Formation constants mobile phase

Jost et al. (212) studied the use of TLC as a pilot technique for transferring retention data to column LC (HPLC). TLC is potentially an inexpensive and convenient method for this purpose if essentially identical phases with the same retention mechanisms are used. However, there are inherent procedural differences in TLC and HPLC, which make exact transfer of data questionable. These differences include a capillary mobile phase driving force in TLC, and forced flow with constant and adjustable rates in HPLC formation of mobile phase gradients (solvent demixing) when multicomponent solvents are used in TLC preloading of the stationary phase with components from the gas phase of the TLC solvent and the presence of binder in layers but not columns. 

The second condition is that the well-dispersed slurry forms a homogeneous bed by formation of the bed under well-controlled conditions. This is achieved by a two-step procedure where the bed is formed using constant velocity of the mobile phase and then stabilizing the bed at a constant pressure (Hagel, 1989). The rationale for the first step at constant velocity is that this will create uniform drag forces from the flowing liquid on the gel particles and thus... 

Several theoretical models, such as the ion-pair model [342,360,361,363,380], the dyneuaic ion-exchange model [342,362,363,375] and the electrostatic model [342,369,381-386] have been proposed to describe retention in reversed-phase IPC. The electrostatic model is the most versatile and enjoys the most support but is mathematically complex euid not very intuitive. The ion-pair model emd dynamic ion-exchange model are easier to manipulate and more instructive but are restricted to a narrow range of experimental conditions for trtilch they might reasonably be applied. The ion-pair model assumes that an ion pair is formed in the mobile phase prior to the sorption of the ion-pair complex into the stationary phase. The solute capacity factor is governed by the equilibrium constants for ion-pair formation in the mobile phase, extraction of the ion-pair complex into the stationary phase, and the dissociation of th p ion-pair complex in the... 

In recent years, for analytical purposes the direct approach has become the most popular. Therefore, only this approach will be discussed in the next sections. With the direct approach, the enantiomers are placed in a chiral environment, since only chiral molecules can distinguish between enantiomers. The separation of the enantiomers is based on the complex formation of labile diastereoisomers between the enantiomers and a chiral auxiliary, the so-called chiral selector. The separation can only be accomplished if the complexes possess different stability constants. The chiral selectors can be either chiral molecules that are bound to the chromatographic sorbent and thus form a CSP, or chiral molecules that are added to the mobile phase, called chiral mobile phase additives (CMPA). The combination of several chiral selectors in the mobile phase, and of chiral mobile and stationary phases is also feasible. 

Formation constant, apparent, 234 of complexes in the mobile phase, 232 Free energy change, 204 for cavity formation in liquid, 204-203 for electrostatic interactions, 208-211 entropic contributions. 212 for retention, 211... 

A cursory review of the literature reveals that the ELC technique with micellar mobile phases has proven to be very beneficial in the characterization of micellar systems (184-186,190-192,227,228). For example, microcolumn exclusion LC has been applied to the determination of the CMC value of surfactants (or micellar-forming proteins), determination of the kinetic rate and equilibrium association constants for surfactant (or protein) micellization (184,192), determination of the size or size distribution of micelles (especially those formed from block copolymers or milk casein) (185,186,191,192,225) as well as for estimation of the time required for formation of micelles (or micelle-forming macromolecules) (186) among others. The size and stability of reversed micelles has also been evaluated using ELC (195). 

Aqueous a-cyclodextrin solutions seem to be generally applicable for TLC separation of a wide variety of substituted aromatics on polyamide thin-layer stationary sheet (13-14). In most cases, the compounds moved as distinct spots and their R, values were dependent on the concentration of the cyclodextrin in tne mobile phase. In a given family of compounds, (o-, m-, and p-nitrophenols, for example) the isomer with the largest stability constant for a-cyclodextrin complex formation had the larger value. In general, the para-substituted isomers have larger R values than the meta-isomers, which in turn have larger R values than the ortho substituted ones. 

The nature of the analyte interactions with liophilic ions could be electrostatic attraction, ion association, or dispersive-type interactions. Most probably all mentioned types are present. Ion association is essentially the same as an ion-pairing used in a general form of time-dependent interionic formation with the average lifetime on the level of 10 sec in water-organic solution with dielectric constant between 30 and 40. With increase of the water content in the mobile phase, the dielectric constant increases and approaches 80 (water) this decrease the lifetime of ion-associated complexes to approximately 10 sec, which is still about four orders of magnitude longer than average molecular vibration time. 

The enantiomeric separation with chiral mobile phases consists of the addition of an active compound in the mobile phase which is constantly pumped though the chromatographic system. The active ingredient contributes to a specific secondary chemical equilibrium, interacting with the enantiomers in the mobile phase as well as in the stationary phase, leading to the formation of diastereomeric complexes potentially in both phases. This affects the overall distribution of the analyte between the stationary phase and the mobile phase, affecting its retention and the overall enantiomeric separation. The rates of formation of the diastereomeric complexes should be similar to the diffusion rates to minimize excessive chemical contribution to the band-broadening. 

The first term of equation (22-16) represents the complexation of the enantiomers in the stationary phase, while the second term represents the complexation in the mobile phase. If the formation constants of the complexes are sufficiently high, the enantioselectivity of the chromatographic system is roughly given by the ratio of the complexation enantioselectivity in the stationary phase and mobile phase. 

In the case of ion-pair complexes between the chiral additive and the enantiomeric analytes, their interaction should be maximized by adjusting the mobile-phase polarity. Solvents of lower dielectric constant favor ion-pair formation. 

Type IV includes chiral phases that usually interact with the enantiomeric analytes through the formation of metal complexes. There are usually used to separate amino acid enantiomers. These types of phases are also called ligand exchange phases. The transient diastereomeric complexes are ternary metal complexes between a transitional metal (usually Cu +), an amino acid enantiomeric analyte, and another compound immobilized on the CSP which is able to undergo complexation with the transitional metal (see also the ligand exchange section. Section 22.5). The two enantiomers are separated based on the difference in the stability constant of the two diastereomeric species. The mobile phases used to separate such enantiomeric analytes are usually aqueous solutions of copper (II) salts such as copper sulfate or copper acetate. To modulate the retention, several parameters—such as the pH of the mobile phase, the concentration of the copper ion, or the addition of an organic modifier such as acetonitrile or methanol in the mobile phase—can be varied. 

In conclusion, CEC has great potential in separation technology. Our theoretical model as well as many published practices in CEC show clearly that the benefit of combining electrophoresis and partitioning mechanisms in CEC is the increase in selectivity for the separation. The intrinsic difference in formation constants is critical, but the experimental factors, such as electric field or the stationary and mobile phases, can also contribute to the improvement of the overall enantioselectivity via increasing the conversion efficiency. However, only when both electrophoretic and partitioning mechanisms act in the positive effects, can high overall enantioselectivity in CEC be obtained. 

The value of ai will depend on the formation constants of the metal ion-ligand formation constants, the concentration of excess ligand in the mobile phase and the pH. Calculation of Oi according to Ringbom [12] follows the equation ... 

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