Distribution of Common Elements

TABLE 2.3 Isotopic Distribution of Common Elements Element Symbol Nominal Mass Exact Mass Relative Abundance 

As a first approximation, the contributions to isotope peak intensities by various elements are additive. Each group of elements can be treated separately and the results summed. 

There is no contribution to isotope peak intensities by these elements. Therefore, weak isotope peak intensities for relatively high-mass ions are evidence for the presence of monoisotopic elements. 

In simple molecules containing only carbon, hydrogen, nitrogen, oxygen, and the monoisotopic elements, elemental compositions of ions may be calculated using isotope peak intensities. As a hrst approximation, one atom in an ion contributes an amount to the intensity of the isotope peaks that is equal to the relative abundances of the isotopes of that atom. When more than one atom is present, the intensity contribution is multiplied by the number of each atom present. 

This approximation is restated by Equations 2.4 and 2.5. If P is the intensity of the ion with no heavy isotope, then (P + 1) is the intensity of the peak 1 mass higher, and (P + 2) is the intensity of the peak 2 masses higher. 

---