Retention hydrogen-bonding effects

Many synthetic water-soluble polymers are easily analyzed by GPC. These include polyacrylamide,130 sodium poly(styrenesulfonate),131 and poly (2-vinyl pyridine).132 An important issue in aqueous GPC of synthetic polymers is the effect of solvent conditions on hydrodynamic volume and therefore retention. Ion inclusion and ion exclusion effects may also be important. In one interesting case, samples of polyacrylamide in which the amide side chain was partially hydrolyzed to generate a random copolymer of acrylic acid and acrylamide exhibited pH-dependent GPC fractionation.130 At a pH so low that the side chain would be expected to be protonated, hydrolyzed samples eluted later than untreated samples, perhaps suggesting intramolecular hydrogen bonding. At neutral pH, the hydrolyzed samples eluted earlier than untreated samples, an effect that was ascribed to enlargement... 

The van der Waals volume can be related to the hydrophobicity of the solutes, and retention of molecular compounds can be predicted from their van der Waals volumes, 7r-energy, and hydrogen-bonding energy effects [72-74], It should be noted that the isomeric effect of substituents cannot be predicted with good precision because this is not simply related to Hammett s a or Taft s other hand, the hydrophobicity is related to enthalpy [75], Retention times of non-ionizable compounds were measured in 70 and 80% acetonitrile/water mixtures on an octadecyl-bonded silica gel at 25-60°C and the enthalpy values obtained from these measurements. 

Retention volumes of monosubstituted benzenes, benzoic acid, phenols, and anilines have been measured in RPLC [76]. Buffered acetonitrile/water and tetrahydrofuran/water eluents were used with an octadecylsilica adsorbent. From the net retention volumes, a substituent interaction effect was calculated and described with the linear free energy relationship developed by Taft. The data was interpreted in terms of hydrogen bonding between the solutes and the eluent. 

The most important parameter for the control of liquid chromatography is the composition of the eluent. Liquid chromatography is a powerful separation method with unlimited possibilities of eluent selection. However, it is not easy to choose a suitable eluent within a short time without a number of trial experiments. The crucial factor is to control the solubility of the analytes in the eluent. Increasing the solubility of analytes in the eluent decreases their retention times. The selection of the components of an eluent is described below, based on the properties of the analytes to be separated. The important properties are hydrophobicity, dipole moment, hydrogen bonding, ionization, and steric effects. 

The selection of the solvent is based on the retention mechanism. The retention of analytes on stationary phase material is based on the physicochemical interactions. The molecular interactions in thin-layer chromatography have been extensively discussed, and are related to the solubility of solutes in the solvent. The solubility is explained as the sum of the London dispersion (van der Waals force for non-polar molecules), repulsion, Coulombic forces (compounds form a complex by ion-ion interaction, e.g. ionic crystals dissolve in solvents with a strong conductivity), dipole-dipole interactions, inductive effects, charge-transfer interactions, covalent bonding, hydrogen bonding, and ion-dipole interactions. The steric effect should be included in the above interactions in liquid chromatographic separation. 

Since the stationary phase is basically nonpolar in RPC, it is not expected to effect solute retention by ionic attraction, hydrogen bonding, formation of charge transfer complexes, or by any of the other strong non-covalent interactions familiar to the chemist. The only attractive force between the stationary phase and the eluite would seem to be van der Waals forces. However, these farces also act between the eluite and the mobile phase so that the net effect is generally not sufficien to account for the strong retention often observed with nonpolar compotinds in RPC. 

Compared to hydrocarbonaceous silica RPC sorbents, not as much commitment has been made to the development of bonded, polar-phase sorbents suitable for the high-performance chromatographic separation of peptides. Due to polar, notably hydrogen bonding, interactions between the peptide and the hydrophilic surface of the sorbent useful selectivity effects can, however, be achieved. In fact, at least two types of separation mechanisms can be identified with bonded polar-phase sorbents. In the first mode, the peptides do not interact per se with the bonded polar-phase sorbent but, rather, are separated on the basis of their ability to permeate into the pores and elute in order of their hydrodynamic volume. In this mode, peptides are separated by steric exclusion effects, with the retention (in terms of elution volume, Ve) of a partial retained peptide, Pb described by the following relationships ... 

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