A New Approach to Determining the Content and 15N Abundance of Total Dissolved Nitrogen in Aqueous Samples: TOC Analyser-QMS Coupling

Abstract

The standard method for determining the ¹⁵N abundance of total dissolved nitrogen (TDN) in aqueous samples (e.g., soil leachate, sewage, urine) is currently Kjeldahl digestion followed by steam distillation or diffusion to isolate the ammonium, and then ¹⁵N measurement using IRMS. However, this technique is both time-consuming and laborious. One way of overcoming these disadvantages could be to couple a TOC analyser to determine the TDN with a sufficient quadrupole MS to determine the ¹⁵N abundance. The highTOC analyser (Elementar Analysensysteme Hanau, Germany), which catalytically oxidises the sample's total nitrogen with a high, constant yield to nitrogen monoxide (NO), appeared particularly suitable. The quadrupole-MS ESD 100 (InProcess Instruments Bremen, Germany) proved to be a suitable mass spectrometer for the ¹⁵N determination of NO. This combination of instruments was found to provide a workable method in numerous measurements of standard and actual samples. The detection limit concerning the N amount required per analysis is 2 μg, corresponding to an N concentration of 0.7mg/l in a maximum sample volume of 3ml. Depending on the N concentration, ¹⁵N abundances starting from 0.5 at.% can be measured with the required precision of better than 3% (simple standard deviation). For example, measuring the abundance of 0.5 at.% requires about 50 μg N, whereas for 1 at.% or more only about 5 μg N is needed per analysis.