Isotopic labelling using basic reagents

By definition, an acid will donate a proton to a base, and it is converted into its conjugate base. Conversely, a base will accept a proton from a 

NaNH2 Sodium amide H2SO4 Sulfuric acid. S  

Ph3CNa Triphenylmethyl sodium. s P-CH3C6H4SO3H p-Toluenesulfonic acid b CD  

C2HsONa Sodium ethoxide Q CD CH3CO2H Acetic acid  

dissolving acetic acid in deuteriated water will rapidly give deuteriated acetic acid by acid-base equilibria. However, if the deuteriated acetic acid were then dissolved in normal water, the reverse process would wash out the label equally rapidly. 

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Until recently, this has been one of the most useful of the isotope dilution methods because it allows the quantitative conversion of a small amount of an unlabelled compound to an isotopically labelled derivative. The unlabelled compound is reacted quantitatively with a radioactive agent of known specific activity and the radioactive derivative is isolated, purified and counted. The activity recorded indicates the amount of radioactive reagent it contains and since the stoichiometry of the reaction is known the amount of compound present can be calculated. There are two basic requirements. First, the compound to be analysed must be quantitatively converted in high and reproducible yield to the radioactive derivative second, there must be a good method of isolating and purifying the labelled derivative. The first experiments to use an isotope derivative technique focused on the determination of amino acids in a mixture [263-265]. The reagent used was I-pipsyl chloride (/ -iodobenzenesulphonyl chloride). 

With the development of radioimmunoassays (RIAs) in the 1960s that used radioactive isotopes as labels (see Chapter 9), the measurement of radioactivity became a common and important practice in clinical laboratories. However, concerns about, and problems with, the safe handhng and disposal of radioactive reagents and waste have led to the development of immunoassays that use nonisotopic labels (see Chapter 9). The rapid acceptance and extensive use of nonisotopic immunoassays by the clinical laboratory have resulted in a decreased use of RIA and ultimately a decreased requirement for them to measure radioactivity. Because of this deemphasis on the necessity to measure radioactivity, only a brief discussion of the topic is presented here. Readers requiring more detail on this topic are referred to the chapter entitled Basic Principles of Radioactivity and Its Measurement that is included in a prior edition of this textbook, ... 

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