Isotopic abundance ratios determination

The mass spectrum of an unknown compound had the following relative intensities for the M, (w/e = 86), M -i- 1, and M H- 2 peaks respectively 18.5, 1.15, and 0.074 (percentage of base peak). From the following partial list of isotopic abundance ratios, determine the molecular formula of the unknown. 

One method for measuring the temperature of the sea is to measure this ratio. Of course, if you were to do it now, you would take a thermometer and not a mass spectrometer. But how do you determine the temperature of the sea as it was 10,000 years ago The answer lies with tiny sea creatures called diatoms. These have shells made from calcium carbonate, itself derived from carbon dioxide in sea water. As the diatoms die, they fall to the sea floor and build a sediment of calcium carbonate. If a sample is taken from a layer of sediment 10,000 years old, the carbon dioxide can be released by addition of acid. If this carbon dioxide is put into a suitable mass spectrometer, the ratio of carbon isotopes can be measured accurately. From this value and the graph of solubilities of isotopic forms of carbon dioxide with temperature (Figure 46.5), a temperature can be extrapolated. This is the temperature of the sea during the time the diatoms were alive. To conduct such experiments in a significant manner, it is essential that the isotope abundance ratios be measured very accurately. 

Catanzaro EJ, Murphy TJ, Garner EL, Shields WR (1966) Absolute isotopic abundance ratios and atomic weight of magnesium. J Res Natl Bur Stand 70A 453-458 Chang VT-C, Makishima A, Belshaw NS, O Nions RK (2003) Purification of Mg from low-Mg biogenic carbonates from isotope ratio determination using multiple collector ICP-MS. J Anal At Spectrom 18 296-301... 

P. Horn, L. Holzl, W. Todt, D. Matthies, Isotope abundance ratios of Sr in wine provenance determination, in a tree-root activity study, and of Pb in a pollution study on tree-rings, Isotopes Environ. Health Studies, 34 (1998), 31-42. 

Quantities of plutonium-241, a beta-particle emitter, can be quantified from (1) assumed isotopic abundance ratios, (2) estimated in-growth of its progeny americion-241 by gamma spectrometry, or by (3) mass spectrometry (Bernhardt 1976). Americium-241 is produced from the beta decay of plutonium-241 and, therefore, can be used to indirectly measure the concentration of plutonium-241 (Metz and Waterbury 1962). Direct determination of plutonium-241 by measurement of its low energy beta-particle decay has been reported using liquid scintillation analysis (Martin 1986). 

Different isotopes differ in their atomic masses. The intensities of the signals from different isotopic ions allow isotopic abundances to be determined with high accuracy. Mass spectrometry reveals that the isotopic abundances in elemental samples from different sources have slightly different values. Isotopic ratios vary because isotopes with different masses have slightly different properties for example, they move at slightly different speeds. These differences have tiny effects at the level of parts per ten thousand (0.0001). The effects are too small to appear as variations In the elemental molar masses. Nevertheless, high-precision mass spectrometry can measure relative abundances of isotopes to around 1 part in 100,000. 

Wasserburg, GJ, Jacobsen SB, DePaolo DJ, McCullock MT, Wen T (198L) Precise determination of Sm/Nd ratios, Sm and Nd isotopic abundances in standard solutions. Geochim Cosmo-chim Acta 45 2311-2323. 

Since only one or two charge states are observed for any element, the elemental identity of each ion is determined from its mass-to-charge ratio by consulting a table of known isotope abundances. 

Fig. 1. Model Spectra re-binned to CRIRES Resolution To demonstrate the potential for precise isotopic abundance determination two representative sample absorption spectra, normalized to unity, are shown. They result from a radiative transfer calculation using a hydrostatic MARCS model atmosphere for 3400 K. MARCS stands for Model Atmosphere in a Radiative Convective Scheme the methodology is described in detail e.g. in [1] and references therein. The models are calculated with a spectral bin size corresponding to a Doppler velocity of 1 They are re-binned to the nominal CRIRES resolution (3 p), which even for the slowest rotators is sufficient to resolve absorption lines. The spectral range covers ss of the CRIRES detector-array and has been centered at the band-head of a 29 Si16 O overtone transition at 4029 nm. In both spectra the band-head is clearly visible between the forest of well-separated low- and high-j transitions of the common isotope. The lower spectrum is based on the telluric ratio of the isotopes 28Si/29Si/30Si (92.23 4.67 3.10) whereas the upper spectrum, offset by 0.4 in y-direction, has been calculated for a ratio of 96.00 2.00 2.00.

Figure 7. Effect of abundance sensitivity on isotope ratio determination...

The masses of isotopes can be measured with accuracies better than parts per billion (ppb), e.g., m40Ar = 39.9623831235 0.000000005 u. Unfortunately, determinations of abundance ratios are less accurate, causing errors of several parts per million (ppm) in relative atomic mass. The real limiting factor, however, comes from the variation of isotopic abundances from natural samples, e.g., in case of lead which is the final product of radioactive decay of uranium, the atomic weight varies by 500 ppm depending on the Pb/U ratios in the lead ore. [8]... 

Limiting ourselves to the observation of isotopic abundances determined by the 4n + 2 and 4n + 3 decay series, we can construct a concordia diagram (Wetherill, 1956) relating (206p /238 j 207pj /235 j ratios developing at... 

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. 

Isotope ratio measurements are performed whenever the exact ratio, or abundance, of two or more isotopes of an element must be known. For example, the isotopic ratios of lead are known to vary around the world, so it is possible to determine the source of lead in paint, bullets and petrol by knowing the isotopic abundances of the four lead isotopes 204, 206, 207, 208. Another example is the use of stable isotopes as metabolic tracers, where an animal is both fed and injected with an element having artificially enriched isotopes and the fractional absorption of the element can be accurately determined. 

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