Compilation of minimum and maximum isotope ratios of selected elements in naturally occurring terrestrial materials and reagents

Documented variations in the isotopic compositions of some chemical elements are responsible for expanded uncertainties in the standard atomic weights published by the Commission on Atomic Weights and Isotopic Abundances of the International Union of Pure and Applied Chemistry. This report summarizes reported variations in the isotopic compositions of 20 elements that are due to physical and chemical fractionation processes (not due to radioactive decay) and their effects on the standard atomic weight uncertainties. For 11 of those elements (hydrogen, lithium, boron, carbon, nitrogen, oxygen, silicon, sulfur, chlorine, copper, and selenium), standard atomic weight uncertainties have been assigned values that are substantially larger than analytical uncertainties because of common isotope abundance variations in materials of natural terrestrial origin. For 2 elements (chromium and thallium), recently reported isotope abundance variations potentially are large enough to result in future expansion of their atomic weight uncertainties. For 7 elements (magnesium, calcium, iron, zinc, molybdenum, palladium, and tellurium), documented isotope-abundance variations in materials of natural terrestrial origin are too small to have a significant effect on their standard atomic weight uncertainties.

This compilation indicates the extent to which the atomic weight of an element in a given material may differ from the standard atomic weight of the element. For most elements given above, data are graphically illustrated by a diagram in which the materials are specified in the ordinate and the compositional ranges are plotted along the abscissa in scales of (1) atomic weight, (2) mole fraction of a selected isotope, and (3) delta value of a selected isotope ratio.

There are no internationally distributed isotopic reference materials for the elements zinc, selenium, molybdenum, palladium, and tellurium. Preparation of such materials will help to make isotope ratio measurements among laboratories comparable.

The minimum and maximum concentrations of a selected isotope in naturally occurring terrestrial materials for selected chemical elements reviewed in this report are given below:

Isotope Minimum
mole fraction Maximum
mole fraction

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2H 0 .000 0255 0 .000 1838
7Li 0 .9227 0 .9278
11B 0 .7961 0 .8107
13C 0 .009 629 0 .011 466
15N 0 .003 462 0 .004 210
18O 0 .001 875 0 .002 218
26Mg 0 .1099 0 .1103
30Si 0 .030 816 0 .031 023
34S 0 .0398 0 .0473
37Cl 0 .240 77 0 .243 56
44Ca 0 .020 82 0 .020 92
53Cr 0 .095 01 0 .095 53
56Fe 0 .917 42 0 .917 60
65Cu 0 .3066 0 .3102
205Tl 0 .704 72 0 .705 06

The numerical values above have uncertainties that depend upon the uncertainties of the determinations of the absolute isotope-abundance variations of reference materials of the elements. Because reference materials used for absolute isotope-abundance measurements have not been included in relative isotope abundance investigations of zinc, selenium, molybdenum, palladium, and tellurium, ranges in isotopic composition are not listed for these elements, although such ranges may be measurable with state-of-the-art mass spectrometry.

This report is available at the url: http://pubs.water.usgs.gov/wri014222.