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Although most of them are relatively small, stable isotope deltas of naturally occurring substances are robust and enable workers in anthropology, atmospheric sciences, biology, chemistry, environmental sciences, food and drug authentication, forensic science, geochemistry, geology, oceanography, and paleoclimatology to study a variety of topics. Two fundamental processes explain the stable isotope deltas measured in most terrestrial systems: isotopic fractionation and isotope mixing. Isotopic fractionation is the result of equilibrium or kinetic physicochemical processes that fractionate isotopes because of small differences in physical or chemical properties of molecular species having different isotopes. It is shown that the mixing of radioactive and stable isotope end members can be modelled to provide information on many natural processes, including (14)C abundances in the modern atmosphere and the stable hydrogen and oxygen isotopic compositions of the oceans during glacial and interglacial times. The calculation of mixing fractions using isotope balance equations with isotope deltas can be substantially in error when substances with high concentrations of heavy isotopes (e.g. (13)C, (2)H, and (18)O ) are mixed. In such cases, calculations using mole fractions are preferred as they produce accurate mixing fractions. Isotope deltas are dimensionless quantities. In the International System of Units (SI), these quantities have the unit 1 and the usual list of prefixes is not applicable. To overcome traditional limitations with expressing orders of magnitude differences in isotope deltas, we propose the term urey (symbol Ur), after Harold C. Urey, for the unit 1. In such a manner, an isotope delta value expressed traditionally as-25 per mil can be written as-25 mUr (or-2.5 cUr or-0.25 dUr; the use of any SI prefix is possible). Likewise, very small isotopic differences often expressed in per meg 'units' are easily included (e.g. either+0.015 ‰ or+15 per meg can be written as+15 μUr.

In light of the proliferation of tree-ring isotope studies, the magnitude and cause of variability of tree-ring δ(13)C, δ(18)O and δ(2)H within individual trees (circumferential) and among trees at a site is examined in reference to field and laboratory sampling requirements and strategies. Within this framework, this paper provides a state-of-knowledge summary of the influence of "juvenile" isotope effects, ageing effects, and genetic effects, as well as the interchangeability of species, choice of ring segment to analyze (whole ring, earlywood or latewood), and the option of sample pooling. The range of isotopic composition of the same ring among trees at a site is ca. 1-3‰ for δ(13)C, 1-4‰ δ(18)O, and 5-30‰ for δ(2)H, whereas the circumferential variability within a tree is lower. A standard prescription for sampling and analysis does not exist because of differences in field environmental circumstances and mixed findings represented in relevant published literature. Decisions in this regard will usually be tightly constrained by goals of the study and project resources. Sampling 4-6 trees at a site while avoiding juvenile effects in rings near the pith seems to be the most commonly used methodology, and although there are some reasoned arguments for analyzing only latewood and developing separate isotope records from each tree, the existence of some contradictory findings together with efforts to reduce cost and effort have prompted alternate strategies (e.g., most years pooled with occasional analysis of rings in the sequence separately for each tree) that have produced useful results in many studies., (Copyright © 2010 Elsevier B.V. All rights reserved.)

In 2007, JRC-IRMM began a series of atmospheric CO2 isotope measurements, with the focus on understanding instrumental effects, corrections as well as metrological aspects. The calibration approach at JRC-IRMM is based on use of a plain CO2 sample (working reference CO2) as a calibration carrier and CO2-air mixtures (in high-pressure cylinders) to determine the method-related correction under actual analytical conditions (another calibration carrier, in the same form as the samples). Although this approach differs from that in other laboratories, it does give a direct link to the primary reference NBS-19-CO2. It also helps to investigate the magnitude and nature for each of the instrumental corrections and allows for the quantification of the uncertainty introduced. Critical tests were focused on the instrumental corrections. It was confirmed that the use of non-symmetrical capillary crimping (an approach used here to deal with small samples) systematically modifies delta13C(CO2) and delta18O(CO2), with a clear dependence on the amount of extracted CO2. However, the calibration of CO2-air mixtures required the use of the symmetrical dual-inlet mode. As a proof of our approach, we found that delta13C(CO2) on extracts from mixtures agreed (within 0.010 per thousand) with values obtained from the 'mother' CO2 used for the mixtures. It was further found that very low levels of hydrocarbons in the pumping systems and the isotope ratio mass spectrometry (IRMS) instrument itself were critical. The m/z 46 values (consequently the calculated delta18O(CO2) values) are affected by several other effects with traces of air co-trapped with frozen CO2 being the most critical. A careful cryo-distillation of the extracted CO2 is recommended. After extensive testing, optimisation, and routine automated use, the system was found to give precise data on air samples that can be traced with confidence to the primary standards. The typical total combined uncertainty in delta13C(CO2) and delta18O(CO2) on the VPDB-CO2 scale, estimated on runs of CO2-air mixtures, is +/-0.040 per thousand and 0.060 per thousand (2-sigma values). Inter-comparison with MPI-BGC resulted in a scale discrepancy of a similar magnitude. Although the reason(s) for this discrepancy still need to be understood, this basically confirms the approach of using specifically prepared CO2-air mixtures as a calibration carrier, in order to achieve scale unification among laboratories. As important practical application and as a critical test, JRC-IRMM took part in the passenger aircraft-based global monitoring project CARIBIC (http://www.caribic-atmospheric.com). In this way, reliable CO2 isotope data for the tropopause region and the free troposphere were obtained. From June 2007 to January 2009, approximately 500 CARIBIC air samples have been analysed. Some flights demonstrated a compact correlation of both delta13C(CO2) and delta18O(CO2) with respect to CO2 concentration, demonstrating mixing of tropospheric and stratospheric air masses. These excellent correlations provide an independent, realistic data quality check., (Copyright (c) 2009 John Wiley & Sons, Ltd.)

