[Modification of a method of determining 15N in biological material].

A method is described for preparation of nitrogen samples and estimation of 15N concentration in free amino acids and urea, isolated from heart muscle, using mass-spectometric analysis. Components of the tissue extracts were separated at the preparative scale; amino acids and urea were desalted by means of ion exchange chromatography. CHN-analyzer was used to measure content of nitrogen in urea and amino acids. The method required about 100 micrograms of nitrogen for the isotope analysis.