Reference materials for use in differential measurements of stable isotope-number ratio (often shortened to “isotope ratio”) determination have been used since the early 1950s. They emerged from the few laboratories that started performing isotopic measurement, mostly in the geosciences. Locally produced isotopic reference materials were disseminated to new research groups to enable results traceable to a common origin. The carbon and oxygen isotopic reference material PDB (Peedee belemnite)  is a good example. The raw carbonate material (Cretaceous belemnite guards) was initially sampled by Heinz Lowenstam and Harold Urey during a field trip to the Peedee formation in South Carolina . By analyzing relative oxygen-18 abundances, they discovered to their disappointment that the material was relatively uniform in oxygen isotopic abundance. However, this uniformity is exactly what is needed for stable isotopic reference materials. Consequently, they collected a substantial amount of material and milled it until it was finely ground. This material was used for years as a reference material for stable carbon and oxygen isotopes in carbonate samples.
In the early 1950s, stable isotope mass spectrometers were not available commercially. These instruments were constructed in university laboratories, and a substantial effort went into the design and maintenance of these manually operated instruments. As isotope-ratio mass spectrometry (IRMS) spread into hydrology, biology, and other fields, computer-controlled instruments became available commercially in the late 1970s and early 1980s. While the routine performance of mass spectrometers continued improving, it became evident that the existing isotopic reference materials in use were not optimal for the task. PDB was comprised of microscopic chunks of calcite belemnite rostra, which were grown by the animal over a span of several years. During growth, water temperature changed throughout the seasons, which was reflected by variations in the oxygen isotopic composition within the rostrum . Hence, on a microscopic scale, PDB was not sufficiently homogeneous. Additionally, the original supply was fully satisfactory for a few laboratories, but as the number of stable isotope applications and laboratories grew, isotopic reference materials were soon in short supply. PDB was exhausted by the end of the 1970s. Consequently, a larger effort was made that finally led to the replacement of PDB in the early 1980s [3–5]. Both new carbon and oxygen scales were termed the “VPDB” (Vienna PDB) scale, in recognition of the leading role and efforts of the International Atomic Energy Agency (IAEA) in Vienna, Austria. The scale origins were defined through fixed offsets from NBS 19, a calcite material with agreed-upon isotopic compositions.
In general, in stable isotopic abundance measurements, the isotope-number ratio of an unknown sample is compared to a sample with well-known and/or agreed-upon properties. [Often, only isotopic homogeneity is well established; in (relative) delta measurements, the “absolute” abundances themselves are less relevant]. Because the differences in isotope-number ratios usually are very small, of the order of 10−3 or even smaller, the delta notation [6–8] is used for conveniently and accurately expressing such small differences.
The relative difference of isotope ratios (also called relative isotope-ratio or in short isotope-delta values) have been reported with the short-hand notation δi/jE, see below. The isotope-delta value is obtained from isotope number-ratios R(iE, jE)P
where iE denotes the higher (superscript i) and jE the lower (superscript j) atomic mass number of element E. The subscript P denotes the substance used to determine the respective values. It is customary to use a more practical short-hand notation R(i/jE)P instead.
The relative difference of isotope ratios (isotope-delta values) is obtained by the relation where Ref indicates a reference material. A more convenient short-hand notation is used as follows:
It is obvious from the short-hand notation that a correct interpretation of δ(i/jE) requires knowledge of the various propositions displayed in the defining relations presented above. Isotope-delta values are small numbers and therefore frequently presented in multiples of 10–3 or per mil (symbol ‰) (see also Coplen  and Wieser et al. ).
Isotope delta is a traditional notation in the geological sciences and has been adopted in many other areas. Reporting of isotope values using isotope-number ratios may still be preferred for a number of elements. The international conventions of scientific symbols suggest clear demarcation of the quantity symbol and the accompanying qualifying contextual information. This demarcation is commonly achieved with the use of subscripts or parentheses [10–12]. Thus, the carbon-isotope delta of the material X against material Y could be written as δ(13/12C)X,Y. However, to avoid clutter in mathematical expressions, publications in the last 60 years have traditionally opted for the simplified shorter form of notation by omitting the brackets and the denominator isotope where possible, e.g., δ13CY. While we follow this traditional short form, the omission of the qualifying brackets throughout this manuscript does not constitute an IUPAC recommendation of such practice; it merely employs a particular notation widely used and understood in the addressed science community.
3 Primary and secondary isotopic reference materials and the “IT Principle”
The delta notation allows small differences in number ratios to be expressed unambiguously beyond the precise knowledge of “absolute” isotopic abundances in the element. Best-measurement results are obtained when a sample and a reference material are similar in their chemical and physical properties, including their isotopic compositions. During measurement, small differences are not likely to be subject to systematic instrumental or preparatory bias; hence, one can measure these with the best accuracy. Moreover, sample and reference materials need to be processed in the same manner through the same sample preparation/conversion system that generates the analyte, which is introduced into the mass spectrometer (often a simple gas such as CO2 or H2). The procedure has been coined the “IT Principle” (IT = Identical Treatment [13, 14]). It has been in practical use since the 1950s as a general guideline for making stable isotopic measurements. As an example, the measurement standard NBS 19, a pure natural calcite powder of uniform, defined grain size , anchors both the stable oxygen and stable carbon isotopic composition scales. This material can be used to compare results with other calcite samples, using, for example, an acid digestion preparation or a high-temperature reaction to release CO2 for subsequent isotopic analysis. However, during such a preparation the isotopic composition of the material may be altered. The CO2 gas evolved from the acid reaction has only two oxygen atoms, whereas the calcite from which it emerged had three oxygen atoms attached to the carbon. Fortunately, because the materials are comparable in nature, the associated isotopic fractionation applies to the sample and to the reference material in the same quantitative fashion. Hence, the isotope relation between reference and sample remains identical.
For samples other than calcite, the situation can be substantially more difficult. For example, a difference in the mineralogy or chemical composition of carbonates can introduce different oxygen isotopic fractionations. This is the case for the oxygen isotopic determination of carbonates, such as siderite or dolomite. The specimen also could be a different chemical compound such as complex oil, or a bio-compound such as whole wood, DNA, a blood sample, or a trace gas component in an air sample, etc. The sample might contain the element being analyzed in both exchangeable and non-exchangeable compartments, such as hydrogen in keratin . A specimen could contain included extraneous water or other contaminant compounds, for example, barium sulfate for oxygen isotopic analysis that could contain pore water and nitrate . The physical/chemical preparation process for producing the pure measurement material can vary considerably, and this often is a significant source of isotopic fractionation and/or contamination. It would be ideal to have dedicated, well-calibrated isotopic reference materials for all types of sample compounds and matrices, but this is not possible. Instead, the practice has emerged to distinguish primary and secondary isotopic reference materials.
Primary isotopic reference materials for delta measurements are “international measurement standards” as defined in the International Vocabulary of Metrology, VIM3 . They define the scale zero or an anchor point and, if applicable, the scale span.1 These materials are assigned stable isotope values by consensus, with no uncertainty when used as reference for the respective delta scale (see Coplen .2) No measurement uncertainty is involved in establishing the reference (zero). “No uncertainty” applies to the consensus reference value for the material as a whole; any subsampling can lead to isotopic variations, which enter the error budget.3 The only remaining uncertainty arises from isotopic inhomogeneity, which is addressed during preparation of the reference materials. This uncertainty has to be considered and propagated accordingly, if accessible from the certificates or from the original literature. In many cases the heterogeneity of the reference material may be hidden in the uncertainty of the analytical measurement of the respective reference material or in the reported repeatability of the measurement of subsamples. The primary isotopic reference materials mark the end members of the respective traceability chains associated with the isotopic measurements.
Secondary isotopic reference materials serve to bridge the materials and chemistry gap. They are designed to be representative of a variety of typical compounds or substances that are analyzed by stable isotope laboratories. The values of secondary isotopic reference materials often are evaluated and compiled as a collaborative effort of several laboratories. These laboratories mostly are selected according to their demonstrated ability to make accurate measurements for the respective type of material; this process commonly is called value assessment. Values of secondary isotopic reference materials cannot be fixed permanently by a single certificate; their values may change as a result of new peer-reviewed, published values based on improved analytical methods and instrumentation. Improvement in value assignment has turned out to be necessary for some secondary isotopic reference materials every few years. A good example is NBS 22 oil (see Table 5). Its evaluated stable carbon isotopic composition had to be changed repeatedly because improved sample conversion procedures and a better understanding of instrumental effects led to a reduction in systematic errors.
For many isotopic reference materials, certificates are available from the respective institution. As an example, Fig. 1 depicts the top section of the NIST certificate (“Report of Investigation”) for NBS 22, Reference Material 8539 . The full certificate is available from the NIST Web site (www.nist.gov/srm/). Apart from reference delta values, the certificate provides information on the origin of the material, the analytical methods used for establishing the certified values, isotopic homogeneity, recommended scaling procedures, and reporting recommendations. Last, but not least, most of the relevant scientific literature concerning the respective reference material is provided.
In addition to traditional light-element stable isotopic abundance analysis (C, H, N, O, and S), accessible by gas-isotope mass spectrometry or more recently by optical (laser) absorption spectroscopy, other predominantly heavier element isotopes are being analyzed by delta measurement techniques. Their isotope-amount ratios increasingly are being measured in geochemistry, archeology, forensics, and food science, owing to improvements in mass spectrometric techniques such as TIMS (thermal ionization mass spectrometry) or SIMS (surface ionization mass spectroscopy), but mainly owing to the advent of MC-ICP-MS (multi-collector inductively coupled plasma mass spectrometry) [19–21]. The latter technique is very versatile in addressing a number of isotope systems in spite of pronounced mass bias effects [22–24]. In addition, isobaric interferences need to be evaluated and corrected for accordingly. For all techniques, the chemical preparation prior to the actual measurement can be laborious and may require treatments from sample digestion to matrix separation (e.g., via ion exchange) [25–28]. If possible, the reference materials are subjected to the same procedures, thus adhering to the IT principle [13, 14]. It is a difficult task to match the matrices of the large variety of samples to be analyzed in any one laboratory with a suitable reference material and matrix. This compromises the ultimately achievable accuracy. For a discussion of the impact of matrix effects on the error budget, see, for instance, Rosner et al. . A review of heavy-element stable isotopic variations in terrestrial materials has been published recently by Tanimizu et al. .
