Isotope abundance, measurements table

Analytical Considerations for Isotope Abundance Measurements (Table 3)... 

Table 1. Natural abundances, measured isotope ratios, and enrichment factors used for Calcium isotopes.

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. 

Dimethylzinc was the first organozinc compound to be studied by mass spectrometry (MS), as part of early MS studies performed by Aston aiming to examine natural abundances of Zn isotopes. Soon thereafter, Bainbridge showed that Aston s data were imprecise because the formation of hydride ions [ZnH]+ was overlooked, and therefore their contribution to the measured abundances of Zn isotopes was not taken into consideration. In the years that followed, numerous studies addressed the subject of Zn isotopic abundances In 2001, the data presented in Table 1 were accepted as the most accurate. It was by mass spectrometry that this best measurement from a single terrestrial source was obtained. 

Their table included isotopic abundances as well as elemental abundances. Since the Suess and Urey table was published, subsequent work has primarily refined the determinations of the cosmic abundances through improved measurements of meteorites, a better understanding of which meteorites should be considered for this work, improved measurements of the solar composition, and a better understanding of nuclear physics. 

Mass spectrometric measurements are based on measuring ion currents of separated ion beams of isotopes. With knowledge of the isotopic composition of the elements investigated (see Table of Isotopic Abundances in Appendix I5), a simple identification of chemical elements using singly... 

From the measured 63Cu/65Cu isotope ratio the isotope abundances of 63Cu and 65Cu are then calculated in a natural sample as roughly 69.2% and 30.8%, respectively. Small deviations from the IUPAC table value10 could be evidence of fine isotope variation in nature. 

Measurement of the isotopic cluster of the molecular ion showed an M + 1 peak of 5.30% and an M + 2 peak of 0.15% of the molecular ion. From tables of isotopic abundance ratios it was found that the expected product C4H8N2 should give M + 1 and M + 2 peaks of 5.21 and 0.11%, respectively, while cyclohexane C6H12 should give M + 1 and M + 2 peaks of 6.68 and 0.19%, respectively. It is clear that the isolated product is most likely the expected cyclic azo compound and not cyclohexane. 

This is readily calculable if the individual elemental abundances are known. Assuming the isotopic abundance of 12-carbon and 13-carbon to be 98.89% and 1.11 and 50-, 52-, 53- and 54-chromium to be 4.31, 83.76, 9.55 and 2.38% respectively, and neglecting any isotopic abundances less than 1%, one can obtain a set of calculated abundances for the Cr(tfa)2+ ion. These and the measured isotopic abundances (by SIM) are listed in Table 16.11. The agreement between the two sets is excellent. The same calculation can be made for the 5 3-chromium spike solution by using isotopic abundances given by the supplier 52-chromium 3.44%, 53-chromium 96.4% and 54-chromium 0.18%. Table 16.12 lists the calculated and the measured isotopic abundances for the spike solution. 

Mathematical correction procedures can be used to remove the contribution of a spectral overlap from a measured signal. However, if the signal due to the spectral overlap is much larger than the analyte signal, the signal-to-noise ratio of the corrected signal may be poor. Furthermore, it may not be easy to predict and account for quantitatively all of the potential sources of spectral overlap, particularly those due to polyatomic ions. For isobaric overlaps (Table 3.2), for which the relative isotopic abundances are predictable, mathematical corrections are straightforward. Instrument software often has built-in correction equations for this case. 

Ideally, we could use the isotopic compositions in Table 12-4 to determine the entire molecular formula of a compound, by carefully measuring the abundances of the M+, M+l, and M+2 peaks. In practice, however, there are background peaks at every mass number. These background peaks are often similar in intensity to the M+l peak, preventing an accurate measurement of the M+l peak. High-resolution mass spectrometry is much more reliable. 

Wexler has experimentally measured (Table 4) the abundance of the ionic species formed from the decay of Tg. A comparison with Table 4 reveals that the dissociation of HeT+ is further reduced with respect to He H+, 94-5% of the daughter ions remaining bound. This observation provides additional evidence for the electron shaking excitation mechanism, since any explanation of the observed isotope effect is of course founded on the premise that at least a fraction of the daughter ions are formed in an excited state. 

Thermal ionization has been used to determine isotopic abundance of virtually all the elements We have recently extnded our own capability in this direction by adapting the silica gel/phosphoric acid filament coating technique (5) to our system Five 1 of a fine silica gel suspension is placed on a filament Five l of the analyte ion solution is coated, dried then coated with 2 pi of a 0 7N phosphoric acid solution and heated until dry again The analysis is performed in a similar manner as before, except that the signal is more transient and somewhat less intense than the calcium analysis With this approach, however, we have made natural abundance isotope ratio measurements on zinc, copper, and magnesium Table II shows our measurements compared to the accepted values, shown in parenthesis, for these elements The isotope used as reference... 

There are four stable isotopes of sulfur as listed in Table 1. The isotopic abundances vary slightly and this is frequently used to distinguish the source of the element. Because measurement of absolute isotope abundance is difficult, relative isotopic ratios are measured by comparison with the abundance of the natural isotopes in a standard sample. The Canyon Diablo meteorite has been used as a standard for sulfur isotopes. 

The measured quantities often are reported in the literature for a specific isotopic molecule. The thermochemical tables in this publication normally use spectroscopic information for a natural isotopic abundance molecule. Care should be exercised in noting to which molecule the spectroscopic data pertains. 

The measurement of isotopic abundances began early this century following the discovery of neon isotopes by J.J. Thompson in 1912. F.W. Aston developed the mass spectrometer into a quantitative instrument for measuring isotopic abundances and by 1935 the isotopic composition of most elements was known. The first International Table of Stable Isotopes was drawn up in 1936, while the latest table of Isotopic Compositions of the Elements appeared recently (4). Lead is an element for which there was early evidence of natural variations in its isotopic composition (5) these were ultimately used to measure the age of the Earth (6). Natural variations have been reported in 43 other elements although many relate to exceptional samples. There are 18 elements in which variations are not uncommon, although most of these elements have relatively light atoms with atomic numbers less than 16 (4). 

Mass spectrometers determine atomic and molecular isotope ratios. Table 2 lists the relative isotopic abundance of elements commonly encountered in pharmaceutical analysis [3,4]. The values in Table 2 have been empirically determined and refinements in the values are necessary as atomic mass measurements improve, but for this discussion any inaccuracies in the table are insignificant. For some elements there are only two naturally occurring isotopes. For example, if you were to randomly sample carbon atoms in nature, 99% of the time you would find 12C, and roughly 1% of the time a 13C would turn up. Other elements, such as chlorine and bromine, have elemental isotope ratios that are not as heavily... 

The noble gas isotope then becomes a proxy for the parent. The isotopes used this way are listed in Table 1. The rest of the analysis is quite similar to a standard three-isotope plot for geochronology systems such as Sm-Nd or Rb-Sr. The major difference is that instead of using an isotope of the parent element, noble gas-based techniques use only isotopes of the daughter element, the noble gas. Because mass spectrometers are generally better at measuring relative isotopic abundances than at measuring absolute amounts, this is a decisive advantage. 

---