Some problems of the determination of the nitrogen mass in samples enriched with $sup 15$N

As a rule, the equivalent weight of nitrogen is said to be 14.008. However, this is right with regard to nitrogen having normal isotopic composition (/sup 14/N = 99.63%, /sup 15/N = 0.37%). The equivalent weight of nitrogen depends on its isoiopic composition; it may rise up io 15.005, i.e., by 7%, when nitrogen is enriched with sup 15/ N. This dependence of nitrogen equivalent weight from its isotopic composition has to be taken into regard when the total nitrogen mass and the /sup 15/N mass are to be determined, or when the mass of compounds labelled /sup 15/N is to be found out. At this, deviations from the known correction formulae, derived for gravimetric determination purpose, arise owing to the fact that the Kjeldahl nitrogen determination method is based on measurement analysis. The related correction formulae are derived, and it is recommended not to use mg or g for characterizing /sup 15/N masses, but to introduce the units mmol or mol which are due to titrimetric determlnation and independent from nitrogen equivalent weight. (auth)