Within the last decade, spatially resolved isotope-ratio analysis using LA-(MC)-ICP-MS or SIMS of solid samples has become a rapidly growing field in isotope analytics. These in situ techniques require homogeneous, well-characterized compact solid reference materials or standards. However, many certified isotopic reference materials for the elements in question are salts or solutions. Solid reference materials with certified isotope-amount ratios or δ values for position-specific isotopic analysis do not exist. The scientific community has mainly used the NIST SRM 600 glass series as a delta standard for various isotope systems in the past. However, when the sample and the NIST glass are compositionally or texturally different, this approach may be problematic (in direct violation of the IT principle ). Differences in instrumental mass fractionation or matrix effects during sampling and sample preparation may lead to substantial errors. To ensure accurate and traceable in situ isotope-ratio determinations, compact, isotopically homogeneous matrix standards with well-characterized δ values are urgently needed.
We have included the newly emerging isotopic reference materials for delta scales in the set of tables presented below. However, it should be emphasized that the use of some of these materials is still at an early stage. The delta-value scales are often not widely agreed upon. Some reference materials were produced only recently and, they still need to demonstrate their merits as scale anchors or even as zero-delta materials, defining the origin of the respective delta scales. Apart from radiogenically altered elemental compositions, the terrestrial isotopic variations found in natural samples are usually small, owing to the fact that the relative mass differences are small as well. Moreover, isotopic fractionation effects via gas/liquid phase transitions are rare, and samples exhibiting enzymatically catalyzed isotopic changes in these elements still need to become more common on the laboratory shelf.
To express the small isotopic signatures, the delta equation  often has been given in the literature with a factor of 10 000. The defined quantity was then called epsilon (ε). However, as a coherent quantity equation, the extraneous factor should be omitted [10, 17]; the delta and epsilon equations become identical. Hence, we recommend that this use of epsilon be abandoned . Instead, and in order to comply with the guidelines for the SI system, the order of magnitude can be expressed using “per meg” or “pptt” . The terminology can also be changed completely to adopt the proposed urey  (symbol Ur) as the unit for delta, enabling one to employ the full range of prefixes permissible in the SI system. We also suggest that authors follow the agreed-upon convention that the heavy isotope should always be in the numerator and the lighter isotope in the denominator of the ratio in question .
In light of the role that secondary isotopic reference materials have played in the past regarding inter-laboratory comparability of data, this compilation sets out to cover comprehensively the secondary isotopic reference materials that have been used in the past for inter-laboratory comparability of data (although the list may still omit some materials accidentally). Arbitrarily selected examples on the most prominent use of the isotope-number ratios are listed for the single elements even though this publication does not intend to be a full review of applications and cannot reflect all publications in the respective fields accordingly.
Scientific publications are made at a particular point in time, while the data presented therein should be valid for a long time. Hence, it is important and has become common practice in scientific publications to provide information about (international) reference materials used and their values measured or employed as a secondary anchor for the respective stable isotope scales. Should a new value assignment of respective material arise after the initial measurements, sample results can be recalculated based on the newly found reference value for the secondary isotopic reference material employed in that publication.
In the following, the history and currently assigned values of isotopic reference materials are provided in the form of commented tables. The isotopic abundances of the elements in naturally occurring terrestrial materials are given as coarse information only with a reduced number of digits. Full values are listed in the recent IUPAC compilations [9, 31], available from the Web site of the IUPAC Commission on Isotopic Abundances and Atomic Weights (CIAAW) (http://www.CIAAW.org). In 2002, an extensive compilation of stable isotope data focusing on minimum and maximum isotope values found in natural samples was assembled by Coplen et al. [32, 33]. Only elements for which a zero-delta scale material has been produced or proposed are listed below. Thus, no information is provided for elements such as barium and tellurium.
To provide an up-to-date comprehensive overview, most isotopic reference materials that have played a significant role and/or were available to a wider scientific audience are included in the tables given below.4 This also applies to materials whose supply is now exhausted or that have been superseded by newer materials for various reasons. The tables are organized according to the periodic table of the elements as appropriate for stable isotopic measurements. Isotopic reference materials are, in general, identified by their original name. If the materials are important for more than one element, comments are given for each element, accepting some redundancy of information. Delta-value assignments are provided in chronological order with a corresponding reference. In the case of multiple entries, the current delta value recommended by CIAAW (if any) is listed in bold font. Values of international measurement standards (primary isotopic reference materials) are underlined in bold font. Multiple entries emphasize the necessity to report both the reference material used for scale anchoring as well as the delta value employed in the presented study. The column “uncertainty” presents uncertainties as provided in the cited literature. These uncertainties are often – especially for certified materials – expanded uncertainties (U) of combined standard uncertainties (uc) with a coverage factor k= 2 (U= k·uc). Data in the scientific literature provide a larger variety of uncertainties and, in many cases, the measurement precision alone, usually expressed as 1-sigma value. The type of uncertainty is not stated in the tables. For further information, readers are recommended to consult the original literature.
Hydrogen has two stable isotopes, 1H and 2H, with isotopic abundances of 99.98 and 0.02 %, respectively, in naturally occurring terrestrial materials. For historical reasons, the stable isotopes of hydrogen bear special names, protium and deuterium.5 The latter, mass number 2 isotope, was discovered in 1931 by Harold C. Urey [51, 52], the same Nobel prize winning chemist who provided the PDB standard calcite and started the new field of isotope geochemistry in his Chicago laboratory around the middle of the last century. Water was a prime medium of the first studies; thus, isotopic reference materials were first needed for water. Beginning in the 1950s, steam condensate from Potomac River Water (NBS 1) and snow melt water (NBS 1a) from Yellowstone, Wyoming, USA, were distributed by the U.S. National Bureau of Standards (NBS, now NIST) for δ2H and δ18O mass-spectrometric measurements of water [53, 54]. To anchor isotope-ratio results from water samples to the major world water pool, a new (virtual) Standard Mean Ocean Water (SMOW) scale was introduced by Craig  in 1961. He recommended that NBS 1 reference water be assigned a δ2H value of –47.62 ‰ relative to SMOW, the new virtual hydrogen-isotope scale, making NBS 1 the first international measurement standard for water. The first secondary measurement standard for water was NBS 1a. The supply of NBS 1 was insufficient to satisfy the expected demand. Therefore, at the request of the International Atomic Energy Agency (IAEA), H. Craig collected ocean water from the equator and 180° longitude, distilled it, and slightly adjusted its δ2H value so that it would agree with the NBS 1 definition . This water reference was first assigned the name SMOW for the acronym Standard Mean Ocean Water, but it was renamed VSMOW (after Vienna-SMOW) at a 1976 IAEA’s Consultants’ Meeting because there could be confusion between the virtual SMOW scale defined in terms of NBS 1 reference water and the scale defined in terms of the water prepared by H. Craig. Stocks of VSMOW were divided between NIST and the IAEA. It was found that the δ2H of NBS 1a reference water (approximately –183 ‰) was not sufficiently negative to encompass the full range of terrestrial water, and ice from the Antarctic Vostok site was obtained by the IAEA. This reference material was initially assigned the acronym SNOW, but it was later renamed with the acronym SLAP for Standard Light Antarctic Precipitation .
As outlined above, SMOW was introduced as a concept first, with a fixed distance from the then used NBS 1 water standard. This was before a common water standard replaced the virtual scale origin with a physically existing reference water, VSMOW . Together with VSMOW, the second 2H-depleted water standard, SLAP, was introduced because it had been recognized that water, in particular, was prone to isotope-scale contraction effects, mainly owing to its surface adhesion properties. The SLAP reference water was the first case of a second (isotopically “light”) anchor point for a stable isotope scale, which has greatly improved comparability of experimental data from laboratories worldwide. This was made an official rule for all hydrogen-bearing compounds at the 37th IUPAC General Assembly in 1993 in Lisbon. The Commission on Atomic Weights and Isotopic Abundances (CAWIA)6 recommended that δ2H values of all hydrogen-bearing materials be measured and expressed relative to VSMOW reference water on a scale normalized by assigning the consensus value of −428 ‰ to SLAP [56, 57]. Previously, this recommendation applied only to water . Authors should discontinue reporting δ2H values relative to SMOW .
For non-water samples, only a few reference materials exist at present. These include NBS 22 oil, NBS 30 biotite, and IAEA-CH-7 polyethylene foil. However, these materials do not differ substantially in their 2H abundances. Therefore, a SLAP-analog reference material is urgently needed to make use of the scaling rule for non-aqueous samples. There are relatively few organic reference materials having exchangeable hydrogen that are available from conventional suppliers of reference materials (Table 1). Two such materials are USGS42 Tibetan human hair and USGS43 Indian human hair. At present, several new materials are not yet characterized completely for hydrogen isotopic composition (IAEA-CH-3, IAEA-CH-6, IAEA-600, USGS40, USGS41). Only provisional data are available.
Lithium has two stable isotopes, 7Li and 6Li, with isotopic abundances of 92.4 and 7.6 %, respectively, in naturally occurring terrestrial materials. Relative lithium isotopic ratios in geochemical and environmental studies commonly are reported as δ7LiRM8545 values relative to the internationally distributed lithium carbonate isotopic reference material NIST RM 8545 (δ7Li = 0; LSVEC) in terms of N(7Li)/N(6Li) ratios. A high variability in lithium isotopic compositions of about 80 ‰ is observed in naturally occurring terrestrial materials, primarily due to the very large difference in mass between 6Li and 7Li.
Some laboratories still are reporting δ6Li values by using N(6Li)/N(7Li) ratios. This is confusing because (i) δ7Li values are opposite in sign to δ6Li values, (ii) the absolute values of δ7Li and δ6Li are not equal, and (iii) samples with more positive δ values are commonly thought of as being more dense or “heavier”, but samples with more positive δ6Li values are less dense. To eliminate possible confusion in the reporting of relative lithium-isotope-ratio data, CIAAW has recommended that relative lithium isotopic measurements of all substances be expressed as δ7Li values in terms of N(7Li)/N(6Li) ratios relative to the internationally distributed lithium carbonate isotopic reference material NIST RM 8545 (δ7Li = 0). Reporting of δ6Li values, expressed in terms of N(6Li)/N(7Li) ratios, should be discontinued. Guidelines for reporting lithium delta values were published in “Atomic weights of the elements 1995” .
