By literal definition, atom % is the concentration of an element based of the number of moles (or atoms) of a particular element relative to the total number of moles (or atoms) of all elements in a given sample (e.g., within an alloy). In the stable isotope world, we use the concentration of the number of atoms of a particular isotope of a specific element divided by the total number of atoms of all the isotopes of that particular element (e.g., for oxygen; O16, O17 and O18) ignoring the radioisotopes of that element, which would be insignificant in an enriched stable isotopic product. The measurement of enriched stable isotopes can be accomplished by several methodologies (e.g., quadrapole MS, time-of-flight MS, GC/MS, LC/MS, ICP/MS, IR, NMR). In the stable isotope community, the terms isotopic enrichment and atom % are, in most respects, used interchangeably. Manufacturers of enriched stable isotopes typically use atom % for building block materials (e.g., CO, CO2, NO) to produce enriched labeled compounds and the subsequent labeled compounds are labeled as % enrichment. I'm sure my former colleagues will have a better explanation than what I have given here. Mark Lemley (former employee of a stable isotope company) Quality Assurance Manager Project Chemist Quality Laboratories, Inc. (EPA SW-846 testing laboratory) Hamilton, OH email: [log in to unmask] --- On Tue, 11/10/09, Aaron Shultis <[log in to unmask]> wrote: From: Aaron Shultis <[log in to unmask]> Subject: [ISOGEOCHEM] isotope enrichment versus atom percent To: [log in to unmask] Date: Tuesday, November 10, 2009, 12:37 PM Hi all, I was curious how the isotope manufacturers calculate the "isotope enrichment" of a labeled reference. For example if the container has "98% isotope enrichment" or "98% 18O" on the label, how is that calculated. and how is that related to atom%. Cheers Aaron -- Aaron Shultis, Research Scientist Water Center School of Natural Resources University of Nebraska-Lincoln 103 Water Sciences Lab Lincoln, NE 68583-0844 U.S.A.