IMAGING AND MICROANALYSIS OF N-14 AND N-15 BY SIMS MICROSCOPY IN YEAST AND PLANT-SAMPLES

We have investigated the usefulness of Secondary Ion Mass Spectrometry (SIMS) for studying the tissue distribution of N-15 labelling in yeas t cells and soybean leaf tissues. The secondary ions best suited for t his are (CN)-C-12-N-14-, and (CN-)-C-12-N-15. Using a mass resolution of 6000, all problems of interference by other ions were avoided. The lateral resolution was of the order of 300 nm, i.e. well suited for su bcellular studies. The sensitivity was good enough to allow the detect ion and the mapping of N-15, even when it was present at its natural v alue concentration of the isotopic ratio of only 0.37%. Using yeast ce lls at isotopic equilibrium with their nutrient medium, the nitrogen i sotopic ratios in the cells were consistent with those in the medium. In the soybean leaf samples, the mapping of N-14 and N-15 was well cor related with the anatomical structures of the tissues. The mean isotop ic ratios (100 N-15/N-14, at/at), measured in the leaf tissues by SIMS , were slightly below those in the nutrient medium as well as those me asured in the leaf tissues by conventional mass spectrometry. This may be explained by differences in the methods of preparation of the leaf samples for SIMS and for mass spectrometry, and by the fact that the plants were probably still not perfectly at isotopic equilibrium with their external medium at the time the experiments were performed.