Medical brain notes 14

  IDENTIFICATION TESTS   The true identification of a drug may be accomplished in a number of ways, namely : determination of physical constants, chromatographic tests and finally the chemical tests. The physical constants essentially include the melting point, boiling point, refractive index, weight per millilitre, specific optical rotation, light absorption, viscosity, specific surface area, swelling power, infra-red absorption, and the like. The chromatographic tests include specific spot-tests by thin-layer chromatography (TLC) of pure drug or its presence in a multi-component system. However, the most specific and reliable are the chemical tests which may be categorized separately under tests for inorganic substances and organic substances. The former may be carried out by well defined general quantitative inorganic analysis and the latter by specific reactions of one or more of the functional moieties present in a drug molecule.

  PHYSICAL CONSTANTS   A wide range of physical constants, for instance : melting point, boiling point, specific gravity, viscosity, refractive index, solubility, polymorphic forms  vis-a-vis  particle size, in addition to characteristic absorption features and optical rotation play a vital role in characterization of pharmaceutical chemicals and drug substances. These physical constants will be discussed briefly with typical examples as under :   1. Melting Point   It is an important criterion to know the purity of a substance ; however, it has a few limitations. The accuracy and precision of melting point is dependent on a number of factors such as—capillary size, sample size, initial temperature of heating-block and the rate of rise of temperature per unit time (minutes). Keeping in view the different manufacturing processes available for a particular drug the melting point has a definite range usually known as the melting range. Thus the melting range t...

  LIMIT TESTS   VIS-A-VIS   QUANTITATIVE DETERMINATIONS   In general, limit tests are quantitative or semi-quantitative tests particularly put forward to identify and control invariably small quantities of impurity that are supposed to be present in a pharmaceutical substance. Obviously the amount of any single impurity present in an official substance is usually small, and therefore, the normal visible-reaction-response to any test for that impurity is also quite small. Hence, it is necessary and important to design the individual test in such a manner so as to avoid possible errors in the hands of various analysts. It may be achieved by taking into consideration the following  three  cardinal factors, namely :   ( a )  Specificity of the Tests :  A test employed as a limit test should imply some sort of selective reaction with the trace impurity. It has been observed that a less specific test which limits a number of possible impurities rat...

  VARIOUS TYPES OF TESTS FOR QUANTITATIVE DETERMINATIONS   In actual practice, it has been observed that different  official compendia  describe a number of detailed types of tests with a view to obtain a constant and regular check that might be possible to maintain the desired degree of optimum purity both in the pure pharmaceutical substances and the respective dosage-forms made therefrom.   A number of such tests shall be discussed here briefly with specific examples wherever possible and necessary :   1. Limits of Insoluble Matter   The limits of insoluble matter present in pharmaceutical substances and stated in various  official com-pendia  are given below : In the same vein, tests for clarity of solution offer another means of limiting insoluble parent drug sub-stances in their correspondingly more highly water-soluble derivatives.   2. Limits of Soluble Matter   In order to detect the presence of some very specific impuritie...