Standards for Isotope Ratios

Because variations in accurate isotope ratio measurements typically concern only a few parts per 1000 by mass and there are no universal absolute ratios, it is necessary to define some standards. For this purpose, samples of standard substances are produced and made available at two major centers IAEA (International Atomic Energy Authority, U.K.) and NIST (National Institute for Standards and Technology, U.S.). Standards from other sources are also available. These primary standards can be used as such, or alternative standards can be employed if the primary ones are not available. However, any alternative standards need to be related accurately to the primary ones (see formulae below). For example, the material PDB (PeeDee belemnite), used particularly as a standard for the ratio of isotopes, is no longer readily available, and a new standard, VPDB, 

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Both absolute quantitation and relative quantitation of species in mixtures is of interest in some circumstances. Quantitation in a 5-minute analysis can be achieved by addition of an internal standard, ideally the target microorganism grown in special media to incorporate heavy isotopes92-95 and determination of the relative peak heights of pairs of proteins from the analyte and the standard. Isotope-labeled proteins or peptides, selected to match proteins or peptides characteristic of target microorganisms, can also serve as internal standards for isotope ratio measurement. The addition of unmatched proteins or peptides is less reliable for either ESI or MALDI measurements because of unpredictable suppression in the variable mixture. 

Standards for isotope ratio measurements are chosen for convenience and assigned 8 = 0%o. Ideally, every standard would be homogeneous, easily obtained in the necessary quantity, have an isotope ratio similar to that in the samples being... 

The production of 13CC>2 from [13C]-deca-BDE degradation was analyzed using the technique of gas chromatography coupled to mass spectrometer for isotope ratio (GC-EMRI). This technique measures the isotopic ratio 13C/12C of CO2 in the gas phase with respect to an international standard V-PDB (Vienna Pee Dee Belem-nite). This isotopic ratio is reported in 8 notation relative to an international standard per mil (%o) ... 

The implicit assumption in this correction is that both mass fractionation factors P, e.g., for Cu and Zn, are different, but will be changed by the same constant upon switching between sample and standard solutions. Again, this assumption has better chances of being correct if purihcation of the sample is efficient. For isotopic ratios not too far from unity (e.g., Cu/ Cu, Zn/ Zn, Te/ Fe), a precision of 0.05%o can be achieved (Marechal et al. 1999 Zhu et al. 2000 Beard et al. 2003). For larger ratios, the obtainable precision is 0.2-0.4%o. 

The tracer-subtraction procedure adds negligible uncertainty to the measured CaJ Ca ratios. However, it is in fact essentially impossible to entirely eliminate the effects of instrumental mass discrimination for the measurements of either the Ca- Ca mixed tracer or for the standard Ca isotope ratios. Hence, it is necessary to have a standard material with an agreed-upon value of 5 Ca. At the time of writing of this article there is no such standard. 

As discussed before, quadrupole based ICP-MS allows multi-element determination at the trace and ultratrace level and/or isotope ratios in aqueous solutions in a few minutes as a routine method with detection limits of elements in the sub pgml-1 range and a precision for determined trace element concentration in the low % range (RSD - relative standard deviation). The precision for isotope ratio measurements varies between 0.1% and 0.5% RSD. This isotope ratio precision is sufficient for a multitude of applications, e.g., for evidence of contamination of sample with depleted or enriched uranium in urine (this technique is used in the author s laboratory in a routine mode14) or the isotope dilution technique for the quantitative determination of trace element and species concentration after doping the sample with enriched isotope spikes. 

One problem is the availability of reliable reference materials for isotope ratio measurements. Certified isotope standard reference materials are available from NIST (National Institute of Standard Reference Materials, http //www.nist.gov/srd/, formerly NBS - National Bureau of Standards) and IRMM (Institute for Reference Materials and Measurements, http //www.irmm.jrc.de). There are isotope standard reference materials for light elements (H, Li, , C, N, O), Mg (magnesium metal isotopic standard NIST SRM 980), and for moderately heavy elements such as (potassium chloride isotopic standard NIST SRM 985), Cr (chromium metal isotopic standard NIST... 

List of Standard Reference Materials for Isotope Ratio Measurements... 

Figure 16.24—Instrument for isotope ratio measurements. The high sensitivity required by these types of analyses is achieved by placing several Faraday-type detectors after the magnet, each recording the current at a single mass. A reference standard is injected with the sample.

