Isotope displacement

Atomic absorption offers a more practical opportunity for determining isotopic composition than atomic emission. Useful reviews of the possibilities of the technique have appeared in two books [233, 234]. Isotopic analysis is in theory possible provided that highly enriched isotope sources are available, the absorption line width available is less than the isotopic displacement and for a given isotope the nuclear spin hyperfine components must be partially resolved from the other isotopic components of the absorption line. In the simplest possible case, for an element with two isotopes, the lamp is prepared from the first isotope and only this isotope in the atom cell will absorb the radiation. The procedure can then be repeated with a lamp prepared with the second isotope. Effectively this is an extension of the impressive selectivity of atomic absorption, because of the classic lock and key effect, treating the different isotopes as different analytes. 

Progesterone (P) Isotope displacement (TLC) Addition of known amounts to HjO (open) 12% 0.5jiig 38% 0.2jag N1... 

Sensitivity is closely related to the specificity and precision of a method. As an extremely important aspect of any method for plasma steroids, and of many for urinary steroids, it has been much discussed. In terms of the best levels of signal-to-noise ratio obtained with pure substances under ideal conditions, the mass spectrometer attached directly to a good GLC column is probably the most sensitive analytical method currently available. In terms of steroids in natural sources the most sensitive methods at present established are those using the isotopic-displacement (protein-binding) technique (0.1-0.5 ng) and electron-... 

The differentiation of normal from abnormal concentrations of plasma testosterone in women requires the use of either electron-capture GLC, isotopic-displacement methods, double-isotope methods, or fluorometric methods. Evidence to date suggests that the isotopic-displacement method is preferable (K3, M3). Existing fluorometric methods are adequate in analytical characteristics, but much more laborious (F2). This differentiation is crucial in cases of simple, provisionally idiopathic, hirsutism (cf. however. Cl). In many cases, however, an elevated excretion of urinary testosterone glucuronide would be equally satisfactory despite the uncertainty of its metabolic precursors (B2, II). This measurement is also desirable in cases of provisionally idiopathic amenorrhea. 

Other problems of differential diagnosis in which chemical measurements are of crucial value all involve moderately sensitive, or at most, highly sensitive methods, with the exception of the acute obstetric problems mentioned previously. In many of these problems a battery of simple and well-established colorimetric and fluorometric methods not involving chromatography could reasonably be used. Thus cortisol could be measured by a variant of the Silber and Busch method (see B17) or an isotope-displacement method (M7, N6) 17-ketosteroids and 17-keto-... 

Isotope-displacement Plasma and urinary estrogens, Estrogens—good ... 

One can reasonably hope that rapid ultrasensitive methods based on the isotope-displacement method will soon be available for the principal estrogens. A promising method has been published by Korenman (Kll). The reliability of this assay and the ease of preparation of the uterine macromolecule used in this assay remain to be established. 

The most complete account of an isotope-displacement method for the ultrasensitive range of testosterone measurements in plasma is that of... 

These methods cannot be recommended for routine use at present in preference to the isotope-displacement methods. The latter are rather more complex than those for progesterone because the chromatographic separation procedure has to be more extensive for testosterone. Thus,... 

G. Racah, Isotopic Displacement and Hyperfine Structure, Nature (London) 129 (1932) 723-733. 

Several different reasons can be considered for He isotope displacements between the xenoliths and the range of MORE values that now serve as a reference ... 

Bartell treats the vibrations as harmonic, so that the linear terms and cross terms in eq. (III-28) drop out, leaving only the square and higher even terms. He then, somewhat arbitrarily, assumes functional forms and assigns attractive and repulsive parameters for the H H, C H, and C C interactions. Since the functional form of the combined interactions is such that the second derivative of the potential energy is positive in the isotopic displacements, i.e., repulsive forces predominate, the smaller mean-square amplitudes in the deuterated molecules lead to a lower average steric potential energy. This means that release of steric compression in the product... 

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