Mobile phase proton-acceptor

Variations in retention and selectivity have been studied in cyano, phenyl, and octyl reversed bonded phase HPLC columns. The retention of toluene, phenol, aniline, and nitrobenzene in these columns has been measured using binary mixtures of water and methanol, acetonitrile, or tetrahydrofuran mobile phases in order to determine the relative contributions of proton donor-proton acceptor and dipole-dipole interactions in the retention process. Retention and selectivity in these columns were correlated with polar group selectivities of mobile-phase organic modifiers and the polarity of the bonded stationary phases. In spite of the prominent role of bonded phase volume and residual silanols in the retention process, each column exhibited some unique selectivities when used with different organic modifiers [84],... 

By appropriate choice of the type (or combination) of the organic solvent(s), selective polar dipole-dipole, proton-donor, or proton-acceptor interactions can be either enhanced or suppressed and the selectivity of separation adjusted [42]. Over a limited concentration range of methanol-water and acetonitrile-water mobile phases useful for gradient elution, semiempirical retention equation (Equation 5.7), originally introduced in thin-layer chromatography by Soczewinski and Wachtmeister [43], is used most frequently as the basis for calculations of gradient-elution data [4-11,29,30] ... 

The mobile phases used in normal-phase chromatography are based on nonpolar hydrocarbons, such as hexane, heptane, or octane, to which is added a small amount of a more polar solvent, such as 2-propanol.5 Solvent selectivity is controlled by the nature of the added solvent. Additives with large dipole moments, such as methylene chloride and 1,2-dichlor-oethane, interact preferentially with solutes that have large dipole moments, such as nitro- compounds, nitriles, amines, and sulfoxides. Good proton donors such as chloroform, m-cresol, and water interact preferentially with basic solutes such as amines and sulfoxides, whereas good proton acceptors such as alcohols, ethers, and amines tend to interact best with hydroxylated molecules such as acids and phenols. A variety of solvents used as mobile phases in normal-phase chromatography are listed in Table 2.2, some of which may need to be stabilized by addition of an antioxidant, such as 3-5% ethanol, because of the propensity for peroxide formation. 

The contribution of solvent-solute hydrogen bonding to selectivity [term (iii) in Eq. (34)] can be generalized for the presence of more than one basic or proton-acceptor solvent in the mobile-phase mixture ... 

A m or challenge to completing a practical model and description of mobile-phase effects in LSC is the further elucidation of hydrogenbonding effects. This will involve a more fundamental classification of solutes and solvents in terms of their proton-donor and proton-acceptor properties, so that values of can be estimated as a function of the molecular structures of solute X and solvent C. It will also require a more precise description of the adsorbate-surface bonding that occurs in the adsorbed monolayer, so that values of can likewise be rationalized and predicted. 

The mobile phase in RPC contains water and one or more water-soluble organic solvents. The most useful are, in order of decreasing polarities, acetonitrile, methanol, dioxane, tetrahydrofuran and propanol. By the choice of the type of the organic solvent, selective polar interactions, dipole-dipole, proton-donor or proton-acceptor, with analytes can be either enhanced or suppressed and the selectivity of the separation adjusted. For simplicity, binary mobile pha.ses are used more often than those containing more than one organic solvent in water, as they often make possible an adequate separation of various samples. However, ternary or less often quaternary mobile phases offer advantage of fine-tuning the optimum selectivity of more difficult separations. This is discussed in more detail in Section 1.4.6. 

Fig. 1.19. Selectivity triangle for three- and four-componeni mobile phases in reversed-phase HPLC MeOH = methanol (predominant proton-donor interactions) ACN = acetonitrile (predominant dipole-dipole interaaions) THF = tetrahydrofuran (predominant proton-acceptor interactions).

Adsorption TLC selection of the mobile phase is conditioned by sample and stationary-phase polarities. The following polarity scale is valid for various compound classes in NPTLC in decreasing order of K values carboxylic acids>amides>amines>alcohols>aldehydes > ketones > esthers > nitro compounds > ethers > hal-ogenated compounds > aromatics >olefins > saturated hydrocarbons > fluorocarbons. For example, retention on silica gel is controlled by the number and functional groups present in the sample and their spatial locations. Proton donor/acceptor functional groups show the greatest retention, followed by dipolar molecules, and, finally, nonpolar groups. 

Solvents can be grouped on the basis of their properties as proton donors (acidic), proton acceptors (basic), and dipole interactions. Solvents can be positioned within the Solvent Selectivity Triangle (Fig. 17) on the basis of the relative involvement of each of these three factors as parameters of solubility. Mobile phases consisting of a mixture of three solvents can be optimized by... 

Selectivity effects are determined by mobile phase interactions between the stationary phase and the solute. Those mobile phases containing proton donors will interact with basic solutes. Conversely, mobile phases containing proton donor acceptors will interact strongly with acidic solvents. Where excessive interactions occur between the solute and the mobile phase, peak tailing can result. However, the inclusion of a basic modifier (triethylamine) can overcome such strong interactions and thereby improve peak shape. 

The retention and the selectivity of separation in RP and NP chromatography depend primarily on the chemistry of the stationary phase and the mobile phase, which control the polarity of the separation systems. There is no generally accepted definition of polarity, but it is agreed that it includes various selective contributions of dipole-dipole, proton-donor, proton-acceptor, tt-tt electron, or electrostatic interactions. Linear Free-Energy Relationships (LFER) widely used to charactaize chemical and biochemical processes were successfiiUy apphed in liquid chromatography to describe quantitative structure-retention relationships (QSRR) and to characterize the stmctural contributions to the retention and selectivity, using multiple linear correlation, such as Eq. 

The nonsteric interactions in ipc depend on the chemical structure of the analyte, and also on nature of stationary and mobile phases. In normal- or reversed-phase hplc, neutral solutes are separated on the basis of their polarity. In the former case, polar stationary phases are employed (eg, bare sihca with polar silanol groups) and less polar mobile phases based on nonpolar hydrocarbons are used for elution of the analytes. Solvent selectivity is controlled by adding a small amoimt of a more polar solvent, such as 2-propanol or acetonitrile or other additives with large dipole moments (methylene chloride and 1,2-dichloroethane), proton donors (chloroform, ethyl acetate, and water), or proton acceptors (alcohols, ethers, and amines). Correspondingly, the more polar the solute, the greater is its retention on the column, yet increasing the polarity of the mobile phase results in decreased solute retention. 

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