The development of an on-chip micro-flow injection analysis of nitrate with a cadmium reductor Original Research Article

The development of a method to determine nitrate by micro-flow injection analysis (μFIA) is described, using electro-osmotic flow (EOF) control in a micro-chip with 300 μm wide and 115 μm deep channel wet etched in borosilicate glass. An in situ miniature copperised cadmium reductor column was produced by immobilisation within a micro-porous silica frit made from 10% formamide and 21% silicon dioxide. The reductor frit was located in the sample introduction reservoir of the device. Nitrate standards were placed in the sampling reservoir and an applied voltage of 50 V relative to a waste reservoir was used to move the nitrate ions through the reductor frit for μFIA. Silica frits without copperised cadmium were also introduced into the interconnecting channels of the μFIA device to promote EOF and minimise hydrodynamic movement of solutions. The nitrite produced on leaving the reductor frit was detected using the Griess diazo-coupling reaction. The reaction was monitored in situ using a micro-spectrometer. A calibration was obtained between 0.5 and 20 μM with a correlation coefficient of 0.985. The R.S.D. at 5 μM NO3− was 8.3% (n=6) and the limit of detection (3σ) was 0.51 μM (0.026 μg ml−1) NO3−.