Reference materials for stable isotope analysis
Isotopic reference materials are compounds with well-defined isotopic compositions and are the ultimate sources of accuracy in mass spectrometric measurements of isotope ratios. Isotopic references are used because mass spectrometers are highly fractionating. As a result, the isotopic ratio that the instrument measures can be very different from that in the sample's measurement. Moreover, the degree of instrument fractionation changes during measurement, often on a timescale shorter than the measurement's duration, and can depend on the characteristics of the sample itself. By measuring a material of known isotopic composition, fractionation within the mass spectrometer can be removed during post-measurement data processing. Without isotope references, measurements by mass spectrometry would be much less accurate and could not be used in comparisons across different analytical facilities. Due to their critical role in measuring isotope ratios, and in part, due to historical legacy, isotopic reference materials define the scales on which isotope ratios are reported in the peer-reviewed scientific literature.
Isotope reference materials are generated, maintained, and sold by the International Atomic Energy Agency, the National Institute of Standards and Technology, the United States Geologic Survey, the Institute for Reference Materials and Measurements, and a variety of universities and scientific supply companies. Each of the major stable isotope systems has a wide variety of references encompassing distinct molecular structures. For example, nitrogen isotope reference materials include N-bearing molecules such ammonia, atmospheric dinitrogen, and nitrate. Isotopic abundances are commonly reported using the δ notation, which is the ratio of two isotopes in a sample relative to the same ratio in a reference material, often reported in per mille . Reference material span a wide range of isotopic compositions, including enrichments and depletions. While the δ values of references are widely available, estimates of the absolute isotope ratios in these materials are seldom reported. This article aggregates the δ and R values of common and non-traditional stable isotope reference materials.
Common reference materials
The δ values and absolute isotope ratios of common reference materials are summarized in Table 1 and described in more detail below. Alternative values for the absolute isotopic ratios of reference materials, differing only modestly from those in Table 1, are presented in Table 2.5 of Sharp , as well as Table 1 of the 1993 IAEA report on isotopic reference materials. For an exhaustive list of reference material, refer to Appendix I of Sharp, Table 40.1 of Gröning, or the website of the International Atomic Energy Agency. Note that the 13C/12C ratio of Vienna Pee Dee Belemnite and 34S/32S ratio of Vienna Canyon Diablo Troilite are purely mathematical constructs; neither material existed as a physical sample that could be measured.| Name | Material | Type of ratio | Isotope ratio: R | δ: | Type | Citation | Notes |
| VSMOW | H2O ' | 2H/1H | 0.00015576 | 0‰ vs. VSMOW | Primary, Calibration | Hagemann et al. ; De Wit et al. | Analogous to SMOW, VSMOW2 |
| SLAP2 | H2O ' | 2H/1H | 0.00008917 | -427.5‰ vs. VSMOW | Reference | Calculated from VSMOW | Used as a second anchor for the δ2H scale |
| GISP | H2O ' | 2H/1H | 0.00012624 | -189.5‰ vs. VSMOW | Reference | Calculated from VSMOW | Stock potentially fractionated during aliquoting |
