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 nonsimultaneous 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 3 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 3 10 4 absorbance (linearity range up to 0.6 absorbance; detection limit of 1.9 3 10 27 m). 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 21 and the RSD at a 6.4 3 10 25 m 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.