Chromatographic solvents, properties

Solvent-property detector A detector which monitors a property of the HPLC mobile phase which is perturbed when an analyte elutes from the chromatographic column. 

Snyder s classification of solvent properties is important in the selection of the chromatographic conditions and the optimization of the chromatographic processes. 

L.R. Snyder, Classification of the solvent properties of common liquids. Journal of Chromatographic Science, 16 (1978) 223-234. 

Poole, C.R, Chromatographic and spectroscopic methods for the determination of solvent properties of room temperature ionic liquids, /. Chromatogr. A, 1037, 49-82, 2004. 

This makes it possible to tune solvent properties to optimize chromatographic separations. Because of the lower viscosity and higher diffusivity of supercritical fluids compared to common solvents, a higher mobile phase velocity can be used in the column, leading to a higher process throughput than that of liquid chromatography. 

All other factors are not equal. Gases, being noninteractive with solutes, contribute neither selectivity nor solvent power (the ability to dissolve solutes) to the chromatographic process (see Section 2.5). GC can therefore be applied only to volatile solutes. Liquids are the most versatile and generally the most effective mobile phases based on their solvent properties. SFs are again intermediate. 

Before discussing the actual pumps, let us consider the mobile phase and its requirements. The mobile liquid must be very pure, and special chromatographic grades are available for most common solvents, including water. Some solvents are routinely stabilized with small amounts of chemicals that can significantly alter their solvent properties for LC use and may absorb in the UV. For example, chloroform is often stabilized with ethanol or pentene. Even particulate matter finds its way into pure solvents requiring the use of a filter in the intake line from the solvent reservoir. 

A substance is said to be in the gaseous state when heated to temperatures beyond its critical point. However, the physical properties of a substance near the critical point are intermediate between those of normal gases and liquids, and it is appropriate to consider such supercritical fluid as a fourth state of matter. For applications such as cleaning, extraction and chromatographic purposes, supercritical fluid often has more desirable transport properties than a liquid and orders of magnitude better solvent properties than a gas. Typical physical properties of a gas, a liquid, and a supercritical fluid are compared in Table 1. The data show the order of magnitude and one can note that the viscosity of a supercritical fluid is generally comparable to that of a gas while its diffusivity lies between that of a gas and a liquid. 

Poole SK, Shetty PH, Poole CF (1989) Chromatographic and spectroscopic studies of the solvent properties of a new series of room-temperature liquid tetraalkylammonium sulfonates. Anal Chim Acta 218 241-264... 

Grant, R. (1992) Properties of chromatographic solvents. LC/GC Int 5, 25 (selected data reproduced with permission). 

SCFs may be used in the same way as other ordinary solvents taking into account their different properties and behaviors. Supercritical fluids can replace liquids solvents in many processes, such as extractions from solids (leaching), countercurrent multistage separations, chromatographic separations, and others, provided the solvent properties of the SCFs are adequate. 

For chromatographic applications, the most useful models of solvent properties are the solubility parameter concept, Snyder s solvent strength and selectivity parameters, solvatochromic parameters and the system constants of the solvation parameter model for gas to liquid transfer. The Hildebrand solubility parameter, 8h (total solubility parameter), is a rough measure of solvent strength, and is easily caleulated from the physical properties of the pure solvent. It is equivalent to the square root of the solvent vaporization energy divided by its molar volume. The original solubility parameter concept was developed from assumptions of regular solution behavior in which the principal intermolecular interactions were dominated by dispersion forces. 

The properties of solvents has been studied extensively by Snyder (5), who created a dassification of the solvent properties of common solvents. It has been found (7) that (excluding proton donors such as alcohols) the maximum difference in mobile-phase selectivity is obtained if the polar solvents have a large difference in basidty. Thus, for maximum selectivity differences, one solvent should have a low basidty. Solvents of this type are acetonitrile, ethyl acetate or other esters, and acetone or other ketones. The other solvent should have a high basidty examples are ethers such as tert-butyl methyl ether, diethyl ether or tetrahydrofuran, or amines such as triethylamine. Between these groups and alcohols, large differences in chromatographic selectivity can be obtained in normal-phase chromatography (10). 

Poole CF (2004) Chromatographic and spectroscopic methods for the determination of solvent properties of... 

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