Percentages and carbon isotopic

Table II. Percentages and Carbon Isotope Composition of Reaction...

Table III. Percentages and Carbon Isotope Composition of Reaction Components During Ethane Oxidation by CuO. The initial 613C value of ethane is -27.9°/oo...

Because of the differing rates of the component reactions, the CNO cycle modifies the initial composition of the material, which typically has O > C > N, making more 14N (and more 13C relative to 12C) at the expense of carbon and (at the higher temperatures) oxygen. Thus in H-buming zones the CN mixture is rapidly modified, and the CNO mixture somewhat more slowly modified, to reach steady-state ratios,4 e.g. at T = 3 x 107 K the steady-state percentages of CNO isotopes, compared to Solar-System values, are as follows ... 

Nearly all lower-mantle diamonds have such low nitrogen that they are classified type II (McCammon, 2001). These features are very distinctive from upper-mantle diamonds, which are chiefly type la and have widely variable percentages of A, A/B, and B centers. The carbon isotopic composition of ultradeep, lower-mantle diamonds is surprisingly homogeneous, most samples having typical upper-mantle values of... 

Nearly all of the carbon on Earth is present as the stable isotope carbon-12. A very small percentage of the carbon in Earth s crust is carbon-14. Carbon-14 undergoes decay to form nitrogen-14. Because carbon-12 and carbon-14 have the same electron configuration, they react chemically in the same way. Both of these carbon isotopes are in carbon dioxide, which is used by plants in photosynthesis. 

All atoms of an element are identical in atomic number but not in mass number. Isotopes of an element are atoms that have different numbers of neutrons and therefore different mass numbers. For example, all carbon atoms (Z = 6) have 6 protons and 6 electrons, but only 98.89% of naturally occurring carbon atoms have 6 neutrons in the nucleus (A = 12). A small percentage (1.11%) have 7 neutrons in the nucleus (A = 13), and even fewer (less than 0.01%) have 8 (A = 14). These are carbon s three naturally occurring isotopes— C, C, and C. Five other carbon isotopes— C, °C, C, C, and —have been created in the laboratory. 

If one has available a cyclotron then there is the possibility of using the radioactive isotope N, providing one can work quickly since its half-life is only 10 min. Although its use is limited it has proved possible to feed N for very short exposure times and to plot the data in the form of percentage of incorporated isotope in different compounds. This method of plotting was used by Calvin (1962) in elucidating the path of carbon in photosynthesis and... 

A further point about mass spectrometry, noticeable in the spectra of both propane (Figure 10.2) and 2,2-dimethylpropane (Figure 10.3), is that the peak for the molecular ion is not at the highest m/z value. There is also a small peak at M+1 because of the presence of different isotopes in the molecules. Although is the most abundant carbon isotope, a small amount (1.10% natural abundance) of is also present. Thus, a certain percentage of the molecules analyzed in the mass spectrometer are likely to contain a atom,... 

Figure 17 Carbon-isotope effects of PAH photolysis. Top line diagram shows negligible shifts in for F, Pa, FI, and P. Only Ae shows a shift in (0.5%c) beyond differences in replicate experiments a = error bar or size of data point). Lower histogram shows percentage of PAHs that were not photodegraded.

As a result of this exchange between C and C isotopes of carbon, the HCN vapor becomes preferentially enriched in the C isotope. Natural carbon contains 1.11% C (Benedict et al., 1981). However, the reaction above will lead to a higher percentage of C in the HCN vapor. If such a behavior could be multiplied in a cascade of many stages... 

For any one element, the abundances (relative amounts) of isotopes can be described in percentage terms. Thus, fluorine is monoisotopic viz., it contains only nuclei of atomic mass 19, and phosphorus has 100% abundance of atoms with atomic mass 31. For carbon, the first two isotopes occur in the proportions of 98.882 to 1.108. 

The presence of one carbon atom in a molecule of carbon dioxide results in registration of the molecular ion peak of m/z 44 and of the A + 1 isotopic peak of m/z 45. The intensity of the latter is 1.1 % of that of M+. It appears due to the presence of 13C02 molecules. An increase in the number of carbon atoms in a molecule leads to an increase of the intensity of the M + 1 ion peak to 1.1 n% of M+, where n is the number of carbon atoms in the molecule. To calculate the number of carbon atoms in a molecule using a mass spectrum one should divide the intensity of the M + 1 peak as a percentage of M by 1.1. The result defines the maximum possible number of carbon atoms. One should remember that calculations may be more complicated if an [M — H]+ ion peak is present. 

To understand the evaluation of a CLE, we need to introduce some terms The word isotopomer is a combination of the terms isotope, and isomer. An isotopomer is one of the different labeling states in which a particular metabolite can be encountered [248] that is, a molecule with n carbon atoms has 2" isotopomers. These are usually either depicted using outlined and filled circles for unlabeled and labeled atoms, respectively (see Fig. 14), or are described in text format for example, C 010 would be the isotopomer of a three-carbon molecule labeled at the second position. An isotopomer fraction is the percentage of molecules in this specific labeling state. The positional enrichment is the sum of all isotopomer fractions in which a specific carbon atom in a specific metabolite is labeled [248]. Consequently, the usage of isotopomers enables to account for more information While a molecule with n carbon atoms will yield n positional enrichments, there are 2 — 1 isotopomer fractions (the 2"th measurement is redundant as, by definition, isotopomer fractions must sum up to unity) [260],... 

In a sample of the element carbon, 98.89% of the atoms have six protons and six neutrons in the nucleus. However, 1.10% have six protons and seven neutrons in the nucleus, and an even smaller percentage have eight neutrons in the nucleus. These atoms are isotopes of carbon. 

Naturally occurring carbon consists of two isotopes, 12C and 13C. What are the percentage abundances of the two isotopes in a sample of carbon whose atomic mass is 12.01112 ... 

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