Determination of trace amounts of nitrogen in steel by gas-diffusion/flow injection-waveguide capillary cell spectrophotometry

We designed a new, simple and precise continuous-flow type analytical method for nitrogen determination combining flow injection analysis (FIA) and waveguide capillary cell (WCC) spectrophotometry. We used a gas-diffusion FIA system for sample preparation and introduction. We prepared steel solution decomposing steel sample with hydrochloric acid. First, nitrogen forms NH4+ in solution changes into NH3 gas in strong alkaline media. Only NH3 gas is separated from the matrix through porous Teflon membrane tubing. Finally, an indophenol type blue dye is produced by the reaction of NH3 gas and some color-producing reagents for spectrophotometric detection. As the detector, we applied the WCC spectrophotometer. A 50 cm long WCC enhances the sensitivity about 50-times compared with a 1 cm conventional cell. By this method, the analytical results of nitrogen determination in four certified reference materials conformed well with the certified values. The variation for 2 μgN per g steel was about 3% (n=3). Moreover, comparing the analytical results of the proposed method with those of the conventional (JIS) method by using two steel sample, we found good agreement.