Determination of (15)N in (15)N-enriched nitrite and nitrate in aqueous samples by reaction continuous flow quadrupole mass spectrometry.

The (15)N tracer method is the most suitable method for studying complex N transformation processes in microbiology and biochemistry. It entails the constant determination of the (15)N abundance of the inorganic nitrogen (N) compounds nitrite and nitrate. However, (15)N analytical methods are time-consuming, difficult to automate, and require at least 10 µg of N per determination. An additional obstacle in the case of nitrite is that it usually only occurs in very small amounts (ppb) dwarfed by much larger quantities of nitrate (ppm). More useful is an approach in which the N compound is selectively converted into a gaseous form suitable for direct measurement by mass spectrometry. By using this 'reaction continuous-flow mass spectrometry' (R/CFMS) we developed methods for the (15)N determination of nitrite and nitrate from tracer experiment samples, i.e. artificially enriched in (15)N. Because both methods are based on the same principle, one continuous flow setup connected directly to a quadrupole mass spectrometer for all determinations was used. Nitrite and nitrate are reduced to NO by iodide and titanium(III) chloride, respectively. The technique developed ensures a precision of relative standard deviation </=3% if at least 0.5 and 2 µg N with an abundance of >/=1 at.% are to be measured for nitrite and nitrate, respectively. Copyright 1999 John Wiley & Sons, Ltd.