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

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 300µm wide and 115µm 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 50V 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 20µM with a correlation coefficient of 0.985. The R.S.D. at 5µM NO3- was 8.3% (n=6) and the limit of detection (3sigma) was 0.51µM (0.026µgml-1) NO3-.