Normal solvent strength

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. 

SOLVENT STRENGTH PARAMETERS FOR SOLVENTS COMMONLY VSBD IN NORMAL PHASE TLC... 

The choice of solvent directly influences the retention of the analyte on the sorbent and its subsequent elution, whereas the solvent polarity determines the solvent strength (or ability to elute the analyte from the sorbent in a smaller volume than a weaker solvent). Dean [272] gave solvent strengths for normal- and reversed-phase sorbents. The elution solvent should be one in which the analytes are soluble and should ideally be compatible with the final analysis technique. For example, for HPLC analysis, a solvent similar to the mobile phase is a good choice of elution solvent. For the elution step it is also important to consider the volume of the solvent. A minimum volume of elution solvent (typically 250 xL per 100 mg of sorbent) allows maximum concentration of the analytes. 

The term polarity refers to the ability of a sample or solvent molecule to interact by combination of dispersion, dipole, hydrogen bonding, and dielectric interactions (see Chapter 2 in reference 5). The combination of these four intermolecular attractive forces constitutes the solvent polarity, which is a measure of the strength of the solvent. Solvent strength increases with polarity in normal phase, and adsorption HPLC decreases with polarity in reversed-phase HPLC. Thus, polar solvents preferentially attract and dissolve polar solute molecules. 

Figure 4.19 Solvent strength of combination of n-pentane and more polar solvents in normal-phase liquid chromatography using alumina. Symbols. A, methyl acetate, 0> acetone, , chloroform, and O, benzene.

Relative elution solvent strength (or eluotropic strength) is depicted in solvent polarity charts (Figure 2.39). The relative elution strength for a solvent on a polar, normal-phase sorbent such as silica or alumina increases in reverse order to that measured on a nonpolar, reversed-phase sorbent. Ac-... 

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. 

The other two parameters are defined similarly the sum of the three parameters is thus normalized to 1. Values for some common solvents are listed in Table 15 (along with the Hildebrand solubility parameters and the Snyder solvent strength parameters). 

Table 3.1 Elutropic series for some modifiers commonly used in normal-phase and reversed-phase liquid chromatography, arranged in order of increasing solvent strength...

Figure 3.5 Solvent strengths (e values) various solvent mixtures in normal-phase liquid 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. 

As in normal phase (see section 3.5.3), the first step in mobile-phase optimization is the determination of the solvent strength that will elute the analytes with a A value between 2 and 10 from the chosen stationary phase. It is not important which modifier is chosen to determine the initial conditions, and methanol-water (50 50, v/v) is a convenient starting place. Once the initial conditions have been established, a variety of techniques may be employed to obtain the optimum separation. Most optimization strategies involve the establishment of the isoelutropic concentrations of methanol-water, acetonitrile-water and tetrahydrofuran-water. The isoelutropic concentrations can be determined by experiment or from tables of isoelutropic mixtures (e.g. Table 3.5) (Wells, 1988). The binary solvent systems A, B, C (Table 3.5, Figure 3.7) define the isoelutropic plane, which is then explored to obtain the optimum combination of water, methanol, tetrahydrofuran, and acetonitrile required for the separation. 

In both normal phase and reversed phase HPLC, the eluting power or solvent strength of the mobile phase is mainly determined by its polarity. Although most analysts have a good idea of what the term polarity implies and could rank most common solvents in order of their polarity, a more quantitative description would be very useful in chromatography. 

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]. 

---