For reporting δ7Li it is recommended that NIST RM 8545 lithium carbonate (LSVEC) be used, which was prepared by H. Svec, Iowa State University  from virgin, North Carolina ores. In comparison to average seawater, NIST RM 8545 is depleted in 7Li by about 30 ‰. Using MC-ICP-MS, repeated analysis of NIST RM 8545 standard solutions can be made with an uncertainty of ~0.2 ‰ . Due to NIST RM 8545 carbonate’s rather negative δ13C value, close to that of atmospheric methane, it frequently has been used as a carbon stable isotopic reference material , and it serves as the anchor for the low isotopic abundance end of the stable carbon-isotope scale .
In addition to the lithium δ–zero material (NIST RM 8545, LSVEC), three certified lithium isotopic reference materials are available. IRMM-015 and IRMM-016 are Li2CO3 materials, and IRMM-615 is a solution made from IRMM-015 base material. While IRMM-015 and IRMM-615 are highly enriched in 6Li (N(6Li)/N(7Li) = ~21.9), IRMM-016 has a natural lithium isotopic composition very close to that of NIST RM 8545. Compared to the δ7LiSRM8545 value of +0.35 ‰ calculated from the certificates, 17 independent studies report consistent δ7LiRM8545 values between –0.8 and +0.5 ‰, with a mean of –0.01 ‰ (± 0.6) for IRMM-016 (georem.mpch-mainz.gwdg.de/).
Within the last 20 years, seawater and rock reference materials, primarily, have been characterized for δ7LiRM8545 values. For seawater, a very large dataset of published δ7LiRM8545 values exists. In 2007, a data compilation for δ7LiRM8545 values of seawater was published that indicated a mean δ7LiRM8545 value of +30.8 ‰ for modern seawater (open ocean) . In addition to seawater samples, δ7LiRM8545 values for the seawater reference materials NRC NASS-5 (+30.7 ‰ (n = 1)), IRMM BCR-403 (+30.8 ‰ ± 0.6 (n = 10)), and the seawater salinity standard OSIL IAPSO (+31.1 ‰ ± 0.3 (n = 4)) have been compiled in the GeoReM database.
For silicate reference materials, a large dataset of published δ7LiRM8545 values is available. The entire NIST SRM 61x silicate glass series was characterized by Kasemann et al. in 2005 for δ7LiRM8545 values . In addition to this complete dataset of δ7LiRM8545 values, three studies published lithium-isotope data for NIST SRM 610 and 612. The reason for the high δ7LiRM8545 value published by Magna et al.  is unclear. Representative of the numerous rock reference materials, the basaltic BHVO materials are mentioned. In March 2013, the mean of 22 published δ7LiRM8545 values for BHVO-2 listed in the GeoReM database yielded a mean value of +4.5 ‰ (±0.5).
The consistency of published δ7LiRM8545 values for isotopic reference and quality control materials suggests an overall expanded analytical uncertainty of most lithium-isotope studies of well below 1 ‰.
Boron has two stable isotopes, 10B and 11B, with isotopic abundances of 19.8 and 80.2 %, respectively, in naturally occurring terrestrial materials. Following the general rule that the heavy isotope should be reported in the numerator of the respective ratio, isotopic measurements are measured relative to NIST SRM 951 and published as δ11BSRM951 values. Due to the large relative mass difference between the two boron isotopes and the special physicochemical behaviour of boron, the variability of boron isotopic composition of naturally occurring terrestrial materials is about 80 ‰. Owing to a high scientific interest in boron-isotope chemistry and the use of boron for nuclear applications, a large number of certified isotopic reference materials exist for boron.
NIST SRM 951 and NIST SRM 952 were both prepared at NIST within the same effort to generate certified reference materials for boron isotopic composition . NIST SRM 951 was made from an original lot of pure H3BO3 of more than 200 kg in 22 containers, which was free from impurities and largely homogeneous throughout the lot (except for one container). NIST SRM 951a is a newly bottled batch of the NIST SRM 951 material. Although not mentioned in the certificate, exactly the same values with their uncertainties have been certified. The raw material for the 11B-depleted NIST SRM 952 was obtained from Oak Ridge National Laboratory. To ensure sample homogeneity and absence of metallic and other impurities, the material was recrystallized twice before further characterization.
The Institute for Reference Materials and Measurements (IRMM) produced two certified boron isotopic reference materials. IRMM-011 consists of 1 g crystalline boric acid aliquots in glass vials. IRMM-610 is an aqueous solution of pure boric acid with an acid content of ~5 mmol·L–1 and boron isotopic composition, which was designed to be close to that of NIST SRM 952. The certified values have been obtained by applying the Na2BO2+ thermal ionization mass spectrometry (TIMS) technique and using a mass spectrometer calibrated via synthetic isotope mixtures . Recently, a δ11BSRM951 value of −0.37 ‰ for IRMM-011 was determined using the Na2BO2+ method .
Beginning in 2001, the BAM Federal Institute of Materials and Testing (BAM) produced a number of certified boron isotopic reference materials with a large variety of isotopic compositions. Six of the BAM materials are enriched in 10B, one is isotopically similar to that of NIST SRM 951 (ERM-AE120), three materials are δ-reference materials (ERM-AE120, 121, 122) with δ11B values of −20, +20, and +40 ‰, and one is a boron carbide matrix material (ERM-ED102). ERM-AE120, ERM-AE121, and ERM-AE122 are the first boron reference materials, which are certified for their δ values.7 They were produced by mixing normal boric acid either with 10B or with 11B solutions to yield specific δ11B values, covering about three-quarters of the boron isotopic variability of naturally occurring materials. To obtain the certified δ11B values listed in Table 4, results from Na2BO2+ TIMS measurements have been combined with those from Cs2BO2+ TIMS measurements and gravimetric preparation.
Apart from certified boron isotopic reference materials, numerous matrix materials are used for quality control of boron-isotope data. Here we focus on internationally recognized natural solution materials from IAEA, NRCC, and OSIL (Ocean Scientific International ltd.), rock and glass materials from IAEA and NIST, and plant reference materials from NIST and IRMM.
In 2003, eight boron isotope quality control materials were produced by the Istituto di Geoscienze e Georisorse, Pisa, Italy  for the IAEA.8 These materials (natural waters, rocks, and one glass) have recommended δ11B values, which originate from an inter-laboratory comparison study:
IAEA-B-1 is a surface seawater sample collected from the north of Elba Island, Ligurian Sea, Italy.
IAEA-B-2 is a groundwater sample collected from an alluvial aquifer in the lower basin of the Cecina River, Italy. After filtration, water samples were acidified with boron-free HCl and distributed in polyethylene (PE) bottles.
IAEA-B-3 is a groundwater sample collected from an alluvial aquifer in the upper basin of the Cecina River, Italy. After filtration, water samples were acidified with boron-free HCl and distributed in PE bottles.
IAEA-B-4 was obtained from a composite sample of euhedral crystals of black tourmaline, near San Piero in Campo, Elba Island. Prior to distribution into PE bottles, the material was ground to a grain size ranging between 5 and 40 μm with a few larger grains of 100 μm, mixed, and homogenized .
IAEA-B-5 is a natural basalt material, originating from the Etna Volcano eruption in 1998. The material was ground to a grain size generally smaller than 5 μm with a few grains up to 40 μm, mixed, and homogenized .
IAEA-B-6 was derived from obsidian collected on the Lipari Island. Homogenization of the material was obtained by alkali fusion .
IAEA-B-7 is a marine limestone collected at Maiella (Abruzzo). The material was ground to a grain size smaller than 5 μm and homogenized .
IAEA-B-8 is a natural clay material collected from a deposit near Montelupo (Tuscany). The clay was heated for three days at 120 °C to remove water, then ground to a grain size smaller than 5 μm, and homogenized .
For NIST SRM 610/612 and the natural seawaters NASS-5 and IAPSO, published δ11BSRM951 values are available. The latter two values are close to the global seawater mean δ11BSRM951 value of (+39.61 ± 0.2) ‰ . Plant reference materials were characterized for the first time in 2011 for their boron isotopic composition. The δ11BSRM951 values of −23.8 ‰ for cabbage (BCR-679), +8.3 ‰ for corn bran (NIST SRM 8433), and +41.1 ‰ for peach leaves point to an extreme variability of δ11BSRM951 values in plants.
Carbon has two stable isotopes, 12C and 13C, with natural isotopic abundances of 98.9 and 1.1 %, respectively. Carbon has one long-lived radioactive isotope, 14C, with an isotopic abundance of ~10–12. (Carbon-14 is constantly produced in the upper atmosphere. With a half-life of ~5700 years, it is important for dating recent artifacts. Owing to its radioactivity, it is not considered further in this compilation.)
The R(13C/12C) ratio, commonly abbreviated as 13C/12C, probably is the most frequently analyzed stable isotope quantity. The corresponding primary scale (VPDB) and its history are discussed above. Today, the scale is realized through two reference materials, NBS 19 (a natural calcite ) and LSVEC (NIST RM 8545; lithium carbonate from natural ores ) with consensus δ13CVPDB values of +1.95 and −46.6 ‰, respectively [72, 103]. The isotopic compositions differ sufficiently to encompass most naturally occurring carbon-bearing materials with the notable exception of biogenic methane, which often is depleted in 13C beyond that of LSVEC . LSVEC lithium carbonate was prepared by H. Svec, Iowa State University , originally to be used as a reference material for lithium isotopic composition. The carbon isotopic composition of LSVEC happened to match that of CH4 in modern air samples. The two materials are used to apply scaling error corrections, thus allowing an improved inter-laboratory comparability of results [72, 103].
When analyzing plant or plant-derived material (including mineral oil), δ13CVPDB values mostly cluster between −25 and −30 ‰, arising from the commonly observed discrimination against 13C during (C−3) photosynthesis. NBS 21 graphite and NBS 22 oil are secondary reference materials that have been in use frequently for these organic materials, but the supply of NBS 21 is exhausted. The relatively large spread of delta values for NBS 22 [93, 94] reflects the analytical history of reference-value assignments, caused by mass-spectrometric cross-contamination effects (“η-effect”) , which before 2000 were largely overlooked and unaccounted for during reference-material-calibration measurements [88, 89, 106, 107]. Another larger source of error in past δ13C measurements has been an inconsistent correction procedure for the 17O-bearing CO2 isotopologue, representing about 7 % of the m/z-45 ion current [11, 86, 108–112]. For improving inter-laboratory comparability, IUPAC now recommends  that the Assonov  parameter set be used for the correction of the 17O contribution. The δ13C-table (Table 5) already has an impressive number of reference materials, which mainly reflects the frequency of this kind of analysis. However, with the ever-increasing relevance of chromatographic techniques, there is a growing need for new, well-calibrated reference materials amenable to and compatible with these techniques.
