Reference materials for stable isotope analysis

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Isotopic reference materials are compounds (solids, liquids, gasses) 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.

Contents

Isotope reference materials are generated, maintained, and sold by the International Atomic Energy Agency (IAEA), the National Institute of Standards and Technology (NIST), the United States Geologic Survey (USGS), the Institute for Reference Materials and Measurements (IRMM), and a variety of universities and scientific supply companies. Each of the major stable isotope systems (hydrogen, carbon, oxygen, nitrogen, and sulfur) has a wide variety of references encompassing distinct molecular structures. For example, nitrogen isotope reference materials include N-bearing molecules such ammonia (NH3), atmospheric dinitrogen (N2), and nitrate (NO3). Isotopic abundances are commonly reported using the δ notation, which is the ratio of two isotopes (R) in a sample relative to the same ratio in a reference material, often reported in per mille (‰) (equation below). Reference material span a wide range of isotopic compositions, including enrichments (positive δ) and depletions (negative δ). While the δ values of references are widely available, estimates of the absolute isotope ratios (R) 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 (2007) [1] (a text freely available online), as well as Table 1 of the 1993 IAEA report on isotopic reference materials. [2] For an exhaustive list of reference material, refer to Appendix I of Sharp (2007), [1] Table 40.1 of Gröning (2004), [3] or the website of the International Atomic Energy Agency. Note that the 13C/12C ratio of Vienna Pee Dee Belemnite (VPDB) and 34S/32S ratio of Vienna Canyon Diablo Troilite (VCDT) are purely mathematical constructs; neither material existed as a physical sample that could be measured. [2]

Table 1: Isotopic parameters of common stable isotope primary reference and calibration materials
NameMaterialType of ratioIsotope ratio:

R (σ)

δ:

(Rsmp/Rstd-1)

TypeCitationNotes
VSMOW H2O (l)2H/1H0.00015576(5)0‰ vs. VSMOWPrimary,

Calibration

Hagemann et al. (1970) [4] (Tse et al. (1980); [5]

De Wit et al. (1980) [6]

Analogous to SMOW (math construct), VSMOW2 (physical solution)
SLAP2 H2O (l)2H/1H0.00008917-427.5‰ vs. VSMOWReferenceCalculated from VSMOWUsed as a second anchor for the δ2H scale
GISP H2O (l)2H/1H0.00012624-189.5‰ vs. VSMOWReferenceCalculated from VSMOWStock potentially fractionated during aliquoting
NBS-19 CaCO3 (s) 13C/12C 0.011202(28)+1.95‰ vs. VPDBCalibrationChang & Li (1990) [7] Defines the VPDB scale, supply is exhausted
VPDB- 13C/12C 0.0111800‰ vs. VPDBPrimaryCalculated from NBS-19

(see also Zhang et al. (1990) [8] )

Supply of PDB (as well as PDB II, PDB III) exhausted

VPDB was never a physical material.

IAEA-603 CaCO3 (s) 13C/12C 0.011208+2.46‰ vs. VPDBCalibrationCalculated from VPDBReplacement for NBS-19
LSVEC Li2CO3 (s) 13C/12C 0.010686-46.6‰ vs. VPDBReferenceCalculated from VPDBUsed as a second anchor for the δ13C scale
AIR N2 (g) 15N/14N 0.003676(4)0‰ vs. AIRPrimary, CalibrationJunk & Svec (1958) [9] Only anchor for the δ15N scale
VSMOW H2O (l) 18O/16O 0.0020052(5)0‰ vs. VSMOWPrimary, CalibrationBaertschi (1976); [10]

Li et al. (1988) [11]

Analogous to SMOW (math construct), VSMOW2 (physical solution)
VSMOW H2O (l) 17O/16O 0.0003800(9)0‰ vs. VSMOWPrimary, CalibrationBaertschi (1976); [10]

Li et al. (1988) [11]

Analogous to SMOW (math construct), VSMOW2 (physical solution)
SLAP2 H2O (l) 18O/16O 0.0018939-55.5‰ vs. VSMOWReferenceCalculated from VSMOWUsed as a second anchor for the δ18O scale
GISP H2O (l) 18O/16O 0.0019556-24.76‰ vs. VSMOWReferenceCalculated from VSMOWStock potentially fractionated during aliquoting
IAEA-S-1 Ag2S (s) 36S/32S 0.0001534(9)Ding et al. (2001) [12] There is no formal definition for the δ33S isotopic scale
IAEA-S-1 Ag2S (s) 34S/32S 0.0441494(70)-0.3‰ vs. VCDTCalibrationDing et al. (2001) [12] Defines the VCDT scale, only anchor for δ34S scale
IAEA-S-1 Ag2S (s) 33S/32S 0.0078776(63)Ding et al. (2001) [12] There is no formal definition for the δ36S isotopic scale
VCDT - 34S/32S 0.04416260‰ vs. VCDTPrimaryCalculated from IAEA-S-1 Canyon Diablo Troilite is isotopically heterogenous [13] VCDT 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 (2004) (discussed below), "Citation" gives the article(s) 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. (2016) [14] 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.