| NBS-19 | CaCO3 ' | 13C/12C | 0.011202 | +1.95‰ vs. VPDB | Calibration | Chang & Li | Defines the VPDB scale, supply is exhausted |
| VPDB | - | 13C/12C | 0.011180 | 0‰ vs. VPDB | Primary | Calculated from NBS-19 | Supply of PDB exhausted VPDB was never a physical material. |
| IAEA-603 | CaCO3 ' | 13C/12C | 0.011208 | Calibration | Calculated from VPDB | Replacement for NBS-19 | |
| LSVEC | Li2CO3 ' | 13C/12C | 0.010686 | Reference | Calculated from VPDB | Used as a second anchor for the δ13C scale | |
| AIR | N2 ' | 15N/14N | 0.003676 | 0‰ vs. AIR | Primary, Calibration | Junk & Svec | Only anchor for the δ15N scale |
| VSMOW | H2O ' | 18O/16O | 0.0020052 | 0‰ vs. VSMOW | Primary, Calibration | Baertschi ; Li et al. | Analogous to SMOW, VSMOW2 |
| VSMOW | H2O ' | 17O/16O | 0.0003800 | 0‰ vs. VSMOW | Primary, Calibration | Baertschi ; Li et al. | Analogous to SMOW, VSMOW2 |
| SLAP2 | H2O ' | 18O/16O | 0.0018939 | -55.5‰ vs. VSMOW | Reference | Calculated from VSMOW | Used as a second anchor for the δ18O scale |
| GISP | H2O ' | 18O/16O | 0.0019556 | -24.76‰ vs. VSMOW | Reference | Calculated from VSMOW | Stock potentially fractionated during aliquoting |
| IAEA-S-1 | Ag2S ' | 36S/32S | 0.0001534 | Ding et al. | There is no formal definition for the δ33S isotopic scale | ||
| IAEA-S-1 | Ag2S ' | 34S/32S | 0.0441494 | -0.3‰ vs. VCDT | Calibration | Ding et al. | Defines the VCDT scale, only anchor for δ34S scale |
| IAEA-S-1 | Ag2S ' | 33S/32S | 0.0078776 | Ding et al. | There is no formal definition for the δ36S isotopic scale | ||
| VCDT | - | 34S/32S | 0.0441626 | 0‰ vs. VCDT | Primary | Calculated from IAEA-S-1 | Canyon Diablo Troilite is isotopically heterogenousVCDT was never a physical material |
In Table 1, "Name" refers to the common name of the reference, "Material" gives its chemical formula and phase, "Type of ratio" is the isotopic ratio reported in "Isotopic ratio", "δ" is the δ value of the material with indicated reference frame, "Type" is the category of the material using the notation of Gröening , "Citation" gives the article reporting the isotopic abundances on which the isotope ratio is based, and "Notes" are notes. The reported isotopic ratios reflect the results from individual analyses of absolute mass fraction, aggregated in Meija et al. and manipulated to reach the given ratios. Error was calculated as the square root of the sum of the squares of fractional reported errors, consistent with standard error propagation, but is not propagated for ratios reached through secondary calculation.
Primary reference materials
Primary reference materials define the scales on which isotopic ratios are reported. This can mean a material that historically defined an isotopic scale, such as Vienna Standard Mean Ocean Water for hydrogen isotopes, even if that material is not currently in use. Alternatively, it can mean a material that only ever existed theoretically but is used to define an isotopic scale, such as VCDT for sulfur isotope ratios.Calibration materials
Calibration materials are compounds whose isotopic composition is known extremely well relative to the primary reference materials or which define the isotopic composition of the primary reference materials but are not the isotopic ratios to which data are reported in the scientific literature. For example, the calibration material IAEA-S-1 defines the isotopic scale for sulfur but measurements are reported relative to VCDT, not relative to IAEA-S-1. The calibration material serves the function of the primary reference material when the primary reference is exhausted, unavailable, or never existed in physical form.Working standards
