Standard addition/absorption detection microfluidic system for salt error-free nitrite determination

• Microfluidic chip-based absorption detection system for salt error-free determination of nitrite in water.
• Standard addition, Griess reaction, and absorption detection play on the microchip in a continuous-flow mode.
• The salt error is caused by the reduction in the yield of Griess reaction.
• The use of black PDMS secures a wide linear range in detection.
• Externally mounted liquid-core waveguide detection flow cell facilitates optical path length change.

A continuous-flow microfluidic chip-based standard addition/absorption detection system has been developed for accurate determination of nitrite 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−1N-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) 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 80-mm liquid-core waveguide mounted on the chip externally has been employed as the absorption detection flow cell. These designs for optics secure a wide linear response range (up to 500 μg L−1N-NO2−) and a low detection limit (0.12 μg L−1N-NO2− = 8.6 nM N-NO2−, S/N = 3). From determination of nitrite in standard samples and real samples collected from an estuary, it has been demonstrated that our microfluidic system is highly accurate (<1% RSD, n = 3) and precise (<1% RSD, n = 3).

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