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) of the whole molecule. This is known as the M+1 peak and comes from the few molecules that contain a C atom in place of a C. 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 molecules will have a C rather than a C. 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 any molecule will contain a C 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:
where C = number of C atoms, X = amplitude of the M ion peak, and Y = amplitude of the M +1 ion peak.
C-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, C is used to quantify proteins (quantitative proteomics). One important application is in stable isotope labeling by amino acids in cell culture (SILAC). C-enriched compounds are used in medical diagnostic tests such as the urea breath test. Analysis in these tests is usually of the ratio of C to C by isotope ratio mass spectrometry.
The ratio of C to C 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 to 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.