Primary, calibration, and reference materials are only available in small quantities and purchase is often limited to once every few years. Depending on the specific isotope systems and instrumentation, a shortage of available reference materials can be problematic for daily instrument calibrations or for researchers attempting to measure isotope ratios in a large number of natural samples. Rather than using primary materials or reference materials, a laboratory measuring stable isotope ratios will typically purchase a small quantity of the relevant reference materials and measure the isotope ratio of an in-house material against the reference, making that material into a working standard specific to that analytical facility. Once this lab-specific working standard has been calibrated to the international scale the standard is used to measure the isotopic composition of unknown samples. After measurement of both sample and working standard against a third material the recorded isotopic distributions are mathematically corrected back to the international scale. It is thus critical to measure the isotopic composition of the working standard with high precision and accuracy because the working standard forms the ultimate basis for accuracy of most mass spectrometric observations. Unlike reference materials, working standards are typically not calibrated across multiple analytical facilities and the accepted δ value measured in a given laboratory could reflect bias specific to a single instrument. However, within a single analytical facility this bias can be removed during data reduction. Because each laboratory defines unique working standards the primary, calibration, and reference materials are long-lived while still ensuring that the isotopic composition of unknown samples can be compared across laboratories.Isotopic reference materials
Traditional isotope systems
The compounds used as isotopic references have a relatively complex history. The broad evolution of reference materials for the hydrogen, carbon, oxygen, and sulfur stable isotope systems are shown in Figure 1. Materials with red text define the primary reference commonly reported in scientific publications and materials with blue text are those available commercially. The hydrogen, carbon, and oxygen isotope scales are defined with two anchoring reference materials. For hydrogen the modern scale is defined by VSMOW2 and SLAP2, and is reported relative to VSMOW. For carbon the scale is defined by either NBS-19 or IAEA-603 depending on the age of the lab, as well as LSVEC, and is reported relative to VPDB. Oxygen isotope ratios can be reported relative to either the VSMOW or VPDB scales. The isotopic scales for sulfur and nitrogen are both defined for only a single anchoring reference material. For sulfur the scale is defined by IAEA-S-1 and is reported relative to VCDT, while for nitrogen the scale is both defined by and reported relative to AIR.Hydrogen
