Carbon-13

Carbon-13 is a natural, stable isotope of carbon and one of the environmental isotopes. It makes up about 1.109% of all naturally-occurring carbon on Earth.

Detection by NMR spectroscopy

Because of its nuclear spin properties, with a spin +1/2, just like the hydrogen atom, this isotope responds to a resonant RF (radiofrequency) 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 C-12 has zero spin, it does not give an NMR signal, and since only 1% of the atoms in a molecule are C-13, it is unlikely that carbon-carbon coupling is seen. Acquiring a carbon-13 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 carbon-13 (and usually nitrogen-15) to facilitate structure determination. This is achieved by growing microorganisms genetically engineered to express the protein on a media with carbon-13 labelled glucose as the only carbon source. In this way proteins with a carbon-13 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 carbon-13 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 carbon-13 rather than carbon-12. 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 carbon-13 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:

$C = \frac{100Y}{1.1X}$

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

Carbon-13-enriched compounds are used in the research of metabolic processes by means of mass spectroscopy. Such compounds are safe because they are non-radioactive.

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Detection by NMR spectroscopy

Detection by mass spectroscopy

See also