Isotopic mass

Each nucleus serves merely to report the behaviour of the same electron orbitals, except for very small effects of isotopic mass on these orbitals. 

A common mistake for beginners in mass spectrometry is to confuse average atomic mass and isotopic mass. For example, the average atomic mass for chlorine is close to 35.45, but this average is of the numbers and masses of Cl and Cl isotopes. This average must be used for instruments that cannot differentiate isotopes (for example, gravimetric balances). Mass spectrometers do differentiate isotopes by mass, so it is important in mass spectrometry that isotopic masses be used... 

Accurate, precise isotope ratio measurements are important in a wide variety of applications, including dating, examination of environmental samples, and studies on drug metabolism. The degree of accuracy and precision required necessitates the use of special isotope mass spectrometers, which mostly use thermal ionization or inductively coupled plasma ionization, often together with multiple ion collectors. 

These isotope masses and their ratio of abundances are characteristic of carbon. Similarly, the isotopes of other elements that occur naturally have fixed ratios of isotopes, as given in Tables 47.1 and 47.2 at the end of the accompanying full text. 

Poisson s ratio at 125—375 K isotopes mass number natural abundance, %... 

Isotope mass number Abundance, % Thermal neutron cross Contribution to the total cross ... 

The physical and thermodynamic properties of elemental hydrogen and deuterium and of their respective oxides illustrate the effect of isotopic mass differences. 

Tritium [15086-10-9] the name given to the hydrogen isotope of mass 3, has symbol or more commonly T. Its isotopic mass is 3.0160497 (1). Moletecular tritium [10028-17-8], is analogous to the other hydrogen isotopes. The tritium nucleus is energetically unstable and decays radioactively by the emission of a low-energy P particle. The half-life is relatively short (- 12 yr), and therefore tritium occurs in nature only in equiUbrium with amounts produced by cosmic rays or man-made nuclear devices. 

In addition to the wet and optical spectrometric methods, which are often used to analyse elements present in very small proportions, there are also other techniques which can only be mentioned here. One is the method of mass spectrometry, in which the proportions of separate isotopes can be measured this can be linked to an instrument called a field-ion microscope, in which as we have seen individual atoms can be observed on a very sharp hemispherical needle tip through the mechanical action of a very intense electric field. Atoms which have been ionised and detached can then be analysed for isotopic mass. This has become a powerful device for both curiosity-driven and applied research. 

Fortunately, isotopic abundances as well as isotopic masses can be determined by mass spectrometry. The situation with chlorine, which has two stable isotopes, 0-35 and 0-37, is shown in Figure 3.2. The atomic masses of the two isotopes are determined in the usual way. The relative abundances of these isotopes are proportional to the heights of the recorder peaks or, more accurately, to the areas under these peaks. For chlorine, the data obtained from the mass spectrometer are... 

Relate the atomic mass of an element to isotopic masses and abundances. 

The sequences of radioactive decays that lead to lead are well-known and the rates of decay have been carefully measured. We shall consider the sequence based upon the relatively slow decomposition of the most abundant uranium isotope, mass 238 (natural abundance, 99%) ... 

When calculating A/ we use the mass of an atom of H instead of the mass of a proton. This strategy allows us to use readily available isotope masses instead of the masses of bare atomic nuclei to calculate Am, because the number of electrons in the isotope will be the same as the total number of electrons in the hydrogen atoms on the other side of the equation and the masses of the electrons cancel. The electron-nucleus binding energy, which contributes to the mass of an atom, is only about 1(Th mu per proton, and so it can be ignored in elementary calculations. 

The distance between the two interacting atoms will be denoted by r, and /g will denote the average distance (or the equilibrium distance in a hypothetical vibrationless state) of the unperturbed system. (The distance will be independent of the isotopic mass.) A displacement... 

The eonclusion to be drawn from equation (6) is that the perturbation energy is equal to the value of the perturbing potential at the equilibrium separation plus terms which are proportional to the even derivatives of V(r) at the equilibrium separation, and also proportional to increasing powers of the mean square of the total deviation from this separation. It is via this mean square that the isotopic mass will affect the perturbation energy. 

Evaporitic sulfur has a range of sulfur isotopic composition from +10%o to +30%o, while sedimentary sulfur is depleted in the heavy isotope and has a range of isotopic composition of about —40%o to +10%o. Most of this variation reflects systematic changes with geological age. The source fractions of a river water can be estimated from an isotopic mass balance ... 

A second kind of model is briefly treated, based on isotopic mass balance arguments, and it is shown that large isotopic discrimination during methano-genesis in ruminants may account for data trends when comparing herbivores and carnivores. A third class of model is sketched at the level of biochemical flows, where some fundamental points are made concerning points where the isotopic composition of metabolites may be altered. The relevance of this to nitrogen isotopic enrichment is considered. 

Influence of Different Physiological States and Digestive Strategies on Isotopic Mass Balance... 

Wolfe, R.R. 1984 Tracers in Metabolic Research Radioisotope and Stable Isotope/Mass Spectrometry Methods. A.R. Liss, NY. 

First, the percentages must be converted to fractions by dividing by 100%. Then we multiply these fractional abundances by the Isotopic masses and add the results. A table helps in organizing these manipulations ... 

A disadvantage of this technique is that isotopic labeling can cause unwanted perturbations to the competition between pathways through kinetic isotope effects. Whereas the Born-Oppenheimer potential energy surfaces are not affected by isotopic substitution, rotational and vibrational levels become more closely spaced with substitution of heavier isotopes. Consequently, the rate of reaction in competing pathways will be modified somewhat compared to the unlabeled reaction. This effect scales approximately as the square root of the ratio of the isotopic masses, and will be most pronounced for deuterium or... 

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