the microfluidic substrate. The application of the proposed device to the determination of nitrite in fresh water provides a linear response (R2 = 0.99) from 0.05 to 0.20 mg NO2− L− 1 and an approximate increase in concentration by four fold was estimated when 3.80 mL of the sample/standard solutions is percolated through the column, providing recoveries from 96% to 101% for tested samples.