Sulfur, relative isotopic abundance

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 ... 

Look for characteristic isotopic abundances that show the presence of bromine, chlorine, sulfur, silicon, and so on. If the deduced molecular ion is of sufficient intensity, the probable molecular formula may be determined using the observed isotopic abundances of the molecular ion region. Set the deduced molecular ion, M, at 100% abundance, and then calculate the relative abundances of M + 1 and M + 2 either manually or using the data system. 

Again because of its high volatility, sulfur is relatively undepleted from interstellar gas by condensation in interstellar grains, so that its isotopic composition has been pardy measured by radioastronomers, who detect emission lines from the molecule CS in the interstellar medium. By comparing the rotational transitions of 13C/32S with those of 12C/34S the interstellar isotopic ratio has been found for sulfur (see 34S, Anomalous isotopic abundance). 

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 relative abundances of isotopic peaks are important for derivation of atomic compositions. It is especially convenient if the studied molecules contain atoms with unique isotopic distributions, such as when irregular changes of isotopic abundance are present. For example, as listed in Table 9.1, the abundances of sulfur isotopes follow the order In the case of potassium it is in the case of chlorine it is... 

Figure 11 Natural variations in sulfur isotope abundances. Large sulfur isotope fractionations are possible during the reduction of sulfates to sulfides by anaerobic bacteria that produce enriched H2S relative to the original sulfate.

Compounds that contain chlorine, bromine, sulfur, or silicon are usually apparent from prominent peaks at masses 2, 4, 6, and so on, units larger than the nominal mass of the parent or fragment ion. Eor example, when one chlorine atom is present, the P + 2 mass peak will be about one-third the intensity of the parent peak. When one bromine atom is present, the P + 2 mass peak will be about the same intensity as the parent peak. The abundance of heavy isotopes is treated in terms of the binomial expansion (a -I- h) , where a is the relative abundance of the light isotope, b is the relative abundance of the heavy isotope, and m is the number of atoms of the particular element present in the molecule. If two bromine atoms are present, the binomial expansion is... 

There are four stable isotopes of sulfur and S, which have relative abundances of 95.1, 0.74, 4.2, and 0.016%, respectively. The relative... 

What are the relative contributions of these two sources Two approaches have been taken. One is to establish the geology and hydrology of a basin in great detail. This has been carried out for the Amazon (Stallard and Edmond, 1981) with the result that evaporites contribute about twice as much sulfate as sulfide oxidation. The other approach is to apply sulfur isotope geochemistry. As mentioned earlier, there are two relatively abundant stable isotopes of S, and The mean 34/32 ratio is 0.0442. However, different source rocks have different ratios, which arise from slight differences in the reactivities of the isotopes. These deviations are expressed as a difference from a standard, in the case of sulfur the standard being a meteorite found at Canyon Diablo, Arizona. 

Table 14.1 Relative abundances of the stable isotopes of hydrogen, carbon, nitrogen, oxygen and sulfur (Hoefs 1996 Schoeller 1999)...

There are many sulfur components in meteorites which may occur in all possible valence states (—2 to +6). TroUite is the most abundant sulfur compound of iron meteorites and has a relatively constant S-isotope composition (recall... 

There are four stable isotopes of sulfur 32S, 33S, 34S, and 3<5S, which have relative abundances of 95.1, 0.74,4.2, and 0.016%, respectively. The relative abundance of the various isotopes varies slighdy, depending on the source of the sulfur the ratio of 32S to 34S is 21.61—22.60. Three radioactive isotopes of masses 31,35, and 37 having half-lives of 2.6 s, 87 d, and 5 min, respectively, have been generated artificially. 

Of course nowadays exact mass measurement could also distinguish these two molecules, as could a variety of other instrumental techniques. The analysis of isotopic clusters is most useful for detecting the presence of halogens, sulfur, and silicon, all of which have abundant isotopes of two atomic weight units higher, thus leading to relatively large M + 2 peaks. 

Review of Isotope Concepts. The average relative concentration of the two most abundant stable isotopes of sulfur, 32S and S, are 95.0% and 4.2%, respectively. Ratios of these two isotopes are measured with specialized isotope ratio mass spectrometers, normalized to standard Canyon Diablo meteoritic sulfur, and expressed as a delta value (634S) according to the relation ... 

The electron impact mass spectra of a series of selenazoles have been compared with those of their thiazole analogs. The results indicate that, in general, the mass spectrometric fragmentation patterns of the selenium heterocycles can be predicted by an examination of the corresponding sulfur analog. One of the characteristics of the mass spectra of selenium compounds is that they are rich in ions due to the presence of the six relatively abundant stable selenium isotopes (81JHC1335). 

The relative mass of one isotope of sulfur is 31.9721 u. Its abundance is 95.02%. Naturally occurring elemental sulfur has a relative atomic mass of 32.066. The mass number of the one other isotope of sulfur is... 

The relative abundances of isotopes in a molecule or in a fragment result from their statistical distribution. Let us consider an example in order to explain the calculation of isotopic clusters. What are the relative intensities of the isotopic peaks accompanying the molecular peak of CS2 Sulfur shows up as a mixture of three main isotopes with nominal masses 32, 33 and 34. These three isotopes occupy two possible positions in the CS2 molecule. The total number of possible combinations is 32, that is 9 ... 

We now have the possible combinations of two sulfur atoms, and the relative abundances that are calculated are those which would be observed in the case of a fragment with composition S2. We must now combine each of these with the carbon isotopes. As there is only one carbon atom in CS2, we have 98.90 % of isotope 12 and 1.10 % of isotope 13. We must now look for all the possible combinations of both of the following isotope series, with their probabilities, while observing that the total mass is the sum of the masses and the probability is the product of the probabilities. Finally, we sum the probabilities of all the ions with equal masses ... 

FIGURE 5.6 The variation with altitude of SO and SO2 abundance and delta values for the same conditions as in Figure 5.5. (a) Volume fraction (or mixing ratio) of 50, S02 and the initial versus altitude, (b) Delta-values relative to initial SO2 sulfur isotope ratios. 

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