A Quick Look Ahead to Simple Uses of Mass Spectra

Virtually every element exists in nature in several isotopic forms. The natural abundance of each of these isotopes is known. Besides giving the mass of the molecular ion when each atom in the molecule is the most common isotope, the mass spectrum also gives peaks that correspond to that same molecule with heavier isotopes. The ratio of the intensity of the molecular ion peak to the intensities of the peaks corresponding to the heavier isotopes is determined by the natural abundance of each isotope. Because each type of molecule has a unique combination of atoms, and because each type of atom and its isotopes exist in a unique ratio in nature, the ratio of the intensity of the molecular ion peak to the intensities of the isotopic peaks can provide information about the number of each type of atom present in the molecule. 

For example, the presence of bromine can be determined easily, because bromine causes a pattern of molecular ion peaks and isotope peaks that is easily identified. If we identify the mass of the molecular ion peak as M and the mass of the isotope peak that is two mass units heavier than the molecular ion as M -t- 2, then the ratio of the intensities of the M and M+2 peaks will be approximately one to one when bromine is present (see Chapter 8, Section 8.5, for more details). When chlorine is present, the ratio of the intensities of the M and M + 2 peaks will be approximately three to one. These ratios reflect the natural abundances of the common isotopes of these elements. Thus, isotope ratio studies in mass spectrometry can be used to determine the molecular formula of a substance. 

Another fact that can be used in determining the molecular formula is expressed as the Nitrogen Rule. This rule states that when the number of nitrogen atoms present in the molecule is odd, the 

PRECISE MASSES FOR SUBSTANCES OF MOLECULAR MASS EQUAL TO 44 AMU 

Since the advent of high-resolution mass spectrometers, it is also possible to use very precise mass determinations of molecular ion peaks to determine molecular formulas. When the atomic weights of the elements are determined very precisely, it is found that they do not have exactly integral values. Every isotopic mass is characterized by a small mass defect, which is the amount by which the mass of the isotope differs from a perfectly integral mass number. The mass defect for every isotope of every element is unique. As a result, a precise mass determination can be used to determine the molecular formula of the sample substance, since every combination of atomic weights at a given nominal mass value will be unique when mass defects are considered. For example, each of the substances shown in Table 1.4 has a nominal mass of 44 amu. As can be seen from the table, their exact masses, obtained by adding exact atomic masses, are substantially different when measured to four decimal places. 

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A QUICK LOOK AHEAD TO SIMPLE USES OF MASS SPECTRA... 

Section 1.6, A Quick Look Ahead to Simple Uses of Mass Spectra, was deleted. 

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