Common Methods of Characterization

Amidon and Houghton [48] completed a comparative study of several common methods of characterizing powder flow. Table 2 contains experimental results for a number of commonly used pharmaceutical excipients. Compressibility index, angle of repose, flow rate through an orifice, and shear cell data are presented. 

This article describes the preparation, properties (including characterization techniques), and potential applications of mesoporous materials. The article begins by examining the chemistry of surfactant/inorganic precursor solutions and discusses its application to the synthesis of mesoporous materials. The most common methods of characterization and the properties of the materials are described and finally potential applications are mentioned. This article is therefore intended to provide a general overview of the synthesis, formation mechanisms, characterization, properties, and applications of mesoporous molecular sieves. 

One of the common methods of characterizing porosity is finding the apparent porosity. This corresponds to the amount of open pores. The total porosity includes both open and closed pores. Open pores affect permeability, vacuum tightness, and the surface available for catalytic reactions and chemical attacks. Permeability increases with porosity, vacuum tightness decreases with it, and the surface available for catalytic reactions and chemical attacks increases. Before the firing process starts, almost the entire pores are open. 

This generalization is of great value. It is based upon exactly the type of experiment you have performed. We have confidence in the rule because this type of experiment has been conducted successfully on hundreds of thousands of substances. The melting behavior is one of the most commonly used methods of characterizing a substance. It leads us to wonder if every solid can be converted to a liquid if the temperature is raised sufficiently. Further, it leads us to wonder... 

The earliest work on dendrimer characterization was concerned with aspects of the organic chemistry - did the proposed chemical reaction take place without side reactions and what was the conversion Since near-100% conversion and near-perfect removal of excess reactants is required for making pure dendrimer, common methods of spectroscopy and chromatography can be used to verify the structure. In a wide variety of dendrimer chemistries, nearly perfect structures have been produced, at least for earlier generations wherein the techniques are more quantitative. 

Dynamic mechanical experiments, where the material is periodically strained, are common methods to characterize the visco-elastic behavior of elastomers by measuring the storage modulus G and loss modulus G". G is a measure for the maximal, reversibly stored energy for a periodical deformation and G" is proportional to the dissipated energy for the oscillation cycle. It is obvious to investigate, whether the l.c. state of the l.c. elastomers influences the dynamic mechanical properties and whether different modes of linking the mesogenic moieties to the backbone can be detected. 

However, for many applications we do not need such a complete characterization of the state of the mixture in practice, simple methods often suffice. Commonly used methods, for example, are color comparison to a standard for qualitative visual homogeneity, or the measurement of some representative physical property. Between the two extremes of complete characterization and qualitative or semiqualitative practical evaluation, there is room for sound quantitative methods of characterization. We must keep in mind, however, that the goodness of mixing is not absolute, but dependent on the required needs. 

Polarity is one of the most important parameters of ILs for its effect on electrochemical reactions. It is important when we characterize ILs to measure not only thermal properties such as melting point but also solvent properties such as polarity [68-70]. The most common method of polarity measurement is a dielectric constant measurement. Weingartner et al. and Hefter et al. have shown, by applying appropriately high frequency methods, that the dielectric constants are uniformly around 10-15. Accordingly, the polarity of ILs should be estimated by other methods. Solvatochromism is heavily applied for this purpose due to its simplicity. 

The properties of a solid which are of greatest interest to an organic chemist are vapor pressure, melting point, and solubility. The most common methods of purifying and characterizing solids depend on these properties. Other properties such as crystal structure, density, and refractive index are now rarely used and will not be considered here. 

As the surface area exposed to the site of administration determines the speed with which a particle dissolves in accordance with the Noyes-Whitney equation, these determinations are important. In addition, in those instances where the particle size is difficult to measure, a gross estimation of the surface area is the second best parameter to characterize the drug. The most common methods of surface area measurement, including gas adsorption (nitrogen or krypton), based on what is most commonly described as the Braunauer, Emmet, and Teller (BET) method, is applied either as a multipoint or single-point determination. 

Spectrophotometric analyses are the most common method to characterize proteins. TTie use of ultraviolet-visible (UV-VIS) spectroscopy is t rpically used for the determination of protein concentration by using either a dye-binding assay (e.g., the Bradford or Lowry method) or by determining the absorption of a solution of protein at one or more wavelengths in the near UVregion (260-280 nm). Another spectroscopic method used in the early-phase characterization of biopharmaceuticals is CD. 

Washington (1992) has discussed the concepts and techniques of particle size analysis and its role in pharmaceutical sciences and other industries. There are many different methods available for particle size analysis. The techniques most readily available include sieving, optical microscopy in conjunction with image analysis, electron microscopy, the Coulter Counter and laser diffractometers. Size characterization is simple for spherical particles, but not for irregular particles where the assigned size will depend on the method of characterization used. Table 6.2 lists particle size measurement methods commonly used and the corresponding approximate useful size range (Mullin 1993). 

The hypotheses and interpretations proposed here need further testing with other zeolite frameworks and additional spectral analysis. Infrared spectroscopy must be used as a supplemental tool to characterize zeolites, along with the more common methods of determining structure and properties e.g., x-ray crystallography, chemical analysis, adsorption, and other characterizations. 

Figure 5. Common methods of particle size characterization. (Adapted from references 2 and 7.)...

There are many different data analysis schemes to estimate the structure and molecular parameters of polymers from the neutron scattering data. Herein, we will present several common methods for characterizing the scattering profiles, depending only on the applicable q range. These methods, which were derived based on different assumptions, have... 

Heats of adsorption can be experimentally measured by calorimetry, temperature programmed desorption (TPD), and adsorption isotherms taken at different temperatures. Calorimetry involves the direct measurement of temperature rise caused by the adsorption of a known amount of gas on to a well-characterized surface. TPD is the most common method of determining the heats of adsorption. In this procedure, molecules are adsorbed on to a clean well-characterized substrate at a fixed temperature. The sample is then heated in a linear fashion while the pressure of the desorbing species is monitored with a mass spectrometer. The desorption rate E(t) is given by... 

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