Carbon-13

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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 on a growth medium with ¹³C labeled glucose as the only carbon source. In this way proteins with a ¹³C content of almost 100% can be produced.

Detection by mass spectroscopy

A mass spectrogram 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 spectroscopy. Such compounds are safe because they are non-radioactive. In addition, ¹³C is used to quantitate proteins (quantitative proteomics). One important application is `stable isotope labeling with amino acids in cell culture´ (SILAC).

See also

• Carbon
• Isotope fractionation

References

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