Isotope ratio measurements accuracy limits

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. 

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-... 

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

Since the introduction of the first commercial instrument in 1983, inductively coupled plasma mass spectrometry (ICP-MS) has become widely accepted as a powerful technique for elemental analysis. Two excellent books on ICP-MS have been published [1,2]. ICP-MS provides rapid, multielement analysis with detection limits at single parts part trillion or below for about 40 to 60 elements in solution and a dynamic range of 104 to 108. These are the main reasons most ICP-MS instruments have been purchased. Two additional, unique capabilities of ICP-MS have also contributed to its commercial success elemental isotope ratio measurements and convenient semiquantitative analysis. The relative sensitivities from element to element are predictable enough that semiquantitative analysis (with accuracy within a factor of 2 to 5) for up to 80 elements can be obtained using a single calibration solution containing a few elements and a blank solution. 

Both ICP-MS and LA-ICP-MS are increasingly replacing other MS techniques that have been dominant analytical methods for precise isotope ratio measurements for many decades. For all types of inorganic IRMS, however, precision and accuracy are primarily limited by sample preparation, introduction, and analytical methodology. 

Mass spectrometry ICP-MS offers an accurate method for sulfur isotopic ratio measurements and a sensitive technique for elemental sulfur determination. The method may be applied as an absolute method, because the analyzed atoms themselves, and not the radiation they emit, produce the analytical signals. The technique is useful for the analysis of high-purity materials, such as metals and semiconductors. The detection limits obtainable in most cases by ICP-MS are considerably better than typical values reported for ICP-AES. However, the analytical precision and accuracy of the method are often poor. The utility of the method to determine sulfur is further complicated, as efficient ionization of sulfur is difficult to obtain because of the high ionization energy of the element. 

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. 

The common mass spectrometers used in ICP-MS today are scanning-based analysers, such as the quadmpole mass filter. Unfortunately, they suffer from important performance limitations when used as detectors of short transient signals (e.g. those generated in spedation analysis), derived from their inability to perform true simultaneous multielemental analysis. With scanning-based instruments, individual mass-to-charge ratios are measured in a sequential mode, from one isotope to the next. As a result, some difficulties (in terms of precision, sensitivity or accuracy of isotopic and isotope ratio measurements) are expected when fast transient or time-dependent signals (such as those... 

Relatively new innovations in ICPMS instrumentation may partly alleviate some of those limitations. For example, some ICPMS instruments are now available with a magnetic sector mass spectrometer rather than a quad-rupole mass spectrometer. These new instruments may provide increased accuracy and precision of lead isotope ratio measurements, which would lead to higher quality measurements by isotope dilution. 

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. 

Accuracy for all thorium measurements by TIMS is limited by the absence of an appropriate normalization isotope ratio for internal correction of instrumental mass fractionation. However, external mass fractionation correction factors may be obtained via analysis of suitable thorium standards, such as the UC-Santa Cruz and IRMM standards (Raptis et al. 1998) for °Th/ Th, and these corrections are usually small but significant (< few %o/amu). For very high precision analysis, the inability to perform an internal mass fractionation correction is probably the major limitation of all of the methods for thorium isotope analysis discussed above. For this reason, MC-ICPMS techniques where various methods for external mass fractionation correction are available, provide improved accuracy and precision for Th isotope determinations (Luo et al. 1997 Pietruszka et al. 2002). 

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