At the Beijing General Assembly in 2005, the Commission recommended that δ13C values of all carbon-bearing materials be measured and expressed relative to the VPDB scale. This scale is to be normalized by assigning consensus values of −46.6 ‰ to LSVEC lithium carbonate and +1.95 ‰ to NBS 19 calcium carbonate, and authors should clearly state so in their reports . Authors are encouraged to report their measurement results for δ13C values of NBS 22 oil, USGS41 l-glutamic acid, IAEA-CH-6 sucrose, or other internationally distributed reference materials, as appropriate for the measurement method concerned, had they been analyzed with the author’s samples. This recommendation supersedes the recommendation made by the Commission in 1993 . Full analytical data supporting this recommendation is found in Coplen et al. . Authors should discontinue reporting their δ13C values relative to PDB . The full recommendation appears in Wieser .
Nitrogen has two stable isotopes, 14N and 15N, with natural isotopic abundances of 99.6 and 0.4 %, respectively. Most of the nitrogen on Earth is in the atmosphere as N2, and its isotopic composition cannot be changed easily. This has earned the R(15N/14N) ratio of N2 in air the status of the zero-delta point for the δ15NAIR scale.
Atmospheric N2 is isotopically homogeneous within current analytical uncertainty, is the international measurement standard for δ15N measurements, and is assigned a δ15N value of zero by international agreement [114, 115, 117]. Intended for use mainly in medical and biological tracer studies, the IAEA distributes ammonium sulfate (IAEA-305A and IAEA-305B), urea (IAEA-310A and IAEA-310B), and ammonium sulfate enriched in 15N (IAEA-311) .
To eliminate possible confusion in the reporting of the nitrogen isotope-amount ratio, n(14N)/n(15N), the Commission recommended in 1991 at the 36th General Assembly in Hamburg that the value 272 be employed for the n(14N)/n(15N) value of N2 in air for calculating n(14N)/n(15N) values. Such quantities are atom fractions or stable isotopic abundance fractions. This recommendation derives from the fact that the Commission’s 1983 Table of Isotopic Compositions rounds the originally reported Junk and Svec n(14N)/n(15N) value of 272.0 ± 0.3 in atmospheric nitrogen to (99.634 ± 0.001)/(0.366 ± 0.001), which is 272.22 ± 0.75. The difference between 272 and 272.22 corresponds to a δ15N difference of 0.8 ‰, which is about 10 times the measurement precision of many stable isotope laboratories. When converting δ15NAIR values to stable isotope-amount ratios, some authors use 272 and some use 272.22. The Commission recommends that 272 always be used in this conversion. The full recommendation appears in Coplen et al. .
Oxygen has three stable isotopes, 16O, 17O, and 18O, with isotopic abundances of 99.76, 0.04, and 0.20 %, respectively, in naturally occurring terrestrial material. The ratio R(18O/16O) is the most commonly measured isotope ratio. The choice of an oxygen-isotope scale depends on the substances measured. Three concurrent delta scales are in use: one for water (δ18OVSMOW), one for carbonates (δ 18OVPDB), and one for O2 gas (δ 18Oair-O2). Measurements of the respective materials can best be made using the appropriate reference material. The relations of the scales are given in the tables; however, they are not fixed permanently. Rather, with new studies or technological advances, these relations might be refined in the future. Nevertheless, the VSMOW and VPDB scales commonly are related for sample x by the quantity equation 
One of the problems with barium sulfate isotopic reference materials is that they may contain inter-crystalline water, trapped during precipitation of barium sulfate. For example, Hannon et al.  report that heating IAEA-SO-6 barium sulfate to 600 °C reduces the yield of oxygen from 105.0 ± 1.2 to 100.0 ± 1.0 %, and the δ 18O value increases from –11.34 ± 0.10 to –10.81 ± 0.08 ‰. This increase in δ18O with heating is consistent with removal of water with a δ18O value of approximately –19 ‰ relative to VSMOW.
In addition to δ18O, the R(17O/16O) ratio, or rather its deviation from the statistical, purely mass-dependent fractionation, has garnered considerable interest. For clarity, a separate table for δ17O reference materials is provided.
At the 37th IUPAC General Assembly in 1993 in Lisbon, the Commission recommended guidelines for δ18O measurements.
Relative N(18O)/N(16O) values (δ18O) of water should be expressed relative to VSMOW water (δ18O = 0) on a scale normalized such that the δ18O of SLAP water is −55.5 ‰ exactly and so stated in the author’s report.
Relative N(18O)/N(16O) values (δ18O) of carbonate should be expressed on a scale normalized such that the δ18O of SLAP reference water is −55.5 ‰ exactly relative to VSMOW water, and so stated in author’s report. The measured value should be expressed either relative to VPDB on a scale such that the δ18O of NBS 19 calcite is −2.2 ‰ exactly, stating the value of the oxygen isotopic fractionation factor used to calculate the δ18O of the carbonate sample and NBS 19 if they are not identical, or relative to VSMOW water (δ18O = 0), stating the values of all isotopic fractionation factors upon which the δ18O measurement depends. If the δ18O values cannot be reported on a normalized scale (perhaps because of the lack of a capability to analyze water samples), the author’s measured δ18O of NBS 18 carbonatite or other internationally distributed isotopic reference material should be reported, as appropriate, had it been analyzed with the samples.
3.6.3 Other substances (oxygen gas, sulfate, silicate, phosphate, etc.)
Relative N(18O)/N(16O) values (δ18O) of all other oxygen-bearing substances should be expressed relative to VSMOW water (δ18O = 0) on a scale normalized such that the δ18O of SLAP water is −55.5 ‰ exactly relative to VSMOW water, and so stated in the author’s report. The author’s report should state either the values of all isotopic fractionation factors upon which a δ18O values depends, or the author’s measured δ18O of NBS 28 quartz, NBS 127 barium sulfate, USGS35 sodium nitrate, or other internationally distributed reference material had it been analyzed with the samples. If the δ18O values cannot be reported on a normalized scale (perhaps because of the lack of a capability to analyze water samples), the author’s measured δ18O of NBS 18 carbonatite or other internationally distributed isotopic reference material should be reported, as appropriate, had it been analyzed with the samples.
Oxygen gas may also be analyzed relative to air-O2 as a reference. Such measurements often require very high precision, which cannot be maintained relative to a water sample like VSMOW [127, 129]. Rather, values are simply measured relative to O2 gas calibrated against air-O2 defining the origin of the δ18Oair-O2 scale . Authors should discontinue reporting their δ18O values relative to SMOW and PDB . The full recommendation appears in Coplen et al. .
Magnesium is comprised of three stable isotopes, 24Mg, 25Mg, and 26Mg, with isotopic abundances of 79.0, 10.0, and 11.0 %, respectively, in naturally occurring terrestrial material. The R(26Mg/24Mg) ratio is studied more frequently, owing to the larger relative mass difference. Isotopic variations in nature extend over a δ26/24Mg range of no more than 6 ‰ [137–139], making an uncertainty of 0.1 ‰ or better desirable.
NIST SRM 980 was certified by NIST in the 1960s by synthetic isotope mixtures [134, 135]. This material served for decades as an isotopic reference material for “absolute” isotope-ratio determinations and as a zero-delta material for the δ26/24MgSRM980 scale. However, in 2003, SRM 980 was demonstrated to be isotopically inhomogeneous by Galy et al. , who found heterogeneities of ~0.35 ‰ for δ25/24Mg and ~0.69 ‰ for δ26/24Mg, both expressed as 2σmean. These heterogeneities are still within the certified uncertainties of NIST SRM 980, but they are much too large for current isotope research, which can achieve uncertainties at the 0.1 ‰ level. Galy et al.  proposed two new magnesium standard solutions, DSM3 and Cambridge1, with DSM3 being recommended as a new zero-delta standard for magnesium, which is what many analysts have been doing since. The drawbacks of these materials are that neither are international reference materials nor are they publicly available in sufficient quantities to satisfy global distribution needs for the next several years. As a possible solution to this dilemma, IRMM-009, being derived from NIST SRM 980, could be used successfully as the zero-delta material for the δ25/24MgSRM980 and the δ26/24MgSRM980 scales, thereby avoiding the known heterogeneity issues of the solid NIST SRM 980 material. Another option could be to use the IAPSO seawater standard with δ25/24Mg = +0.44 ‰ as an interim reference until a replacement for DSM3 has been found. The IAPSO seawater isotopic composition is very close to the global mean seawater delta values of δ25/24Mg = +0.43 ± 0.04 ‰ and δ26/24Mg = +0.82 ± 0.06 ‰, respectively . The magnesium delta scales need to be clarified in the near future.
In 2000 and 2001, together with IRMM-009, two additional isotopic reference materials were released by IRMM, ERM-AE637 (formerly IRMM-637), and ERM-AE638 (formerly IRMM-638). ERM-AE637 has an isotopic composition in the range of terrestrial materials, and ERM-AE638 is highly enriched in 26Mg. Both materials have been certified using NIST SRM 980 for correcting mass fractionation and/or discrimination. The delta values have been calculated from the certified ratios, which is not very precise. They are given without certified uncertainties.
Silicon has three stable isotopes, 28Si, 29Si, and 30Si, with isotopic abundances of 92.2, 4.7, and 3.1 %, respectively, in naturally occurring terrestrial material. While being the second most abundant element on Earth, silicon isotopes have found only limited applications. This may be due to the fact that the variation in silicon isotopic composition is small (δ30/28Si ~ ±3.5 ‰) , thus requiring rather high precision and limiting a wider spread of interest. In addition, preparation of a gaseous compound like SiF4 is more cumbersome than a simple oxidation step for other elements.
More recently, MC-ICP-MS with a medium mass resolution (m/Δm> 2000) has been introduced as a solution to this experimental challenge . Replacing the fluorination step with a NaOH-SiO2 fusion reaction, silicon-isotope values can now be compared between laboratories with improved uncertainty . Additionally, materials covering a wider range of isotopic compositions have been investigated [141, 142].
