Carbon, isotopic abundance

Organic substances such as collagen must be converted to CO2 for high precision determination of stable carbon isotopic abundances. Solid organic samples are converted to CO2 by combustion in the presence of an oxidizing agent (33. 39). Quartz tubes (9 mm) were cut to 30 cm lengths, sealed at one end, and loaded with 2 g of CuO and a 9 mm piece... 

At about C90 the X+1 peak reaches the same intensity as the X peak, and at higher carbon number w, it becomes the base peak of the pattern, because the probability that an ion contains at least one C becomes larger than that for the monoisotopic ion. A further increase in w makes the X+2 signal stronger than the X and X+1 peak and so on. A table with representative carbon isotopic abundances is provided in the Appendix. 

Carbon has two stable isotopes and C) of -98.89% and -1.11% abundance, respectively. Carbon compounds are exchanged between the oceans, atmosphere, biosphere and lithosphere. Significant isotope ratio variations exist within these groups due to both kinetic and equilibrium isotope effects. Representative carbon isotope abundance variations are shown in Figure 9. 

Figure 9 Natural variations in carbon isotope abundances. North American diet to a large extent is derived from maize, a C4 plant. In countries like Japan, where fish is a major part of the average diet, values are in between the European and North American values.

Standards The standard for carbon isotope abundance measurements is based on a Cretaceous belem-nite sample from the Peedee formation in South Carolina, USA. The original material is no longer available. It has been replaced by the convention that NBS 19, a carbonate material, has a value of -i-1.95 l versus PDB. This new scale is termed V-PDB (Vienna-PDB). The IAEA distributes a number of secondary standards including graphite (USGS24) with a d C value of - 15.994 V-PDB, oil (NBS-22) at -29.741 V-PDB, and calcium carbonate (NBS-18) with a value of - 5.014 V-PDB. 

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... 

One method for measuring the temperature of the sea is to measure this ratio. Of course, if you were to do it now, you would take a thermometer and not a mass spectrometer. But how do you determine the temperature of the sea as it was 10,000 years ago The answer lies with tiny sea creatures called diatoms. These have shells made from calcium carbonate, itself derived from carbon dioxide in sea water. As the diatoms die, they fall to the sea floor and build a sediment of calcium carbonate. If a sample is taken from a layer of sediment 10,000 years old, the carbon dioxide can be released by addition of acid. If this carbon dioxide is put into a suitable mass spectrometer, the ratio of carbon isotopes can be measured accurately. From this value and the graph of solubilities of isotopic forms of carbon dioxide with temperature (Figure 46.5), a temperature can be extrapolated. This is the temperature of the sea during the time the diatoms were alive. To conduct such experiments in a significant manner, it is essential that the isotope abundance ratios be measured very accurately. 

For example, if a carbonaceous sample (S) is examined mass spectrometrically, the ratio of abundances for the carbon isotopes C, in the sample is Rg. This ratio by itself is of little significance and needs to be related to a reference standard of some sort. The same isotope ratio measured for a reference sample is then R. The reference ratio also serves to check the performance of the mass spectrometer. If two ratios are measured, it is natural to assess them against each other as, for example, the sample versus the reference material. This assessment is defined by another ratio, a (the fractionation factor Figure 48.2). 

Boron [7440-42-8] B, is unique in that it is the only nonmetal in Group 13 (IIIA) of the Periodic Table. Boron, at wt 10.81, at no. 5, has more similarity to carbon and siUcon than to the other elements in Group 13. There are two stable boron isotopes, B and B, which are naturally present at 19.10—20.31% and 79.69—80.90%, respectively. The range of the isotopic abundancies reflects a variabiUty in naturally occurring deposits such as high B ore from Turkey and low °B ore from California. Other boron isotopes, B, B, and B, have half-Hves of less than a second. The B isotope has a very high cross-section for absorption of thermal neutrons, 3.835 x 10 (3835 bams). This neutron absorption produces alpha particles. 

A further point about mass spectrometry, noticeable in the spectrum of propane (Figure 12.2), is that the peak for the molecular ion is not at the highest m/z value. There is also a small peak at M + l because of the presence of different isotopes in the molecules. Although 12C is the most abundant carbon isotope, a small amount (1.10% natural abundance) of 13C is also present. Thus, a certain... 

In some ways, it s surprising that carbon NMR is even possible. After all, 12Q the most abundant carbon isotope, has no nuclear spin and can t be seen b> NMR. Carbon-13 is the only naturally occurring carbon isotope with a nucleai spin, but its natural abundance is only 1.1%. Thus, only about 1 of ever) 100 carbons in an organic sample is observable by NMR. The problem of low abundance has been overcome, however, by the use of signal averaging anc Fourier-transfonn NMR (FT-NMR). Signal averaging increases instrument sensitivity, and FT-NMR increases instrument speed. 

The most abundant isotope is which constitutes almost 99% of the carbon in nature. About 1% of the carbon atoms are There are, however, small but significant differences in the relative abundance of the carbon isotopes in different carbon reservoirs. The differences in isotopic composition have proven to be an important tool when estimating exchange rates between the reservoirs. Isotopic variations are caused by fractionation processes (discussed below) and, for C, radioactive decay. Formation of takes place only in the upper atmosphere where neutrons generated by cosmic radiation react with nitrogen ... 

Keeling, C.D. 1961 The concentration and isotopic abundances of atmospheric carbon dioxide in rural and marine air. Geochimica et Cosmochimica Acta 24 277-298. 

Figure 4.11. Average collagen ( c ) and carbonate hydroxylapatite ( a ) 8 C values in modem and Pleistocene herbivores, carnivores and bears. Isotopic abundances for modem samples are from this paper, those for Kent s Cavern samples are from Bocherens et al. (1995b) and those for other Pleistocene localities are from Bocherens et al. (1994).

From now onward, we shall assume natural abundance for carbon isotopes, so that the signal we have to deal with arises from a single carbon-13 in the molecule under investigation. Moreover, most of the experiments described below imply, at one stage or another, proton... 

Jim, S., Ambrose, S. H. and Evershed, R. P. (2003b) Natural abundance stable carbon isotope evidence for the routing and de novo synthesis of bone FA and cholesterol. Lipids 38, 179 186. 

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