Isotopic Enrichment Using Chemical Reactions

The Synthesis of Deuterium Derivatives 1.5.7.2. by isotopic Enrichment Using Chemical Reactions... 

Formation of Bonds between Hydrogen and C, Si, Ge, Sn, Pb 123 1.6.7. The Synthesis of Deuterium Derivatives 1.6.7.2. by Isotopic Enrichment Using Chemical Reactions... 

Chemical shifts of bulk and coordinated water differ and as a result, the line width at half-height can be used to calculate the rate of chemical exchange. If reactions are slow, NMR methods can be used directly to determine reaction rates since one need only follow the isotopic enrichment of a complex. 

Elements.—The use of a tunable narrow-frequency laser to produce an isotopically selected chemical reaction has been described by Leone and Moore 1 this approach should permit efficient isotope separations to be performed. In the example reported, natural Br2 (79Br/81Br = 1) is photo-predissociated by selective excitation into the 3n0+ state. Bromine atoms, enriched in one isotope, react with HI before scrambling occurs, to produce 80—85% enriched H81Br. 

One example of a separation cascade is the liquid/liquid extraction system where the desired isotopic enrichment takes place in phase A. Using systems with crown ethers or cryptands, these ligands should have a good solubility in one phase, whereas the same compounds should be insoluble or difficult to dissolve in the other phase. This also applies to chemical reactions in solid/liquid systems, e.g. in the system ion... 

The chemical enrichment of non-metal isotopes on a technical scale has been carried out for a long time whereas it has not been possible to use chemical separation of metal isotopes technically until today because of very small fractionation effects in those chemical systems. This is the main reason why metal isotopes are usually enriched by means of expensive physical methods instead of chemical exchange reactions. However, for a number of years chemical systems which could be used for the separation of metal isotopes in a technical scale have been intensively sought. Therefore, it is not surprising that in this connexion chemical systems with crown ethers and cryptands became of interest after an experiment by Jepson and DeWitt in 1976 They found a significantly higher isotopic separation factor for calcium in an extraction system with crown ethers than had been achieved by other chemical reactions up to that time. 

A. Chemical vs. Isotopic Competitive Methods Two types of competitive methods can and have been used. They are the chemical competitive and the isotopic fractionation techniques. In the chemical competitive method, the isotopic compounds A or A compete with a chemically different species, B, for reaction with C. The method is, therefore, not applicable to unimolecolar reactions and requires samples of A and A of appreciable isotopic enrichment. Furthermore, the species B must react with C at a rate of similar order of magnitude s A or A do. Consider for simplicity reactions first order in each of the reactants... 

The separation of isotopes of alkaline earth metals by ion-exchange chromatography (Be and Ca), using the band elution technique, and by chemical exchange reactions (Ca), using macrocyclic polyether complexes, has been assessed. The separation factors for Be and Ca decrease with increase in mass of the isotopes, and were found to be of the same order as those determined previously. Enrichment of the heavier isotopes of Ca by reaction (1), where L represents a macrocyclic polyether (c.g. DCH18C6, DB18C6), has also been shown to be effective. ... 

Mass spectrometric approaches are also very useful for the measurement of stable isotopes in drug metabolism studies. The application of MS to the quantitative measurement of stable isotope has been limited due to the high cost and sophistication of the instruments necessary for stable isotope enrichment studies. Nonetheless, recent improvements in instrument design and performance, as well as computer software for instrument control, data acquisition, and analysis, have increased the sensitivity and reliability of stable isotopic enrichment studies. These new MS instruments, including continuous-flow isotope ratio mass spectrometry (CF-IRMS) and HPLC-chemical reaction interface mass spectrometry (HPLC-CRIMS) are increasingly less expensive, easier to operate, and accessible for mass balance/ metabolite identification studies with stable isotopes. 

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