Two alternative methods are available to measure 15N abundance in samples: mass spectrometry and emission spectrometry. Mass spectrometry is the only sufficiently precise method for measurements at the level of natural abundance, while emission spectrometry can only be used with enriched materials. Hence, mass spectrometry is the most commonly used technique (Knowles and Blackburn 1993).
Before analysis, N in samples needs to be converted to N2. Two possible methods are available: the Dumas and the Rittemberg methods. The Dumas method consists in a complete combustion of the sample followed by a complete reduction of N oxides to N2. This method is commonly used when a mass spectrometer is coupled with an elemental analyser. The same procedure, without combustion and only with reduction, can be used for N oxides in gaseous samples (Mosier and Schimel 1993). On the other hand, the Rittemberg method consists in an oxidation of NH4+ to N2 by hypobromide. When N is present in chemical forms different from ammonium, a Kjeldahl digestion with a nitrate reduction is usually carried out first (Knowles and Blackburn 1993). If the Kjeldahl method is used, corrections must be made (a) to calculate the total N in samples using the true average atomic mass of N; (b) for the extraneous N that contaminates the sample (N dissolved in sulphuric acid) and decreases the 15N enrichment of a labelled sample (Powlson and Barraclough 1993). Examples of 15N abundance calculations from mass/charge (m/e) ratios obtained with mass spectrometry can be found in the study of Mulvaney (1993) and from emission spectra in the study by Preston (1993).
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