The isotopic reference frame of Standard Mean Ocean Water was established by Harmon Craig in 1961 by measuring δ2H and δ18O in samples of deep ocean water previously studied by Epstein & Mayeda. Originally SMOW was a purely theoretical isotope ratio intended to represent the mean state of the deep ocean. In the initial work the isotopic ratios of deep ocean water were measured relative to NBS-1, a standard derived from the steam condensate of Potomac River water. Notably, this means SMOW was originally defined relative to NBS-1, and there was no physical SMOW solution. Following the advice of an IAEA advisory group meeting in 1966, Ray Weiss and Harmon Craig made an actual solution with the isotopic values of SMOW which they called Vienna Standard Mean Ocean Water. They also prepared a second hydrogen isotope reference material from firn collected at the Amundsen-Scott South Pole Station, initially called SNOW and later called Standard Light Antarctic Precipitation. Both VSMOW and SLAP were distributed beginning in 1968. The isotopic characteristics of SLAP and NBS-1 were later evaluated by interlaboratory comparison through measurements against VSMOW. Subsequently, VSMOW and SLAP were used as the primary isotopic reference materials for the hydrogen isotope system for multiple decades. In 2006 the IAEA Isotope Hydrology Laboratory constructed new isotopic reference materials called VSMOW2 and SLAP2 with nearly identical δ2H and δ18O as VSMOW and SLAP. Hydrogen isotope working standards are currently calibrated against VSMOW2 and SLAP2 but are still reported on the scale defined by VSMOW and SLAP relative to VSMOW. Additionally, Greenland Ice Sheet Precipitation δ2H has been measured to high precision in multiple labs, but different analytical facilities disagree on the value. These observations suggest GISP may have been fractionated during aliquoting or storage, implying that the reference material should be used with care.| Name | Material | δ2H | Standard deviation | Reference | Link |
| VSMOW2 | H2O | 0‰ | 0.3‰ | VSMOW | |
| SLAP2 | H2O | -427.5‰ | 0.3‰ | VSMOW | |
| GISP | H2O | -189.5‰ | 1.2‰ | VSMOW | |
| NBS 22 | Oil | -120‰ | 1‰ | VSMOW |
Carbon
The original carbon isotope reference material was a Belemnite fossil from the PeeDee Formation in South Carolina, known as the Pee Dee Belemnite. This PDB standard was rapidly consumed and subsequently researchers used replacement standards such as PDB II and PDB III. The carbon isotope reference frame was later established in Vienna against a hypothetical material called the Vienna Pee Dee Belemnite. As with the original SMOW, VPDB never existed as a physical solution or solid. In order to make measurements researchers use the reference material NBS-19, colloquially known as the Toilet Seat Limestone, which has an isotopic ratio defined relative to the hypothetical VPDB. The exact origin of NBS-19 is unknown but it was a white marble slab and has a grain size of 200-300 micrometers. To improve the accuracy of carbon isotope measurements, in 2006 the δ13C scale was shifted from a one-point calibration against NBS-19 to a two point-calibration. In the new system the VPDB scale is pinned to both the LSVEC Li2CO3 reference material and to the NBS-19 limestone. NBS-19 is now also exhausted and has been replaced with IAEA-603.| Name | Material | δ13C | Standard deviation | Reference | Link |
| IAEA-603 | CaCO3 | 2.46‰ | 0.01‰ | VPDB | |
| NBS-18 | CaCO3 | 0.035‰ | VPDB | ||
| NBS-19 | CaCO3 | 1.95‰ | - | VPDB | |
| LSVEC | Li2CO3 | 0.2‰ | VPDB | ||
| IAEA-CO-1 | Carrara marble | 0.030‰ | VPDB | ||
| IAEA-CO-8 | CaCO3 | 0.032‰ | VPDB | ||
| IAEA-CO-9 | BaCO3 | 0.057‰ | VPDB | ||
| NBS 22 | Oil | 0.043‰ | VPDB |
Oxygen
