Development of Microfluidic Analysis Systems for Water Quality Monitoring

Abstract

A continuous-flow microfluidic chip-based standard addition/absorption detection system has been developed for accurate determination of nitrogen in water of varying salinity. The absorption detection of nitrite is made via color development using the Griess reaction. We have found the yield of the reaction is significantly affected by salinity (e.g., ?12% error for 30‰ NaCl, 50.0 μg L?1 N-NO2? solution). The microchip has been designed to perform standard addition, color development, and absorbance detection in sequence. To effectively block stray light, the microchip made from black poly(dimethylsiloxane) (PDMS) is placed on the top of a compact housing that accommodates a light-emitting diode, a photomultiplier tube, and an interference filter, where the light source and the detector are optically isolated. An liquid-core waveguide (50?80 mm) mounted on the chip externally has been employed as the absorption detection flow cell. These designs for optics secure the wide linear response range (up to 500 μg/L N-NO2? and 7.0 mg/L N) and the low detection limit (0.12 μg/L N-NO2? and 0.26 mg/L N, S/N = 3). Capillary-based on an UV-photooxidation reactor, and a copperized cadmium packed column have been developed for total nitrogen determination. From determination of nitrite and total nitrogen in standard samples and real samples collected from an estuary and coastal sea, it has been demonstrated that our microfluidic system is highly accurate (< 1.5% relative standard deviation (RSD), n = 3) and precise (< 1.0% RSD, n = 3). A simple, sensitive microfluidic chip-based chemiluminescence (CL) detection system has been developed for accurate chromium (Cr) speciation in water. The CL detection is based on the Cr(III)-catalyzed oxidation of luminol (5-amino-2,3- dihydro-1,4-phthalazinedione) by hydrogen peroxide in basic solution. To effectively block ambient light, a microfluidic chip is made from black PDMS, and a photon detector exists in a compact housing. The microchip has been designed to perform the reduction reaction of Cr(VI) to Cr(III), the CL reaction, and the selective removal of interfering metal ions (e.g., Fe(II), Co(II), Cu(II), Ni(II), and Fe(III)). The system can determine accurately Cr(III) and Cr(VI) over a wide pH range, and also has the low detection limit (Cr(III): 0.13 μg/L, Cr(VI): 0.33 μg/L, S/N = 3) and the wide linear range (Cr(III): 0?1000 μg/L (R2 = 1.000), Cr(VI): 0?2000 μg/L (R2 = 1.000)). The high performance allows it to apply Cr speciation in natural water and wastewater. The accuracy, precision of Cr determination in our system has been demonstrated by determining Cr(III), Cr(VI), and total Cr in wastewater taken from steel factories, and by comparing the results determined with using commercial, benchtop flame atomic absorption spectrometer and UV-visible spectrophotometer adopting standard methods (Standard Method 3110B and Standard Method 3500-Cr B).