Reference terminology

The terminology of isotopic reference materials is not applied consistently across subfields of isotope geochemistry or even between individual laboratories. The terminology defined below comes from Gröening et al. (1999) [15] and Gröening (2004). [3] Reference materials are the basis for accuracy across many different types of measurement, not only the mass spectrometry, and there is a large body of literature concerned with the certification and testing of reference materials.

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 (VSMOW) 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.

Reference materials

Reference materials are compounds which are carefully calibrated against the primary reference or a calibration material. These compounds allow for isotopic analysis of materials differing in chemical or isotopic composition from the compounds defining the isotopic scales on which measurements are reported. In general these are the materials most researchers mean when they say "reference materials". An example of a reference material is USGS-34, a KNO3 salt with a δ15N of -1.8‰ vs. AIR. In this case the reference material has a mutually agreed upon value of δ15N when measured relative to the primary reference of atmospheric N2 (Böhlke et al., 2003). [16] USGS-34 is useful because it allows researchers to directly measure the 15N/14N of NO3 in natural samples against the standard and report observations relative to N2 without having to first convert the sample to N2 gas.

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 (commonly called the working gas or the transfer gas) 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 (as well as possible given the precision of the instrument and the accuracy of the purchased reference material) 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.

Figure 1: The development of modern stable isotope reference materials. The materials shown in red are used commonly as the reference for reporting isotopic ratios in natural materials, while those shown in blue are commercially available and are used to calibrate working references materials for measuring isotopic ratios. The N isotope system is not included because the reference material has never changed from atmospheric N2. StandardHistory.svg
Figure 1: The development of modern stable isotope reference materials. The materials shown in red are used commonly as the reference for reporting isotopic ratios in natural materials, while those shown in blue are commercially available and are used to calibrate working references materials for measuring isotopic ratios. The N isotope system is not included because the reference material has never changed from atmospheric N2.

Hydrogen

The isotopic reference frame of Standard Mean Ocean Water (SMOW) was established by Harmon Craig in 1961 [17] by measuring δ2H and δ18O in samples of deep ocean water previously studied by Epstein & Mayeda (1953). [18] 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 (VSMOW). [15] 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 (SLAP). [2] 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 (Gonfiantini, 1978). [19] 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 (GISP) 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.

Table 2: Hydrogen Isotope Reference Materials
NameMaterialδ2HStandard

deviation

ReferenceLink
VSMOW2 H2O 0‰0.3‰ VSMOW Link
SLAP2 H2O -427.5‰0.3‰ VSMOW Link
GISP H2O -189.5‰1.2‰ VSMOW Link
NBS 22Oil-120‰1‰ VSMOW Link

Carbon

The original carbon isotope reference material was a Belemnite fossil from the PeeDee Formation in South Carolina, known as the Pee Dee Belemnite (PDB). 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 (VPDB). [2] 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, [20] 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 (Coplen et al., 2006a; Coplen et al., 2006b). [21] [22] NBS-19 is now also exhausted and has been replaced with IAEA-603.

Table 3: Carbon Isotope Reference Materials
NameMaterialδ13CStandard

deviation

ReferenceLink
IAEA-603 CaCO3 2.46‰0.01‰ VPDB Link
NBS-18 CaCO3 -5.014‰0.035‰ VPDB Link
NBS-19 CaCO3 1.95‰- VPDB Link
LSVEC Li2CO3 -46.6‰0.2‰ VPDB Link
IAEA-CO-1 Carrara marble +2.492‰0.030‰ VPDB Link
IAEA-CO-8 CaCO3 -5.764‰0.032‰ VPDB Link
IAEA-CO-9 BaCO3 -47.321‰ 0.057‰ VPDB Link
NBS 22Oil-30.031‰0.043‰ VPDB Link

Oxygen

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‰ (Brand et al., 2014). [23]

