MULTI-INSERTION OF SMALL CONTROLLED VOLUMES OF SOLUTIONS IN A FLOW ASSEMBLY FOR DETERMINATION OF NITRATE (PHOTOREDUCTION) AND NITRITE WITH PROFLAVIN SULFATE

A simple, maintenance-free, non-contaminating and completely automated flow system was developed for nonsimultaneous determinations of nitri te and nitrate, in which spectrophotometric measurements at 328 nm wer e performed on the nitrite reaction with proflavin sulfate [bis(3,6-di aminoacridinium)sulfate]. The sample solutions were automatically inse rted into phosphate buffer (pH 6.8) containing 10 mM EDTA by using a p rogrammable solenoid valve; then they flowed through a 6.4 m x 0.5 mm id PTFE coil, wrapped around an 8 W low pressure mercury lamp, and fin ally 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 x 10(4) absorbance (linearity range up to 0.6 abs orbance; detection limit of 1.9 x 10(-7) M). With the lamp on, to dete rmine 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 x 10(-5) M l evel 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.