Molecular ion for carbon monoxide:

For instance, the molecular ion for carbon monoxide is build up of ¹²C and ¹⁶O only. There are no molecules present consisting of ¹³C or ¹⁷O as these would take place at a higher position in the mass spectrum. Hence, the theoretical values for the molecular weight are calculated by using the atomic weights for particular isotopes and not the accurate atomic weights of the elements as they take place in nature. The latter (relative atomic weights) take the relative abundances of the dissimilar isotopes into account and will be dissimilar in value. For instance, the accurate atomic weight of the carbon isotope ¹²C is 12.0000 and this is the value employed for measuring the accurate mass of a molecular ion. The accurate relative atomic weight of carbon is higher at 12.011 because of the existence of the isotope ¹³C.

Note: The peak being measured in the mass spectrum is for the molecular ion. This ion consists of the most abundant isotope of all the elements present.