Table 4: Oxygen Isotope Reference Materials
NameMaterialδ18OStandard

deviation

ReferenceLink
VSMOW2 H2O 0‰0.02‰ VSMOW Link
SLAP2 H2O -55.50‰0.02‰ VSMOW Link
GISP H2O -24.76‰0.09‰ VSMOW Link
IAEA-603 CaCO3 -2.37‰0.04‰ VPDB Link
NBS-18 CaCO3 -23.2‰0.1‰ VPDB Link
NBS-19 CaCO3 -2.20‰- VPDB Link
LSVEC Li2CO3 -26.7 ‰0.2‰ VPDB Link
IAEA-CO-1 Carrara marble -2.40.1‰ VPDB Link
IAEA-CO-8 CaCO3 -22.70.2‰ VPDB Link
IAEA-CO-9 BaCO3 -15.6 ‰0.2‰ VPDB Link

Nitrogen

Nitrogen gas (N2) 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.

Table 5: Nitrogen Isotope Reference Materials
NameMaterialδ15NStandard

deviation

ReferenceLinkSource/derivation of material
IAEA-N-1 (NH4)2SO4 0.4‰0.2‰ AIR Link
IAEA-N-2 (NH4)2SO4 20.3‰0.2‰ AIR Link
IAEA-NO-3 KNO3 4.7‰0.2‰ AIR Link
USGS32 KNO3 180‰1‰ AIR Link
USGS34 KNO3 -1.8‰0.2‰ AIR Link from nitric acid
USGS35 NaNO3 2.7‰0.2‰ AIR Link purified from natural ores
USGS25 (NH4)2SO4 -30.4‰0.4‰ AIR Link
USGS26 (NH4)2SO4 53.7‰0.4‰ AIR Link
NSVEC N2 gas -2.8‰0.2‰ AIR Link
IAEA-305 (NH4)2SO4 39.8‰

375.3‰

39.3 - 40.3‰

373.0 - 377.6‰

AIR Link 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 Link derived from urea

SD given as 95% confidence interval

IAEA-311 (NH4)2SO4 2.05 ‰2.03 - 2.06‰ AIR Link SD given as 95% confidence interval

Sulfur

The original sulfur isotopic reference material was the Canyon Diablo Troilite (CDT), 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‰ (Beaudoin et al., 1994). [13] 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 (VCDT) 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 (originally called IAEA-NZ1) to be -0.30‰ relative to VCDT. [2] These fairly recent changes to the sulfur isotope reference materials have greatly improved interlaboratory reproducibility (Coplen & Krouse, 1998). [24]

Table 6: Sulfur Isotope Reference Materials
NameMaterialδ34SStandard

deviation

ReferenceLinkSource/derivation of material
IAEA-S-1 Ag2S -0.30‰- VCDT Link from sphalerite (ZnS)
IAEA-S-2 Ag2S 22.7‰0.2‰ VCDT Link from gypsum (Ca2SO4*2H2O)
IAEA-S-3 Ag2S -32.3‰0.2‰ VCDT Link from sphalerite (ZnS)
IAEA-S-4 S 16.9‰0.2‰ VCDT Link from natural gas
IAEA - SO-5: BaSO4 0.5‰0.2‰ VCDT Link from aqueous sulfate (SO4)
IAEA - SO-6 BaSO4 -34.1‰0.2‰ VCDT Link from aqueous sulfate (SO4)
NBS - 127 BaSO4 20.3‰0.4‰ VCDT Link from sulfate (SO4) 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. [25] These reference materials span a large range of δ2H (-210.8‰ to +397.0‰), δ13C (-40.81‰ to +0.49‰), and δ15N (-5.21‰ to +61.53‰), and are amenable to a wide range of analytical techniques. The organic reference materials include caffeine, glycine, n-hexadecane, icosanoic acid methyl ester (C20 FAME), L-valine, methylheptadecanoate, polyethylene foil, polyethylene power, vacuum oil, and NBS-22. [25]

Table 7: Isotope Reference Materials for Organic Molecules [25]
NameChemicalδDVSMOW-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 ± 0.0420.17 ± 0.06
USGS63 caffeine 174.5 ± 0.9-1.17 ± 0.0437.83 ± 0.06
IAEA-600 caffeine -156.1 ± 1.3-27.73 ± 0.041.02 ± 0.05
USGS64 glycine --40.81 ± 0.041.76 ± 0.06
USGS65 glycine --20.29 ± 0.0420.68 ± 0.06
USGS66 glycine --0.67 ± 0.0440.83 ± 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-
USGS70icosanoic acid methyl ester-183.9 ± 1.4-30.53 ± 0.04-
USGS71icosanoic acid methyl ester-4.9 ± 1.0-10.5 ± 0.03-
USGS72icosanoic acid methyl ester348.3 ± 1.5-1.54 ± 0.03-
USGS73 L-valine --24.03 ± 0.04-5.21 ± 0.05
USGS74 L-valine --9.3 ± 0.0430.19 ± 0.07
USGS75 L-valine -0.49 ± 0.0761.53 ± 0.14
USGS76methylheptadecanoate-210.8 ± 0.9-31.36 ± 0.04-
IAEA-CH-7polyethylene foil-99.2 ± 1.2-32.14 ± 0.05-
USGS77polyethylene power-75.9 ± 0.6-30.71 ± 0.04-
NBS 22oil-117.2 ± 0.6-30.02 ± 0.04-
NBS 22avacuum oil-120.4 ± 1.0-29.72 ± 0.04-
USGS782H-enriched vacuum oil397.0 ± 2.2-29.72 ± 0.04-