Sulfur has four stable isotopes, 32S, 33S, 34S, and 36S, with isotopic abundances of 95.0, 0.75, 4.2, and 0.015 %, respectively, in naturally occurring terrestrial material. Of these, the R(34S/32S) ratio is the most common target for stable isotope determinations. Isotopic measurements are made relative to VCDT (“Vienna Cañon Diablo Troilite”; see below) and expressed as δ34SVCDT values. The traditional measurement gas is SO2[155, 156], which is easy to generate by combustion, but also has several drawbacks.
SO2 does not represent the highest oxidation state. SO3 gas is also formed during oxidation, which chemically is highly reactive. It can form solid needles at lower temperatures and vanish from a reaction stream, resulting in apparent isotopic alteration. Optimization of an intermediate stage is required to ensure that sulfur is converted quantitatively to SO2.
SO2 easily dissolves in water or on water-covered surfaces, forming sulfurous acid, H2SO3. Likewise, SO3 generates sulfuric acid, H2SO4 under such conditions. These acids, in turn, can damage surfaces in inlet systems and in a gas source mass spectrometer. SO2 can produce other gaseous compounds, which may interfere with the sulfur isotopic analysis.
In mass spectrometry, the SO2 ion current on the m/z-64 signal is comprised of 34S16O16O+, 32S18O16O+, and 33S17O16O+. To extract the pure 34S signature, the m/z-64 ion current needs to be corrected. This requires that the sample and reference materials have identical oxygen isotopic compositions, which can be achieved by a common combustion procedure.
Due to the high surface activity of SO2, measured isotopic differences between samples often are too small, and a cross-contamination (between-sample memory) is difficult to avoid even when heating all transfer lines as well as the mass spectrometer ion source.
For establishing delta values for isotopic reference materials, it has therefore become common practice to use SF6 instead of SO2. The chemistry is difficult to master, but the mass-spectrometric measurement is facilitated by the inertness of SF6 and by the fact that fluorine has only one stable isotope . Once appropriate reference values are assigned, the respective materials can be used for scaling measured isotopic distances using isotope bracketing. More recently, sulfur isotopic information has also been obtained from MC-ICP-MS measurements, with a major advantage of a significantly reduced sample size [153, 158, 159].
The Commission wanted to eliminate possible confusion in the reporting of relative sulfur isotope-amount-ratio data. Thus, in 1995 at the 38th IUPAC General Assembly in Guildford, UK, in agreement with an IAEA Consultants’ Meeting, the Commission recommended that δ34S measurements of all sulfur-bearing materials be expressed relative to VCDT. The VCDT scale is defined by assigning a δ34S value of –0.3 ‰ exactly (relative to VCDT) to the silver sulfide reference material IAEA-S-1. This recommendation derives from the determination by Beaudoin et al.  that the troilite from the Cañon Diablo meteorite, CDT, is isotopically inhomogeneous, having a δ34S variability of 0.4 ‰. Reporting of δ34S measurements relative to CDT should be discontinued. The full recommendation appears in Krouse et al. .
Chlorine has two isotopes, 35Cl and 37Cl, with isotopic abundances of 75.8 and 24.2 %, respectively, in naturally occurring terrestrial material. The relative mass difference is similar to that of oxygen, and the corresponding isotope effects could be large enough for routine measurements (neglecting the differences in valence states). This is, however, only true for some special cases, in particular when isotopic fractionations between a liquid and a gas phase are involved. Most chlorine is in the world oceans, where the signature does not change by much more than 1 ‰. Hence, high precision is a requirement for studying δ37Cl signatures in natural samples. Industrially produced organic chlorine compounds exhibit a wider range of δ37ClSMOC values (−7 to +6 ‰) [162, 163]. Only recently, a set of natural, soil-organic compounds highly depleted in 37Cl [164, 165] has been found with δ37ClSMOC values extending to −13 ‰. Typically, these phenolic compounds arise from enzymatically catalyzed reactions with chloro-peroxidases (CPO)  in forest soils.
Before approximately 2002, most delta measurements of chlorine isotopes were expressed relative to seawater chloride (SMOC for Standard Mean Ocean Chloride), which was thought to be homogeneous in chlorine isotopic composition to within approximately ±0.15 ‰ . However, the δ37Cl value of seawater chloride can vary by more than 1 ‰ depending upon geographic location of specimen , and seawater chloride itself cannot serve as an isotopic reference material. Xiao et al.  measured a δ37Cl value of +0.43 ‰ for NIST SRM 975. This value has been internationally accepted as the new anchor for the SMOC scale. As a replacement material, NIST SRM 975a has been assigned a δ37Cl value of +0.2 ‰ exactly .Closer to the SMOC scale origin is ISL-354 sodium chloride, which has been produced from seawater by Y. Xiao of the Qinghai Institute of Salt Lakes .ISL-354 is intended to be used in addition to NIST SRM 975a as a secondary reference material. The relation between the different reference materials and their implications for the atomic weight of chlorine has recently been investigated thoroughly by Wei et al. .
Calcium has five naturally stable isotopes, 40Ca, 42Ca, 43Ca, 44Ca, and 46Ca, with isotopic abundances of 96.9, 0.6, 0.1, 2.1, and 0.004 %, respectively, in naturally occurring terrestrial material. It also has one very long lived radioactive isotope, 48Ca (half-life 4.4 × 1019 years ), with a characteristic terrestrial isotopic composition, amounting to an isotopic abundance of 0.2 %. Terrestrial isotopic variations are largest for biological systems, whereas inorganic materials exhibit only small calcium isotopic fractionations . For an orca bone, Skulan et al.  found a δ44/40Ca value of −3.2 ‰ relative to dissolved calcium in seawater.
NIST SRM 915a is used most often as a reference for the respective isotope ratios. Calcium isotope-amount ratios n(44Ca)/n(40Ca) commonly are measured to determine δ44/40Ca values. However, 40Ca may be a poor choice for the denominator in this ratio because 40Ca is a product of 40K radioactive decay; thus, the mole fraction of 40Ca will vary with the age and the N(K)/N(Ca) ratio of a material [33, 170]. In addition, 40Ca cannot be used in (hot-plasma) MC-ICP-MS studies due to the overwhelming 40Ar interference. Here, analysis of δ44/42Ca at a resolution of m/Δm> 2500 is the only choice. By multiplying with 1.9995, these data may be converted to δ44/40Ca values [168, 172]. This technique can also be used to exclude the small radiogenic contribution in 40Ca. The δ44/42Ca values can be measured in this manner with an uncertainty below 0.2 ‰ . For comprehensive reviews, see DePaolo  and Boulyga (2010) .
Chromium has four stable isotopes, 50Cr, 52Cr, 53Cr, and 54Cr, with isotopic abundances of 4.3, 83.8, 9.5, and 2.4 %, respectively, in naturally occurring terrestrial material. Only 52Cr and 53Cr are used for δ-value measurements; 50Cr and 54Cr suffer from interference of 50V and 54Fe. The R(53Cr/52Cr) ratio is about 0.113 and should, in principle, be easy to measure precisely. However, considerable difficulties due to the redox chemistry of chromium can arise during sample preparation for TIMS, limiting attainable uncertainty to about 0.1 ‰.
Usually, NIST SRM 979 serves as the δ53/52Cr reference point defining the scale origin. IRMM-012 has been made from NIST SRM 979 by dissolution of the nitrate salt in nitric acid. Variations in nature of up to +6 ‰ relative to NIST SRM 979 have been observed, most notably in groundwater samples . These enrichments seem to be related to chromium(VI) compound cycling. With the newer MC-ICP-MS instrumentation, operating with a mass resolution of m/Δm ~ 10 000, sample preparation is improved and results can have uncertainties as low as 0.06 ‰ . Reproducibility of a local laboratory reference solution has been reported to be as low as 0.024 ‰ . IRMM-625 is a 53Cr-enriched material (atom fraction = ~95.5 %) with R(53Cr/52Cr) = 23.95 .
Chromium stable isotopic distributions have been studied in a variety of fields including geochemistry [178, 179], cosmo-chemistry , and nutrition . In almost all cases, NIST SRM 979 has been used as a reference, either as an isotopic reference material for correcting mass fractionation or as the zero-delta material for the δ53/52Cr scale.
Iron has four stable isotopes, 54Fe, 56Fe, 57Fe, and 58Fe, with isotopic abundances of 5.8, 91.8, 2.1, and 0.3, respectively, in naturally occurring terrestrial material. Isotopic variations are usually reported on the R(56Fe/54Fe) ratio (~15.7) relative to the elemental iron reference material IRMM-014, which has been certified with synthetic isotope mixtures. IRMM-014 generally is accepted as the zero-delta reference for the δ56/54Fe scale, but stocks of this material are now exhausted, and a replacement is urgently needed. Variations of δ56/54FeIRMM014 values in natural materials range from −3.0 to + 2.5 ‰. Using TIMS, isotopic measurements can be made with an uncertainty of 0.15 ‰. Using high-resolution MC-ICP-MS with m/Δm>9000, routine measurement uncertainty for δ56/54FeIRMM014 of 0.1 ‰ and below has become achievable [184, 185]. With further refinement of chemical methods, this value has been further optimized, and a routine precision of 0.03 ‰ can be obtained .
BHVO-1 is a Hawaiian basalt reference material from the U.S. Geological Survey (USGS) with a mole fraction of iron >12 % (as Fe2O3, see http://crustal.usgs.gov/geochemical_reference_standards/basaltbhvo1.html). Iron-isotope studies are carried out in a variety of fields [186, 187]; the majority of studies focus on medical , nutritional , and biological issues.
Nickel has five stable isotopes, 58Ni, 60Ni, 61Ni, 62Ni, and 64Ni, with isotopic abundances of 68.1, 26.2, 1.1, 3.6, and 0.9 %, respectively, in naturally occurring terrestrial material. Isotopic variation among major inorganic compartments is very small, vanishing in the measurement uncertainty of 0.1−0.2 ‰ for δ60/58NiSRM986 values. As a consequence, nickel has been used rarely in stable isotope studies. The majority of reports on nickel isotopes have focused on radiogenic isotope studies , on nutrition studies , and some on mass-dependent isotopic fractionation studies . Recent work on methanogen biomarkers with variations in nickel isotopic composition suggests that there is more to learn from these types of experiments .
