Isotopes of carbon

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Carbon has two stable, naturally occurring isotopes, carbon-12 and carbon-13. There is also a naturally-occuring radioactive isotope, carbon-14 (t½ = 5700 years), that is formed by the action of cosmic rays on the Earth's atmosphere. Carbon-11 is an artificial isotope used in positron emission tomography (PET), a medical imaging technique.

Atomic weight

Ar(6C) = 12.0107(8)
Isotope Mass/u Amount
fraction 		Normal
range
12C 12 (by definition)[note 1] 0.9893(8) 0.9885–0.9902
13C 13.003 354 837 78(98) 0.0107(8) 0.0115–0.0098
References:[1][2][3]

The standard atomic weight of carbon has been fixed at its current value since 1995.[1] The range of natural isotopic compositions makes a more precise standard value unobtainable, and some terrestrial geological specimens may show compositions outside of the normal range.[1] Natural amounts of carbon-14 (x < 10−12) are insignificant in the calculation of atomic weight.[1]

Carbon-13 is systematically depleted in samples of biological origin (compared to atmospheric carbon dioxide and most carbonate rocks) due to a kinetic isotope effect that is particularly pronounced in photosynthetic reactions.[note 2] The effect is strongest in C4-plants, which are predominant in tropical and subtropical climates, and measurements of carbon isotopic compositions are important in geochemistry, paleoclimatology and paleoceanography. Such measurements have also attracted interest for detecting cases of "doping" in sport:[4][5] steroid hormones produced by biotechnological methods would be expected to have higher amount fractions of 13C than those produced by an athlete's body.[note 3]

The primary standard for the isotopic composition of carbon is Pee Dee Belemnite (PDB), a fossile marine carbonate from the Pee Dee Formation, South Caroline, USA: as with most carbonate rocks, this has a relatively high amount franction of 13C, and so most δ13C values are negative. Supplies of standard PDB have long been been exhausted, so modern measurements of isotopic composition are referred to a secondary standard, NBS 19, a crushed marble of unknown geographical origin that is also referred to as "TS-limestone".[6] NBS 19 forms the basis of the V-PDB (for "Vienna PDB") scale of δ13C, which is designed to give equivalent δ13C values to the older PDB scale: NBS 19 has a defined value of δ13C = +1.95 on the V-PDB scale. A two-point scale based on NBS 19 and LSVEC has also been proposed.[7]

Radiologically significant isotopes

Isotope Half life Decay mode Energy
MeV 		Weighted mean
energies/MeV 		Daughter
nuclide 		Ref.
11C 1223.1(12) s
(20.39(2) min) 		β+ (99.759(15)%) 0.3856(4) γ, X: 1.02
β, Auger: 0.385 		115B (stable) [8][9]
γ± (2×99.759%) 0.5110
X (ec) (0.0002%) 1.0 × 10−4
K-Auger (0.241(15)%) 1.7 × 10−4
14C 5.70(3) × 103 a β− (100%) 0.04947 β: 0.04947 147N (stable) [10][11]

All isotopes

Symbol Z(p) N(n) Mass/u Excess energy
MeV 		Binding energy/A
MeV 		β−-decay energy
MeV 		Spin Half life Decay mode,
proportion
Excitation energy/MeV
8C 6 2 8.037 675(25) 35.094(23) 3.0978(29) — 0 2.0(4) zs 2p (100%)
9C 6 3 9.031 0367(23) 28.9105(21) 4.337 48(24) — (−3⁄2) 126.5(9) ms β+ (100%)
10C 6 4 10.016 853 23(43) 15.698 68(40) 6.032 041(40) −23.10(40) 0 19.290(12) s β+ (100%)
11C 6 5 11.011 433 61(102) 10.650 34(95) 6.676 370(86) −13.653(46) −3⁄2 20.39(2) min β+ (100%)
12C 6 6 12 (by definition)[note 1] 0 (by definition)[note 1] 7.680 144 −17.338 08(100) 0 STABLE
13C 6 7 13.003 354 837 78(98) 3.125 011 29(91) 7.469 849 −2.220 47(27) −1⁄2 STABLE
14C 6 8 14.003 241 9887(41) 3.019 8931(38) 7.520 319 0.156 476(4) 0 5.70(3) × 103 a β− (100%)
15C 6 9 15.010 5992(86) 9.873 14(80) 7.100 169(53) 9.771 71(80) +1⁄2 2.449(5) s β− (100%)
16C 6 10 16.014 7013(38) 13.6941(36) 6.922 05(22) 8.0105(44) 0 747(8) ms β− (100%)
17C 6 11 17.022 5861(187) 21.0388(174) 6.557 62(102) 13.167(23) (+3⁄2) 193(5) ms β− (100%)
18C 6 12 18.026 759(32) 24.926(30) 6.425 75(167) 11.812(35) 0 92(2) ms β− (100%)
19C 6 13 19.034 805(106) 32.421(98) 6.1179(52) 16.559(99) (+1⁄2) 46.2(23) ms β− (100%)
20C 6 14 20.040 32(26) 37.56(24) 5.9587(120) 15.79(25) 0 16(3) ms β− (100%)
21C 6 15 21.049 34(54)# 45.96(50)# 5.659(24)# 20.71(51)# +1⁄2# <30 ns ?n
22C 6 16 22.057 20(97)# 53.28(90)# 5.436(41)# 21.24(92)# 0 6.2(13) ms β− (100%)
Values marked # are estimated from systematic trends rather than experimentally measured.
Spins quoted in parentheses are uncertain in value and/or parity.
Sources: Except as otherwise noted,
isotopic masses and associated energies are taken from the AME 2003 dataset;[2]
nuclear spins and decay properties are taken from NUBASE 2003.[12]