The information in Table 7 comes directly from Table 2 of Schimmelmann et al. (2016). [25]

Non-traditional isotope systems

Heavy isotope systems

Isotopic reference materials exist for non-traditional isotope systems (elements other than hydrogen, carbon, oxygen, nitrogen, and sulfur), 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 (which is often the same as the material defining the scale, but not always), the calculated absolute isotopic ratio, and links to lists of isotopic reference materials prepared by the Commission on Isotopic Abundances and Atomic Weight (part of the International Union of Pure and Applied Chemistry (IUPAC)). A summary list of non-traditional stable isotope systems is available here, and much of this information is derived from Brand et al. (2014). [23] In addition to the isotope systems listed in Table 8, ongoing research is focused on measuring the isotopic composition of barium (Allmen et al., 2010; [26] Miyazaki et al., 2014; [27] Nan et al., 2015 [28] ) and vanadium (Nielson et al., 2011). [29] Specpure Alfa Aesar is an isotopically well-characterized vanadium solution (Nielson et al., 2011). [29] 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.

Table 8: Heavy Isotope Reference Materials
ElementSymbolδType of ratioName

(material for δ = 0)

Material

(material for δ = 0)

Name (material with

'best' measurement)

Isotope Ratio:

R (σ)

CitationLink
Lithium Li δ7Li7Li/6LiLSVEC (NIST RM 8545) Li2CO3 IRMM-01612.17697(3864)Qi et al. (1997) [30] Link
Boron B δ11B11B/10BNIST SRM 951(a) Boric acid IRMM-0114.0454(42)De Bièvre & Debus (1969) [31] Link
Magnesium Mg δ26/24Mg26Mg/24MgDMS-3 NO3 solutionDSM-30.13969(13)Bizzarro et al. (2011) [32] Link
Silicon Si δ30/28Si30Si/28SiNBS 28 (NIST RM 8546)Si sandWASO-17.20.0334725(35)De Bievre et al. (1997) [33] Link
Chlorine Cl δ37Cl37Cl/35ClSMOC-NIST SRM 9750.319876(53)Wei et al. (2012) [34] Link
Calcium Ca δ44/42Ca44Ca/42CaNIST SRM 915a CaCO3 NIST SRM 9153.21947(1616)Moore & Machlan (1972) [35] Link
Chromium Cr δ53/52Cr53Cr/52CrNIST SRM 979Cr(NO3)3 saltNIST SRM 9790.113387(132)Shields et al. (1966) [36] Link
Iron Fe δ56/54Fe56Fe/54FeIRMM-014 elemental Fe IRMM-01415.69786(61907)Taylor et al. (1992) [37] Link
Nickel Ni δ60/58Ni60Ni/58NiNIST SRM 986 elemental Ni NIST SRM 9860.385198(82)Gramlich et al. (1989) [38] Link
Copper Cu δ65Cu65Cu/63CuNIST SRM 976 elemental Cu NIST SRM 9760.44563(32)Shields et al. (1965) [39] Link
Zinc Zn δ68/64Zn68Zn/64ZnIRMM-3702ZN (II) solutionIRMM-37020.375191(154)Ponzevera et al. (2006) [40] Link
Gallium Ga δ71Ga71Ga/69GaNIST SRM 994 elemental Ga NIST SRM 9940.663675(124)Machlan et al. (1986) [41] Link
Germanium Ge δ74/70Ge74Ge/70GeNIST SRM 3120a elemental Ge Ge metal1.77935(503)Yang & Meija (2010) [42] Link
Selenium Se δ82/76Se82Se/76SeNIST SRM 3149 Se solution NIST SRM 31490.9572(107)Wang et al. (2011) [43] Link
Bromine Br δ81Br81Br/79BrSMOB-NIST SRM 9770.97293(72)Catanzaro et al. (1964) [44] Link
Rubidium Rb δ87Rb87Rb/85RbNIST SRM 984 RbCl NIST SRM 9840.385706(196)Catanzaro et al. (1969) [45] Link
Strontium Sr δ88/86Sr88Sr/86SrNIST SRM 987 SrCO3 NIST SRM 9878.378599(2967)Moore et al. (1982) [46] Link
Molybdenum Mo δ98/95Mo98Mo/95MoNIST SRM 3134solutionNIST SRM 31341.5304(101)Mayer & Wieser (2014) [47] Link
Silver Ag δ109Ag109Ag/107AgNIST SRM 978a AgNO3 NIST SRM 9780.929042(134)Powell et al. (1981) [48] Link
Cadmium Cd δ114/110Cd114Cd/110CdNIST SRM 3108solutionBAM Cd-I0122.30108(296)Pritzkow et al. (2007) [49] Link
Rhenium Re δ187Re187Re/185ReNIST SRM 989 elemental Re NIST SRM 9891.67394(83)Gramlich et al. (1973) [50] Link
Osmium Os δ187/188Os187Os/188OsIAG-CRM-4solutionK2OsO40.14833(93)Völkening et al. (1991) [51] Link
Platinum Pt δ198/194Pt198Pt/194PtIRMM-010 elemental Pt IRMM-0100.22386(162)Wolff Briche et al. (2002) [52] Link
Mercury Hg δ202/198Hg202Hg/198HgNRC NIMS-1solutionNRC NIMS-12.96304(308)Meija et al. (2010) [53] Link
Thallium Tl δ205Tl205Tl/203TlNRC SRM 997 elemental Tl NIST SRM 9972.38707(79)Dunstan et al. (1980) [54] Link
Lead Pb δ208/206Pb208Pb/206PbERM-3800solutionNIST SRM 9812.168099(624)Catanzaro et al. (1968) [55] Link
Uranium U δ238/235U238U/235UNIST SRM 950-A uranium oxide Namibian ore137.802321(688638)Richter et al. (1999) [56] Link

