Carbon-13 is a stable isotope of carbon. It makes up about 1.11% of all naturally-occurring carbon on earth.

Uses in NMR spectroscopy

Because of its spin properties (it can have positive and negative spin, just like the hydrogen atom), the presence of this isotope can be detected using NMR Spectroscopy, giving information on the structure of an organic compound, namely, the number of carbon atoms in said compound. Because of its trace presence, running a carbon-13 NMR scan can take much time, often a couple of weeks, because many scans have to be run and then combined using a Gaussian interpolation in order to have results distinguishable from background noise.

Uses in mass specroscopy

A mass spectrogram of an organic compound will usually contain a small peak of one mass unit greater than the apparant molecular ion peak (M). This is known as the M+1 peak and orginates 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 effectivly to give the number of carbon atoms for small to medium sized organic compounds. The following formula is appropriate and the result should be rounded to the nearest integer:

C = number of C atoms X = size of molecular ion peak Y = molecular ion plus one peak

Carbon 13 derived compounds are used in the determination of some metabolic processes in the human body, this analysis requires mass spectroscopy. They are used because they are non-radioactive.

See also

• Carbon
• isotope