Stable isotope measurement 8-value

To facilitate the d values being discussed in an international forum it is necessary to cross-calibrate the data obtained from individual laboratories. This is accomplished by knowing the 6 value of each laboratory s reference material relative to an internationally accepted standard. The standards used for stable isotopic measurements are given in Table 2. In the case of carbon, where the standard is Pee Dee be-lemnite (PDB),... 

At equilibrium, the fractionation factor is related to the temperature of exchange. Measured fractionations (differences in delta values) are therefore proportional to temperature of exchange. Harold Urey s recognition of these properties led to the application of stable isotopic measurements to fossils containing carbonate to infer the temperature of the ancient oceans. 

Abundances of lUPAC (the International Union of Pure and Applied Chemistry). Their most recent recommendations are tabulated on the inside front fly sheet. From this it is clear that there is still a wide variation in the reliability of the data. The most accurately quoted value is that for fluorine which is known to better than I part in 38 million the least accurate is for boron (1 part in 1500, i.e. 7 parts in [O ). Apart from boron all values are reliable to better than 5 parts in [O and the majority arc reliable to better than I part in 10. For some elements (such as boron) the rather large uncertainty arises not because of experimental error, since the use of mass-spcctrometric measurements has yielded results of very high precision, but because the natural variation in the relative abundance of the 2 isotopes °B and "B results in a range of values of at least 0.003 about the quoted value of 10.811. By contrast, there is no known variation in isotopic abundances for elements such as selenium and osmium, but calibrated mass-spcctrometric data are not available, and the existence of 6 and 7 stable isotopes respectively for these elements makes high precision difficult to obtain they are thus prime candidates for improvement. 

Dietary studies, based on stable isotope values of animal tissues and foodstuffs, can be successful only insofar as the complexities of molecular metabolism manage to give rise to relatively simple relationships in what is measured. Many of these relationships have been explored, and have in part been described by observational regularities, and in part summarised by physiological or biochemical explanations. Some questions can and have now been clearly posed for example ... 

Ra is soluble and therefore tends to be released to deep waters when it is formed by °Th decay in marine sediments. Substrates which capture the resulting excess of Ra found in seawater can potentially be dated using the decay of this Ra excess ( Raxs). Unfortunately there is no stable isotope of Ra with which to normalize measured Ra values but the marine chemistry of Ba is sufficiently close to that of Ra that it can be used as a surrogate for a stable Ra isotope and seawater Ra/Ba ratios are constant throughout the oceans, except in the deep North Pacific (Chan et al. 1976). The half life of Ra is only 1600 years so Raxs/Ba chronology is limited to the Holocene but it nevertheless has potential for use in several regions. 

Using the above laboratory relations we compute the values of 6D and 618 expected for rain made by a single distillation from the ocean and for rain made by a subsequent distillation for sea rain which fell on the land. These values show that when rain and snow exhibit very large depletions of the heavy isotopes, e.g., 6D of -200 and -300 ppt, two or three distillations have occurred. It is known from measurements of tritium in rain [76] that two or three distillations occur in the U.S. between evaporation from the Pacific Ocean and precipitation in the eastern U.S. The agreement between the number of distillations deduced from stable isotopes and deduced from tritium is gratifying. 

Although GC-C-IRMS systems that can measure the chlorine isotopic composition of individual chlorinated hydrocarbons are currently unavailable, it is clear that chlorine isotope analysis is also a useful technique to consider for study [614,677,678]. Measurement of chlorine stable isotope ratios in natural samples such as rocks and waters has become routine [626,679,680], but few measurements of chlorine isotopes in chlorinated aliphatic hydrocarbons have been reported [614]. A chlorine isotope effect was found in ferf-butyl chloride [681], demonstrating that 37Cl is more strongly bound to carbon than is 35Cl. Significant differences in the <5i7Cl values of some atmospheric chlorinated... 

Because the natural variations in stable isotope abundances are usually very small (see above), and since routine measurements are usually made in an isotope ratio mass spectrometer which compares the relative intensities of the mass resolved beams of the sample with those of some standard material (Section 7.2.2), it is standard practice to report abundance ratios using the dimensionless 8-value notation. 

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