Isotopic labeling, categories

With regard to isotopomers, tw o sub-categories arise. The first is the natural abundance isotopomer distribution of the organics and organometallics. The second is specific isotopic labeling. Normally, the pure component spectra recovered are lumped para-... 

Dianions of the above types may not fall into the category of homoenolate in a strictly formal sense. Nevertheless the amide dianion does show a behavior typical of the homoenolate. Thus, the reaction of the isotopically labeled stannylpropionate results in scrambling of the label probably via a cyclopropane intermediate Eq. (47) [44]. As the result of such an equilibration, isomerization of a-methyl and a-phenyl substituted propionate homoenolates may occur to give the thermodynamically more favorable isomers, respectively. 

In the general field of reaction kinetics isotopic techniques are of considerable value for determining fine details in the mechanism of chemical reactions. Such isotopic studies in mercury-photosensitized reactions may be divided into three distinct categories the investigation of the kinetic isotope effects in the primary energy-transfer interactions, isotopic labeling and kinetic isotope effect in processes other than the primary one, and monoisotopic photoexcitation. 

The other major category of D values are termed interdiffusion or chemical diffusion coefficients. Experimentally, Ci does change significantly and D is a function of Ci. Diffusion of one or more chemical species is dependent on the opposing diffusion of another species in order to maintain a constant matrix volume and/or electrical neutrality. The diffusion in olivine of Mg in one direction and the complementary diffusion of Fe in the opposite direction represent one example. Rarely does this type of experiment employ the use of isotopically labeled species. However, in some cases isotopically-enriched H2O (T, D and/or O) has been used where the composition of the solid (melt) became significantly modified by incorporation of water into the structure. 

The sensitivity of hybridization assays is a function of the detection limit of the label, some of which are not only as sensitive as radiolabelled assays, but are much faster in exposure times to X-ray film. There are four broad categories of detection for non-isotopic labels. These are bi-oluminescence, colorimetry, electrochemiluminescence, and fluorescence. 

Some of the intended categories of use of radioisotopic reference material have been reviewed recently by Fajgelj et al. (1999). They include assignment of property values, establishing the traceability of a measurement result, determining the uncertainty of a measurement result, calibration of an apparatus, assessment of a measurement method, use for recovery studies and use for quality control purposes. It should be noted however that, in general, natural matrix reference materials are not recommended for calibration purposes. This should preferably be done with pure chemical forms of the element labelled with the isotope of interest. Calibrated isotopic sources of this kind are available from a number of commercial suppliers and are not the subject of this review. 

Most compounds in this category are tin hydrides or stannanes where a hydrogen isotope is bonded to the tin atom. The synthesis of these labelled stannanes will be presented in detail. However, the synthesis of other labelled tin compounds is also discussed. 

Methods based on chemiluminescent labels are other non-isotopic IAs and ILAs that continue to be the focus of active research. Most common approaches under this category are direct measurements of the chemiluminescence accomplished by the action of the imprinted polymer (e.g., catalytic effect) or by indirect methods, such as competitive CILAs. 

The internal standard should show physical and chemical properties that are as close as possible to those of the molecule that has to be measured. It must be pure, absent from the sample and, of course, inert towards the compounds in the sample. The internal standards can be classified into three categories structural analogues that are labelled with stable isotopes, structural homologues and compounds from the same chemical family. These various types of internal standards are classified here in descending order according to their usefulness and their price. In fact, the starting material for labelled compounds is fairly... 

Three categories of enantioselective procedures are the focus of this section. Two of them can be used with isotopic hydrogen, thereby introducing both deuterium or tritium and chirality into unlabeled substrates. These procedures can also be used with unlabeled reagent to introduce chirality into already labeled compounds. The third procedure is used to transform an achiral or prochiral labeled compound into a chiral one, but cannot introduce isotopic hydrogen or carbon. 

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