Secondary chemical equilibria

The popularity of reversed-phase liquid chromatography (RPC) is easily explained by its unmatched simplicity, versatility and scope [15,22,50,52,71,149,288-290]. Neutral and ionic solutes can be separated simultaneously and the rapid equilibration of the stationary phase with changes in mobile phase composition allows gradient elution techniques to be used routinely. Secondary chemical equilibria, such as ion suppression, ion-pair formation, metal complexatlon, and micelle formation are easily exploited in RPC to optimize separation selectivity and to augment changes availaple from varying the mobile phase solvent composition. Retention in RPC, at least in the accepted ideal sense, occurs by non-specific hydrophobic interactions of the solute with the... 

Secondary Chemical Equilibria in Keversed-Phase Liquid Cliromatograpby... 

Ion-pair chromatography (IPC) is a further example of the use of secondary chemical equilibria to control retention and... 

Uf course, the enhancement of chromatographic selectivity by secondary chemical equilibria is neither new nor confined to reversed-phase systems. Most widespread probably has been the exploitation of protonic equilibria by appropriately ati usting the pH of the eluent so that the degree of ionization of the eluite is altered. Generally the ionized and neutral forms of an eluite are retained differently (2( 7. 208). Formation of metal complexes of certain eluites has also been utilized for modulating retention behavior for higher selectivity. 

The use of hetaerons such as those employed in ion-pair chromatography is not extensively discussed here it is covered in Section VI. The reader is referred to that section and Table XVIII, in particular, for a partial listing of separations effected by exploiting secondary chemical equilibria. 

Secondary chemical equilibria, 230,280 see also Secondary equilibria with diprotic acids and zwitterions, equilibrium constants and retention. 241 with micelle ftmtiation, 236... 

Secondary Chemical Equilibria in High-Performance Liquid Chromatography... 

B. L. Karger, J. N. LePage, N. Tanaka, Secondary chemical equilibria in HPLC, in Cs. Horvath (Ed.) High-Performance Liquid Chromatography, Vol. 1, Academic Press, New York, 1980. 

Slant of the mobile phases decreases. The dielectric constant is expected to influence the position of the equilibrium in ionic secondary chemical equilibria of acidic compounds [80-83], The solvent has the ability to disperse electrostatic charges via ion-dipole interactions, which is inversely proportional to the dielectric constant of the solvent composition. The lower the dielectric constant, the lower the ionization constant of the acid, Ka, and consequently greater Ka values are obtained. 

Some other elution modes have been described. They are induced by various factors — cyclical field, secondary chemical equilibria, adhesion chromatography, asymmetrical electro-osmotic flow for a review, see Ref. 2. However, the number of their implementations is rather limited, and for this reason, these modes are not discussed here. 

There are numerous variations on free solution CE (FSCE), such as micellar electrokinetic capillary chromatography (MECC or MEKC), where a moving, pseudostationary phase is added to the CE buffer, and secondary chemical equilibria or interactions ensue that effect separations of even neutral compounds, as well as ionic analytes. However, in general, CE utilizes truly homogeneous, solution phase separation approaches, without a stationary (permanent, fixed) phase, making it perhaps ideally suited for molecular recognition in searching combinatorial libraries. 

Then there are other parameters whose choice is not so compelling stationary phase, temperature, pH (or other ionic effects) and secondary chemical equilibria,... 

Certainly one of the major advantages of the aqueous-based mobile phases used in reversed-phase liquid chromatography is the ability to control secondary chemical equilibria. In liquid chromatography the primary equilibrium is the distribution of the solute between the stationary and mobile phases. Any other equilibrium involving the solute in the mobile or stationary phases is considered secondary. These secondary equilibrium processes change the chemical form of the solute, and can be used advantageously to change retention of solutes that... 

They showed that the selectivity of secondary chemical equilibria-based separations had been substantially underestimated because nonoptimum mobile-phase conditions were employed in previous selectivity estimates. 

BL Karger, JN LePage, N Tanaka. Secondary chemical equilibria. In Cs. Horvath, ed. High Performance Liquid Chromatography Advances and Perspectives. Vol. 1. New York Academic Press, 1980. 

Table 8.11 Secondary chemical equilibria used to optimize selectivity in capillary electrophoresis ...

For difficult separations, the selectivity can be further modified by employing secondary chemical equilibria, and solvation effects by adding appropriate reagents or solvents to the electrolyte system. Table 8.11 [404]. Increasing the ionic strength... 

The retention of an eluite in liquid chromatography is based upon the distribution of the eluite between the stationary and mobile phase (primary equilibrium). By convention, any other equilibria that takes place in the mobile phase, or stationary phase, or both, are considered secondary . In the past, manipulation of the mobile-stationary phase equilibrium distribution of the solute by using secondary chemical equilibria (SCE) was widely utilized in order to overcome low column efficiencies [32]. In spite of the fact that we now have columns with higher inherent efficiencies, SCE is still a widely practiced technique. The wide range of different chemistries that can be used to alter the mobile-stationary phase equilibrium to achieve better resolution or... 

If the pH and concentration of each ligand in solution are known, then these reactions and constants can be used to determine the equilibrium concentration of each metal species. In those instances in which the solution is also a liquid chromatographic eluent, such qualitative and quantitative information relates to the different types of eluite forms that are present in the mobile phase—free and complexed metal. This information is significant because the retention that is observed for a particular eluite is dependent upon the relative proportion of each different eluite forms present in solution. Those readers having utilized one of the more well known modes of secondary chemical equilibria (i.e., acid-base, ion-pairing, solute-micelle) may be well versed in how changes in the composition of the mobile phase can influence the relative proportions of eluite forms, and ultimately, the overall appearance of the chromatogram [33-35]. 

Foley [33] has indicated that for any eluite which can exist in more than one chemical form due to secondary chemical equilibria, retention is given by the summation of the weighted average capacity factors for each eluite form. The mole fraction of each of the eluite species is calculated and used as weighting factors in this treatment. For the trivalent metal ion system described above, M , and... 

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