Abundance Ratios of Isotopes

The apparatus used by J. J. Thomson in discovering two isotopes of neon, described in Section 4-1, was a simple form of mass spectrograph. Modern mass spectrographs have been used in attacking many physical and chemical problems, including that of determining nucleidic masses and the abundance ratios of isotopes. 

Information of this type is provided by the quantitative analysis of various constituents, determination of species-specific compounds, and by the determination of abundance ratios of isotopes. 

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. 

Before measurement it must be decided exactly which isotopes are to be compared. For oxygen, it is usually the ratio of 0 to 0, and for hydrogen it is H to H. Such isotope ratios are measured by the mass spectrometer. For example, examination of a sample of a carbonaceous compound provides abundances of ions at two m/z values, one related to C and one to C (it could be at m/z 45 and COj at m/z 44). By convention, the heavier isotope is always compared with the lighter isotope. The ratio of isotopes is given the symbol R (Figure 48.1). 

These isotope masses and their ratio of abundances are characteristic of carbon. Similarly, the isotopes of other elements that occur naturally have fixed ratios of isotopes, as given in Tables 47.1 and 47.2 at the end of the accompanying full text. 

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. 

Routine mass spectrometry can be used to identify many elements from their approximate ratios of isotope abundances. For example, mercury-containing compounds give ions having the seven isotopes in an approximate ratio of 0.2 10.1 17.0 23.1 13.2 29.7 6.8. 

Other important areas of mass spectrometric investigation of isotope ratios need accurate, not approximate values. For example, for some investigations in archaeology, pharmaceuticals, and chemistry, very accurate precise ratios of isotope abundances are needed. 

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). 

For many applications in geochemistry and archaeology, the information desired from mass spectrometry is a precise measure of the abundance ratio of two or more isotopes of the same element - 12C/13C, or 160/180, or 206Pb/207Pb and 208Pb/207Pb at the heavier end of the mass scale. In these... 

Note. Not all naturally occurring isotopes are stable - some are radioactive but with long half-lives. See (for example) WebElements [http //www.webelements.com/webelements/index. html] for abundance ratios of naturally occurring isotopes and half-lives of radioisotopes. 

Also, since ratios of isotopic ratios are commonly used to report isotope effect data (see Equations 7.17 and 7.18 for example), the actual numerical value of the abundance ratio of the standard is not important since it drops out from the final equation expressing the isotope effect ... 

Figure 5. Intensity ratios of isotopically substituted O4 ions as a function of ionizing electron energy. 1(64) corresponds to O4 ions containing four O atoms and, because the experiments were performed under natural abundance conditions, 1(65) and 1(66) correspond to ions where one O atom is replaced with an O and 0, respectively. The dotted lines indicated the expected ratios based on natural abundance.

Fig. 8. Variation of the abundance ratio of the m/z 340 and 338 ions (isotopic pattern of the [M — Azj ions from N3P3AZ5CI) relative to the composition of the mixture...

Ion fragments consistent with the formation of the bonding between the inorganic tin and acrylic acid are present. Most of these are of low intensity, but many give isotopic abundances for the two most abundant tin isotopes in near the expected relative abundance (the abundance ratio of 120 to 118 is 1.4). Where possible, the relative abundance of 120 to 118 are cited. For the... 

The mass spectra of chlorine- and bromine-containing compounds clearly show the abundance ratios of the stable isotopes 35C1 37C1 = 3 1 and 79Br 81Br = 1 1 in the molecular ions and those ionic fragments which contain halogens (see Section 9-11). 

HOC+, the metastable isomer to the well-known HCO+ molecule, has probably been detected in the galactic centre source Sgr B2 (Woods et al. 1983). This identification rests on only the J = 1 - 0 transition, which has been measured in the laboratory by Gudeman and Woods (1982). Since this line lies in the galactic centre near several rotational transitions of HCOOH, there remains some doubt as to the proper identification. An unambiguous interstellar identification of HOC + would therefore have to await the detection of higher rotational transitions, which have been measured in the laboratory by Blake et al. (1983), or isotopically substituted species. In addition, the HCO+/HCO+ abundance ratio of 330 obtained from the observation is at odds witt theoretical determinations (De Frees et al. 1984 Jarrold et al. 1986). 

The rich chemistry of carbon also makes it a constituent of a large number of interstellar molecules in addition to CO. Radioastronomical studies of these interstellar molecules reveal variations in the abundance ratio of the two stable C isotopes. Such studies also illuminate the field of interstellar chemistry. 

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