Isotope selection requirements

S is the selectivity of photon absorption under the particular experimental conditions, and 8 is the relative abundance of the desired isotope. Equation 8.19 shows, for example, that S values on the order of 103 are required before more than about 10% of photons are used to excite D in natural abundance H/D mixtures (8 1.5 x 10-4). The selectivity required for uranium separation is less because > 8(D). 

Radionuclides. Isotope selection was based primarily on three experimental requirements (1) both alpha and... 

Quantitation using unlabeled compounds as internal standards and GC-FID detection lacks the high sensitivity and the high selectivity required for aroma compounds present in the ppb level. For chemically stable compounds and those in higher concentration (i.e., >1000 ppb), however, this method gives reliable data. In all other cases, using isotope labeled compounds as internal standards is the method of choice if the they are available. 

Since the dissociation of the abundant H2 and CO molecules result from photoabsorption at discrete wavelengths, isotopically selective photodissociation based on self-shielding is possible. Two conditions are required for this (1) dissociation via line absorption for each isotopically substituted molecule, and (2) differential photolysis that depends upon the isotopic abundances. Self-shielding occurs when the spectral lines leading to dissociation of the major isotopic species optically saturate, while the other residual lines relevant for dissociation of the minor isotopes remain transparent. As a consequence, such photolysis depends on nucleic abundance rather than the mass of a molecule see Fig. 4.4 (Langer 1977 Thiemens Heidenreich 1983). 

During its lifetime, a fusion reactor presents little radiation hazard. The internal structure, particularly the vacuum containment vessel and the heat exchanger, will be subject to intense neutron bombardment. The neutrons will convert some of the elements of the structure into long-lived radioactive isotopes. Selecting construction materials that do not easily become activated can minimize radioisotope production. No material is entirely resistant to neutron activation, thus the decommissioning of a fusion reactor will require the handling and disposal of potentially hazardous radioactive isotopes. Because of the lack of uranium, plutonium, and fission products, the total radiation exposure hazard from the decommissioned fusion reactor is 10,000 to 1,000,000 less than from a decommissioned fission reactor. 

Modern analytical techniques usually have sufficient sensitivity to determine the concentration of uranium in aqueous environmental samples and in most cases mass spectrometric techniques can also provide isotopic composition data. However, in some samples, especially where the precise content of minor uranium isotopes is required then preconcentration, separation, and purification can improve the accuracy of the measurement. Several methods have been developed for this purpose based on solid phase extraction (SPE), electro-analytical selective absorption techniques, liquid-extraction, ion-exchange and chromatographic columns, co-precipitation, and selective sorption. Other methods, like single-drop microextraction, are being developed and may serve for microanalysis (Jain and Verma 2011). Some of these techniques are discussed in the context of the specific sample preparation procedures throughout the book, so in this section only a few select methods will be discussed. 

Magnetic hyperfine fields observed by Mbssbauer spectroscopy are useful information to characterize condensed matters. If a sample includes a certain amount of Fe or Sn, Mbssbauer measurements are easily applied. If the sample is able to be enriched with Mbssbauer isotope Fe or Sn, the limit of concentration required for the measurements can be greatly reduced. Specific samples for interface studies are prepared by doping with isotope selectively at the interface sites. In this article, several examples of Mbssbauer studies using interface-selectively enriched samples are introduced. 

Schemes for laser isotope enrichment all have some general requirements in common. Firstly, the isotomers of the starting material must have some discrete spectral absorptions that do not overlap. Secondly, the laser has to be sufficiently tunable and monochromatic to excite only one of these isotomers. Thirdly, a chemical or photochemical process is needed that acts selectively on the excited species, preferably to lock up the selected isotope in a new stable species. Finally, the effects of processes that destroy the isotopic selectivity must be minimized. One of the chief ways in which isotopic selectivity can be lost is by transfer of the excitation energy from one isotomer to another. The interplay between reactions of selectively excited species and inelastic collisions is a recurring theme in this chapter (see Section 1.3.4). Because their energy states are very close, the exchange of energy between different isotomers is nearly resonant and is likely to be rapid. In isotope enrichment experiments, these processes are detrimental, since they destroy the isotopic selectivity of the initial excitation.

Following the movement of airborne pollutants requires a natural or artificial tracer (a species specific to the source of the airborne pollutants) that can be experimentally measured at sites distant from the source. Limitations placed on the tracer, therefore, governed the design of the experimental procedure. These limitations included cost, the need to detect small quantities of the tracer, and the absence of the tracer from other natural sources. In addition, aerosols are emitted from high-temperature combustion sources that produce an abundance of very reactive species. The tracer, therefore, had to be both thermally and chemically stable. On the basis of these criteria, rare earth isotopes, such as those of Nd, were selected as tracers. The choice of tracer, in turn, dictated the analytical method (thermal ionization mass spectrometry, or TIMS) for measuring the isotopic abundances of... 

Conversion electron Mossbauer spectroscopy (CEMS) measurements with back scattering geometry have the merit that spectra can be obtained from a sample with much less isotope content compared with transmission measurements. Another merit is that a sample, deposited on a thick substrate, could be measured, and that because of the limited escape depth of the conversion electrons, depth-selective surface studies are possible. The CEMS technique was found to be best applicable to specimens of 10-100 pg Au cm, i.e., about two orders of magnitudes thinner than required for measurements in transmission mode [443]. This way (1) very thin films of gold alloys, as well as laser- and in beam-modified surfaces in the submicrometers range of depth [443], and (2) metallic gold precipitates in implanted MgO crystals [444] were investigated. 

There are methods available to quantify the total mass of americium in environmental samples. Mass spectrometric methods provide total mass measurements of americium isotopes (Dacheux and Aupiais 1997, 1998 Halverson 1984 Harvey et al. 1993) however, these detection methods have not gained the same popularity as is found for the radiochemical detection methods. This may relate to the higher purchase price of a MS system, the increased knowledge required to operate the equipment, and the selection by EPA of a-spectrometry for use in its standard analytical methods. Fluorimetric methods, which are commonly used to determine the total mass of uranium and curium in environmental samples, have limited utility to quantify americium, due to the low quantum yield of fluorescence for americium (Thouvenout et al. 1993). 

---