Relative abundance of natural isotopes

Natural Isotopic Abundances. The relative abundances of natural isotopes produce peaks one or more mass units larger than the parent ion (Table 7.75a). For a compound C H O N, a formula allows one to calculate the percent of the heavy isotope contributions from a monoisotopic peak, Pto the Pm + 1 peak ... 

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... 

RELATIVE ABUNDANCES OF NATURALLY OCCURRING ISOTOPES Table 4.18 Relative Abundances of Naturally Occurring Isotopes... 

Enrichment, Isotopic—An isotopic separation process by which the relative abundances of the isotopes of a given element are altered, thus producing a form of the element that has been enriched in one or more isotopes and depleted in others. In uranium enrichment, the percentage of uranium-235 in natural uranium can be increased from 0.7% to >90% in a gaseous diffusion process based on the different thermal velocities of the constituents of natural uranium (234U, 235U, 238U) in the molecular form UF6. 

Walker EC, Cuttitta F, Senftle FE (1958) Some natural variations in the relative abundance of copper isotopes. Geochim Cosmochim Acta 15 183-194... 

The relative atomic mass of an element is the weighted mean of the isotopic masses. The weighted mean is calculated from the masses of all the possible isotopes of the element, taking into account the natural relative abundance of each isotope. 

Their relative abundance of various isotopes of hydrogen in nature is as follows ... 

Boron exists as two naturally occurring isotopes 1°B (10.01 u) and 1 lB (11.01 u). Calculate the relative abundance of each isotope of boron. 

Stable carbon isotope methods exploit natural variations in the relative abundances of the isotopes and and are valuable archaeological tools for the study of paleonutrition ... 

In nature most elements are found as a mixture of isotopes. Usually, no matter where a sample of an element is obtained, the relative abundance of each isotope is constant. For example, in a banana, which is a rich source of potassium, 93.25% of the potassium atoms have 20 neutrons, 6.7302% will have 22 neutrons, and a scant 0.0117% will have 21 neutrons. In another banana, or in a totally different source of potassium, the percentage composition of the potassium isotopes will still be the same. 

With atomic number 92 and atomic weight 238.03, uranium is the heaviest naturally occurring element. There are eleven known isotopes, of which three — with atomic weights 234, 235 and 238 — occur in nature. They are all radioactive, with half-lives (in years) of 2.35 X 10 , 7 X 10 and 4.5 X 10 , respectively. The relative abundance of the isotopes varies depending on the age and geological history of the uranium occurrence a typical distribution is U 99.28% U 0.71% U 0.005%. A quite exceptional deviation from this distribution has been found at Oklo in Gabon as a result of spontaneous fission chain reactions in the remote past (Anon., 1975). At Oklo U concentrations as low as 0.29% have been found. 

Stable isotope (see Box 1.1) abundances cannot at present be determined with sufficient accuracy to be of use in studies of their natural variations. Mass spectrometers can, however, measure the relative abundances of some isotopes very accurately, resulting in stable isotope ratio measurements, for example oxygen —180/160, carbon — 13C12C and sulphur—3%/32S. Stable isotope ratios are reported in delta notation (8) as parts per thousand (%o per mil) relative to an international standard, i.e. ... 

Naturally occurring iron consists of four isotopes with the abundances indicated here. From the masses and relative abundances of these isotopes, calculate the atomic weight of namrally occurring iron. 

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