Separation of stable isotopes

The stable isotopes of carbon, nitrogen, oxygen, and sulphur are available commercially in a variety of chemical forms. In this section, an outline is given of the various methods that are currently used for the separation of stable isotopes. It will be clear that the primary source of isotope, and therefore, in most cases the cheapest available form, is determined by the separation procedure adopted. A discussion of some aspects of the synthesis of labelled compounds follows. Although in a review of this size, this cannot be comprehensive, the methods used and the problems that are raised will be outlined. Some recent developments that increase the choice of chemical form available will be mentioned. 

Isotope content is expressed as atom%, defined as the ratio of the number of atoms of the isotope to the total number of atoms of that element expressed as a percentage. It only refers to the atoms in the labelled position(s). For example, in sodium [1- C] acetate at 90 atom%, 90% of the carbonyl carbons are C, while occurs only to the extent of the natural abundance in the methyl group. Mole% is sometimes used where atoms are replaced by moles in the above ratio. Isotopic enrichment is an expression that is frequently loosely used. Strictly speaking it indicates the change in atom% that has occurred in a separation process and most commonly represents the increase in abundance above the natural level it is expressed as atom% excess. 

This review is conflned to the separation of the isotopes of carbon, nitrogen, oxygen, and sulphur. Although the natural abundance of these isotopes is low, the substitution of an isotope in a molecule produces changes, albeit small, in both the chemical and physical properties. It is these changes that have been utilised in the different methods that have been adopted and successfully exploited for the separation of isotopes on the commercial scale. The basic principles have been well documented [22] and the present commercial processes have been reviewed [23]. 

The fractionation of two isotopes (A and A ) in equilibrium between two phases (1 and II) is determined by the equilibrium constant of the reversible process illustrated in Equation (1). 

Most of the carbon-13 used is separated using the cryogenic distillation of carbon monoxide. The separation factor at the operating temperature is 1.008. This method is adopted at the ERDA facility at Los Alamos, USA [24] and by Pro-chem in London, England. Although claims have been made that carbon-13 can be produced more cheaply by chemical exchange [25], this has yet to be proved commercially. The reliability of the system and the simplicity of the technology ensures that the distillation of carbon monoxide retains its dominant role for the separation of carbon-13 at 90—93 atom% for some time to come. 

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G. H. Clewlett, Chemical Separation of Stable Isotopes, Annu. Rev. Nucl. Sci. 4, 293 (1954)... 

H2. Healy, R. M., A. A. PaDco, E. F. Joseph, and J. S. Drury Chemical Separation of Stable Isotopes, Proceedings of the International Symposium on Isotope Separation, Interscience, New York, 1958, p. 199. 

The time for running-in is therefore roughly proportional to the square of the number of theoretical plates or to the column length [152], [169]. The equilibration time will be discussed more fuUy in connection with the separation of stable isotopes (section... 

The macro separation of stable isotopes on ion exchange columns. J. Chim. Phys. (France) 60, 89. with D.E. Hegland and S. Legvold, Magnetization and electrical resistivity of terbium single crystals. Phys. Rev, 131, 158-162,... 

With the application of FIA in the mixture analytical mode for the analysis of environmental samples and after a marginal sample pretreatment by SPE, matrix effects are a high probability. But, as cited previously [27—31], matrix effects were not only observed with FIA but also in LC-MS and MS—MS modes. Advice to overcome these problems by, e.g. an improved sample preparation, dilution of the analyte solution, application of stable isotopic modification of LC conditions [29] or even application of two-dimensional LC separations [27], postcolumn standard addition [29], addition of additives into the mobile phase (e.g. propionic acid, ammonium formate) [34,35] or even matrix compounds [32] were proposed and discussed. 

A quite reliable method involves the addition of stable isotope-labelled analytes, which exhibit the same chromatographic and ionisation behaviour as unlabelled analogues present in the sample, and which can therefore be distinguished from the latter with the aid of the MS detector separating the different ion masses. However, whereas for many environmental pollutants such as pesticides, pharmaceuticals,... 

Finally, there is the question of how accurately the theory thus proposed can predict. For without the ability to predict, a theory is only a theory. The theory has already predicted the correlation between Tc and N, the number of naturally occurring isotopes as shown in Fig. 9. Subsequent prediction to this correlation is that if and when the number of stable isotopes in an element such as Mo, were to be separated and isolated into only one isotope with the same atomic mass its Tc shall rise. 

Sometimes it is necessary to obtain an isotope from solution (or merely study its behavior), but stable nuclides of the element to be studied are nonexistent or unavailable. In such cases, an element having chemistry similar to that of the radioisotope may serve as a carrier. Thus, ReO has been used as a carrier for TcO, I" as a carrier for At, and ThO2 as a carrier for Np02+. (If isolation of the new isotope is desired, there remains the problem, often difficult, of separation of the isotope from its carrier.)... 

Separation and Applications of Stable Isotopes, Avona and Spicer, American Laboratory, April 1987. 

Molecular weight determinations were consistent with a dimeric structure, [U(OCH2CH3)5]2, and the compound can be distilled at 123 °C (0.001 Torr). Species that are thermally stable, distillable or sublimable are desirable for use in the separation of metal isotopes. The number of ensuing reports describing various synthetic routes to [U(OCH2CH3)5]2 are evidence of that motive.Some of these methods are described by Equations (38) to (43) ... 

In Table 12 the distribution of sodium ions between water and chloroform (referred to g HjO/g CHCI3) is presented in dependence on the different polyethers. All results were obtained under analogous conditions with 0.1 mmol Na and 0.1 mmol polyether in the system where the pH-value was established to be 8 by adding 10 mmol of tetraethylammonium chloride. The establishment of the equilibrium requires less than 60 min in all systems and was followed by the y-activity of the sodium isotopes and the P-activity of C-labeled polyethers. The enrichment of one of the sodium isotopes in a practical scale from a Na/ Na-mixture can only be achieved in a system where the distribution ratio (Na ),/(Na ) g is not too high. However, in contrast to the enrichment of stable isotopes from a sample with natural isotope abundance, the enrichment of Na or of Na from an isotopic mixture is not of great importance because these two isotopes can be produced by nuclear reactions. On the other hand, the investigations on sodium isotopic separations are of common interest in respect to further knowledge about isotopic effects. 

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