Normal-phase chromatography solvent strength

Adsorption chromatography on bare silica is an example of normal-phase chromatography, in which we use a polar stationary phase and a less polar solvent. A more polar solvent has a higher eluent strength. Reversed-phase chromatography is the more common... 

The strength of the solvent is defined by the solvent strength parameter, e°, as listed in Table 2.2. A solvent with a low e° is chosen, and quantities of a second solvent with a greater s° are added until the desired separation is achieved. If the desired separation does not result from altering the concentration of the second solvent, either the nature of the second solvent can be changed or another additive can be introduced. Readers are directed to Refs. 1, 6, and 7 for in-depth discussions on the development of mobile phases for normal-phase chromatography. 

Both the solvent-interaction model (Scott and Kucera, 1979) and the solvent-competition model (Snyder, 1968, 1983) have been used to describe the effects of mobile-phase composition on retention in normal-phase liquid chromatography. The solvent interaction model on the one hand provides a convenient mathematical model for describing the relationship between retention and mobile phase composition. The solvent competition model on the other hand provides a more complete, quantitative description of the relative strengths of adsorbents and organic solvents used in normal-phase chromatography. 

The solvent-strength parameters for the common solvents used in normal phase chromatography on carbon are quite different from those of silica or alumina. Thus carbon offers quite different selectivities than alumina and silica for normal-phase chromatography. However, the lack of a reproducible commercial source for carbon was for many years a significant limitation to its widespread application. In addition the sensitivity of carbon to changes in solvent strength is much less than that of silica or alumina. 

Retention in normal-phase chromatography increases as the polarity of the mobile phase decreases. The selectivity of the analytes may arise from the differences in solvent strengths (eq), acidity, basicity, and dipolar nature of the mobile phase. Furthermore, solvent localization of the mobile phase plays a major role in the retention of the analytes [15,16]. These solvent strengths have been shown to be different when used with varied stationary-phase packings such as alumina, diol, and silica [3,17]. 

In general, the eluting strength of commonly used solvents for normal phase chromatography is stationary phase = silica gel neutral alumina increasing order proceeds as follows petroleum ethers < hexanes < cyclohexane < toluene < diethyl ether < dichloromethane < chloroform < ethyl acetate < acetone < ethanol < methanol < acetic acid. 

Solvents commonly used in normal phase chromatography are aliphatic hydrocarbons, such as hexane and heptane, halogenated hydrocarbons (e.g., chloroform and dichloromethane), and oxygenated solvents such as diethyl ether, ethyl acetate, and butyl acetate. More polar mobile phase additives such as isopropanol, acetone, and methanol are frequently used see Table 2). The technique is particularly suited to analytes that are very hydrophobic, e.g., fat-soluble vitamins such as tocopherols (6J and other hydrocarbon-rich metabolites that exhibit poor solubility in the water-miscible solvents employed in other separation modes. In addition, since the geometry of the polar adsorbent surface is fixed, the technique is useful for the separation of positional isomers the proximity of functional groups to the adsorbent surface, and hence the strength of interaction, may well differ between isomers. 

The order of solvents is according to increasing eluotropic strength in normal phase chromatography [2). 

The retention mechanism and solvent selectivity have been studied most carefully with alumina or silica as stationary phases. The knowledge of both for bonded phases used in normal-phase chromatography is much more limited. Nevertheless, it is safe to assume that similar selectivity rules for solvent strength and selectivity can be applied, especially since the results obtained for alumina and silica correlate well with each other. 

Why does eluent strength increase in normal-phase chromatography when a more polar solvent is added ... 

Normal Phase. As rationalized in III.A.2, the strength of normal-phase chromatography for vitamin E lies in its ability to separate all tocopherols and tocotrienols, including the positional isomers, particularly in connection with the analysis of vegetable oils and foods. Silica continues to be the most popular column material for this purpose, but polar bonded phases have increasingly gained a foothold (Table 3). Diol phases in particular can be readily substituted for silica and can be eluted with similar binary mobile phases containing a hydrocarbon as a base solvent and an alcohol, an aliphatic ether, or a cyclic ether as... 

Recently, Janjic et al. published some papers [33-36] on the influence of the stationary and mobile phase composition on the solvent strength parameter e° and SP, the system parameter (SP = log xjx, where and denote the mole fractions of the modiher in the stationary and the mobile phase, respectively) in normal phase and reversed-phase column chromatography. They established a linear dependence between SP and the Snyder s solvent strength parameters e° by performing experiments with binary solvent mixtures on silica and alumina layers. 

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