Mobile Phases for Normal-Phase Chromatography

The mobile phases used in normal-phase chromatography are based on nonpolar hydrocarbons, such as hexane, heptane, or octane, to which is added a small amount of a more polar solvent, such as 2-propanol.5 Solvent selectivity is controlled by the nature of the added solvent. Additives with large dipole moments, such as methylene chloride and 1,2-dichlor-oethane, interact preferentially with solutes that have large dipole moments, such as nitro- compounds, nitriles, amines, and sulfoxides. Good proton donors such as chloroform, m-cresol, and water interact preferentially with basic solutes such as amines and sulfoxides, whereas good proton acceptors such as alcohols, ethers, and amines tend to interact best with hydroxylated molecules such as acids and phenols. A variety of solvents used as mobile phases in normal-phase chromatography are listed in Table 2.2, some of which may need to be stabilized by addition of an antioxidant, such as 3-5% ethanol, because of the propensity for peroxide formation. 

Saturated hydrocarbons olefins aromatic hydrocarbons = organic halides sulfides ethers nitro compounds esters = aldehydes = ketones alcohols = amines sulfones amides carboxylic acids 

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. 

---

In normal-phase chromatography, the retention is governed by the interaction of the polar parts of the stationary phase and solute. For retention to occur in normal phase, the packing must be more polar than the mobile phase with respect to the sample. Therefore, the stationary phase is usually silica and typical mobile phases for normal phase chromatography are hexane, methylene chloride, chloroform, diethyl ether, and mixtures of these. In reverse phase the packing is nonpolar and the solvent is polar with respect to the sample. Retention is the result of the interaction of the nonpolar components of the solutes and the nonpolar stationary phase. Typical stationary phases are nonpolar hydrocarbons, waxy liquids, or bonded hydrocarbons (such as Ci8, Q, etc.) and the solvents are polar aqueous-organic mixtures such as methanol-water or acetonitrile-water. In the strictest interpretation, normal and reverse phase are terms which only relate to the polarity of the column and mobile phase with respect to the sample as shown in Table 3-3 and drawn schematically in Figure 3-14. 

M.A. Hemandez-Torres, J.S. Landy and J.G. Dorsey, Reversed Micellar Mobile Phases for Normal Phase Chromatography, Anal. Chem., 58 744 (1986). 

---