Solvents increasing solvent strength

Type of organic modifier Retention decreases with increasing solvent strength. 

TABLE 9.6 Solvents of Chromatographic Interest (arranged in order of increasing solvent strength)... 

Gases such as carbon dioxide, nitrous oxide, and ammonia are commonly used. Organic solvents such as methanol, isopropanol, methylene chloride, tetrahydrofuran, and acetonitrile are frequently employed in HPLC as modifiers to increase solvent strength. 

Both tR and k increase exponentially with decreasing percentage of organic solvents (or solvent strength) in the mobile phase. 

Solid-phase extraction is a sample preparation technique that has been derived from liquid chromatography technology and has been applied extensively to the analysis of biological samples as well as pharmaceutical products. It is a step gradient technique in which the analyte, dissolved in a weak solvent, is retained on a stationary phase and subsequent additions of various moving phases of increasing solvent strength results in selective and controlled elution of the interferences and analytes. 

As a result of increasing solvent strength, the absorption band of molecule I undergoes a blue shift, and that of molecule II a red shift. These shifts, serving as the basis of the solvent scale, were denoted Xb Xr- The two independent parameters... 

Figure 5 Stepwise gradient elution in sandwich chamber with a glass distributor (A) of the eluent (a) 0.4 ml portions of eluents of increasing solvent strength are introduced under the distributor and from the edge of the layer (b) developed chromatogram with zones of the mobile phase and a stepwise profile of the gradient (c) corresponding graphical representation of the (approximated) continuous gradient (reprinted from Ref. 22 with permission).

The solvent characteristics of supercritical fluids have been extensively investigated over the past two decades (2). Supercritical fluids have increased solvent strength versus gases due to their liquid-like densities. The pressure and temperature within the supercritical region can be adjusted to regulate the density and therefore the solvent strength of a supercritical fluid. In addition to the liquid-like density, supercritical fluids exhibit gas-like diffusivity and viscosity. 

Physical and ionic adsorption may be either monolayer or multilayer (12). Capillary stmctures in which the diameters of the capillaries are small, ie, one to two molecular diameters, exhibit a marked hysteresis effect on desorption. Sorbed surfactant solutes do not necessarily cover ah. of a sohd iaterface and their presence does not preclude adsorption of solvent molecules. The strength of surfactant sorption generally foUows the order cationic > anionic > nonionic. Surfaces to which this rule apphes include metals, glass, plastics, textiles (13), paper, and many minerals. The pH is an important modifying factor in the adsorption of all ionic surfactants but especially for amphoteric surfactants which are least soluble at their isoelectric point. The speed and degree of adsorption are increased by the presence of dissolved inorganic salts in surfactant solutions (14). 

Increased water resistance Increased tensile strength Increased block resistance Increased solvent resistance Increased adhesion to hydrophilic surfaces... 

Fig. 3. Effects of composition on physical properties. A, acetyl B, butyryl C, cellulose. 1, increased tensile strength, stiffness 2, decreased moisture sorption 3, increased melting point 4, increased plasticizer compatibiUty 5, increased solubiUties in polar solvents 6, increased solubiUties in nonpolar...

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