Chromatography - introduction

Chromatography is used to separate the individual components of a mixture on the basis of differences in their physical characteristics, e.g. molecular size, shape, charge, volatility, solubility and/or adsorption properties. The essential components of a chromatographic system are  

The individual substances in the mixture interact with the stationary phase to different extents, as they are carried through the system, enabling separation to be achieved. 

In a chromatographic system, those substances which interact strongly with the stationary phase will be retarded to the greatest extent, while those which show little interaction will pass through with minimal delay, leading to differences in distances travelled or elution times. 

Chromatography is sub-divided according to the mechanism of interaction of the solute with the stationary phase. 

This is based on the partitioning of a substance between two liquid phases, in this instance the stationary and mobile phases. Substances which are more soluble in the mobile phase will pass rapidly through the system while those which favour the stationary phase will be retarded (Fig. 31.2). In normal phase partition chromatography the stationary phase is a polar solvent, usually water, supported by a solid matrix (e.g. cellulose fibres in paper chromatography) and the mobile phase is an immiscible, non-polar organic solvent. For reversed-phase partition chromatography the stationary phase is 

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A critical review of the history of the ion-pair concept indicates that considerations other than electrostatic were scarcely provided by model makers. Clearly, for chromatography, solvophobic interactions usually neglected by ion-pair model makers are aucial. Chromatographers involved in ion-pair strategies realized that the theoretical description of the ion-pair was critical at the time of the ion-pair chromatography introduction in the late 1970s and subsequently, the electrostatic model of ion association dominated scientific debate. 

Ion-interaction chromatography is an intermediate between reversed-phase and ion-exchange chromatography. Introduction of amphiphilic and Uo-philic ions into the mobile phase causes their adsorption on the hydrophobic surface of packing material with subsequent transformation into a pseudo ion-exchange surface. Ionic interactions with charged analytes can occur in the mobile phase and with counterions that may be adsorbed on the stationary-phase surface. 

Dorsey JG. Liquid Chromatography Introduction. In Meyers, RA (ed). Encyclopedia of analytical chemistry, Chichester John Wiley Sons, 2000 11231-3,... 

Stop-flow injection In high-performance liquid chromatography, introduction of the sample at the head of the column while solvent flow is temporarily discontinued. 

KEYWORDS selenium, selenocyanate, selenite, selenate, ICP-MS, and ion chromatography Introduction... 

Polysaccharides (such as agarose and dextrin) are used in chromatography. Introduction of dyes permits certain materials to be selectively separated. 

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