Abundance ratio

The possibility that an even larger impact caused the P T extinction received support when Becker and Poreda found that helium and argon atoms were present in the inner cores of some of the fullerenes from the P T boundary sediments (The cover of this book shows a helium atom inside a mol ecule of Ceo) What is special about the fullerene trapped atoms is that the mixtures of both helium and argon isotopes resemble extraterrestrial isotopic mixtures more than earthly ones The He/ He ratio in the P T boundary fullerenes for example is 50 times larger than the natural abundance ratio... 

In a process similar to that described in the previous item, the stored data can be used to identify not just a series of compounds but specific ones. For example, any compound containing a chlorine atom is obvious from its mass spectrum, since natural chlorine occurs as two isotopes, Cl and Cl, in a ratio of. 3 1. Thus its mass spectrum will have two molecular ions separated by two mass units (35 -i- 2 = 37) in an abundance ratio of 3 1. It becomes a trivial exercise for the computer to print out only those scans in which two ions are found separated by two mass units in the abundance ratio of 3 1 (Figure 36.10). This selection of only certain ion masses is called selected ion recording (SIR) or, sometimes, selected ion monitoring (SIM, an unfortunate... 

The two isotopes of chlorine are Cl and y3l, which occur naturally in the abundance ratio of 3 1. 

A diagrammatic illustration of the effect of an isotope pattern on a mass spectrum. The two naturally occurring isotopes of chlorine combine with a methyl group to give methyl chloride. Statistically, because their abundance ratio is 3 1, three Cl isotope atoms combine for each Cl atom. Thus, the ratio of the molecular ion peaks at m/z 50, 52 found for methyl chloride in its mass spectrum will also be in the ratio of 3 1. If nothing had been known about the structure of this compound, the appearance in its mass spectrum of two peaks at m/z 50, 52 (two mass units apart) in a ratio of 3 1 would immediately identify the compound as containing chlorine. 

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. 

Two further expressions are used in discussions on isotope ratios. These are the atom% and the atom% excess, which are defined in Figure 48.6 and are related to abundance ratios R. It has been recommended that these definitions and some similar ones should be used routinely so as to conform with the system of international units (SI). While these proposals will almost certainly be accepted by mass spectrometrists, their adoption will still leave important data in the present format. Therefore, in this chapter, the current widely used methods for comparison of isotope ratios are fully described. The recommended Sl-compatible units such as atom% excess are introduced where necessary. 

Many artificial (likely radioactive) isotopes can be created through nuclear reactions. Radioactive isotopes of iodine are used in medicine, while isotopes of plutonium are used in making atomic bombs. In many analytical applications, the ratio of occurrence of the isotopes is important. For example, it may be important to know the exact ratio of the abundances (relative amounts) of the isotopes 1, 2, and 3 in hydrogen. Such knowledge can be obtained through a mass spectrometric measurement of the isotope abundance ratio. 

Approximate ratios of isotope abundance ratios are important in identifying elements. For example, the naturally occurring Cl, Cl isotopes exist in an abundance ratio of about 3 1, and C, exist in a ratio of about 99 1. 

A 6-value is used to compare a measured isotope ratio in a sample with that for a standard substance containing the same isotopes but in known abundance ratio. 

For marble provenance studies, the most successful technique seems to be the measurement, through mass spectrometry, of the abundance ratios of the stable isotopes of carbon and oxygen (116). However, no single technique appears to provide unequivocal results, especially in cases such as the different Mediterranean sources, and a combination is often necessary to arrive at an approximate place of origin (117). 

FIG. 20-10 Fraction of mineral B that is liberated as a function of volumetric abundance ratio D of gangue to mineral B (1/grade), and ratio of grain size to particle size of broken fragments (1/Bneness). [ V egel and Li, Trans. Soc. Min. Eng.-Am. Inst. Min. Metall. Pet. Eng., 238, 179 (1967).]... 

Miyake, Y. and Wada, E. 1967 The abundance ratio of N/ N in marine environments. Records of Oceanographic Works, Japan 9 (new series) 32-53. 

Sulfur also shows variations in the abundance ratio of S and S. Tiny differences in the rates of... 

Mohler FL i960) Isotopic abundance ratios reported for reference samples stocked by the National Bureau of Standards. NBS Technical Note 51 US Dept of Commerce Wash DC. 

Quetel CR, Prohaska T, Hamester M, Kerl W, Taylor PDP (2000b) Examination of the performance exhibited by a single detector double focusing magnetic sector ICP-MS instrument for uranium isotope abundance ratio measurements over almost three orders of magnitude and down to pg g-1 concentration levels. J Anal At Spectrom 15 353-358... 

In this paper I discuss overall metallicity, a measure of the overall heavy-element abundance in the star, and direct determination of elemental abundances and abundance ratios of Fe, O, the a-elements Mg, Si, Ca, and Ti, and also the light elements Na and Al. 

Determination of the Age of the Galactic Thin Disk from Th/Eu Stellar Abundance Ratios... 

Comparison of our stellar abundance data with [Th/Eu] vs. [Fe/H] curves obtained from the GCE models, calculated for four different Galactic disk ages - 6, 9, 12, and 15 Gyr Tq = 8.2 1.9 Gyr. The uncertainty is relative only to the abundance ratio uncertainties, not considering the uncertainties intrinsic to the GCE model itself, which are difficult to evaluate. 

Does the [a/Fe] Abundance Ratio Trend Reverse at Super-Solar Regime A Test on the Role of Accurate Knowledge of Atmospheric Parameters... 

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