This chapter defines the terms and procedures used in chromatography and provides general information.Specific requirements for chromatographic tests and assays of drug substances and dosage forms,including adsorbent and developing solvents,are given in the individual monographs.
Chromatography is defined as a procedure by which solutes are separated by a dynamic differential migration process in a system consisting of two or more phases,one of which moves continuously in a given direction and in which the individual substances exhibit different mobilities by reason of differences in adsorption,partition,solubility,vapor pressure,molecular size,or ionic charge density.The individual substances thus obtained can be identified or determined by analytical methods.
The general chromatographic technique requires that a solute undergo distribution between two phases,one of them fixed (stationary phase),the other moving (mobile phase).It is the mobile phase that transfers the solute through the medium until it eventually emerges separated from other solutes that are eluted earlier or later.Generally,the solute is transported through the separation medium by means of a flowing stream of a liquid or a gaseous solvent known as the “eluant.”The stationary phase may act through adsorption,as in the case of adsorbents such as activated alumina,silica gel,and ion-exchange resins,or it may act by dissolving the solute,thus partitioning the latter between the stationary and mobile phases.In the latter process,a liquid coating held on an inert support serves as the stationary phase.Partitioning is the predominant mechanism of separation in gas-liquid chromatography,paper chromatography,and forms of column chromatography designated as liquid-liquid chromatography.In practice,separations frequently result from a combination of adsorption and partitioning effects.
The types of chromatography useful in qualitative and quantitative analysis that are employed in the USPassays and tests are Column,Gas,Paper,Thin-Layer,and Pressurized Liquid Chromatography (commonly called high-pressure or high-performance liquid chromatography).Paper and thin-layer chromatography are ordinarily more useful for purposes of identification,because of their convenience and simplicity.Column chromatography offers a wider choice of stationary phases and is useful for the separation of individual compounds,in quantity,from mixtures.Both gas chromatography and pressurized liquid chromatography require more elaborate apparatus and usually provide high-resolution methods that will identify and quantitate very small amounts of material.