Chemical precipitation separation method principle

A number of analytical methods are used to detect and determine the radiochemical impurities in a given radiopharmaceutical. Most commonly used are methods like paper (PC), thin-layer (TLC), and gel chromatography, paper and gel electrophoresis, HPLC, and precipitation. A common principle for the different methods is that they can chemically separate the different radiolabeled components in the radiopharmaceutical. It may sometimes be necessary to perform more than... 

Advantages and Limitations of Radiometric Titrations. Radiometric detection of the equivalence point is a general method that does not depend on the chemical reaction employed. This contrasts with other methods of detection, which depend on specific chemical or physical transitions at the equivalence point. Amperometric titrations are applicable only to electrochemically active systems conductometric titrations apply only to ionic solutions, and so on. In principle, any titration system in which a phase separation can be effected is amenable to radiometric detection, provided there exist suitable radioactive labels. The major limitation of the method is the requirement for phase separation. In precipitation titrations, the phase separation is automatic and the method is well suited to this class of titrations. For other classes of titrations, special phase-separation methods, such as solvent extraction, need to be applied. At the present time, the method suffers from a lack of phase-separation techniques suitable for continuous monitoring of the titration curves. 

The fundamental principle of the new method 9-11) is to separate the precipitation operation into two stages. The first stage is to make three superposed layers of salt solutions and then to mix them into a supersaturated solution instantaneously. The second step is to form a homogeneous precipitate from this supersaturated solution (PFHS) (12). The amount of time between the first step and the second one is varied with the chemical species of the salts and their concentration in the supersaturated solution but usually is from a few seconds to several minutes and sometimes to a few hours. This duration is an induction period of precipitation, during which the supersaturated solution keeps... 

After separation from excess reagent (by liquid-liquid distribution, chromatography, precipitation, etc.), the mass or concentration of this product is determined from activity measurement. The determination is based on a radioactive substance chemically different from the analyte substance (in contrast to isotope dilution analysis) therefore, the chemical reaction is of prime importance. By variation of this key reaction, the principle can be adapted to various procedures. The superiority of radio-reagent methods over classical separation techniques arises from the use of an inactive carrier and the high sensitivity of the activity measurements, which are not subject to interference by the carrier or other substances. 

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