Despite these caveats, many analyses are done on land either because the sample can be stored without changing the concentrations of the analytes it contains or because the apparatus required for the analysis cannot be operated on board a ship. For instance, some radionuclides are measured on land for both reasons. Samples analyzed on land can be spiked immediately with another (artificial) isotope of the same element to fill the adsorption sites on container walls and to serve as an internal standard. The mass spectrometers required for isotope ratio analyses are often too sensitive to vibration and motion for shipboard use. Analytes present at greater than trace levels, or which can be stabilized with some pretreatment, may also be analyzed successfully on shore. 

Certified isotopic standards are available for some elements but by no means all. Such standards are available from the National Institute of Standards and Technology [52] and New Brunswick Laboratory [53] in the United States, and from the Institute for Reference Materials and Measurements [54] in Belgium. De Bievre et al. have published a review of reference materials available for isotope ratio measurements [55]. It is clearly desirable to use certified materials for instrument calibration if at all possible. If no standard is available for the element in question, the analyst has little choice but to obtain a pure sample of it and assume it has the composition listed by IUPAC [1]. A few elements, with lead being the outstanding example, have isotopic compositions that vary widely in... 

Isotope ratio mass spectrometry provides information for the isotopic abundance of an analyte gas relative to the known isotope ratio of a reference compound. Primary standards are available for calibration and to provide traceability to facilitate comparison of results from different laboratories. The peak area for each isomer is integrated and the corresponding ratios calculated. Since the small variations of the heavier isotopes are of the order of 0.001 to 0.05 atom percent, the S-notation in units of parts per thousand (°/oo) is used to report changes in isotope abundance, where 8s = 1000 (Rs/Rstd - 1), and Rs is the measured isotope ratio for the sample and Rstd the isotope ratio for the reference standard [2,182,194,195]. Minimum sample quantities for isotope ratio measurements vary tremendously with the operating conditions and other factors, but typically carbon isotope ratios can be determined for samples containing about 10 ng of carbon. 

Blood collected at each time point was allowed to remain on ice for 15 min prior to centrifugation at 1800g. Plasma was transferred to 5-ml cryogenic vials and stored at -8(PC until analyzed. Plasma lipids were extracted from duplicate 2.2-g plasma aliquots for isotope ratio analysis or from 0.25-g aliquots for HPLC quantification of retinol and /3C after addition of internal standard (retinyl acetate). Plasma aliquots were de-proteinized with 1 vol of ethanol and lipids extracted with 3 vol of hexane (Optima Grade, Fisher Scientific, Rochester, NY). 

As seen in the specific examples given later, numerous analytical methods are used for the determination of uranium in environmental samples. The most popular among them are ICPMS and alpha spectrometry, but neutron activation analysis, gamma spectrometry, and XRF are often deployed and even simple spectrophoto-metric (like colorimetric aresnazo-III) techniques are sometimes still used. For the precise determination of total uranium and its isotopic composition, isotope dilution (ID) methods can be used. One example is a comparison of ID-TIMS and ID-SIMS for isotope ratios in soil standards where two separation and preconcentration chromatographic techniques were also compared (Adriaens et al. 1992). 

An overview of the uranium and plutonium certified reference materials that are used for safeguards and fissile material control for bulk and particle analysis was presented at a conference organized by the European Joint Research Centre (JRC) (Jakopic et al. 2012). There are three main types of CRMs single isotope standards for isotope dilution measur ents CRMs for isotope ratio calibration and mass discrimination correction and reference materials for age dating. As far as uranium is concerned, there are (almost pure) mono-isotopic standards and... 

The best precision is obtained for isotope ratios near one, however, if the element to be determined is near the detection limit when the ratio of spike isotope to natural isotope should be between 3 and 10 so that noise contributes only to the uncertainty of natural isotojje measurement. Errors also become large when the isotope ratio in the spiked sample approaches the ratio of the isotopes in the spike (overspiking), or the ratio of the isotopes in the sample (underspiking). The accuracy and precision of the isotope dilution analysis ultimately depends on the accuracy and precision of the isotope ratio measurement, so all the precautions that apply to isotope ratio analysis also apply in this case. Isotope dilution analysis is attractive because it can provide very accurate and precise results. The analyte acts as its own de facto internal standard. For instance, if the isotopic spike is added prior to any sample preparation then the spike will behave in exactly the same way as the analyte because it is chemically identical, providing it has been properly equilibrated by a series of oxi-dation/reductions. Hence, any losses from the sample can be accounted for because the analyte and spike will be equally affected. 

Baxter, D.C., Rodushkin, 1., Engstrom, E., and Malinovsky, D. (2006) Revised exponential model for mass bias correction using an internal standard for isotope abundance ratio measurements by multi-collector inductively coupled plasma mass spectrometry. J. Anal. At. Spectrom., 21, 427-430. 

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