Formulae and Isotopic Abundances

If you look at the mass spectmm for methane (Fig. 10.2) and the corresponding Table 10.2, you note from the marked peak on the spectrum that the molecular ion is at m/z = 16. The molecular weight to the units place for methane is, of course, equal to the atomic weight of carbon plus four times the atomic weight of hydrogen, or (1)(12) + (4)(1) = 16. The table shows the presence of an additional small peak at m/z= 17. The peak at 17 exists because the element carbon has more than one isotope. The two stable isotopes of carbon are and constitutes 98.90% of all naturally occurring carbon atoms  

If our molecule contains only carbon and hydrogen, since there is 1.1% relative natural abundance of compared to 100% and our mass spectrum shows a 1.1% abundance of M -I-1 to M, only one carbon atom can be present in the molecule. That is, if we are dealing with a hydrocarbon and only one carbon atom is present, then the 

There is a definite relationship between the peaks at M + 1 and M that is directly related to the number of carbons present in hydrocarbon molecules  

This relationship is very valuable in characterizing an unknown compound. The calculation just presented is for hydrocarbons (compounds containing only carbon and hydrogen) and ignores the contribution from the isotope of hydrogen, called deuterium. In rigorous calculations, the contribution from deuterium would be taken into account. 

A list of common isotopes found in organic compounds and their relative abundance in nature is given in Table 10.4. A complete table of the relative abundance of natural isotopes for all elements is located in Appendix 10.1. As can be seen in Table 10.4, the natural abundance of deuterium is only 0.016% of the abundance, so it can usually be ignored 

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