Carbon Isotopes

Carbon is element number 6 in the periodic table and has valence states of 2, 3, or 4 (1). Thirteen isotopes of carbon have been identified (2), ranging in atomic mass number from ⁸C (half-life = 3 × 10–²² s) to 20C (half-life = 0.01 s) (see Radioactivity and Radioisotopes). Two stable isotopes of carbon are found in nature: ¹²C at 98.9%, and ¹³C at 1.1% abundance.

 The most important radioactive isotope of carbon is ¹⁴C (half-life = 5730 years). Carbon-14 decays by beta-minus emission to nitrogen-14, which is stable. Carbon-14 decay yields one beta particle, with an energy of 0.156 MeV maximum, 0.0495 MeV on average. Carbon-14 is produced naturally in the earth's atmosphere by cosmic ray interactions with stable nitrogen-14 according to the reaction ¹⁴N(n,p)¹⁴C. Atmospheric ¹⁴CO₂ is inhaled by animals and respired by plants in photosynthesis, incorporating small amounts of carbon-14 in all living organisms. Consequently, carbon-14 dating is useful for determining the age of organic matter such as wood and archeological specimens. Hundreds of different organic molecules have been labeled with carbon-14 as a tracer for studying biochemical processes and are available from commercial suppliers.

Another radioactive isotope, carbon-11 (half-life = 20.38 min), is used in nuclear medicine diagnostics with positron-emission tomography. Carbon-11 decays by beta-plus (positron) emission to boron-11, which is stable. Carbon-11 decay yields 0.98 beta particles, with an energy of 0.960MeV maximum, 0.386 MeV on average. Positron emission is characterized by twin 0.511-MeV photons that result from the annihilation of a positron and an electron and allow it to be detected externally. Carbon-11 is produced by proton accelerators according to the reaction ¹¹B(p,n(11C. Carbon-11 positrons are useful for ¹¹C-acetate (3) and ¹¹C-palmitate (3) metabolism kinetic imaging in the assessment of heart disease (4.(

 Carbon-13 is very useful in studies involving nuclear magnetic resonance (NMR), because it gives an NMR signal, whereas the more abundant isotope, carbon-12, does not.

Carbon-14 was first produced artificially for metabolism studies in 1939. At present, it is produced commercially by neutron irradiation of beryllium nitride or aluminum nitride according to the reaction ¹⁴N(n,p)¹⁴C. The low cross section of this reaction and the long half-life of carbon-14 results in low specific activities, which can make carbon-14 labeling of organic materials quite difficult.

 Carbon-14 labeling of organic molecules has numerous applications in the biomedical sciences. Examples include studies of glucose metabolism and conversion to ¹⁴CO₂, galactose oxidation, carbohydrate metabolism, the Krebs cycle, nucleic acid synthesis and hybridization, autoradiography, or fluorography, and for studying many other biochemical pathways. Among the most important carbon-14 compounds used in biochemistry and metabolism studies are the carbon dioxides and monoxide, carboxyl-labeled acids, methanols, cyanides, carbides, formic acids, and cyanamides (5). Carbon-14 is detected by beta-particle liquid scintillation counting, autoradiography, and fluorography.

References

1. D. R. Lide and H. Pr. Frederikse, eds. (1995) CRC Handbook of Chemistry and Physics, CRC Press, Boca Raton, Fla. 

2. Knolls Atomic Power Laboratory (1966) Chart of the nuclides, 15th ed., available from General Electric Company, San Jose, Calif. 

3. H. R. Schön, H. R. Schelbert, A. Najafi et al. (1982) Am. Heart J. 103, 532–547. 

4. T. L. Rosamond, D. R. Abendschein, B. E. Sobel et al. (1987) J. Nucl. Med. 28, 1322–1329. 

5. The Radiochemical Manual, 2nd ed. (1966) Amersham, Bucks, England.