Solvent strength in reversed-phase

The relationship between retention and solvent strength in reversed-phase chromatography is described by the linear solvent strength model 19,20... 

Khaledi, M. G., J. K. Strasters, A. H. Rodgers, and E. D. Breyer. 1990. Simultaneous enhancement of separation selectivity and solvent strength in reversed-phase liquid chromatography using micelles in hydro-organic solvents nal. Chem62 130-136. 

Separations as a function of temperature and binary mobile solvent strength in reversed-phase chromatography can be optimized based on four initial experiments. 

Solvent optimization in reversed-phase liquid chromatography is commenced by selecting a binary mobile phase of the correct solvent strength to elute the seuaple with an acceptable range of capacity. factor values (1 < k <10 in general or 1 < k < 20 when a larger separation capacity is required). Transfer rules (section 4.6.1) are then used to calculate the composition of other isoeluotropic binary solvents with complementary selectivity. In practice, methanol, acetonitrile and tetrahydrofuran are chosen as the selectivity adjusting solvents blended in different... 

In reversed-phase thin-layer chromatography (RP-TLC), the choice of solvents for the mobile phase is carried out in a reversed order of strength, comparing with the classical TLC, which determines a reversed order of values of compounds. The reversed order of separation assumes that water is the main component of the mobile phase. Aqueous mixmres of some organic solvents (diethyl ether, methanol, acetone, acetonitrile, dioxane, i-propanol, etc.) are used with good results. 

Solvent strength determines the value, but not the selectivity. The mobile phase can be established by using the polarity index P proposed by Snyder. The highest values of P represent the strongest solute adsorbed in conventional TLC but represent the weakest for the separation in reversed phases. Sometimes aqueous polar mixtures cannot totally wet the chemically bonded layer. For this reason, checking... 

Figure 4.29 An example of the use of ternary solvents to control mobile phase strength and selectivity in reversed-phase liquid chromatography. A, methanol-water (50 50) B, tetrahydrofuran-water (32 68) C, methanol-tetrahydrofuran-water (35 10 55). Peak identification 1 - benzyl alcohol 2 phenol 3 3-phenylpropanol 4 2,4-dimethylphenol 5 benzene and 6 -diethylphthalate. (Reproduced with permission from ref. 522. Copyright Elsevier Scientific Publishing Co.)...

Snyder L.R., Dolan J.W., Molnar I., and Djordjevic, N.M., Selectivity control in reversed-phase HPLC methods development varying temperature and solvent strength to optimize separations, LC-GC, 15 (2), 136, 1997. 

The k values in HPLC can be controlled by means of solvent strength. When it is necessary to increase k value, a weaker solvent is used. For example, in reversed-phase separations, solvent strength is greater for pure methanol than for pure water. The right proportionality of these solvents has to be found to get optimum separation. 

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. 

A. Wang and P.W. Carr, Comparative study of the linear solvation energy relationship, linear solvent strength theory, and typical conditions model for retention prediction in reversed-phase liquid chromatography. J. Chromatogr.A 965 (2002) 3-23. 

Electrospray requires low-ionic-strength solvent so that buffer ions do not overwhelm analyte ions in the mass spectrum. Low-surface-tension organic solvent is better than water. In reversed-phase chromatography (Section 25-1), it is good to use a stationary phase that strongly retains analyte so that a high fraction of organic solvent can be used. A flow rate of 0.05 to 0.4 mL/min is best for electrospray. 

The optimization procedure is similar to that used in reverse phase. The concentration of methylene chloride in hexane is adjusted to get partition ratios in the range of 1 to 10. The solvent strength e° of this solution is calculated and used in preparing the other solvent mixtures to the same value. Consequently, all mixtures used in this process will have the same... 

S. D. West, The prediction of reversed-phase HPLC retention indices and resolution as a function of solvent strength and selectivity,/. Chromatogr. Set. 25 (1987), 122-129 and S. D. West, Correlation of retention indices with resolution and selectivity in reversed-phase HPLC and GC, J. Chromatogr. Set. 27 (1989), 2-12. 

Kennedy 1990). This form of chromatography uses hydrophilic gel-based material as a matrix, which has been partially substituted on the surface with non-polar alkyl (e.g., methyl or octyl) or aryl (e.g., phenyl) groups (Table 4-3). Similar materials are used in reverse-phase chromatography (RPC), but the degree of substitution used for HIC chromatography (10-50 pmol ml-1 gel) is much lower than that used in RPC (100-500 pmol ml-1 gel). The solvent used for elution in HIC is low ionic strength buffer and not organic solvents which are characteristic of RPC. 

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