Sub-Parts-Per-Billion Nitrate Method: Use of an N(2)O-Producing Denitrifier to Convert NO(3) or NO(3) to N(2)O

A more sensitive analytical method for NO(3) was developed based on the conversion of NO(3) to N(2)O by a denitrifier that could not reduce N(2)O further. The improved detectability resulted from the high sensitivity of the Ni electron capture gas chromatographic detector for N(2)O and the purification of the nitrogen afforded by the transformation of the N to a gaseous product with a low atmospheric background. The selected denitrifier quantitatively converted NO(3) to N(2)O within 10 min. The optimum measurement range was from 0.5 to 50 ppb (50 mug/liter) of NO(3) N, and the detection limit was 0.2 ppb of N. The values measured by the denitrifier method compared well with those measured by the high-pressure liquid chromatographic UV method above 2 ppb of N, which is the detection limit of the latter method. It should be possible to analyze all types of samples for nitrate, except those with inhibiting substances, by this method. To illustrate the use of the denitrifier method, NO(3) concentrations of <2 ppb of NO(3) N were measured in distilled and deionized purified water samples and in anaerobic lake water samples, but were not detected at the surface of the sediment. The denitrifier method was also used to measure the atom% of N in NO(3). This method avoids the incomplete reduction and contamination of the NO(3) -N by the NH(4) and N(2) pools which can occur by the conventional method of NO(3) analysis. N(2)O-producing denitrifier strains were also used to measure the apparent K(m) values for NO(3) use by these organisms. Analysis of N(2)O production by use of a progress curve yielded K(m) values of 1.7 and 1.8 muM NO(3) for the two denitrifier strains studied.