Carbon isotope tracers discussion

Adsorbents, extraction procedures, organic pollutants, 113 Aerosol, problems in measuring, coal combustion, 312-317 Aerosol carbon data, results and discussion, carbon isotope tracers, 270-275... 

This section presents a discussion of the fundamentals of three major solute isotope systems carbon, nitrogen, and sulfur. Recent comprehensive reviews of the geochemistry of several other solute isotope tracers in hydro-logic systems include those by Faure (1986),... 

New information on this problem has come from enzyme studies and isotope tracer studies. The work discussed in the preceding section disclosed that a great many different products were synthesized from carbon dioxide during photosynthesis. The nearest approach to a first product of photosynthesis is n-glyceric acid 3-phosphate. From this compound, through a whole series of enzyme-catalyzed reactions, the plant carries out the synthesis of the carbohydrates. 

Isotope tracer studies have proved that long-chain fatty acids may be built up in mammalian tissues entirely from 2-carbon units which are presumably identical with Co A—S—COCH3. As is the case with protein synthesis, it has been found very difficult to study fatty acid synthesis in the mammal in cell-free preparations. However, as discussed above, Stadt-man and Barker have succeeded in carrying out both fatty acid oxidation and fatty acid synthesis in cell-free soluble enzyme preparations of CL kluyveri. The data obtained in these investigations fit in well with the indirect evidence from tracer experiments concerning fatty acid synthesis in the mammal. Kennedy and Barker have suggested a scheme for the oxidation and synthesis of butyrate in CL kluyveri, based on the assumption that free intermediates do not occur as such in this process, but rather as conjugates with an unidentified coenzyme, probably coenzyme A. 

One of the most sensitive tracers recording the composition of ancient sea water is the isotopic composition of chemical sediments precipitated from sea water. The following discussion concentrates on the stable isotope composition of oxygen, carbon, and sulfur, but in recent years other isotope systems have been included such as Ca (De La Rocha and De Paolo 2000 Schmitt et al. 2003 Fantle and de Paolo 2005 Farkas et al. 2007) and B (Lemarchand et al. 2000, 2002 Joachimski et al. 2005) and Li (Hoefs and Sywall 1997). One of the fundamental questions in all these approaches is which kind of sample provides the necessary information, in the sense that it represents the ocean water composition at its time of formation and has not been modified subsequently by diagenetic reactions. 

Elemental carbon has many important applications. The diamond is a precious gem, known to mankind for ages graphite is used as an electrode and has numerous other applications carbon-14 isotope is used in carbon dating and the isotope carbon-13 in tracer studies and NMR. Carbon black is used in paints, pigments and inks. Activated carbon is used as an adsorbent for purification of water and separation of gases. Coke is used for electrothermal reduction of metal oxides to their metals. These applications are discussed below in more detail. 

Carbon-13. The increasing availability of C-labelled substrates has provided a marked impetus in the use of these tracers in clinical studies. Most recent interest centres round the use of C02 breath tests for the in vivo estimation of the rate of oxidation of specific C-labelled substrates. Some clinical implications of these tests will be discussed later. Mention should be made of the available methods for respiratory COj collection. They include the collection of whole breath in evacuated glass tubes prior to CO2 separation [92], the direct cryogenic trapping of exhaled CO2 in liquid nitrogen [93] and the precipitation of CO2 as carbonate in sodium hydroxide [94]. The latter method is suspect in that it results in isotopic fractionation which may not be reproducible. 

---