Accuracy in isotope ratio measurements

Table 13.2 Precision and accuracy in isotope ratio measurement of toxic metals by GC-MS ...

The advantages and limitations of these mass spectrometers are the same as described earlier. ICPMS has a very high throughput compared to TIMS, and the MC-ICPMS has quite similar accuracy in isotopic ratio measurements and sensitivity as the SIMS and TIMS methods. 

Heumann, K. G., Gallus, S. M., Radlinger, G., and Vogl, J. (1998) Precision and accuracy in isotope ratio measurements by plasma source mass spectrometry. J. Anal. At. Spectrom., 13, 1001. 

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. 

Accuracy of isotope ratio measurement is critically dependent on having the instrument properly calibrated and following correct analytical protocol. Mass bias is present to some degree in all thermal ionization analyses, and a lot of ingenuity has been invested in mitigating its effect. Mass bias arises from a... 

Figure 1.11. The effect of sample size and isotope abundance on the accuracy of isotope ratio measurements in GC-MS using a quadrupole instrument and computer data system. The curves are based on numerous separate studies (reproduced from reference [110])...

Over the past decade, awareness has grown significantly that isotopic analysis by ICP-MS is full of potential sources of bias that need to be controlled carefully to generate not only precise but also accurate data. Whereas in the early days the speed advantage of ICP-MS over TIMS in isotope ratio measurements has been emphasized has been emphasized because it permits direct analysis of digested samples, this advantage has now mostly vanished. Because of much better control of spectral and non-spectral interferences, it is now common practice in ICP-MS as in TIMS to separate the element chemically from the matrix when high analytical accuracy is of concern. This does not equalize the two... 

In this equation the index al represents the interfered isotope of the element a while the index p 1 represents the interfering isotope of element p. /fpi/pa represents the measured isotope amount ratio P l/p2, where P2 is a non-interfered isotope of element p. Since the interference is present, this ratio cannot be accurately measured in the sample and therefore has to be replaced with a value derived from an alternative source. Most ICP-MS instruments simply replace the ratio I3i/I32 by the reported lUPAC ratio value. ° They do not consider mass discrimination effects, despite the fact that equation (4.1) only deals with measured intensities, which are intrinsically affected by mass discrimination. For measurements requiring higher accuracy, especially isotope ratio measurements, this fact has to be taken into account and the interference correction has to be modified accordingly. 

Moens and co-workers used the fluoride transfer reaction frommethyl fluoride to Sr+ (resulting in the formation of SrF+) to separate the strontium isotope pattern from a rubidium-containing background matrix, with the aim of making Rb-interference free isotope ratio measurements of strontium. However, it has been proposed that mass discrimination effects due to the ion-molecule reaction can alter the experimentally determined isotope pattern, thereby degrading the accuracy of isotope ratio measurements made using this approach. Consider the reaction... 

Accurate, precise isotope ratio measurements are important in a wide variety of applications, including dating, examination of environmental samples, and studies on drug metabolism. The degree of accuracy and precision required necessitates the use of special isotope mass spectrometers, which mostly use thermal ionization or inductively coupled plasma ionization, often together with multiple ion collectors. 

Accurate, precise isotope ratio measurements are used in a variety of applications including dating of artifacts or rocks, studies on drug metabolism, and investigations of environmental issues. Special mass spectrometers are needed for such accuracy and precision. 

The measurements that have been made at Rochester and the experience that has been gathered over the years on the operation of sputter ion sources [38] indicate that an analytical tool of unprecedented sensitivity and accuracy for isotopic ratio determinations can be constructed by coupling SIMS technology with the new accelerator technique. The only difference in principle between the experiments that have been conducted to date and the technique as it would be applied in secondary ion mass spectrometry is that the primary beam of cesium would be focussed to a fine probe of pm dimensions rather than the spot diameters of approximately 1 mm that have been used to date. 

TI is a very precise and accurate method in stable isotope ratio measurements and quantification of inorganic elements, for example, by isotope dilution mass spectrometry [8]. Because TI is a continuous ion source, it could be coupled to any analyzer that is suitable for such sources. However, because the strength of TI lies in the quantitative precision and accuracy, sector analyzers are preferred to ensure maximum quality. 

The best precision is obtained for isotope ratios near unity (unless 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 isotope 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 two situations being illustrated in Fig. 5.11. The accuracy and precision of the isotope dilution analysis ultimately depend 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. 

In addition, one of the main features of mass spectrometry is, and this is the major advantage in comparison to other atomic and molecular non-mass spectrometric techniques, that it offers the possibility of determining isotope ratios and abundances of isotopes with high precision and accuracy in all types of samples (in solid, liquid and gaseous materials as well). Isotope ratio measurements have applied increasingly for stable isotopes in nature, especially for investigating... 

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. 

Precise and accurate isotope analyses by mass spectrometry have attained growing importance in the last few years due to instrumental improvements with respect to sensitivity, detection limits, precision and accuracy.1 As mentioned before, because the isotope abundances of several elements are not constant and vary as a result of nuclear, biological, chemical, geochemical and physical processes, isotope ratio measurements are required for different research and application fields. Isotope ratio measurements are therefore necessary for elements with two or more isotopes for inves-... 

For many decades, TIMS was the isotope analytical technique of choice, but due to instrumental developments in ICP-MS, especially with multiple ion collectors (MC-ICP-SFMS), and the advantages of ICP-MS in comparison to TIMS (e.g., higher element sensitivities, faster isotope ratio measurements, comparable precision and accuracy, practically no restriction on the ionization potential of chemical elements, time independent mass fractionation and the possibility of additional multi-element analysis at trace and ultratrace level and fewer, less time-consuming sample preparation steps75), TIMS will be replaced in future by powerful ICP-MS to an ever greater extent. 

Of all the different mass spectrometric techniques for isotope analysis (such as ICP-MS, LA-ICP-MS, TIMS, GDMS, AMS, SIMS, RIMS and isotope ratio mass spectrometry of gases), the greatest proportion of pubhshed papers today concern ICP-MS with single and multiple ion collection.19 Due to its benefits, ICP-MS has now become a widely accepted method for isotope analysis and allows isotope ratios to be measured in a short time with good accuracy and precision.9,19,75 78 As discussed above, as a powerful and universal tool, ICP-MS has opened up new applications for isotope ratio measurements of elements with a high first ionization potential, which are difficult to analyze with TIMS (such as Mo, Hf, Fe). Of all the heavy metals studied, uranium was favoured by ICP-MS and LA-ICP-MS. 

An analytical procedure has been proposed for precise uranium isotope ratio measurements in a thin uranium layer on a biological surface by LA-ICP-MS using a cooled laser ablation chamber.125 One drop of uranium isotope standard reference materials NIST, 350, NIST 930, of our isotopic laboratory standard CCLU 500 (20p.l, U concentration 200 ng 1) and of uranium with natural isotopic pattern were deposited on the leaf surface and analyzed by LA-ICP-MS at well defined laser crater diameters of 10, 15, 25 and 50 p.m. A precision for measurements of isotope ratios in the range of 2.1-1.0% for 235U/238U in selected isotope standards was observed whereby the precision and the accuracy of isotope ratios compared to the non-cooled laser ablation chamber was improved.125... 

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