Isotope ratio mass spectrometers are not truly specialized to measure isotope ratios at natural abundance only. The resistors of the amplifiers are only selected to make such measuremetns as good as possible. Exchanging resistors is easy and therefore any given ratio can be measured with very good precision. But; what is good precision. This mostly depends on the question you are investigating. Technically there is only one major problem. THERE ARE NO STANDARDS FOR NON-NATURAL ABUNDANCE that are well enough characterized and have the support of the community. Regards Willi -- ‰‰‰‰‰‰‰‰‰‰‰‰‰‰‰‰‰‰‰‰‰‰‰‰‰‰‰‰‰‰‰‰‰‰ Willi A. Brand, Stable Isotope Laboratory [log in to unmask] Max-Planck-Institute for Biogeochemistry Carl-Zeiss-Promenade 10, 07745 Jena, Germany Tel +49-3641-643718 P.O.Box 100164, 07701 Jena, Germany Fax: +49-3641-643710 ‰‰‰‰‰‰‰‰‰‰‰‰‰‰‰‰‰‰‰‰‰‰‰‰‰‰‰‰‰‰‰‰‰‰