Isotope ratio calibration

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... 

Uncertainty evaluation is often perceived as a passive (a posteriori) part of the analysis process. The double-spike method of isotope ratio calibration, however, has demonstrated that uncertainty evaluation is an active research tool and the results obtained this way are significant in steering the entire field of mass bias correction. 

Isotope dilution mass spectrometry (IDMS) can be applied with most of the ionisation methods used in mass spectrometry to determine isotope ratios with greater or lesser accuracy. For calibration by means of isotope dilution, an exactly known amount of a spike solution, enriched in an isotope of the element(s) to be determined, is added to an exactly known amount of sample. After isotopic equilibration, the isotope ratio for the mixture is determined mass spectrometrically. The attraction of IDMS is its potential simplicity it relies only on the measurement of ratios. The... 

In isotope dilution inductively coupled plasma-mass spectrometry (ID-ICP-MS) the spike, the unspiked and a spiked sample are measured by ICP-MS in order to determine the isotope ratio. Using this technique, more precise and accurate results can be obtained than by using a calibration graph or by standard addition. This is due to elimination of various systematic errors. Isotopes behave identically in most chemical and physical processes. Signal suppression and enhancement due to the matrix in ICP-MS affects both isotopes equally. The same holds for most long-term instrumental fluctuations and drift. Accuracy and precision obtained with ID-ICP-QMS are better than with other ICP-QMS calibration... 

Merritt, D. A., Brand, W. A. and Hayes, J. M. (1994) Isotope ratio monitoring gas chromatography mass spectrometry methods for isotopic calibration. Organic Geochemistry 21, 573 583. 

Figure 12B. Oxygen isotope ratios in C. japonica calibrated for temperature change using temperatures obtained from (a) time of cherry tree blooms, (b) first freezing of Lake Biwa, and (c) number of snowy days per year (26).

Natural abundance data are nearly always reported as delta values, 5 in units of per mil ( mil = 1000), written %o. This is a relative measurement made against a laboratory s own reference material, a working standard , calibrated against an international standard. Delta values are calculated from measured isotope ratio as ... 

Fitzsimons et al. (2000) have reviewed the factors that influence the precision of SIMS stable isotope data. All sample analyses must be calibrated for instrumental mass fractionation using SIMS analyses of a standard material. Under favorable circumstances, precision can reach a few tenths of a per mill. The latest version of ion-microprobe is the Cameca-lMS-1280 type, allowing further reduction in sample and spot size and achieving precise analysis of isotope ratios at the 0. %o level (Page et al. 2007). 

Clayton RN, Epstein S (1958) The relationship between 0/ 0 ratios in coexisting quartz, carbonate and iron oxides from various geological deposits. J Geol 66 352-373 Clayton RN, Kieffer SW (1991) Oxygen isotope thermometer calibrations. In Taylor HP, O Neil JR, Kaplan IR (eds.) Stable isotope geochemistry A tribute to Sam Epstein. Geochem Soc Spec Publ 3 3-10... 

Isotope ratios are given as deviations, in relation to a defined primary standard (zero point). The polyethylene foils CH 7 and NBS 22-oil are commercially available secondary standards, certificated and managed by the International Atomic Energy Agency. However, GC-lRMS systems cannot be calibrated without the aid of alternative peripheries like an elemental analyser (EA) or a dual inlet, owing to the lack of commonly accepted reference materials applicable in GC-IRMS techniques (Fig. 17.11). 

Calibration curves for all BAs subjected to the sample work-up procedure were linear in the range 5-200 pmoles. The linear regression correlation coefficients varied from 0.991 to 0.999 and the slopes varied from 0.83 to 1.32, when the measured isotope ratios were plotted against the calculated ratios. Detection limits for all BAs investigated were 1 pg (2.5 fmol) injected onto the column. 

An excellent possibility for quantifying analytical data in inorganic mass spectrometry is isotope dilution analysis (IDA) based on more precise isotope ratio measurements. IDA uses highly enriched isotope spikes of analytes of known concentration for calibration and is the method of choice if a high accuracy for element concentrations is required. The principles and applications of this method will be discussed below. 

Further details of different strategies in solution calibration are described in the literature.1 29 76 79 Precise and accurate measurements of isotope ratios, which is one of the major advantages of mass spectrometric techniques, are a requirement for the application of isotope dilution techniques in trace analysis, which is also the main goal of the application of isotope dilution in solution based calibration in LA-ICP-MS. 

For measurements of isotope ratios or isotope abundances, any of the mass spectrometers discussed in the previous chapters, such as SSMS, LIMS, GDMS56 and LA-ICP-MS,6 are of benefit for the direct isotope analysis of solid samples. SSMS and LIMS are rarely applied in isotope analysis due to their relatively low precision. Several applications of the isotope dilution technique as a calibration strategy in SSMS, mostly on geological samples, are known.57-59 GDMS has been mostly applied in multi-element trace analysis and depth profiling and plays only a minor role... 

Whereas TIMS suffers from the difficulties of time dependent mass fractionation in MC-ICP-MS, the mass fractionation is relatively constant. Its magnitude is often greater than that of TIMS and matrix effects are possible84. It is known that isotopic reference materials used for the calibration of isotope ratio measurements are available only as a pure metal or compound. Possible matrix effects during isotope ratio measurements, especially in solid-state... 

Valkiers calibrated the isotope ratio measurement using synthetically prepared... 

Furthermore, isotope standard reference materials are required in different chemical forms for a multitude of elements in order to calibrate and evaluate isotope ratio measurements by mass spectrometry. In addition, reliable analytical data required in science and routine work will only be obtained by improving the quality assurance of the measurement procedures and also using interlaboratory comparisons of analytical results in round robin tests. 

CRMs issued by NIST (previously NBS) and IRMM (previously CBNM) based on calibrated isotope ratio mass spectrometry. The indicates the sample is a spike (i.e. can be used for isotope dilution). Further information are presented in www.irmm.jrc.be and www.nist.gov. 

In order to overcome, or at least minimise, such drawbacks we can resort to the use of chemometric techniques (which will be presented in the following chapters of this book), such as multivariate experimental design and optimisation and multivariate regression methods, that offer great possibilities for simplifying the sometimes complex calibrations, enhancing the precision and accuracy of isotope ratio measurements and/or reducing problems due to spectral overlaps. 

As the precision of the ICP-MS isotope ratio is poor compared with the precision using TIMS, the range of applications for ICP-MS have traditionally been limited to measuring induced changes in the isotopic composition of a target element (for example, to calibrate by means of isotope dilution). However, the introduction of multicollector ICP-MS systems to enhance precision and accuracy in isotopic analysis opened up novel applications. 

IDMS is based on measurements of masses and isotope ratios only. Some important advantages, compared with other calibration strategies, such as external calibration and standard additions, are that instrumental instabilities such as signal drift and matrix effects will have no influence in the final concentration in the sample, high accuracy and small measurement uncertainties are enabled, possible loss of substance of the isotope-diluted sample will have no influence on the final result and there is no need to resort to an external instrumental calibration or standard additions to the sample. 

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