Whenever δ60/58Ni (sometimes also δ62/58Ni or δ61/58Ni) values are determined, NIST SRM 986 is used as the zero-delta material. Using high-resolution MC-ICP-MS, Gall et al.  recently improved measurement procedures and reached a routine δ60/58NiSRM986 uncertainty of 0.07 ‰ for USGS reference materials like BHVO-2 (basalt). For a synthetic (pure) nickel-oxide powder, long-term precision (observed over one year) was improved by a factor of two (0.034 ‰), showing the role of unresolved interfering components on natural samples like BHVO-2.
Copper has two stable isotopes, 63Cu and 65Cu, with isotopic abundances of 69.2 and 30.8 %, respectively, in naturally occurring terrestrial material. Isotopic variations are measured relative to NIST SRM 976 and reported as δ65CuSRM976 values. Natural samples cover a range of roughly 16 ‰ with the most positive values found in carbonates and the most negative values in copper chlorides . The preferred mass-spectrometric technique today is MC-ICP-MS at a medium mass resolution, resulting in a routine analytical uncertainty of ~0.05 ‰ . Most studies focus on geochemical topics  or, in special cases, on financially driven topics, such as the provenance of minerals .
Nearly all studies use NIST SRM 976 as the zero-delta material for the δ65Cu-scale; only in laser ablation techniques has NIST SRM 610 been used as a reference . Regrettably, the supply of NIST SRM 976 is exhausted. However, this material may still be in use at some institutions. Several units of NIST SRM 976 have been dissolved by IRMM and are now offered as ERM-AE633. ERM-AE47 has been prepared by dissolving the primary material (BAM-Y001) from the BAM Federal Institute for Materials Research and Testing, which is certified for its purity. For future studies, it is recommended that both ERM-AE633 and ERM-AE647 be used in order to assign values on the δ65CuSRM976 scale using the values given in Table 17.
Zinc has five stable isotopes, 64Zn, 66Zn, 67Zn, 68Zn, and 70Zn, with isotopic abundances of 49.2, 27.7, 4.0, 18.4, and 0.6 %, respectively, in naturally occurring terrestrial material. Both R(68Zn/64Zn) and R(66Zn/64Zn) ratios routinely are used in stable isotope studies.
The natural variation of zinc isotopes has been investigated in geochemical, biological, and environmental research projects . Isotopic variations in natural samples are rather small, exhibiting a range of only ~1 ‰ for δ66/64ZnIRMM−3702 measurements, and because of this, the measurement precision must be very high. The first Zn-isotope studies were performed by Maréchal et al. in 1999  on an early MC-ICP-MS with a mass resolution of m/Δm~500, enabling an uncertainty of 0.05 ‰ for δ66/64Zn measurements. The authors used an in-house zinc metal (JMC 3-0749, “JMC Lyon”) from Johnson and Matthey as their first reference material. This material is now exhausted. Since 2006, the isotopic reference material IRMM-3702 has been available . It should be used as the zero-delta anchor for both the δ66/64Zn and δ68/64Zn scales. In order to establish a firm bridge, a thorough re-evaluation of the isotopic ratios of the JMC material relative to IRMM-3702 has been made recently by Moeller et al. , establishing a δ66/64ZnIRMM-3702 value of −0.29 ‰ for JMC Lyon. IRMM-651 and IM-1009 are alternative reference materials. The δ66/64ZnIRMM-3702 and δ68/64ZnIRMM-3702 values in Table 18 are calculated from the certified isotopic abundance ratios .
Gallium has two stable isotopes, 69Ga and 71Ga, with isotopic abundances of 60.1 and 39.9 %, respectively, in naturally occurring terrestrial material. The standard reference material, NIST SRM 994, has been characterized using TIMS by Machlan et al. in 1986 [203, 204]. According to these authors, the R(71Ga/69Ga) ratio is 1.50676 ± 0.00039. Gallium isotopes have been used primarily for correcting mass bias effects in ICP-MS [25, 27, 205]. NIST SRM 994 is suggested as the zero-delta material.
Germanium has five stable isotopes, 70Ge, 72Ge, 73Ge, 74Ge, and 76Ge, with isotopic abundances of 20.6, 27.5, 7.8, 36.5, and 7.7 %, respectively, in naturally occurring terrestrial material. Stable isotope-ratio measurements using MC-ICP-MS have recently found a wider interest [206, 208–211]. While all isotopes can be used for stable isotope studies, subtle isotopic variations require an uncertainty of better than 0.1 ‰ using the R(74Ge/70Ge) ratio; both isotopes have a relatively high abundance, and they exhibit the largest atomic mass difference. In naturally occurring terrestrial materials, the corresponding δ74/70Ge values cover an interval between –5 and +5 [206, 207] relative to NIST SRM 3120a, which has been proposed as the zero-delta reference by Escoube et al. . The most negative δ74/70GeSRM3120a values are found in natural germanium sulfide materials .
Selenium has six stable isotopes, 74Se, 76Se, 77Se, 78Se, 80Se, and 82Se, with isotopic abundances of 0.9, 9.4, 7.6, 23.8, 49.6, and 8.7 %, respectively, in naturally occurring terrestrial material. Measurements of selenium-isotope ratios began in 1989 by Wachsmann and Heumann using negative TIMS [214, 215].
Natural variations are now best measured by MC-ICP-MS using the 82Se/76Se ratio, which exhibits a natural δ82/76Se range of almost 15 ‰ . The NIST SRM 3149 reference solution (10 mg/g Se) has been proposed as the zero-delta material [212, 213, 216–218].
Bromine has two stable isotopes, 79Br and 81Br, with isotopic abundances of 50.7 and 49.3 %, respectively, in naturally occurring terrestrial material. Measurements of bromine-isotope ratios began by 1920, and since this time a variety of techniques have been developed including negative ion TIMS, positive ion TIMS, IRMS, and MC-ICP-MS [219, 221, 222]. Most investigations focus on volatile organic compounds. Standard Mean Ocean Bromine (SMOB) has been proposed as an international reference material for δ81Br measurements because variations in bromine isotopic composition of seawater bromide were not discernible . Sample preparation usually involves precipitation with Ag+ solutions, followed by conversion of bromine to methyl bromide, which is measured directly using gas IRMS. As an alternative, (GC-)MC-ICP-MS has also been used for analyzing bromine isotopes [222–224]. The range of terrestrial δ81BrSMOB values is not large (−0.8 to +3.3 ‰), with the largest variations found in oil-field formation waters [219, 225]. (Industrially produced brominated organic compounds can have a much larger range with δ81BrSMOB values as low as −4.3 ‰ ). The measurement uncertainty, therefore, is critical; values of 0.06 ‰ have been achieved for seawater samples [220, 227]. A review of the techniques has been compiled recently by Cincinelli et al. .
SMOB itself is not available as an isotopic reference material. Instead, NIST SRM 977 could be used for anchoring the δ81BrSMOB scale, using an assigned δ81BrSMOB value of −0.64 ‰ for NIST SRM 977 . This will become necessary as instrumentation improves and authors report variations in bromine isotopic composition of seawater bromide with geographic location.
Rubidium has one stable isotope, 85Rb, accounting for 72.2 % of the terrestrial isotopic abundance, and it has one very long-lived radioactive isotope, 87Rb, adding to the terrestrial isotopic composition with an abundance of 27.8 %. The half-life of 87Rb is ~5 × 1010 years. In both terrestrial materials and chondrites, δ87/85Rb values usually do not vary by more than 1–2 ‰, indicating a very homogenous mixture of these isotopes throughout the solar system [229, 231]. Owing to this isotopic invariability, changes in the R(87Rb/85Rb) ratio have only rarely been studied and expressed using the delta notation. Using MC-ICP-MS, materials enriched in 87Rb with δ87/85RbSRM984 > 14 ‰ can be analyzed with an uncertainty of 0.2 ‰ or better. Owing to its radioactivity, 87Rb is assessed by measuring R(85Rb/87Rb) together with R(87Sr/86Sr) ratio in order to determine the radiogenic 87Sr abundance in rocks for age determination. IRMM-619 is a solution of 0.5 μmol rubidium dissolved in a 4-mL acid solution, and it has a certified R(85Rb/87Rb) ratio of 2.5930(20), reflecting that of terrestrial materials.
Strontium has four stable isotopes, 84Sr, 86Sr, 87Sr, and 88Sr, with isotopic abundances of 0.6, 9.9, 7.0, and 82.6 %, respectively, in naturally occurring terrestrial material. Best suited for stable isotope studies is the R(88Sr/86Sr) ratio. However, the major part of strontium isotopic analysis is focused on the determination of the radiogenic 87Sr (see rubidium), more specifically the R(87Sr/86Sr) ratio, commonly abbreviated as 87Sr/86Sr, for Rb/Sr dating or studies of material origin. Stable isotopic variations of strontium are used in provenancing water  and food [245, 246], for studying biological migration  and environmental cycles [248, 249], and in archaeometry  and forensic science . Marine calcium carbonates have been used to investigate equilibrium or kinetic isotope effects . Additionally, δ88/86SrSRM values can be used as a paleo-thermometer  (actually studying both 87Sr/86Sr and 88Sr/86Sr isotope pairs in order to account for the δ88/86Sr variations in natural terrestrial materials). The values found in seawater corals reflect the water temperature during coral growth with a slope of +0.033(5) ‰ per kelvin, whereas inorganic aragonite has a much smaller dependence (+0.0054(5) ‰ per kelvin) . Routine measurement uncertainty for δ88/86Sr on a MC-ICP-MS is ~0.025 ‰. NIST SRM 987 is recommended for radiogenic and stable strontium-isotope studies as the zero-delta reference.
The IAPSO seawater standard has been analyzed by several groups [234–236], reporting very similar values. The most recent δ88/86SrSRM987 value of 0.386 ‰ by Krabbenhoeft seems to be the most precise measurement.
Recently, the first biological reference materials were characterized for their strontium isotopic composition. DOLT-4 (dogfish liver)  and TORT-3 (lobster hepatopancreas) were characterized for their strontium isotopic composition including δ87/86SrSRM987 values of −1.377 ± 0.018 and −1.363 ± 0.036 ‰, respectively.
