Hydrogen relative isotopic abundance

In all these applications, sensitive instruments for the determination of isotope abundances are required. In this connection, the conventional ionic-type mass spectrometer is used almost exclusively for the determination of absolute and relative isotope abundances with the exception of the wide use of infrared methods for the determination of hydrogen to deuterium ratios. 

Many artificial (likely radioactive) isotopes can be created through nuclear reactions. Radioactive isotopes of iodine are used in medicine, while isotopes of plutonium are used in making atomic bombs. In many analytical applications, the ratio of occurrence of the isotopes is important. For example, it may be important to know the exact ratio of the abundances (relative amounts) of the isotopes 1, 2, and 3 in hydrogen. Such knowledge can be obtained through a mass spectrometric measurement of the isotope abundance ratio. 

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. 

Due to the distinctive mass spectral patterns caused by the presence of chlorine and bromine in a molecule, interpretation of a mass spectrum can be much easier if the results of the relative isotopic concentrations are known. The following table provides peak intensities (relative to the molecular ion (M+) at an intensity normalized to 100%) for various combinations of chlorine and bromine atoms, assuming the absence of all other elements except carbon and hydrogen.1 The mass abundance calculations were based on the most recent atomic mass data.1... 

The relative amounts of the individual isotope species in each element, expressed in percent, are called the isotopic abundances. For example, in seawater the relative abundances of hydrogen isotopes are... 

The product of isotopic abundance and relative sensitivity determines the overall sensitivity to detection of the isotope relative to hydrogen with its 99.985% natural abundance and unit relative sensitivity. Low y nuclei may be quite difficult to detect as sensitivity is proportional to y (e.g., Fe which is 3 x 10 times less sensitive than protons for equal numbers of nuclei). Sensitivity may be enhanced by repetitive spectral accumulation as the signal-to-noise (S/N) ratio is proportional to the square root of the number of accumulations. It requires 10 accumulations to give a Fe signal 1/50 the S/N of protons - a daunting task. 

Heavy atom and secondary hydrogen kinetic isotope effects are often quite small, so they can be difficult to measure due to the error values often associated with rate constants. However, as any reaction occurs, the reactants are incrementally enriched in the slower reacting components. Thus, for reactants with the natural abundance of heavy isotopes, near the end of the reaction the proportion of heavy isotopes in the reactants has increased relative to the proportion present at the beginning of the reaction. 

This M + 1 peak is not the result of an impurity, however it is always there no matter how carefully the cyclohexane is purified. This M + 1 peak exists because the carbon and hydrogen have naturally occurring isotopes, and the mass spectrometer can detect molecules containing these heavy isotopes. Table 15.1 gives the relative natural abundance of some common isotopes. 

Once the patterns of possible combinations of atoms are calculated (and there are extensive compilations of accurate masses for assemblies of atoms, along with their common isotope abundances, to aid this process [8]), it is very easy to spot them in a spectrum. Problems will occur when several such patterns overlap, and this is particularly severe when hydrogen atoms are present, so that several patterns, shifted relative to one another by just one mass unit, could be superimposed. In fact, because atomic masses are known so accurately, it is often possible to determine the constitution of an ion just from its mass alone, without any prior knowledge of what elements are present in the sample. 

The natural variations in isotopic abundances can be large, depending on the relative elemental mass differences hydrogen (100%) > oxygen (12.5%) > carbon (8.3%) > nitrogen (7.1%), see also Table 2.48 (Rossmann, 2001 Rosman and Taylor,... 

Hydrogen exhibits the largest natural isotope abundance variations due to the relatively large mass difference between its two stable isotopes JH and (also written commonly as H and D, respectively). H is approximately 99.985% and D 0.015% naturally abundant. The three isotopes of oxygen O and are -99.763%, -0.0375% and -0.1995% abundant, respectively. Examples of isotope abundance variations measured for H and O are summarized in Figures 6 and 7. 

Ans. Hydrogen occurs as diatomic molecules, and it would be easy to separate H , H H, and 2H , but not the individual atoms. But there would be relatively little "H, since in abundance the heavy isotope accounts for only 0.015% of naturally occurring hydrogen atoms. 

Fig. 1. Evolution of 3He/H in the solar neighborhood, computed without extra-mixing (upper curve) and with extra-mixing in 90% or 100% of stars M < 2.5 M (lower curves). The two arrows indicate the present epoch (assuming a Galactic age of 13.7 Gyr) and the time of formation of the solar system 4.55 Gyr ago. Symbols and errorbars show the 3He/H value measured in meteorites (empty squares) Jupiter s atmosphere (errorbar) the local ionized ISM (filled triangle) the local neutral ISM (filled circle) the sample of simple Hll regions (empty circles). Data points have been slightly displaced for clarity. The He isotopic ratios has been converted into abundances relative to hydrogen assuming a universal ratio He/H= 0.1. See text for references.

Table 14.1 Relative abundances of the stable isotopes of hydrogen, carbon, nitrogen, oxygen and sulfur (Hoefs 1996 Schoeller 1999)...

Water is a mixture of varying isotopic composition (Franks, 2000). In addition to the two most common isotopes, 160 and there are two stable oxygen isotopes (170, lsO), one stable hydrogen isotope (2H, deuterium), and one radioactive hydrogen isotope (3H, tritium, half-life = 12.6 years). Water also contains low concentrations of hydronium (H30+) and hydroxide ions (OH-) and their isotopic variants. In total, water consists of more than 33 chemical variants of HOH however, these variants occur in relatively minor amounts (Fennema, 1996). Table II gives the natural abundance isotopic composition of the four major water species. 

---