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    Isotopic Reference Materials

    In science, isotope ratio measurements are often performed in a differential mode by comparing the measured isotope amount ratio in a samples to that of a standard. The ratio of both measurements, the isotope delta, allows small isotopic differences to be expressed unambiguously beyond the exact knowledge of absolute isotopic abundances.

    Reference materials for use in differential measurements of stable isotope amount ratios have been used since the early 1950s. Since then, the number of international measurement standards for anchoring stable isotope delta scales has mushroomed from three to more than thirty, expanding for more than 25 chemical elements.

    To provide an up-to-date comprehensive overview, the Commission has evaluated all isotopic composition reference materials that have played a significant role and/or were available to a wider scientific audience. This also applies to materials whose supply is now exhausted or that have been superseded by newer materials for various reasons. Below is a table of elements for which the Commission has evaluated isotopic composition reference materials. Click on individual elements to find out more information.

    Z Symbol Element Common delta reference material
    1HhydrogenVSMOW, SLAP
    5BboronNIST SRM 951a
    6CcarbonVPDB, LSVEC
    7NnitrogenAir N2, USGS32
    8OoxygenVPDB, VSMOW, SLAP
    16SsulfurVCDT, IAEA-S-1
    17ClchlorineSMOC, NIST SRM 975
    20CacalciumNIST SRM 915a
    24CrchromiumNIST SRM 979
    28NinickelNIST SRM 986
    29CucopperNIST SRM 976
    31GagalliumNIST SRM 994
    32GegermaniumNIST SRM 3120a
    34SeseleniumNIST SRM 3149
    37RbrubidiumNIST SRM 984
    38SrstrontiumNIST SRM 987
    42MomolybdenumNIST SRM 3134
    47AgsilverNIST SRM 978a
    48CdcadmiumNIST SRM 3108, BAM-I012
    75RerheniumNIST SRM 989
    80HgmercuryNRC NIMS-1
    81TlthalliumNIST SRM 997
    82PbleadNIST SRM 981, ERMM-3800
    92UuraniumNIST SRM 950a
    Z Symbol Element Common delta reference material


    Isotope delta is defined as

      δref(iE, sample) = Rsample/Rref – 1,

    where E is the element of interest, Rsample and Rref are the measured isotope amount ratios in the sample and reference, whereas i is the mass number of the numerator isotope in both isotope ratios (denominator isotope is the same for both the sample and the reference).
    For example, isotope delta for boron is commonly measured against NIST SRM 951 isotopic reference material of boric acid and, for substance P, it has a symbol δSRM951(11B, P).

    For certain elements (such as hydrogen, oxygen, and carbon) the isotope delta scale is defined using two reference materials.

    In 2014, the Commission published a report on international isotopic reference materials.
    Assessment of international reference materials for isotope-ratio analysis by W.A. Brand, T.B. Coplen, J. Vogl, M. Rosner, and T. Prohaska. Pure Appl. Chem. 86, 425-467 (2014)