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. (2016). "Material" gives chemical formula, "Type of ratio" is the isotopic ratio reported in "Isotope ratio", and "Citation" gives the article(s) 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. (2016), [14] and manipulated to reach the reported ratios. Error was calculated as the square root of the sum of the squares of fractional reported errors.

Clumped isotopes

Clumped isotopes present a distinct set of challenges for isotopic reference materials. By convention the clumped isotope composition of CO2 liberated from CaCO347) [57] [58] [59] and CH41813CH3D12CH2D2) [60] [61] [62] are reported relative to a stochastic distribution of isotopes. That is, the ratio of a given isotopologue of a molecule with multiple isotopic substitutions against a reference isotopologue is reported normalized to that same abundance ratio where all isotopes are distributed randomly. In practice the chosen reference frame is almost always the isotopologue with no isotopic substitutions. This is 12C16O2 for carbon dioxide and 12C1H4 for methane. Standard isotopic reference materials are still required in clumped isotope analysis for measuring the bulk δ values of a sample, which are used to calculate the expected stochastic distribution and subsequently to infer clumped isotope temperatures. However, the clumped isotope composition of most samples are altered in the mass spectrometer during ionization, meaning that post-measurement data correction requires having measured materials of known clumped isotope composition. At a given temperature equilibrium thermodynamics predicts the distribution of isotopes among possible isotopologues, and these predictions can be calibrated experimentally. [63] To generate a standard of known clumped isotope composition, current practice is to internally equilibrate analyte gas at high temperatures in the presence of a metal catalyst and assume that it has the Δ value predicted by equilibrium calculations. [63] Developing isotopic reference materials specifically for clumped isotope analysis remains an ongoing goal of this rapidly developing field and was a major discussion topic during the 6th International Clumped Isotopes Workshop in 2017. It is possible that researchers in the future will measure clumped isotope ratios against internationally distributed reference materials, similar to the current method of measuring the bulk isotope composition of unknown samples.

Certifying reference materials

Overview

The certification of isotopic reference materials is relatively complex. Like most aspects of reporting isotopic compositions it reflects a combination of historical artifacts and modern institutions. As a result, the details surrounding the certification of isotopic reference materials varies by element and chemical compound. As a general guideline, the isotopic composition of primary and original calibration reference materials were used to define the isotopic scales and so have no associated uncertainty. Updated calibration materials are generally certified by IAEA and important reference materials for two-point isotopic scales (SLAP, LSVEC) were reached through interlaboratory comparison. The isotopic composition of additional reference materials are either established through individual analytical facilities or through interlaboratory comparisons but often lack an official IAEA certification. There are certified values for most of the materials listed in Table 1, about half of the materials listed in Tables 2–7, and few of the materials in Table 8.