isotopic ratios are commonly compared to both the VSMOW and the VPDB references. Traditionally oxygen in water is reported relative to VSMOW while oxygen liberated from carbonate rocks or other geologic archives is reported relative to VPDB. As in the case of hydrogen, the oxygen isotopic scale is defined by two materials, VSMOW2 and SLAP2. Measurements of sample δ18O vs. VSMOW can be converted to the VPDB reference frame through the following equation: δ18OVPDB = 0.97001*δ18OVSMOW - 29.99‰.| Name | Material | δ18O | Standard deviation | Reference | Link |
| VSMOW2 | H2O | 0‰ | 0.02‰ | VSMOW | |
| SLAP2 | H2O | 0.02‰ | VSMOW | ||
| GISP | H2O | 0.09‰ | VSMOW | ||
| IAEA-603 | CaCO3 | 0.04‰ | VPDB | ||
| NBS-18 | CaCO3 | 0.1‰ | VPDB | ||
| NBS-19 | CaCO3 | - | VPDB | ||
| LSVEC | Li2CO3 | 0.2‰ | VPDB | ||
| IAEA-CO-1 | Carrara marble | 0.1‰ | VPDB | ||
| IAEA-CO-8 | CaCO3 | 0.2‰ | VPDB | ||
| IAEA-CO-9 | BaCO3 | 0.2‰ | VPDB |
Nitrogen
makes up 78% of the atmosphere and is extremely well mixed over short time-scales, resulting in a homogenous isotopic distribution ideal for use as a reference material. Atmospheric N2 is commonly called AIR when being used as an isotopic reference. In addition to atmospheric N2 there are multiple N isotopic reference materials.| Name | Material | δ15N | Standard deviation | Reference | Link | Source/derivation of material |
| IAEA-N-1 | 2SO4 | 0.4‰ | 0.2‰ | AIR | ||
| IAEA-N-2 | 2SO4 | 20.3‰ | 0.2‰ | AIR | ||
| IAEA-NO-3 | KNO3 | 4.7‰ | 0.2‰ | AIR | ||
| USGS32 | KNO3 | 180‰ | 1‰ | AIR | ||
| USGS34 | KNO3 | 0.2‰ | AIR | from nitric acid | ||
| USGS35 | NaNO3 | 2.7‰ | 0.2‰ | AIR | purified from natural ores | |
| USGS25 | 2SO4 | 0.4‰ | AIR | |||
| USGS26 | 2SO4 | 53.7‰ | 0.4‰ | AIR | ||
| NSVEC | N2 gas | 0.2‰ | AIR | |||
| IAEA-305 | 2SO4 | 39.8‰ 375.3‰ | 39.3 - 40.3‰ 373.0 - 377.6‰ | AIR | derived from ammonium sulfate SD given as 95% confidence interval | |
| IAEA-310 | CH4N2O | 47.2‰ 244.6‰ | 46.0 - 48.5‰ 243.9 - 245.4‰ | AIR | derived from urea SD given as 95% confidence interval | |
| IAEA-311 | 2SO4 | 2.05 ‰ | 2.03 - 2.06‰ | AIR | SD given as 95% confidence interval |
Sulfur
The original sulfur isotopic reference material was the Canyon Diablo Troilite, a meteorite recovered from Meteor Crater in Arizona. The Canyon Diablo Meteorite was chosen because it was thought to have a sulfur isotopic composition similar to the bulk Earth. However, the meteorite was later found to be isotopically heterogeneous with variations up to 0.4‰. This isotopic variability resulted in problems for the interlaboratory calibration of sulfur isotope measurements. A meeting of the IAEA in 1993 defined Vienna Canyon Diablo Troilite in an allusion to the earlier establishment of VSMOW. Like the original SMOW and VPDB, VCDT was never a physical material that could be measured but was still used as the definition of the sulfur isotopic scale. For the purposes of actually measuring 34S/32S ratios, the IAEA defined the δ34S of IAEA-S-1 to be -0.30‰ relative to VCDT. These fairly recent changes to the sulfur isotope reference materials have greatly improved interlaboratory reproducibility.| Name | Material | δ34S | Standard deviation | Reference | Link | Source/derivation of material |
| IAEA-S-1 | Ag2S | - | VCDT | from sphalerite | ||
| IAEA-S-2 | Ag2S | 22.7‰ | 0.2‰ | VCDT | from gypsum | |
| IAEA-S-3 | Ag2S | 0.2‰ | VCDT | from sphalerite | ||
| IAEA-S-4 | S | 16.9‰ | 0.2‰ | VCDT | from natural gas | |
| IAEA - SO-5: | BaSO4 | 0.5‰ | 0.2‰ | VCDT | from aqueous sulfate | |
| IAEA - SO-6 | BaSO4 | 0.2‰ | VCDT | from aqueous sulfate | ||
| NBS - 127 | BaSO4 | 20.3‰ | 0.4‰ | VCDT | from sulfate from Monterey Bay |
Organic molecules
