Liquid-solid chromatography solvent selection

L. R. Snyder, J. L. Glajch, and J. J. Kirkland, Theoretical basis for systematic optimization of mobile phase selectivity in liquid-solid chromatography solvent-solute localization effects, /. Chromatogr. 218 (1981), 299-326. 

Fig. 22. Hypothetical example of retention optimization in liquid-solid chromatography. (a) Solvent strength k is optimized (b) selectivity a.

For most samples, which are soluble in organic solvents and of low molecular weight, adsorption chromatography (also called liquid-solid chromatography, LSC) on silica gel, aluminum oxide, or other inorganic oxides is suitable. Separation is based on specific interaction of the functional groups on the surface of the adsorbent with those of the sample. Silica gel is by far the most widely used adsorbent, available in consistent quality and at comparatively low cost. Due to its ionic nature aluminum oxide offers additional selectivity however, it is more difficult to obtain products with consistent separation properties. Therefore, separations on aluminum... 

Sample preparation for analysis by hyphenated methods requires some additional planning when compared to nonhyphenated methods. All steps, extraction, concentration, and final solvent selection must take into consideration and be compatible with all the components of the hyphenated instrumentation. For gas chromatographic methods, all the components in the mixture must be in the gaseous state. For liquid chromatography (LC) or high-performance liquid chromatography (HPLC), the samples of the analytes of interest can be solids or liquids, neutral or charged molecules, or ions, but they must be in solution. If the follow-on analysis is by MS, then each of the analytes may require a different method of introduction into the MS. Metals and metal ions may be introduced by HPLC if they are in solution but commonly are introduced via AAS or inductively coupled plasma (ICP). Other analytes may be directly introduced from HPLC to MS [2],... 

LIQUID CHROMATOGRAPHY. An analytical method based on separation of the components of a mixture in solution by selective adsorption. All systems include a moving solvent, a means of producing solvent motion (such us gravity or a pump I, a means ol sample introduction, a fractionating column, and a detector. Innovations in functional systems provide the analytical capability for operating in three separation modes (1) liquid-liquid partition in which separations depend on relative solubilities of sample components in two immiscible solvents (one of which is usually water) 12) liquid-solid adsorption where the differences in polarities nf sample components and their relative adsorption on an active surface determine tile degree ol separation (2) molecular size separations which depend on the effective molecular size of sample components ill solution. 

In recent years h.p.l.c. has become a valuable chromatographic tool for analytical and preparative scale work. In this latter area the separation of isomers (structural, diastereoisomeric, and enantiomeric) has been possible by the selection of appropriate column packing material and solvent systems. However, the equipment, operating costs, and column packing materials are more expensive than those in t.l.c., g.l.c. and conventional liquid-solid column chromatography. 

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