Primary and original calibrations

The agreed-upon isotopic composition of primary reference and the original calibration materials were generally not reached through interlaboratory comparison. In part this is simply because the original materials were used to the define the isotopic scales and so have no associated uncertainty. VSMOW serves as the primary reference and calibration material for the hydrogen isotope system and one of two possible scales for the oxygen isotope system, and was prepared by Harmon Craig. VSMOW2 is the replacement calibration standard and was calibrated by measurements at five selected laboratories. The isotopic composition of SLAP was reached through interlaboratory comparison. [19] NBS-19 is the original calibration material for the carbon isotope scale made by I. Friedman, J. R. O’Neil and G. Cebula [64] and is used to define the VPDB scale. IAEA-603 is the replacement calibration standard and was calibrated by measurements at three selected laboratories (GEOTOP-UQAM in Montreal, Canada; USGS in Reston, USA; MPI-BGC in Jena, Germany). The isotopic composition of LSVEC was reached through interlaboratory comparison. [19] IAEA-S-1, the original calibration material for the sulfur isotope scale and still in use today, was prepared by B. W. Robinson. [2]

International Atomic Energy Agency

IAEA issues official certificates of isotopic composition for most new calibration materials. The IAEA has certified isotopic values for VSMOW2/SLAP2 [65] and IAEA-603 [66] (the replacement for the NBS-19 CaCO3 standard). However, the isotopic composition of most reference materials distributed by IAEA are established in the scientific literature. For example, IAEA distributes the N isotope reference materials USGS34 (KNO3) and USGS35 (NaNO3), produced by a group of scientists at the USGS and reported in Böhlke et al. (2003), [16] but has not certified the isotopic composition of these references. Moreover, the cited δ15N and δ18O values of these references were not reached through interlaboratory comparison. A second example is IAEA-SO-5, a BaSO4 reference material produced by R. Krouse and S. Halas and described in Halas & Szaran (2001). [67] The value of this reference was reached through interlaboratory comparison but lacks IAEA certification. Other reference materials (LSVEV, IAEA-N3) were reached through interlaboratory comparison [2] and are described by the IAEA but the status of their certification is unclear.

National Institute of Standards and Technology

As of 2018 NIST does not provide certificates for the common stable isotope reference materials. As seen at this link [68] showing the light stable isotope references currently available from NIST, this category includes all of the isotopic references critical for isotopic measurement of hydrogen, carbon, oxygen, nitrogen, and sulfur. However, for most of these materials NIST does provide a report of investigation, which gives a reference value that is not certified (following the definitions of May et al. (2000)). [69] For the above examples of USGS34 and USGS35, NIST reports reference values [70] but has not certified the results of Böhlke et al. (2003). [16] Conversely, NIST has not provided a reference value for IAEA-SO-5. As seen at this link, [71] NIST does certify isotopic reference materials for non-traditional "heavy" isotopic systems including rubidium, nickel, strontium, gallium, and thallium, as well as several isotopic systems that would normally be characterized at "light" but non-traditional such as magnesium and chlorine. While the isotopic composition of several of these materials were certified in the mid-1960s, other materials were certified as recently as 2011 (for example, Boric Acid Isotopic Standard 951a).

Uncertainty and error in reference materials

Uncertainty in absolute isotope ratios

Because many isotopic reference materials are defined relative to one another using the δ notation, there are few constraints on the absolute isotopic ratios of reference materials. For dual-inlet and continuous flow mass spectrometry uncertainty in the raw isotopic ratio is acceptable because samples are often measured through multi-collection and then compared directly with standards, with data in the published literature reported relative to the primary reference materials. In this case the actual measurement is of an isotope ratio and is rapidly converted to a ratio or ratios so the absolute isotope ratio is only minimally important for attaining high-accuracy measurements. However, the uncertainty in the raw isotopic ratio of reference materials is problematic for applications that do not directly measure mass-resolved ion beams. Measurements of isotope ratios through laser spectroscopy or nuclear magnetic resonance are sensitive to the absolute abundance of isotopes and uncertainty in the absolute isotopic ratio of a standard can limit measurement accuracy. It is possible that these techniques will ultimately be used to refine the isotope ratios of reference materials.