A recent international project has developed and determined the hydrogen, carbon, and nitrogen isotopic composition of 19 organic isotopic reference materials, now available from USGS, IAEA, and Indiana University. These reference materials span a large range of δ2H, δ13C, and δ15N, and are amenable to a wide range of analytical techniques. The organic reference materials include caffeine, glycine, n-hexadecane, icosanoic acid methyl ester, L-valine, methylheptadecanoate, polyethylene foil, polyethylene power, vacuum oil, and NBS-22.| Name | Chemical | δDVSMOW-SLAP | δ13CVPDB-LSVEC | δ15NAIR |
| USGS61 | caffeine | 96.9 ± 0.9 | -35.05 ± 0.04 | -2.87 ± 0.04 |
| USGS62 | caffeine | -156.1 ± 2.1 | -14.79 ± 0.04 | 20.17 ± 0.06 |
| USGS63 | caffeine | 174.5 ± 0.9 | -1.17 ± 0.04 | 37.83 ± 0.06 |
| IAEA-600 | caffeine | -156.1 ± 1.3 | -27.73 ± 0.04 | 1.02 ± 0.05 |
| USGS64 | glycine | - | -40.81 ± 0.04 | 1.76 ± 0.06 |
| USGS65 | glycine | - | -20.29 ± 0.04 | 20.68 ± 0.06 |
| USGS66 | glycine | - | -0.67 ± 0.04 | 40.83 ± 0.06 |
| USGS67 | n-hexadecane | -166.2 ± 1.0 | -34.5 ± 0.05 | - |
| USGS68 | n-hexadecane | -10.2 ± 0.9 | -10.55 ± 0.04 | - |
| USGS69 | n-hexadecane | 381.4 ± 3.5 | -0.57 ± 0.04 | - |
| USGS70 | icosanoic acid methyl ester | -183.9 ± 1.4 | -30.53 ± 0.04 | - |
| USGS71 | icosanoic acid methyl ester | -4.9 ± 1.0 | -10.5 ± 0.03 | - |
| USGS72 | icosanoic acid methyl ester | 348.3 ± 1.5 | -1.54 ± 0.03 | - |
| USGS73 | L-valine | - | -24.03 ± 0.04 | -5.21 ± 0.05 |
| USGS74 | L-valine | - | -9.3 ± 0.04 | 30.19 ± 0.07 |
| USGS75 | L-valine | - | 0.49 ± 0.07 | 61.53 ± 0.14 |
| USGS76 | methylheptadecanoate | -210.8 ± 0.9 | -31.36 ± 0.04 | - |
| IAEA-CH-7 | polyethylene foil | -99.2 ± 1.2 | -32.14 ± 0.05 | - |
| USGS77 | polyethylene power | -75.9 ± 0.6 | -30.71 ± 0.04 | - |
| NBS 22 | oil | -117.2 ± 0.6 | -30.02 ± 0.04 | - |
| NBS 22a | vacuum oil | -120.4 ± 1.0 | -29.72 ± 0.04 | - |
| USGS78 | 2H-enriched vacuum oil | 397.0 ± 2.2 | -29.72 ± 0.04 | - |
The information in Table 7 comes directly from Table 2 of Schimmelmann et al..
Non-traditional isotope systems
Heavy isotope systems
Isotopic reference materials exist for non-traditional isotope systems, including lithium, boron, magnesium, calcium, iron, and many others. Because the non-traditional systems were developed relatively recently, the reference materials for these systems are more straightforward and less numerous than for the traditional isotopic systems. The following table contains the material defining the δ=0 for each isotopic scale, the 'best' measurement of the absolute isotopic fractions of an indicated material, the calculated absolute isotopic ratio, and links to lists of isotopic reference materials prepared by the Commission on Isotopic Abundances and Atomic Weight. A summary list of non-traditional stable isotope systems is available , and much of this information is derived from Brand et al.. In addition to the isotope systems listed in Table 8, ongoing research is focused on measuring the isotopic composition of barium and vanadium. Specpure Alfa Aesar is an isotopically well-characterized vanadium solution. Furthermore, fractionation during chemical processing can be problematic for certain isotopic analyses, such as measuring heavy isotope ratios following column chromatography. In these cases reference materials can be calibrated for particular chemical procedures.| Element | Symbol | δ | Type of ratio | Name | Material | Name | Isotope Ratio: R | Citation | Link |
| Lithium | Li | δ7Li | 7Li/6Li | LSVEC | Li2CO3 | IRMM-016 | 12.17697 | Qi et al. | |