The three-isotope tracer Delta(17)O is increasingly used in atmospheric chemistry and other research areas. Thanks to the development of isotope-ratio mass spectrometry (IRMS), delta(17)O and delta(18)O can be determined with a precision of a few 0.01 per thousand, and values for Delta(17)O may be calculated with similar precision. However, interpreting small and precisely determined Delta(17)O values as a deviation from an expected mass-dependent fractionation process is not straightforward. Several aspects are of high importance. In the present paper we review existing definitions, formulas and some other aspects of Delta(17)O reporting. One of the most confusing aspects is a variance of definitions and corresponding formulas. While Delta(17)O is traditionally defined to characterise a data point, i.e. Delta(17)O is considered as a deviation from an expected mass-fractionation line, the recently introduced definition (Miller MF. Geochim. Cosmochim. Acta 2002; 66: 188) characterises a fractionation line itself, in terms of its ordinate intercept. The formulas corresponding to this definition gives a characteristic for a specific process. When the 'traditionally defined' Delta(17)O is in use, an expected fractionation processes--the key point for Delta(17)O reporting--should be defined and parameterised with the same accuracy as intended for reporting Delta(17)O. When Delta(17)O is reported for a data point, not only a value for lambda but an ordinate intercept of a reference fractionation line should be given with high accuracy. We note that defining a single fractionation process is hardly possible for many natural compounds. For such compounds we propose to use a phenomenological reference line, namely an isotope composition range of natural sources. Next, aspects of Delta(17)O comparison and mass-balance calculations are considered. All the aspects considered for Delta(17)O may be relevant for others three-isotope tracers, e.g. Delta(33)S.

Recycling of [(18)O]water for [(18)F]fluoride production can be accomplished with reliable results. We have developed sensitive, robust, and rapid analyses of impurities in [(18)O]water. Anions were quantitated by capillary electrophoresis and organic residuals were quantitated by gas chromatography using methods with excellent reproducibility and linearity. Kryptofix 222 (K-222) was quantitated by a sensitive LC-MS-MS technique. Isotopic composition was determined by GC-MS with satisfactory accuracy and precision. These methods were employed to evaluate recovered [(18)O]water purified by a novel electrolysis method. 2-[(18)F]FDG yields using purified [(18)O]water with very low levels of impurities are indistinguishable from newly purchased [(18)O]water. High (> 300 ppm) carbonate concentration reduces the fluoride trapping efficiency of QMA. The analyses of anions, organics, and isotopic enrichment were applied routinely for quality control of [(18)O]water to predict a satisfactory outcome of 2-[(18)F]FDG production.

Despite a rapidly growing literature on analytical methods and field applications of O isotope-ratio measurements of NO(3)(-) in environmental studies, there is evidence that the reported data may not be comparable because reference materials with widely varying delta(18)O values have not been readily available. To address this problem, we prepared large quantities of two nitrate salts with contrasting O isotopic compositions for distribution as reference materials for O isotope-ratio measurements: USGS34 (KNO(3)) with low delta(18)O and USGS35 (NaNO(3)) with high delta(18)O and 'mass-independent' delta(17)O. The procedure used to produce USGS34 involved equilibration of HNO(3) with (18)O-depleted meteoric water. Nitric acid equilibration is proposed as a simple method for producing laboratory NO(3)(-) reference materials with a range of delta(18)O values and normal (mass-dependent) (18)O:(17)O:(16)O variation. Preliminary data indicate that the equilibrium O isotope-fractionation factor (alpha) between [NO(3)(-)] and H(2)O decreases with increasing temperature from 1.0215 at 22 degrees C to 1.0131 at 100 degrees C. USGS35 was purified from the nitrate ore deposits of the Atacama Desert in Chile and has a high (17)O:(18)O ratio owing to its atmospheric origin. These new reference materials, combined with previously distributed NO(3) (-) isotopic reference materials IAEA-N3 (=IAEA-NO-3) and USGS32, can be used to calibrate local laboratory reference materials for determining offset values, scale factors, and mass-independent effects on N and O isotope-ratio measurements in a wide variety of environmental NO(3)(-) samples. Preliminary analyses yield the following results (normalized with respect to VSMOW and SLAP, with reproducibilities of +/-0.2-0.3 per thousand, 1sigma): IAEA-N3 has delta(18)O = +25.6 per thousand and delta(17)O = +13.2 per thousand; USGS32 has delta(18)O = +25.7 per thousand; USGS34 has delta(18)O = -27.9 per thousand and delta(17)O = -14.8 per thousand; and USGS35 has delta(18)O = +57.5 per thousand and delta(17)O = +51.5 per thousand.

The differential measurement of the abundance of oxygen isotopes based on reference materials, such as VSMOW for the case of water, was used because the precision of the absolute mass-spectrometric determination of the abundance fell short of the differences to be measured. Since then these measurements have been much improved, so that a calibration scheme of the oxygen isotope abundance in water, carbonates, silica, phosphates, sulfates, nitrates and organic materials is suggested, based on an accredited primary standard of oxygen in air and using standard fluorination and O(2) to CO(2) conversion techniques., (Copyright 2002 John Wiley & Sons, Ltd.)

Stable isotope ratios are reported in the literature in terms of a deviation from an international standard (delta-values). The referencing procedures, however, differ from instrument to instrument and are not consistent between measurement facilities. This paper reviews an attempt to unify the strategy for referencing isotopic measurements. In particular, emphasis is given to the importance of identical treatment of sample and reference material ('IT principle'), which should guide all isotope ratio determinations and evaluations. The implementation of the principle in our laboratory, the monitoring of our measurement quality, the status of the international scales and reference materials and necessary correction procedures are discussed., (Copyright 2001 John Wiley & Sons, Ltd.)