A new procedure for the conversion of bound nitrogen into nitrogen gas, applicable to solid material, is described. It embodies two main stages: the catalytic ammonification of the sample and the consecutive transformation of ammonium to molecular nitrogen by the modified Dumas method. The first step of the preparation is carried out in a quartz reactor, filled with granular nickel, at a temperature of 700°C, the ammonia gas produced being trapped in sulphuric acid. In the second step, this solution is neutralised, dried and subjected to the Dumas combustion.The addition of reagents does not influence the results of the measurements. Spectrometric 15N percentage abundances are converted into mass spectrometric "true" 15N percentage abundance by using two straight calibration lines, one over the lower range of abundances (0.36-2.4% 15N) and the other over the range 2.4-11% 15N. A relative error smaller than 2% for the recalculation of the standards substantiates the accuracy of the conversion .The method has been applied to the study of nutrient uptake by marine phytoplankton. The assimilation rate of the labelled compound was determined on samples of 2.5 mg of particulate matter with a maximum nitrogen content of 0.2 mg.
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