| Boron | B | δ11B | 11B/10B | NIST SRM 951 | Boric acid | IRMM-011 | 4.0454 | De Bièvre & Debus | |
| Magnesium | Mg | δ26/24Mg | 26Mg/24Mg | DMS-3 | NO3− solution | DSM-3 | 0.13969 | Bizzarro et al. | |
| Silicon | Si | δ30/28Si | 30Si/28Si | NBS 28 | Si sand | WASO-17.2 | 0.0334725 | De Bievre et al. | |
| Chlorine | Cl | δ37Cl | 37Cl/35Cl | SMOC | - | NIST SRM 975 | 0.319876 | Wei et al. | |
| Calcium | Ca | δ44/42Ca | 44Ca/42Ca | NIST SRM 915a | CaCO3 | NIST SRM 915 | 3.21947 | Moore & Machlan | |
| Chromium | Cr | δ53/52Cr | 53Cr/52Cr | NIST SRM 979 | Cr3 salt | NIST SRM 979 | 0.113387 | Shields et al. | |
| Iron | Fe | δ56/54Fe | 56Fe/54Fe | IRMM-014 | elemental Fe | IRMM-014 | 15.69786 | Taylor et al. | |
| Nickel | Ni | δ60/58Ni | 60Ni/58Ni | NIST SRM 986 | elemental Ni | NIST SRM 986 | 0.385198 | Gramlich et al. | |
| Copper | Cu | δ65Cu | 65Cu/63Cu | NIST SRM 976 | elemental Cu | NIST SRM 976 | 0.44563 | Shields et al. | |
| Zinc | Zn | δ68/64Zn | 68Zn/64Zn | IRMM-3702 | ZN solution | IRMM-3702 | 0.375191 | Ponzevera et al. | |
| Gallium | Ga | δ71Ga | 71Ga/69Ga | NIST SRM 994 | elemental Ga | NIST SRM 994 | 0.663675 | Machlan et al. | |
| Germanium | Ge | δ74/70Ge | 74Ge/70Ge | NIST SRM 3120a | elemental Ge | Ge metal | 1.77935 | Yang & Meija | |
| Selenium | Se | δ82/76Se | 82Se/76Se | NIST SRM 3149 | Se solution | NIST SRM 3149 | 0.9572 | Wang et al. | |
| Bromine | Br | δ81Br | 81Br/79Br | SMOB | - | NIST SRM 977 | 0.97293 | Catanzaro et al. | |
| Rubidium | Rb | δ87Rb | 87Rb/85Rb | NIST SRM 984 | RbCl | NIST SRM 984 | 0.385706 | Catanzaro et al. | |
| Strontium | Sr | δ88/86Sr | 88Sr/86Sr | NIST SRM 987 | SrCO3 | NIST SRM 987 | 8.378599 | Moore et al. | |
| Molybdenum | Mo | δ98/95Mo | 98Mo/95Mo | NIST SRM 3134 | solution | NIST SRM 3134 | 1.5304 | Mayer & Wieser | |
| Silver | Ag | δ109Ag | 109Ag/107Ag | NIST SRM 978a | AgNO3 | NIST SRM 978 | 0.929042 | Powell et al. | |
| Cadmium | Cd | δ114/110Cd | 114Cd/110Cd | NIST SRM 3108 | solution | BAM Cd-I012 | 2.30108 | Pritzkow et al. | |
| Rhenium | Re | δ187Re | 187Re/185Re | NIST SRM 989 | elemental Re | NIST SRM 989 | 1.67394 | Gramlich et al. | |
| Osmium | Os | δ187/188Os | 187Os/188Os | IAG-CRM-4 | solution | K2OsO4 | 0.14833 | Völkening et al. | |
| Platinum | Pt | δ198/194Pt | 198Pt/194Pt | IRMM-010 | elemental Pt | IRMM-010 | 0.22386 | Wolff Briche et al. | |
| Mercury | Hg | δ202/198Hg | 202Hg/198Hg | NRC NIMS-1 | solution | NRC NIMS-1 | 2.96304 | Meija et al. | |
| Thallium | Tl | δ205Tl | 205Tl/203Tl | NRC SRM 997 | elemental Tl | NIST SRM 997 | 2.38707 | Dunstan et al. | |
| Lead | Pb | δ208/206Pb | 208Pb/206Pb | ERM-3800 | solution | NIST SRM 981 | 2.168099 | Catanzaro et al. | |
| Uranium | U | δ238/235U | 238U/235U | NIST SRM 950-A | uranium oxide | Namibian ore | 137.802321 | Richter et al. |
Table 8 gives the material and isotopic ratio defining the δ = 0 scale for each of the indicated elements. In addition, Table 8 lists the material with the 'best' measurement as determined by Meija et al.. "Material" gives chemical formula, "Type of ratio" is the isotopic ratio reported in "Isotope ratio", and "Citation" gives the article reporting the isotopic abundances on which the isotope ratio is based. The isotopic ratios reflect the results from individual analyses of absolute mass fraction, reported in the cited studies, aggregated in Meija et al., and manipulated to reach the reported ratios. Error was calculated as the square root of the sum of the squares of fractional reported errors.