Natural abundance of isotopes

The greater the mass of an individual atom, the greater the molar mass of the substance. However, most elements exist in nature as a mixture of isotopes. We saw in Section B, for instance, that neon exists as three isotopes, each with a different mass. In chemistry, we almost always deal with natural samples of elements, which have the natural abundance of isotopes. So, we need the average molar mass, the molar mass calculated by taking into account the masses of the isotopes and their relative abundances in typical samples ... 

Fig. 35. C MAS NMR spectra of methylated zeolite Y ( (Ils-Y) recorded after loading with toluene (natural abundance of ( -isotopes) and thermal treatments at temperatures from 298 (a) to 493 K (e). Asterisks denote spinning sidebands. Reproduced with permission from (263). Copyright 2003 American Chemical Society.

Table 15.3 Natural Abundances of Isotopes of Some Common Elements...

T able 2 Nuclear spin and % natural abundance of isotopes commonly encountered in biological samples... 

The mjz value of the molecular ion is the summation of all the atomic masses in the molecule, including the naturally occurring isotopes. For organic molecules you will find a small peak M + 1) above the apparent molecular ion mass (M ) value due to the presence of C. The importance of isotope peaks is the detection of chlorine and bromine in molecules since these two elements have large natural abundances of isotopes, e.g. Cli Cl = 3 1 and Br Br = 1 1. The mass spectra produced by molecules containing these atoms are very distinctive with peaks at M + 2 and even M + 4 and M + 6 depending on how many chlorine or bromine atoms are present. The identification of the number and type of halogen atoms is illustrated in Box 30.1. 

The number of protons in an atom is sort of like the characteristic that distinguishes a human as male. If a person has this characteristic, then he is male. If an atom has a certain number of protons, then that decides what element it is. But different males can have different masses. We could easily find the average mass of a sample of males by adding all their individual masses and dividing by the number in the sample. The same procedure is followed for elements on the periodic table. As can be seen in the appendix, the mass for chlorine is given as 35.45 amu, which is not the mass of either isotope. It is an average over the natural abundance of isotopes. However, it is the number of protons that determines the element, so even though isotopes may... 

It is these isotopic peaks that make it possible for us to determine the molecular formula of the compound. Knowing the relative natural abundances of isotopes, one can calculate for any molecular formula the relative intensity to be expected for each isotopic peak M + 1, M + 2, etc. The results of such calculations are available in tables. Consider, for example, a compound for which is 44. 

Stable isotopes occurring naturally in a molecule are detected by presence of satellite peaks in a mass spectrum. These satellite peaks appear at values higher than that calculated from standard tables for parent or fragment ions constituted of the naturally more abundant mass. The shift in values and intensity of peaks depend on the number and natural abundance of isotopic variants in the molecule and are fixed within limits of experimental error [2,3]. When stable-labeled compounds are synthesized, the mass spectra of the parent or fragment ion show peaks shifted to m/ values equal to (M + x) depending on the nature... 

H, C, Si, and using two cardinal types of pulse sequences based on the HNCO and HNCA experiments which constitute standard schemes routinely employed for structure elucidation of peptides and proteins. However, whereas work on biomolecules is generally carried out with uniformly and labelled molecules, these studies were performed in natural abundance of isotopes, which required some adaptations of the experiments in order to improve the suppression of signals from isotopomers with magnetically inactive and Si nuclei. 

Fluorine, F at, wt 18.998403 at. no. 9 valence l elemental state F2. A halogen. Occurrence in earth s crust 0 065% by wt. Natural abundance of isotopes , 9F 100% i F 0l 4 109.7 min) is prepered in nuclear reactors. Does not occur in elemental state in nature. Most important sources are fluorite, cryolite and florapatite, q.v, Finger, "Fluorine Resources and Fluorine Utilization4 in Advances in Fluorine... 

Table 13.2 The Natural Abundance of Isotopes Commonly Found in Organic Compounds...

Natural abundance of isotopes In nature, most elements are found as mixtures 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, 93.26% of the potassium atoms have 20 neutrons, 6.73% have 22 neutrons, and 0.01% have 21 neutrons. In another banana, or in a different source of potassium, the percentage composition of the potassium isotopes will still be the same. The three potassium isotopes are summarized in Figure 4.17. 

I Table 14.2 Precise Masses and Natural Abundances of Isotopes Relative to 100 Atoms of the Most Abundant Isotope... 

FIGURE 20-4 Electron-impact mass spectra of (a) methylene chloride and (b) 1-pentanol. TABLE 20-3 Natural Abundance of Isotopes of Some Common Elements... 

The percent natural abundance of isotopes is the relative amount of each different isotope in a naturally occurring sample of a given element. 

The relative amount of each different isotope in a naturally occurring sample of a given element is roughly constant. For example, in any natural sample of neon atoms, 90.48% of them are the isotope with 10 neutrons, 0.27% are the isotope with 11 neutrons, and 9.25% are the isotope with 12 neutrons. These percentages are called the natural abundance of the isotopes. Each element has its own characteristic natural abundance of isotopes. However, advances in mass spectrometry (see Section 2.8) have allowed accurate measurements that reveal small but significant variations in the natural abundance of isotopes for many elements. 

What are isotopes What is percent natural abundance of isotopes ... 

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