Relative isotopic abundance

Carbon has two common isotopes, and with relative isotopic abundances of, respectively, 98.89% and 1.11%. (a) What are the mean and standard deviation for the number of atoms in a molecule of cholesterol (b) What is the probability of finding a molecule of cholesterol (C27H44O) containing no atoms of... 

Burnett DS, Lippolt HJ, Wasserburg GJ (1966) The relative isotopic abundance of K in terrestrial and meteoritic samples. J Geophys Res 71 1249-1259... 

Figure 1. Schematic representation of the calcium mass spectrum in (a) natural materials, (b) a Ca- Ca tracer solution used for separating natural mass dependent isotopic fractionation from mass discrimination caused by thermal ionization, and (c) a typical mixture of natiwal calcium and tocer calcium used for analysis. The tracer solution has roughly equal amounts of Ca and Ca. In (c) the relative isotopic abundances are shown with an expanded scale. Note that in the mixed sample, masses 42 and 48 are predominantly from the tracer solution, and masses 40 and 44 are almost entirely from natural calcium. This situation enables the instrumental fractionation to be gauged from the Ca/ Ca ratio, and the natural fractionation to be gauged from the sample Ca/ Ca ratio.

Table 2.1 Relative Isotope Abundances of Common Elements ...

Different isotopic species of a molecule have different reduced masses and different rotational constants. Hence each such species has its own pure-rotation spectrum, the intensity of the spectrum being determined by the relative isotopic abundance. 

While significant uncertainty exists in determining the absolute stellar abundances of the heavy elements, the relative isotopic abundance of an atomic species can be determined somewhat independently of the atmospheric modelling uncertainties [4, 29]. Observations of different ZrO bands in a star do produce slightly different surface abundance values, but the uncertainty this introduces into our analysis of stellar nucleosynthesis is small compared to other uncertainties we have already discussed. In addition, the heavy element... 

The J = 2-l, v-0 emissions of Z8SiO,Z9SiO, and 30Si0 from three late-type stars were simultaneously observed with the Nobeyama 45-m telescope in Jaunuary 1987. The relative intensities of [z9SiO ] / C3 0 S i 0 ] were measured to be 2.4 for X Cyg, 1.5 for NML Tau, and 2.9 for Villi Oph. These values are lower limits for the relative isotope abundance of (z,Si3 / (30Si), and are larger than... 

We should not leave our discussion of nuclear reactors without mentioning the Oklo phenomenon. In 1972, French scientists analyzing uranium ore from the Oklo uranium mine in Gabon found ore that was depleted in 235U. Further investigation showed the presence of high abundances of certain Nd isotopes, which are formed as fission products. The relative isotopic abundances of these isotopes were very different from natural abundance patterns. The conclusion was that a natural uranium chain reaction had occurred 1.8 billion years ago. 

Grange, A. H., and Sovocool, G. W. (2008). Automated determination of precursor ion, product ion, and neutral loss compositions and deconvolution of composite mass spectra using ion correlation based on exact masses and relative isotopic abundances. Rapid Commun. Mass Spectrom. 22 2375-2390. 

Even when highly enriched compounds are used in the synthesis of a labeled molecule, the labeling reaction never will be 100% complete. This results in the presence of a number of unlabeled and partially labeled molecules in the IS, which will give a response at the same m/z value as the unlabeled analyte. An exact knowledge of the incorporation efficiency is required as it influences both the detection limit and precision of an assay (Dehennin et al., 1980). Furthermore, the relative isotopic abundances of labeled and unlabeled molecule should be known to allow accurate calibration (cf. Section 3). 

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. 

This mathematical equation corresponds to the equation of a curve. The straight line represents only a special case. The nature of the calibration curve that is obtained depends on the normalized relative isotopic abundances and thus on the nature of the isotopes that are introduced, on the increase in the molecular weight and on how enriched the labelled compound is. 

Several methods that do not require chemical separation are available for measuring uranium in urine (in units of total mass or total activity). These methods include spectrophotometric (total mass), fluorometric (total mass), kinetic phosphorescence analysis (KPA) (total mass), and gross alpha (total activity) analyses (Wessman 1984). The most widely used methods for routine uranium analysis are a-spectrometry and liquid scintillation spectrometry. These methods utilize the natural radioactivity of uranium and are sensitive and require little sample preparation. Photometric techniques such as fluorometry and phosphorometry are less widely used, but kinetic phosphorescence analysis is becoming more widely used. Measurements of total uranium do not provide the relative isotopic abundance of the uranium isotopes, but this may only be important when converting between activity and mass when the isotopic ratios are uncertain. 

Early work by Thode et al. (1949) established that there are large variations in the isotopic composition of sulfur compounds in nature. Since then sulfur isotope abundance data have been frequently used to elucidate many terrestrial processes including the genesis of sulfide ore bodies. The mechanisms of isotopic fractionation (alteration of relative isotopic abundances) can be broadly categorised under exchange processes or the kinetic isotope effects discussed on pp. 324ff. Isotopic exchange may be represented by the reaction ... 

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 fact drat radioisotopes decay to produce ionizing radiation does not influence their biological role. Radioisotopes are taken up and assimilated in the same way as are stable isotopes. Enzymes may be sufficiently sensitive to discriminate slightly between isotopes of the same element, leading to changes in relative isotopic abundance, but the effect is small (Marechal et al., 1999 Anbar et al., 2000 Marechal and Albarede, 2002 Zhu et al., 2002 Beard et al., 2003 Weiss et al.. 

The nature of the nucleus is the most important consideration in whether a useful level of nuclear magnetic resonance occurs. The most sensitive nuclei are those that behave like strong magnets, have a high relative isotope abundance, and have nuclear charge distribution approaching spherical. In Table V, the nuclear resonance susceptibility of some common atoms is summarized. 

In all these applications, sensitive instruments for the determination of isotope abundances are required. In this connection, the conventional ionic-type mass spectrometer is used almost exclusively for the determination of absolute and relative isotope abundances with the exception of the wide use of infrared methods for the determination of hydrogen to deuterium ratios. 

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. 

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