Carbon-13 Full table
General
Name, symbol	Carbon-13,13C
Neutrons	7
Protons	6
Nuclide data
Natural abundance	1.109%
Isotope mass	13.00335 u
Spin	−½

Carbon-13 (¹³C) is a natural, stable isotope of carbon and one of the environmental isotopes. It makes up about 1.1% of all natural carbon on Earth.^[1]

Contents 1 Detection by NMR spectroscopy 2 Detection by mass spectrometry 3 Uses in Earth Science 4 See also 5 Notes

Detection by NMR spectroscopy

Because of its nuclear spin properties, this isotope responds to a resonant radio frequency (RF) signal. The absorption and emission of the RF signal by the nuclei can be monitored and detected using nuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy. This is a technique that gives information on the identity and number of atoms adjacent to other atoms in said molecule, thereby giving clues to the structure of an organic molecule. Since ¹²C has zero spin, it does not give an NMR signal, and since only 1% of the atoms in a molecule are ¹³C, it is unlikely that carbon-carbon coupling is seen. Acquiring a ¹³C NMR spectrum can take from a couple of minutes to hours because many scans have to be summed together in order to have results distinguishable from background noise.

In biological NMR, proteins can be deliberately labelled with ¹³C (and usually nitrogen-15) to facilitate structure determination. This is achieved by growing microorganisms genetically engineered to express the protein, but instead of the usual media, the bacteria are grown on a growth medium with ¹³C-containing carbon source, such as glucose, glycerol, pyruvic acid, etc. In this way proteins can be produced that are labeled with ¹³C uniformly (close to 100%), partially, or at specific sites.

Detection by mass spectrometry

A mass spectrum of an organic compound will usually contain a small peak of one mass unit greater than the apparent molecular ion peak (M). This is known as the M+1 peak and originates due to the presence of ¹³C atoms. A molecule containing one carbon atom will be expected to have an M+1 peak of approximately 1.1% of the size of the M peak as 1.1% of the carbon atoms will be ¹³C rather than ¹²C. Similarly a molecule containing two carbon atoms will be expected to have an M+1 peak of approximately 2.2% of the size of the M peak, as there is double the previous likelihood that a molecule will contain a ¹³C atom.

In the above the mathematics and chemistry have been simplified, however it can be used effectively to give the number of carbon atoms for small to medium sized organic molecules. In the following formula the result should be rounded to the nearest integer:

C = number of C atoms X = amplitude of the M ion peak Y = amplitude of the M+1 ion peak

¹³C-enriched compounds are used in the research of metabolic processes by means of mass spectrometry. Such compounds are safe because they are non-radioactive. In addition, ¹³C is used to quantitate proteins (quantitative proteomics). One important application is in Stable isotope labeling with amino acids in cell culture or SILAC.

The ratio of ¹³C to ¹²C is slightly higher in plants employing C4 carbon fixation than in plants employing C3 carbon fixation. Because the different isotope ratios for the two kinds of plants propagate through the food chain, it is possible determine if the principal diet of a human or other animal consists primarily of C3 plants or C4 plants by measuring the isotopic signature of their collagen and other tissues. Deliberate increase of proportion of ¹³C in diet is the concept of i-food, a proposed way to increase longevity.

Uses in Earth Science

Due to differential uptake in plants as well as marine carbonates of ¹³C, it is possible to use these isotopic signature in earth science. In aqueous geochemistry, by analyzing the δ¹³C value of surface and ground waters the source of the water can be identified. This is due to the fact that atmospheric, carbonate, and plant derived δ¹³C values all differ with respect to Pee Dee Belemnite (PDB) standard.

To calculate δ¹³C:

See also

• Carbon
• Isotopes of carbon
• Isotope fractionation

Notes

Lighter: carbon-12	Carbon-13 is an isotope of carbon	Heavier: carbon-14
Decay product of: boron-13, nitrogen-13	Decay chain of Carbon-13	Decays to: stable