Carbon isotopes, definition

In 1808 John Dalton proposed his atomic theory, which included the statement that when atoms of two or more elements combine to form a compound, they combine in a definite ratio by number of atoms and by mass. This is called the law of definite proportions. This provided a means to determine the mass of one atom relative to another. It was necessary to assign a mass to one element to find the mass of another element in a compound. Today we use the most common carbon isotope, assigned a mass of 12.00 atomic mass units (amu), as the basis for comparative weights of the atoms. 

Natural Abundance Stable Carbon Isotopes (I3C). There are trends in 13C of plant, litter, and organic constituents in soil that can be used to investigate carbon cycling, but to date the trends have proven too subtle, the variation too high, or the mechanisms too poorly understood to exploit these patterns definitively. The 813C of plant litter and particulate SOM is typically close to that of the plant source,... 

The masses for the elements listed in the table inside the back cover of this text are relative masses in terms of atomic mass units (amu) or daltons. The atomic mass unit is based on a relative scale in which the reference is the C carbon isotope, which is assigned a mass of exactly 12 amu. Thus, the amu is by definition 1/12 of the mass of one neutral c atom. The molar mass of is then... 

We report here measurements of the stable hydrogen Isotopic composition of methane and water, the stable carbon Isotopic composition of methane and carbon dioxide, and ancillary parameters from several freshwater environments and from a few locations within the Tampa Bay estuary. The stable Isotopic compositions determined In this study are reported as a parts-per-mll ( /oo) deviation (5) from a standard with a known stable isotopic ratio. The definition of the 6 value Is ... 

Bailey, N. J. L., Burwood, R. Harriman, G. E. 1990. Application of pyrolysate carbon isotope and biomarker technology to organofacies definition and oil correlation problems in North Sea Basins. Organic Geochemistry, 16, 1157-1172. 

The above conclusion must be remembered when considering carbon isotope effects for elimination reactions. Certainly the alpha- and beta-carbon isotope effects for elimination from propyl trimethylammonium ion indicate a different extent of rehybridisation at the two carbon atoms in the transition state, but a more definite conclusion requires additional kinetic evidence. 

Fig. 6. Carbon isotope compositions of total oils, Pr and Ph versus thermal maturity (Evolution Index) for oils from northeast Japan (Ishiwatari et aL, 2001). Data for total oils are quoted from Waseda (1993). Data for biodegraded oils are omitted. Definition of Evolution Index is a ratio of n-heptane to 1,3-dimethyl-cyclopentane (Hiratsuka, 1976). A thermal maturity index of L/(H L)-sterane (definition is given in the text) is quoted from Sakata et al. (1990) for comparison. Open circles total oils closed circles Pr and Ph (average values).

The mass of an atom depends on the number of electrons, protons, and neutrons it contains and all atoms of a given isotope are identical in mass. The SI unit of mass (the kilogram) is too large to function as a convenient unit for the mass of an atom, thus a smaller unit is desirable. In 1961, the International Union of Pure and Applied Chemistry (lUPAC) defined the atomic mass unit (u) to be exactly equal to one-twelfth the mass of one carbon-12 atom. Carbon-12 ( C) is the carbon isotope that has six protons, six neutrons, and six electrons. Using this definition, we have that 1 u = 1.660539 X 10 kg. The atomic mass (sometimes called atomic weight) of an atom is then defined, relative to this standard, as the mass of the atom in atomic mass units (u). For example, the two naturally occurring isotopes of hefium, He and " He, have atomic masses of 3.01602931 u and 4.00260324 u, respectively. This means that a helium-4 (" He) atom is 4.00260324/12 = 0.33355027 times as massive as a carbon-12 atom. ... 

Quaternary salts, isolation of, 10 Quaternization, by alkyl halides, 2-7 by aryl halides, 7-9 on carbon, 53 definition of, 2 by dimethyl sulfate, 9 electronic effects in, 11 in JV-heterocycles, 16, 38 by heterocyclyl halides, 7—9 isotope effect on, 55 mechanism of, 53-56 by methyl euyl-sulfonates, 9, 10 on oxygen, 52 rates of, 55 reagents for, 2-10 by self-condensation, 8 solvent effect on, 10, 55 solvents for, 10 steric effects on, 12, 13 substituents, influence on, 11, 19, 23 on sulfur, 51 Quinaldine, 4-amino-, 4 Quinazolines, 2-alkyl-, salt formation of, 6... 

Atomic Weight. As of this writing (ca 1994) the definition of atomic weights is based on carbon-12 [7440-44-0], the most abundant isotope of carbon, which has an atomic weight defined as exactiy 12 (21). 

An important benefit of the isotopic approach is that below-ground biomass values are not required. The 13C natural abundance isotopic carbon budget approach is based on C3 plants having lower 13C value than C4 plants (Ehleringer 1991 Clay et al. 2006) and that the signatures can be tracked by placing C3 plant residue into a soil derived from C4 plants or vice versa. In these calculations, several definitions are needed. These definitions include... 

Let us first introduce some important definitions with the help of some simple mathematical concepts. Critical aspects of the evolution of a geological system, e.g., the mantle, the ocean, the Phanerozoic clastic sediments,..., can often be adequately described with a limited set of geochemical variables. These variables, which are typically concentrations, concentration ratios and isotope compositions, evolve in response to change in some parameters, such as the volume of continental crust or the release of carbon dioxide in the atmosphere. We assume that one such variable, which we label/ is a function of time and other geochemical parameters. The rate of change in / per unit time can be written... 

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],... 

A reliable authenticity assessment is concluded from the simultaneous consideration of multielement IRMS and enantioselective analysis. The differences of the stable isotope ratios of linalool and linalyl acetate are depicted as a three-dimensional plot of A values (d values of linalool minus d values of linalyl acetate for oxygen, hydrogen and carbon) (Fig. 17.15). This plot shows that the commercial samples S1-S5 are different from all the other samples investigated. Linalool and linalyl acetate of S1-S5 definitely are not genuine lavender oil compounds. 

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