δ-scales with two anchoring reference materials

Measuring isotopic ratios by mass spectrometry includes multiple steps in which samples can undergo cross-contamination, including during sample preparation, leakage of gas through instrument valves, the generic category of phenomena called 'memory effects', and the introduction of blanks (foreign analyte measured as part of the sample). [1] As a result of these instrument-specific effects the range in measured δ values can be lower than the true range in the original samples. To correct for such scale compression researchers calculate a "stretching factor" by measuring two isotopic reference materials (Coplen, 1988). [72] For the hydrogen system the two reference materials are commonly VSMOW2 and SLAP2, where δ2HVSMOW2 = 0 and δ2HSLAP2 = -427.5 vs. VSMOW. If the measured difference between the two references is less than 427.5‰, all measured 2H/1H ratios are multiplied by the stretching factor required to bring the difference between the two reference materials in line with expectations. After this scaling, a factor is added to all measured isotopic ratios so that the reference materials attain their defined isotopic values. [1] The carbon system also uses two anchoring reference materials (Coplen et al., 2006a; 2006b). [21] [22]

See also

Related Research Articles

The molecular mass (m) is the mass of a given molecule. The unit dalton (Da) is often used. Different molecules of the same compound may have different molecular masses because they contain different isotopes of an element. The derived quantity relative molecular mass is the unitless ratio of the mass of a molecule to the atomic mass constant (which is equal to one dalton).

The dalton or unified atomic mass unit is a non-SI unit of mass defined as 1/12 of the mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state and at rest. The atomic mass constant, denoted mu, is defined identically, giving mu = 1/12m(12C) = 1 Da.

Relative atomic mass, also known by the deprecated synonym atomic weight, is a dimensionless physical quantity defined as the ratio of the average mass of atoms of a chemical element in a given sample to the atomic mass constant. The atomic mass constant is defined as being 1/12 of the mass of a carbon-12 atom. Since both quantities in the ratio are masses, the resulting value is dimensionless. These definitions remain valid even after the 2019 redefinition of the SI base units.

Isotope geochemistry is an aspect of geology based upon the study of natural variations in the relative abundances of isotopes of various elements. Variations in isotopic abundance are measured by isotope-ratio mass spectrometry, and can reveal information about the ages and origins of rock, air or water bodies, or processes of mixing between them.

Vienna Standard Mean Ocean Water (VSMOW) is an isotopic standard for water, that is, a particular sample of water whose proportions of different isotopes of hydrogen and oxygen are accurately known. VSMOW is distilled from ocean water and does not contain salt or other impurities. Published and distributed by the Vienna-based International Atomic Energy Agency in 1968, the standard and its essentially identical successor, VSMOW2, continue to be used as a reference material.

Isotopic labeling is a technique used to track the passage of an isotope through chemical reaction, metabolic pathway, or a biological cell. The reactant is 'labeled' by replacing one or more specific atoms with their isotopes. The reactant is then allowed to undergo the reaction. The position of the isotopes in the products is measured to determine what sequence the isotopic atom followed in the reaction or the cell's metabolic pathway. The nuclides used in isotopic labeling may be stable nuclides or radionuclides. In the latter case, the labeling is called radiolabeling.

In chemistry, isotopologues are molecules that differ only in their isotopic composition. They have the same chemical formula and bonding arrangement of atoms, but at least one atom has a different number of neutrons than the parent.

There are three known stable isotopes of oxygen (8O): 16
O
, 17
O
, and 18
O
.

An isotopic signature is a ratio of non-radiogenic 'stable isotopes', stable radiogenic isotopes, or unstable radioactive isotopes of particular elements in an investigated material. The ratios of isotopes in a sample material are measured by isotope-ratio mass spectrometry against an isotopic reference material. This process is called isotope analysis.

<span class="mw-page-title-main">Isotope dilution</span>

Isotope dilution analysis is a method of determining the quantity of chemical substances. In its most simple conception, the method of isotope dilution comprises the addition of known amounts of isotopically enriched substance to the analyzed sample. Mixing of the isotopic standard with the sample effectively "dilutes" the isotopic enrichment of the standard and this forms the basis for the isotope dilution method. Isotope dilution is classified as a method of internal standardisation, because the standard is added directly to the sample. In addition, unlike traditional analytical methods which rely on signal intensity, isotope dilution employs signal ratios. Owing to both of these advantages, the method of isotope dilution is regarded among chemistry measurement methods of the highest metrological standing.

<span class="mw-page-title-main">Isotope-ratio mass spectrometry</span> Usage of mass spectrometry to measure remaining isotopes

Isotope-ratio mass spectrometry (IRMS) is a specialization of mass spectrometry, in which mass spectrometric methods are used to measure the relative abundance of isotopes in a given sample.

