Appendix B Overview of Isotopes

B.1 Isotopes

B

Overview Many elements in their natural state exist as one of several isotopes. An isotope is one of two or more atoms with the same atomic number but a different mass. The most abundant isotope of carbon is 12C, but natural carbon also contains 13C and 14C.

Because a mass spectrometer measures mass-to-charge ratios, isotopes appear in the mass spectrum. Isotopes of low abundance, such as 14C, do not affect the appearance of a mass spectrum. However, isotopes that occur in greater abundance, such as 13C, which occurs in a natural abundance of approximately 1.1 percent1 in carbon, do affect the appearance of a mass spectrum.

The mass spectrum of methane (Figure B-1)illustrates the impact of an isotope on the appearance of a mass spectrum. Methane includes a peak representing the molecular ion at

16Da (12CH4) and a peak representing the isotope at 17 Da (13CH4). The relative abundance of the ions is about 99:1.

12C

13C

Figure B-1 Mass Spectrum of Methane

1. Meth. Enzymol., McCloskey, J.A, ed., 1990, 193, 869.

B-2 Applied Biosystems