Accuracy, of isotope ratio measurements

Limits for Precision and Accuracy of Isotope Ratio Measurements and How to Solve the Problems... 

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])...

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... 

From a series of isotope ratio measurements, the precision of measurement can be assessed statistically, as shown here. Precision reveals the reproducibility of the measurement method, but it does not provide information on the accuracy of the measurement (see also Figures 48.8 and 48.9). 

Q. What are the factors which most affect the accuracy and precision of isotope ratio measurements ... 

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... 

Grogler et al. (19) were hampered by the low accuracy of isotope composition measurement that was then attainable (the absolute 2a errors in the isotope ratios were about 1%), by the lack of a sufficient data base for ore samples, and by the lack of a coherent archaeological strategy. Nevertheless, this early work provided a base for the subject of lead isotope archaeology. 

In addition, the occurrence of isobaric interferences of analyte ions with isobaric polyatomic ions can hamper the accuracy and precision of isotope ratio measurements (see also Section 6.1.3). The main factors affecting the accurate and precise determination, for example, of using ICP-MS, are the isobaric interference of the molecular ion on 236pj+ analyte ions, and... 

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. 

Mass-dependent fractionation processes that occur during the ionization process limit the ultimate accuracy and precision of isotope ratio measurements. As the sample is heated and ions are formed, the lighter isotopic species will evolve from the filament at a faster rate. The remaining (unionized) sample becomes relatively more enriched in the heavier isotopes and no longer has a representative isotope composition. Corrections to this fractionation are based on empirical calculations according to exponential or Rayleigh distillation models. 

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. 

Gases and vapors of volatile liquids can be introduced directly into a plasma flame for elemental analysis or for isotope ratio measurements. Some elements can be examined by first converting them chemically into volatile forms, as with the formation of hydrides of arsenic and tellurium. It is important that not too much analyte pass into the flame, as the extra material introduced into the plasma can cause it to become unstable or even to go out altogether, thereby compromising accuracy or continuity of measurement. 

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-... 

---