Multi-insertion of small controlled volumes of solutions in a flow assembly for determination of nitrate (photoreduction) and nitrite with proflavin sulfate

Abstract

A simple, maintenance-free, non-contaminating and completely automated flow system was developed for non- simultaneous determinations of nitrite and nitrate, in which spectrophotometric measurements at 328 nm were performed on the nitrite reaction with proflavin sulfate [bis(3,6-diaminoacridinium)sulfate]. The sample solutions were automatically inserted into phosphate buffer (pH 6.8) containing 10 mM EDTA by using a programmable solenoid valve; then they flowed through a 6.4 m × 0.5 mm id PTFE coil, wrapped around an 8 W low pressure mercury lamp, and finally they merged with a hydrochloric acid solution of proflavin. With the lamp off the nitrite in the sample was selectively measured with a sensitivity of 0.75 × 10⁴ absorbance (linearity range up to 0.6 absorbance; detection limit of 1.9 × 10^–7M). With the lamp on, to determine the sum of nitrite + nitrate there was a decrease in sensitivity of 41% versus that of the selective variant for nitrite determination. The sample throughput was 15 h^–1 and the RSD at a 6.4 × 10^–5M level of nitrate was 0.4% (n = 15). Many interferences were studied and simple methods of correcting for them were applied to analyse samples such as waters, soils, fertilisers and meat.