Notes and references

Notes

  1. ↑ 1.0 1.1 1.2 The relative atomic mass of carbon-12 is exactly twelve atomic mass units, without any measurement uncertainty, by the definition of the atomic mass unit.
  2. ↑ Marine organisms that obtain carbonate from sea water to construct exoskeletons do not show the depletion of 13C in the exoskeletons: indeed, when such exoskeletons become carbonate rocks, they often show anomalously high amount fractions of 13C, an indication that the carbon did not pass through a photosynthetic pathway.
  3. ↑ It was shown by Aguilera, Hatton & Catlin (2002) that their 2002 samples of artificial epitestosterone had significantly (P < 0.0001) lower δ13C values than natural epitestosterone from human urine.

References

  1. ↑ 1.0 1.1 1.2 1.3 Atomic weights of the elements. Review 2000. Pure Appl. Chem., 75 (6), 683–800. DOI: 10.1351/pac200375060683.
  2. ↑ 2.0 2.1 Wapstra, A. H.; Audi, G.; Thibault, C. The AME2003 atomic mass evaluation (I). Evaluation of input data, adjustment procedures. Nucl. Phys. A 2003, 729, 129–336. DOI: 10.1016/j.nuclphysa.2003.11.002. Wapstra, A. H.; Audi, G.; Thibault, C. The AME2003 atomic mass evaluation (II). Tables, graphs, and references. Nucl. Phys. A 2003, 729, 337–676. DOI: 10.1016/j.nuclphysa.2003.11.003. Data tables.
  3. ↑ Isotopic Compositions of the Elements 1997. Pure Appl. Chem., 70 (1), 217–35. DOI: 10.1351/pac199870010217.
  4. ↑ Aguilera, Rodrigo; Hatton, Caroline K.; Catlin, Don H. Detection of Epitestosterone Doping by Isotope Ratio Mass Spectrometry. Clin. Chem. 2002, 48, 629–36. PMID 11901061.
  5. ↑ Aguilera, Rodrigo; Chapman, Thomas E.; Pereira, Henrique; Oliveira, Giselle C.; Illanes, Renata P.; Fernandes, Telma F.; Azevedo, Débora A.; Aquino Neto, Francisco Drug testing data from the 2007 Pan American Games: δ13C values of urinary androsterone, etiocholanolone and androstanediols determined by GC/C/IRMS. J. Steroid Biochem. Mol. Biol. 2009, 115 (3–5), 107–14. DOI: 10.1016/j.jsbmb.2009.03.012.
  6. ↑ NBS 19, TS-Limestone; International Atomic Energy Agency, <http://curem.iaea.org/catalogue/SI/SI_002190000.html>. (accessed 14 March 2010).
  7. ↑ Coplen, Tyler B.; Brand, Willi A.; Gehre, Matthias; Gröning, Mannfred; Meijer, Harro A. J.; Toman, Blaza; Verkouteren, R. Michael New Guidelines for δ13C Measurements. Anal. Chem. 2006, 78 (7), 2439–41. DOI: 10.1021/ac052027c.
  8. ↑ ENSDF Decay Data in the MIRD (Medical Internal Radiation Dose) Format for 11C; National Nuclear Data Center, <http://www.nndc.bnl.gov/useroutput/11c_mird.html>. (accessed 13 March 2010).
  9. ↑ Ajzenberg-Selove, F. Energy levels of light nuclei A = 11–12. Nucl. Phys. A 1990, 506 (1), 1–158. DOI: 10.1016/0375-9474(90)90271-M.
  10. ↑ ENSDF Decay Data in the MIRD (Medical Internal Radiation Dose) Format for 14C; National Nuclear Data Center, <http://www.nndc.bnl.gov/useroutput/14c_mird.html>. (accessed 13 March 2010).
  11. ↑ Ajzenberg-Selove, F. Energy levels of light nuclei A = 13–15. Nucl. Phys. A 1991, 523 (1), 1–196. DOI: 10.1016/0375-9474(91)90446-D.
  12. ↑ Audi, G.; Bersillon, O.; Blachot, J.; Wapstra, A. H. The NUBASE evaluation of nuclear and decay properties. Nucl. Phys. A 2003, 729, 3–128. doi:10.1016/j.nuclphysa.2003.11.001, <http://amdc.in2p3.fr/nubase/Nubase2003.pdf>.
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