Absolute isotope ratios

The accuracy with which absolute isotope abundances can be measured is substantially poorer than the precision with which relative differences in isotope abundances between two samples can be determined. Nevertheless, the determination of absolute isotope ratios is very important, because these numbers form the basis for the calculation of the relative differences, the 5-values. Table 1.6 sununarizes absolute isotope ratios of primary standards used by the international stable isotope community. 

Table 1.6 Absolute isotope ratios of international standards. (After Hayes 1983)...

Rstandard IS the Corresponding rate in a standard. Nevertheless, the determination of absolute isotope ratios (Table 10.1) is essential, since these numbers form the basis for the calculation of the relative differences. 

De Bievre, P. (1976). Absolute isotope ratio mass spectromet some problems and possibilities, 7th Triennal Intern. Conf. Mass Spectrometry, Firenze, Sept. 1976, Adv. Mass Spec. 7A, 395-447, Heyden and Son Ltd. London. 

The isotope ratio of interest is obtained by comparing the sample measurement with that from the reference. Actually, several comparisons are made -sample and reference gases are repeatedly and alternately admitted to the mass spectrometer, with analyses taking 10 min and involving 20 or so sam-ple/reference comparisons. Since the sample and reference gases are analyzed identically, there is no need for the mass spectrometer to determine the absolute isotope ratio the comparative process ensures... 

Because the changes in the isotopic composition due to isotope fractionation are small and a difference in isotope ratios relative to one another can be determined more easily than an absolute isotope ratio, an isotope ratio for a sample is usually expressed as the difference between the specific isotope ratio and that of a selected standard ... 

Description The isotopic composition is characterized via isotope amount ratios or isotope amount fractions. These data have been obtained from measurements which have been corrected for all bias occurring in mass spectrometry. In terms of isotope ratio data, one can often read about absolute isotope ratio data or absolute measurements. Here it is meant that the measurements leading to isotope ratio data are a best estimate for the true value. From a metrological point of view, all published isotope ratio data should be corrected for all bias and should be accompanied by an uncertainty budget. Isotope amount fractions and atomic weights should only be calculated from fiilly corrected isotope amount ratios. Anything else is observed data only. Thus, isotope amount fractions and molar masses are always true or absolute data [35]. 

Table 3. Absolute isotopic ratios of some standard samples... 

As research into absolute isotope ratio measurements by MC-ICP-MS continues, some new limitations have been identified recently. Although in MC-ICP-MS, mass bias is generally considered to be mass-dependent fractionation (MDF) and is corrected by various mass-dependent correction models, mass-independent fractionation (MIF) in MC-ICP-MS was described by Lu Yang et al., of the National Research Council Canada, at the 2013 European Winter Conference on Plasma Spectrochemistry. Their study looked at numerous elements and concluded that this appears to be a common phenomenon, and consequently has serious implications on the absolute isotope amount ratio measurements. 

Table 2.51 IRMS primary reference materials for irm-GC-MS. Absolute isotope ratios R in 1 o standard precision.

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