Molybdenum has six stable isotopes, 92Mo, 94Mo, 95Mo, 96Mo, 97Mo, and 98Mo, with isotopic abundances of 14.5, 9.2, 15.8, 16.7, 9.6, and 24.4 %, respectively, in naturally occurring terrestrial material, and it has one radioactive isotope, 100Mo, with a characteristic terrestrial isotopic composition having an isotopic abundance of 9.8 % [31, 251, 254]. The half-life of 100Mo is ~7 × 1018 years. Of these, R(97Mo/95Mo) and R(98Mo/95Mo) ratios have been used primarily for studies of small isotopic variations using delta notation (92Mo, 94Mo, and 96Mo often are not analyzed because of possible isobaric interferences from residual zirconium isotopes). In accordance with mass-dependent fractionation, the largest isotope effects are observed for R(98Mo/95Mo) ratios. The corresponding natural δ98/95Mo isotopic variations cover a range from (−1.5 to + 3) ‰ on the δ98/95MoSRM3134 scale [253, 255].
The first determinations of stable molybdenum isotopic variations were published in 2001 by Anbar et al.  and Siebert et al. . Although both groups used different delta notations, the δ98/95Mo value used by Siebert et al. has been accepted widely since publication. Unfortunately, no internationally accepted reference material was available, and therefore each group used its own isotopic reference material. This problem has been recognized, and recently different reference materials have been analyzed relative to each other [251, 252], including the material used as the best measurement for defining the Mo atomic weight (NIST SRM 3134) [253, 254, 258]. This material, which is an atomic spectrometry standard provided by NIST, has been proposed as an anchor point for the δ98/95Mo 9. Mean Ocean Molybdenum (MOMo) δ98/95Mo has been measured as +2.09 ± 0.07 ‰ relative to NIST SRM 3134  by analysis of five IAPSO ampoules (four from the Atlantic and one from the Mediterranean). Another value has been given in the same paper as +2.34 ‰ relative to JMC-Bern, a local reference material, thus revealing a small offset between the two reference materials employed. The scales can be converted with the relation δ98/95MoSRM 3134 = δ98/95MoJMC Bern – 0.25 ‰ .
Silver has two stable isotopes, 107Ag and 109Ag, with isotopic abundances of 51.8 and 48.2 %, respectively, in naturally occurring terrestrial material. Only a few studies involving silver stable isotopes have been published. The majority of these studies focus on radiogenic 107Ag (β– decay of 107Pd with a half-life 6.5 million years) in cosmological materials [260, 261], with some studies investigating isotopic fractionation of terrestrial environmental samples .
The most common delta notation is δ107/109AgSRM978a with the radiogenic isotope in the numerator in order to express the variability of this isotope directly. However, we prefer and follow the general rule to put the heavier isotope in the numerator and the lighter one in the denominator so that characterizations like “heavy” or “light” can be used without confusion .
Using MC-ICP-MS, a δ109/107AgSRM978a measurement uncertainty of 0.05 ‰ or better (as low as 0.01 ‰, depending upon the material) can be achieved [259, 262]. This is suitable for detecting commercial products fortified with silver by identifying variations in silver isotopic composition. Most natural samples exhibit only very small deviations from the reference NIST SRM 978a.
Cadmium has seven stable isotopes, 106Cd, 108Cd, 110Cd, 111Cd, 112Cd, 114Cd, and 116Cd, with isotopic abundances 1.2, 0.9, 12.5, 12.8, 24.1, 28.7, and 7.5 %, respectively, in naturally occurring terrestrial material, and it has one radioactive isotope, 113Cd, with a characteristic terrestrial isotopic composition having an isotopic abundance of 12.2 %. The half-life of 113Cd is 8 × 1015 years. The most commonly measured isotope ratio is R(114Cd/110Cd) because both isotopes have abundances greater than 10 %, and there is a substantial difference in mass between the two isotopes. Variations in δ114/110Cd values of terrestrial materials range from −3.6 ‰ to +3.4 ‰ . The primary technique for analyzing cadmium isotopes is by MC-ICP-MS, where a routine δ114/110Cd uncertainty of ~0.4 ‰ can be achieved . For the isotopic composition of reference materials in standard solutions, uncertainty values lower than 0.07 ‰ have been reported by different laboratories .
BAM-I012 is a primary isotopic reference material for which “absolute” isotope-amount ratios have been determined using synthetic isotope mixtures . Unfortunately, the base material is isotopically fractionated relative to the mean Earth’s crust by −1.3 ‰ [264, 269]; therefore, the scientific community is searching for a new zero-delta material. In the meantime, this criterion has been achieved with NIST SRM 3108 . As NIST SRM 3108 is an atomic spectrometry standard, it is suggested that BAM-I012 be assigned a δ114/110CdSRM3108 value of −1.3 ‰, which effectively retains NIST SRM 3108 as the zero-delta material.
A number of additional secondary reference materials have been in use, including “Münster Cd”, “JMC Cd Mainz”, “Alfa Cd Zürich”, and “JMC Cd Münster” . Most of these local laboratory materials are not available as a general resource internationally. Pritzkow et al.  prepared and characterized a material named Cd-2211, which they suggested as the zero-delta material, but which is not yet commercially available. On this scale, the BAM-I012 material (named Cd-I012 herein) is listed with δ114/110CdCd-2211 = –1.66 ‰.
Rhenium has one stable isotope, 185Re, with an isotopic abundance of 37.4 %, and it has one radioactive isotope, 187Re, with a characteristic terrestrial isotopic composition with an isotopic abundance of 62.6 %. The half-life of 187Re is 4.16 × 1010 years. Using MC-ICP-MS, the isotopic composition of rhenium (δ187/185ReSRM989) can be measured relative to the NIST SRM 989 elemental rhenium with an external reproducibility of 0.04 ‰ . The range in natural materials of δ187/185Re values is small, extending from 0 to 0.3 ‰ . Analytical complications arise from the fact that 187Re is long-lived, undergoing β– decay with a half-life of 4.16 × 1010 years , thereby producing its isobar, 187Os, which must be removed quantitatively before analysis.
The majority of rhenium-isotope measurements are for rhenium-osmium chronology in geochemistry and cosmo-chemistry . For these uses and other geochemical research, however, only R(187Re/186Os) or R(187Re/188Os) ratios are used . One application in which rhenium-isotope ratios are measured is the quantification of rhenium by isotope-dilution mass spectrometry (IDMS) . For the determination of the desired isotope-amount ratios (absolute or “true” isotope ratios), an isotopic reference material such as NIST SRM 989 (or SRM 3143 with δ187/185ReSRM989 = +0.29 ‰) is helpful. Otherwise, tabulated IUPAC data for natural rhenium may be used to make corrections for mass fractionation and/or discrimination.
Osmium has six stable isotopes, 184Os, 187Os, 188Os, 189Os, 190Os, and 192Os, with isotopic abundances of 0.02, 2.0, 13.2, 16.1, 26.3, and 40.8 %, respectively, in naturally occurring terrestrial material, and it has one radioactive isotope, 186Os, with a characteristic terrestrial isotopic composition with an isotopic abundance of 1.6 %. The half-life of 186Os is 2 × 1015 years. Due to the β– decay of 187Re to 187Os and the radiogenic production of 186Os from 190Pt, the most often studied isotope ratios are R(187Os/188Os) and R(186Os/188Os). These are used for dating meteorites or rhenium-bearing terrestrial minerals. In addition, the R(187Os/188Os) ratio is used primarily to obtain information on the origin of igneous rocks, the evolution of the Earth’s crust and mantle , mixing scenarios , and climate-related processes .
The osmium-isotope ratios of interest have been analyzed primarily using N-TIMS [276, 280–282]. With the advent of MC-ICP-MS, the corresponding R(187Os/188Os) and R(186Os/188Os) ratios can now be measured with uncertainties of 0.016 and 0.017 %, respectively . Ratios of the more abundant osmium isotopes, R(192Os/188Os) and R(189Os/188Os), are used mainly for normalizing other ratios of interest . These ratios are considered largely invariant across most terrestrial materials. A number of reference materials have been used for osmium stable isotope studies (DTM, UMd, LOsST, and DROsS) ; however, no reliable δ1xx/188OsIAG-CRM-4 values, where xx = 84, 87, 89, 90, or 92, are yet available for these materials.
Platinum has five stable isotopes, 192Pt, 194Pt, 195Pt, 196Pt, and 198Pt, with isotopic abundances of 0.8, 32.9, 33.8, 25.2, and 7.4 %, respectively, in naturally occurring terrestrial material, and it has one radioactive isotope, 190Pt, with a characteristic terrestrial isotopic composition with an isotopic abundance of 0.01 %. The half-life of 190Pt is 4.5 × 1011 years.
The first use of platinum stable isotopes measured with a double spike technique on a MC-ICP-MS was published only recently [285–287]. A delta scale has been introduced as μ198Pt with an extraneous factor of 106, which we recommend be abandoned. Instead, in order to unify terminology and avoid inconsistencies, we suggest μ198Pt to be replaced by δ198/194PtIRMM010 and values be expressed in “per meg”. No further studies on platinum isotopic variation or fractionation in natural samples have been found in the recent literature (except those related to the platinum-osmium method for mineral dating). Apparently, platinum-isotope-ratio measurements have only been made in IDMS studies for quantifying platinum concentrations in biological, environmental, and geological samples [274, 288].
Mercury has seven stable isotopes, 196Hg, 198Hg, 199Hg, 200Hg, 201Hg, 202Hg, and 204Hg, with isotopic abundances of 0.1, 10.0, 16.9, 23.1, 13.2, 29.9, and 6.9, respectively, in naturally occurring terrestrial material. None of the isotopes of mercury are radiogenic, and isotopic variations largely follow mass-dependent isotopic fractionation laws, with some notable exceptions. The R(202Hg/198Hg) ratio, with a nominal value of 2.963, can be measured with the most reliable precision . Natural isotopic variations  encompass a δ202/198Hg range of about ±4 ‰. The most precise method for assessing stable mercury isotopic variations today is MC-ICP-MS, which has a reported routine external precision for δ202/198Hg measurements as low as 0.08 ‰ [19, 290].
Triggered by the rapid development in MC-ICP-MS, mercury-isotope studies have increased significantly within the last decade. Mercury-isotope research is carried out in many disciplines, and a major part involves investigating isotopic fractionation in the biogeochemical mercury cycle [294–297]. Within this research, even mass-independent isotopic fractionation of mercury has been observed [298–300]. Differentiation between mass-dependent and mass-independent isotopic fractionation requires higher accuracies than usual . Therefore, mercury isotopic reference materials supporting measurement uncertainties in the sub-permil range are required .
