In the example above the atomic mass of carbon is given as 12. 011, not 12. This is because naturally occurring carbon is a mixture of the isotopes 12C, 13C and 14C which have relative atomic masses of 12, 13 and 14 respectively. Moreover, the proportion of the isotopes varies between samples, so 12. 011 is an average value. By contrast, there is less variation in naturally occurring hydrogen so the average atomic mass is known more precisely. The precision of the molecular mass is determined by precision of the least precise atomic mass value, in this case that of carbon. In high-resolution mass spectrometry the isotopomers 12C1H4 and 13C1H4 are observed as distinct molecules, with molecular masses of 16 and 17, respectively. The intensity of the mass-spectrometry peaks is proportional to the isotopic abundances in the molecular species. 12C 2H 1H3 can also be observed with molecular mass of 17.