ABSTRACT
A general discussion of isotopes appears in Chapter 46. For naturally occurring elements, the ratios of abundances of isotopes are well known and, for convenience, are listed in Tables 47.1 and 47.2. These ratios can be used in routine mass spectrometry to identify various elements. For example, molybdenum has the seven isotopes listed here (the percent abundances are given in parentheses):
Mo (14.8),
Mo (9.25),
Mo (15.9),
Mo (16.7),
Mo (9.55),
Mo (24.1),
Mo (9.63); it would not be very difficult to identify the presence of molybdenum in a sample from the pattern of isotopic peaks in a mass spectrum. Although listed in Tables 47.1 and 47.2 as fixed ratios, the actual ratios of isotopes differ slightly, depending on the source of the elements being investigated. Thus, although
C and
C are listed as occurring in a ratio of 98.882 to 1.108, if this ratio is measured in metabolic products, as in drug testing, the ratio will be found to be slightly different from that given in Tables 47.1 and 47.2 because metabolic reactions differ somewhat between
C and
C. Such accurate measurement of precise isotope ratios is important in many areas, including chemistry, geology, environmental science, the nuclear industry, and medicine. Special instruments are needed to measure these precise levels. It also becomes necessary to stipulate that certain materials should be regarded as having standard isotope ratios, and these are used to gauge any changes appearing in nonstandard substances.
