Snyders Solvent Parameter

The gradient system should be capable of producing a variety of gradient shapes. Linear gradients are common, but the Snyder solvent parameter produced will not likely be linear, as was shown earlier in Figure... 

Va Adsorbent surface volume (see Snyder solvent parameter)... 

Polarity is a key word in many chromatographic separations since a polar mobile phase will give rise to a low solute retention in normal phase LC (liquid-solid chromatography, LSC, of adsorption chromatography), or to a high solute retention in reversed-phase LC (RPLC). Nevertheless, it is often unclear exactly what this term means. One way to define the concept of polarity is to consider the Hildebrand solubility parameter another is to consider the Snyder solvent parameter. 

However, not withstanding the above objections, further discussion of the Snyder solvent triangle classification method is justified by its common use in many solvent optimization schemes in liquid chromatography. The polarity index, P, is given by the sum of the logarithms of the polar distribution constants for ethanol, dioxane and nltromethane and the selectivity parameters, X, as the ratio of the polar distribution constant for solute i to... 

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). 

Effect of Solvent Strength on Retention Factors. Solvents that interact strongly with. solutes are often termed strong" solvents. Strong solvents are often, but not always, polar solvents. Solvent strength depends on the nature of the analyte and stationary phase. Several indexes have been developed fur quantitatively describing the polarity of solvents. The most useful of these for partition chromatography is the polarity index P which was developed by Snyder." This parameter is based on. solubility measurements for the substance in question in three solvents dioxane (a low ... 

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. 

One of the most important problems of planar chromatography is that of the optimization of solvent systems for the separation of mixtures of different samples. An analyst is interested in obtaining the expected result using a minimum number of experiments. Snyder has introduced a new system for solvent classification that permits a logical selection of solvents both in term of polarity indices (F ) and selectivity parameters (Xj), proving theoretically the validity of such universal solvent systems [18,38,41,42]. 

The correlation of Snyder s solvent strength e° with molecular dipolarity and polarizability (7t ) and the hydrogen-bond acidity (a) and the hydrogen-bond basicity ((3) solvatochromic parameters for adsorption chromatography can be achieved, although most papers on solvatochromic parameters deal with reversed-phase systems [18]. 

Eluotropic series can be expressed more precisely by the Snyder parameter e°, given in Table 4.2 for alumina and silica gel. It is important to take into account eluotropic series and know some rules when solvents are used according to this table [16,17] ... 

Seleetion of basic parameters of chromatographic process (stationary phase, proper pure solvents according to Snyder s classification, and vapor phase)... 

A. de Juan, G. Fonrodona and E. Casassas, Solvent classification based on solvatochromic parameters a comparison with the Snyder approach. Trends Anal. Chem., 16(1) (1997) 52-62. T. Naes, The design of calibration in near infra-red reflectance analysis by clustering. J. Chemom., 1 (1987) 129-134. 

A number of models have been proposed to describe the solution formation process [505-509], some of which can be extended to Include chromatographic processes and other solvent-dependent phenomena. In terms of chromatographic aiqplications the most useful are the solubility parameter concept, solvatochromic parameters and Snyder s solvent strength and selectivity... 

Snyder et al. (146) derived a rather simple relationship between the elution time of an eluite and the rate of change of solvent composition in gradient elution. They found the elution time, is related to the column dead-time, and an experimental parameter b by... 

The most important features of this equation are the emergence of a eluotropic series, i.e., an eluent-strength series based on e°. The key solvent-strength parameter, e°, which relates to the eluotropic series, was calculated by Snyder (14) for a wide variety of pure solvents. Table 2 lists some values. 

Extraction Parameters. The challenge in SFE is to determine the best extraction parameters for the analytes of interest in a particular matrix. These parameters are listed in Table I. The solvent power of the extraction fluid as controlled by the density, temperature, and composition is the primary parameter which must be controlled to obtain optimal recoveries. The definition of solvent power used here is that of Snyder and Kirkland (1). As previously mentioned low densities will extract non-polar volatile compounds and high densities will extract relatively more polar, less volatile compounds. 

Section 2.3 describes various methods for the characterization and classification of mobile and stationary phases for chromatography. In section 2.3.1 the solubility parameter is introduced as a quantitative definition of the word polarity . Section 2.3.2 describes the characterization of GC stationary phases according to Rohrschneider and section 2.3.3 the classification of LC solvents according to Snyder. The applicability of the different methods is summarized in section 2.3.4. 

Solvent classification parameters according to Snyder. Data taken from ref. [215]. 

The main strength of the Snyder scheme is for the classification of solvent selectivity. We have seen from table 2.8 that solvents that are chemically similar yield similar selectivity parameters. This type of classification can be made on the basis of structural information alone. However, the Snyder scheme goes one step further, in that it classifies different chemical classes into a single selectivity group. From the definition equations (2.14 through 2.17) we see that the three selectivity parameters are correlated by the equation... 

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