NIMS-1 has been certified as isotopic reference material  and is recommended for use as an anchor for the δ202/198Hg scale. Before NIMS-1 was certified, NIST SRM 3133 was used as a mercury isotopic reference material. However, NIST SRM 3133 was prepared and certified for quantitative analysis only, not for isotopic measurements. NIMS-1 has been made from NIST SRM 3133, and it is now specifically recommended for future mercury isotopic analysis. While both materials are listed with δ202/198HgNIMS1 = 0 in Table 31, the exact values may still differ slightly.
Thallium has only two stable isotopes, 203Tl and 205Tl, with isotopic abundances of 29.5 and 70.5 %, respectively, in naturally occurring terrestrial material. The corresponding δ205/203Tl values are expressed relative to NIST SRM 997 and can be determined with an uncertainty of 0.1 ‰ using MC-ICP-MS . Values of δ205/203Tl in terrestrial materials cover an interval of no more than 2 ‰ , which is large when considering the relative mass differences.
Before the widespread use of MC-ICP-MS, the main interest in thallium-isotope studies was the search for anomalies in the 205Tl abundance in meteorites due to the decay of the now-extinct 205Pb . With the advent of MC-ICP-MS, the precisions of R(205Tl/203Tl) ratio determinations have improved such that investigations of mass-dependent thallium stable isotopic fractionation can now be carried out . In these studies, R(205Tl/203Tl) ratios have been expressed relative to NIST SRM 997. Although ε notation is still commonly employed, it is recommended that isotopic compositions be communicated as δ205/203TlSRM 997 values in publications. If desired, values can be expressed in parts per ten thousand, using the abbreviation pptt, with explanation of the abbreviation in a footnote. More details on this topic can be found in Coplen .
ERM-AE649 is a thallium nitrate solution with a 205Tl isotope-amount fraction (isotopic abundance) of 0.704766(89)k=2 that is indistinguishable from that of NIST SRM 997, which is 0.704765(88)k=2 .
Lead has four stable isotopes, 204Pb, 206Pb, 207Pb, and 208Pb, with isotopic abundances of 1.4, 24.1, 22.1, and 52.3 %, respectively, in naturally occurring terrestrial material. Only 204Pb is primordial; the three heavier isotopes are radiogenic. 206Pb is the end member of the 238U decay chain and 207Pb that of the 235U chain. 208Pb is generated from 232Th. The corresponding isotope-abundance variations in naturally occurring terrestrial materials cover a wide range, e.g., ~100 ‰ for δ207/206Pb and ~60 ‰ for δ208/206Pb. For isotopic variations generated by mass-dependent isotopic fractionation processes, the common isotope preferred in the denominator is 204Pb . With an isotopic abundance of only 1.4 %, mass-spectrometric measurements are rather difficult and sometimes lack the necessary measurement precision. Compared to the radiogenic abundance alterations, the mass-dependent fractionation changes are small, of the order of 1 ‰ . The IRMM has made an attempt to produce calibration reference materials explicitly for lead isotope-δ measurements (“δ-iCRM”)  with ERM-3800 proposed as the zero-delta material. With these materials used as references in MC-ICP-MS direct comparisons, an uncertainty between 0.01 and 0.05 ‰ has been achieved .
Lead is one of the most frequently studied isotope systems in geochemistry (for a review, see Faure and Mensing ). Based on geochemical findings and archaeological applications, lead-isotope signatures have, for instance, been used to trace the origin of archeological artifacts [313–315]. The common control reference for these studies, NIST SRM 981, has been measured by many laboratories (see Weiss et al.  and Baker et al. ). The accepted R(208Pb/206Pb) ratio for NIST SRM 981 is 2.1681(8), and that for NIST SRM 610 has been determined as 2.1694(1) [310, 311].
Only a limited number of studies have published lead-isotope variations as δ values, e.g., the investigation of lead isotopic fractionation during smelting and refining . In this study, mass-dependent isotopic fractionation was investigated, and δ208/206Pb values reported relative to NIST SRM 981 were published. Additionally, lead-isotope ratios from a number of reference materials can be found in Baker et al. .
Natural uranium has three isotopes, 234U, 235U, and 238U, having isotopic abundances of 54 × 10–6, 0.7 %, and 99.3 %, respectively, in naturally occurring terrestrial material. All three are radioactive isotopes with characteristic terrestrial isotopic compositions. 234U is an intermediate product of the 238U decay chain with a half-life of about 2.5 × 105 years. Large natural variations are seen in the isotope-amount ratio n(234U)/n(238U) due to the relative rates of release of these isotopes from minerals . The half-lives of 235U and 238U of (7 × 108 and 4.6 × 109) years, respectively, are sufficiently long to have preserved these materials since the formation of the solar system. Uranium disequilibrium dating methods are based on the uranium activity and measurement of the corresponding radiogenic products (for more information, see the reviews by Ivanovich and Harmon [321, 322]).
For stable isotopic measurements, only the isotope-number ratio R(238U/235U), commonly abbreviated as 238U/235U, is of interest. Until recently, no natural variation in this ratio with a value of 137.88 had been observed. This value recently was investigated in an inter-laboratory effort with eight participants, organized by the IRMM in Geel (see Richter et al. ). They found a significantly lower average ratio of 137.837. Hiess et al.  confirmed the basic finding and published results for a large number of terrestrial minerals. They suggest that the 238U/235U ratio be revised to 137.818(45). With new and improved instrumentation allowing for much smaller sample sizes (MC-ICP-MS), δ238/235U variations of the order of 1 ‰ have been observed in naturally occurring terrestrial materials . This variation could be a result of uranium oxidation–reduction reactions and/or to a nuclear field shift which would cause preferential separation of some uranium isotopomers [318, 325]. Delta measurements are reported relative to NIST SRM 950-A, which is the zero-delta material for the δ238/235USRM950A scale [317, 318]. Uranium in sea water differs by −0.41(2) ‰ from that of SRM 950-A .
4 Membership of sponsoring bodies
Membership of the Inorganic Chemistry Division Committee for the period 2012–2013 was as follows:
President: R. D. Loss (Australia); Vice President: J. Reedijk (Netherlands); Secretary: M. Leskela (Finland); Titular Members: M. Drábik (Slovakia); N. E. Holden (USA); P. Karen (Norway); S. Mathur (Germany); K. Sakai (Japan); L. R. Öhrström (Sweden); E. Tshuva (Israel); Associate Members: J. Buchweishaija (Tanzania); A. Kiliç (Turkey); D. Rabinovich (USA); J. Garcia-Martinez (Spain); T. Ding (China); R.-N. Vannier (France); National Representatives: N. Trendafilova (Bulgaria); V. Chandrasekhar (India); Y. Abdul Aziz (Malaysia); S. Ali (Pakistan); B. Prugovečki (Croatia); H. Toma (Brazil); S. Youngme (Thailand).
Membership of the Commission on Isotopic Abundances and Atomic Weights for the period 2012–2013 was as follows:
Chair: W. A. Brand (Germany); Secretary: J. Meija (Canada); Titular Members: T. Prohaska (Austria); M. Gröning (Austria); T. Hirata (Japan); R. Schoenberg (Germany); Associate Members: M. Wieser (Canada); M. Berglund (Belgium); G. O’Connor (USA); X. K. Zhu (China).
This manuscript is the result of a collaborative effort of the IUPAC Subcommittee on Stable Isotopic Reference Materials Assessment, IUPAC Project “Assessment of Stable Isotopic Reference and Inter-Comparison Materials”, Project No. 2009-027-1-200 (Web access: http://www.ciaaw.org/membership4.htm).
Valuable comments and suggestions were made by Robert Vocke (National Institute of Standards and Technology, Gaithersburg, MD, USA), John Karl Böhlke (U.S. Geological Survey, Reston, VA, USA), Michael Berglund (Institute of Reference Materials and Measurements, Geel, Belgium), Paul de Bièvre (Independent Consultant on MiC, Geel, Belgium), Manfred Gröning (International Atomic Energy Agency, Vienna, Austria), and Glenda O’Connor (U.S. Department of Energy, New Brunswick Laboratory, Argonne, IL, USA).
Careful reviews were provided by Juris Meija (National Research Council of Canada, Institute for National Measurement Standards, Ottawa, Canada), Lauren Tarbox (U.S. Geological Survey, Reston, VA, USA), Michael E. Wieser (Department of Physics and Astronomy, University of Calgary, Calgary, Canada), and Johanna Irrgeher (University of Natural Resources and Life Sciences Vienna, Tulln, Austria).
For his valuable recommendations regarding the Notations section we are indebted to Ron D. Weir (Kingston, Canada), chair of the IUPAC Interdivisional Committee on Terminology, Nomenclature and Symbols (ICTNS).
Please note that all values and statements given in this publication remain the sole responsibility of the authors.
The support of the U.S. Geological Survey National Research Program is acknowledged. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government, IUPAC, or the institutions of the coauthors.
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About the article
Published Online: 2014-03-04
Published in Print: 2014-03-20
In metrology the usual aim is to produce SI-traceable results. Here, primary reference materials are those having the highest metrological quality for SI-traceable quantity values like concentrations or (absolute) isotopic abundances with associated uncertainty. For primary isotope reference materials defining delta scales, this is different. These materials are given without uncertainty by definition because these values have been determined by international agreement. Isotope ratios are measured relative to these materials. The corresponding (absolute) isotopic abundances cannot be measured with the required precision.
Full article freely available at http://onlinelibrary.wiley.com/doi/10.1002/rcm.5129/abstract.
This is in contrast to “absolute” isotope-ratio assessment, where also the uncertainty of the isotope-number ratio determination has to be accounted for in the certificate.
While striving for a complete set, we certainly may have missed some of the important materials.
The 3H isotope also has a popular name, tritium. Because it is radioactive (half-life ~12.3 years), it is not listed in this compilation of stable isotopic reference materials.
Now the Commission on Isotope Abundances and Atomic Weights, CIAAW (see www.ciaaw.org).
More isotopic reference materials, which are certified for their isotopic composition, but not for delta values, are available from BAM, featuring boron-10 isotopic abundances from 20 to 96 %; see Vogl and Rosner  and https://www.webshop.bam.de. An overview on available boron isotope reference materials is given in Vogl et al. .
Additional information on these materials is available at the IAEA Web site, http://nucleus.iaea.org/rpst/ReferenceProducts/ReferenceMaterials/Stable_Isotopes/index.htm.
According to the NIST Web page (http://www.nist.gov/srm/), SRM 3134 is out of stock. Hence, a replacement for the scale defining material is necessary in the near future.