Carbon-13

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. Pure carbon-13 costs about 700 USD per gram.

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:

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

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).