In a chemical analysis, the internal standard method involves adding the same amount of a chemical substance to each sample and calibration solution. The internal standard responds proportionally to changes in the analyte and provides a similar, but not identical, measurement signal. It must also be absent from the sample matrix to ensure there is no other source of the internal standard present. Taking the ratio of analyte signal to internal standard signal and plotting it against the analyte concentrations in the calibration solutions will result in a calibration curve. The calibration curve can then be used to calculate the analyte concentration in an unknown sample.

Oxygen-16 is a nuclide. It is a stable isotope of oxygen, with 8 neutrons and 8 protons in its nucleus, and when not ionized, 8 electrons orbiting the nucleus. Oxygen-16 has a mass of 15.99491461956 u. It is the most abundant isotope of oxygen and accounts for 99.762% of oxygen's natural abundance.

<span class="mw-page-title-main">Standard atomic weight</span> Relative atomic mass as defined by IUPAC (CIAAW)

The standard atomic weight of a chemical element (symbol Ar°(E) for element "E") is the weighted arithmetic mean of the relative isotopic masses of all isotopes of that element weighted by each isotope's abundance on Earth. For example, isotope 63Cu (Ar = 62.929) constitutes 69% of the copper on Earth, the rest being 65Cu (Ar = 64.927), so

<i>δ</i><sup>13</sup>C Measure of relative carbon-13 concentration in a sample

In geochemistry, paleoclimatology, and paleoceanography δ13C is an isotopic signature, a measure of the ratio of the two stable isotopes of carbon—13C and 12C—reported in parts per thousand. The measure is also widely used in archaeology for the reconstruction of past diets, particularly to see if marine foods or certain types of plants were consumed.

<span class="mw-page-title-main">Atomic mass</span> Rest mass of an atom in its ground state

The atomic mass (ma or m) is the mass of an atom. Although the SI unit of mass is the kilogram (symbol: kg), atomic mass is often expressed in the non-SI unit dalton (symbol: Da) – equivalently, unified atomic mass unit (u). 1 Da is defined as 112 of the mass of a free carbon-12 atom at rest in its ground state. The protons and neutrons of the nucleus account for nearly all of the total mass of atoms, with the electrons and nuclear binding energy making minor contributions. Thus, the numeric value of the atomic mass when expressed in daltons has nearly the same value as the mass number. Conversion between mass in kilograms and mass in daltons can be done using the atomic mass constant .

<span class="mw-page-title-main">Commission on Isotopic Abundances and Atomic Weights</span> International scientific committee

The Commission on Isotopic Abundances and Atomic Weights (CIAAW) is an international scientific committee of the International Union of Pure and Applied Chemistry (IUPAC) under its Division of Inorganic Chemistry. Since 1899, it is entrusted with periodic critical evaluation of atomic weights of chemical elements and other cognate data, such as the isotopic composition of elements. The biennial CIAAW Standard Atomic Weights are accepted as the authoritative source in science and appear worldwide on the periodic table wall charts.

<span class="mw-page-title-main">Nanoscale secondary ion mass spectrometry</span>

NanoSIMS is an analytical instrument manufactured by CAMECA which operates on the principle of secondary ion mass spectrometry. The NanoSIMS is used to acquire nanoscale resolution measurements of the elemental and isotopic composition of a sample. The NanoSIMS is able to create nanoscale maps of elemental or isotopic distribution, parallel acquisition of up to seven masses, isotopic identification, high mass resolution, subparts-per-million sensitivity with spatial resolution down to 50 nm.

The δ34S value is a standardized method for reporting measurements of the ratio of two stable isotopes of sulfur, 34S:32S, in a sample against the equivalent ratio in a known reference standard. Presently, the most commonly used standard is Vienna-Canyon Diablo Troilite (VCDT). Results are reported as variations from the standard ratio in parts per thousand, per mil or per mille, using the ‰ symbol. Heavy and light sulfur isotopes fractionate at different rates and the resulting δ34S values, recorded in marine sulfate or sedimentary sulfides, have been studied and interpreted as records of the changing sulfur cycle throughout the earth's history.

Sulfur isotope biogeochemistry is the study of the distribution of sulfur isotopes in biological and geological materials. In addition to its common isotope, 32S, sulfur has three rare stable isotopes: 34S, 36S, and 33S. The distribution of these isotopes in the environment is controlled by many biochemical and physical processes, including biological metabolisms, mineral formation processes, and atmospheric chemistry. Measuring the abundance of sulfur stable isotopes in natural materials, like bacterial cultures, minerals, or seawater, can reveal information about these processes both in the modern environment and over Earth history.

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