Relative isotopic abundance common elements

Mass spectrometers determine atomic and molecular isotope ratios. Table 2 lists the relative isotopic abundance of elements commonly encountered in pharmaceutical analysis [3,4]. The values in Table 2 have been empirically determined and refinements in the values are necessary as atomic mass measurements improve, but for this discussion any inaccuracies in the table are insignificant. For some elements there are only two naturally occurring isotopes. For example, if you were to randomly sample carbon atoms in nature, 99% of the time you would find 12C, and roughly 1% of the time a 13C would turn up. Other elements, such as chlorine and bromine, have elemental isotope ratios that are not as heavily... 

This book presents a unified treatment of the chemistry of the elements. At present 112 elements are known, though not all occur in nature of the 92 elements from hydrogen to uranium all except technetium and promethium are found on earth and technetium has been detected in some stars. To these elements a further 20 have been added by artificial nuclear syntheses in the laboratory. Why are there only 90 elements in nature Why do they have their observed abundances and why do their individual isotopes occur with the particular relative abundances observed Indeed, we must also ask to what extent these isotopic abundances commonly vary in nature, thus causing variability in atomic weights and possibly jeopardizing the classical means of determining chemical composition and structure by chemical analysis. 

Table 2.1 Relative Isotope Abundances of Common Elements ...

Table 16.2. Relative Isotopic Abundances of Some Common Elements... 

The molecular ion, represented by M", is the intact molecule with one electron missing. This should be the peak in the spectrum with the largest mass, but it is not always observed. All spectra have an ion that we call the base peak. This is the most abundant peak in the spectrum m/z 44 in the spectrum of N2O or m/z 69 in the spectrum of PFTBA). In the next section you will find that more than one peak may correspond to the M" or fragment ion when that ion contains elements with different isotopes. We use the term nominal mass to describe the mass of the molecule in terms of the most abundant (and, generally, the lightest) isotopes of the element. Relative isotopic abundances for common elements are summarized in Table 8.36. 

Table 4.2 Masses and relative abundances of some isotopes of commonly occurring elements...

Interactions between one or more nuclei and the unpaired electron yield a wealth of information concerning molecular structure. In addition, they have proven invaluable in the identification of paramagnetic species. As indicated in Table II many of the common elements have isotopes with nuclear magnetic spins which distinguish them from the other elements. If the isotopes of interest are not sufficiently abundant in the natural form, enriched samples may be purchased. The quantity used in surface studies is usually quite small, so relatively expensive isotopes such as 170 can be studied. In fact, it is possible to recover most of the isotope following an experiment, should the cost require it. 

Table 2.1 lists the principal stable isotopes of the common elements and their relative abundance calculated on the basis of 100 molecules containing the most common isotope. Note that this presentation differs from many isotope abundance tables, in which the sum of all the isotopes of an element adds up to 100%. 

Table 15.3 lists the relative abundances of the major isotopes of the elements that are commonly encountered in organic compounds. In principle, it is possible to use the... 

This book concerns the common atoms of our natural world. How many of each element exist, and why What variations are found in the relative numbers of the isotopes of each element, and how are those variations interpreted If I could write an epic poem, I would lyricize over the history of the universe writ small by their natural abundances. I would rhapsodize over the puzzling arrangements at different times and places of the thousand or so different isotopes of some ninety chemical elements. These different arrangements speak of distant past events. 

The composition of the Earth was determined both by the chemical composition of the solar nebula, from which the Sun and planets formed, and by the nature of the physical processes that concentrated materials to form planets. The bulk elemental and isotopic composition of the nebula is believed or usually assumed to be identical to that of the Sun. The few exceptions to this include elements and isotopes such as lithium and deuterium that are destroyed in the bulk of the Sun s interior by nuclear reactions. The composition of the Sun as determined by optical spectroscopy is similar to the majority of stars in our galaxy and, accordingly, the relative abundances of elements in the Sun are referred to as "cosmic abundances". Although the cosmic abundance pattern is commonly seen in other stars, there are dramatic exceptions, such as stars composed of iron or solid nuclear matter, as is the case with neutron stars. The best estimation of solar abundances is based on data from optical spectroscopy and meteorite studies and in some cases extrapolation and nuclear theory. The measured solar abundances are listed in Fig. 2-1 and Table 2-1. It is believed to be accurate to about 10% for the majority of elements. The major features of the solar abundance distribution are a strong decrease in the abundance of heavier elements, a large deficiency of Li, Be, and B, and a broad abundance peak centered near Fe. The factor of 10 higher... 

Nitrogen is the most abundant terrestrial element in an uncombined state, as it makes up 78 percent of Earth s atmosphere as N2, but it is a minor component (19 parts per million) of Earth s crust. Nitrogen exists as two isotopes N (99.63% relative abundance) and N (0.4% abundance). Both isotopes are nuclear magnetic resonance (NMR) active, with the rarer N isotope being utilized more commonly in NMR spectroscopy because of its nuclear spin of one-half... 

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... 

Table 10.4 Relative Natural Isotope Abundances of Common Elements in Organic...

Table 2.3 shows the elements commonly found in organic compounds and the relative abnndances of their isotopes. Of these elements, only fluorine, phosphorus, and iodine are monoisotopic. Most elements are mixtures of two or more stable isotopes, differing in mass by 1 or 2 Da. Most elements include one major isotope (greater than 90 percent relative abundance), but chlorine and bromine have two rather abundant isotopes separated by 2 Da. 

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

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