Adsorption Mechanisms in Normal-Phase Chromatography. Mobile-Phase Modifier Adsorption from Dilute Solutions and Displacement Effect

Abstract

The adsorption equilibria of methanol from diluted solutions of the alcohol in dichloromethane (the weak solvent of the binary mixture) have been investigated in normal-phase conditions on an hybrid polymeric chiral stationary phase. This was done by analyzing the shapes of nonlinear perturbations of methanol, measured with a refractive index detector from a column in equilibrium with pure dichloromethane. Experiments have been set up to demonstrate the displacement effect, predictable from the adsorption data, by a nonlinear pulse of methanol on a racemic mixture of temazepam injected at trace level. A most used method in fundamental studies of band behavior in chromatography has been used for modeling the system. An extraordinary increase of the apparent column efficiency has been observed due to the displacement phenomenon, with the peak of the second eluted enantiomer noticeably higher and narrower than that of the first eluted component.

Determination of adsorption isotherms by means of HPLC: Adsorption mechanism elucidation and separation optimization

“…However, when strong interactions between mobile phase modifiers and active site on the stationary phase are present, the traditional PM can lead to serious inconveniences. This was observed by Cavazzini et al [54] by investigating the adsorption behavior of methanol on a polymeric hybrid chiral stationary phase. With data obtained only by the traditional PM method it was not possible to model the adsorption behavior of methanol from very diluted solutions of methanol in dichloromethane.…”

“…Cavazzini et al [54] used the IM to model the adsorption behavior of methanol and temazepam enantiomers on a chiral stationary phase. The study was aimed at demonstrating the competition for the adsorption between the mobile phase additive (methanol) and the enantiomers of temazepam.…”

Analytical Separation of Closantel Enantiomers by HPLC

“…There are explanations for these findings: MeOH is a protic solvent, which can strongly be engaged with the chiral selector due to its high polarity that contributes to breaking the hydrogen bonding interaction between the analyte and ADMPC polymer [ 32 ]. For this reason, the MeOH could reduce the chiral recognition and separation [ 33 , 34 ], in contrast with EtOH and IPA that have lower polarity. In addition to the poor adsorption of MeOH in the chiral polymeric structure, the distribution peak of MeOH in the ADMPC backbone was much lower than those of EtOH and IPA, as mentioned in [ 35 ].…”

Adsorption of Binary Solvents on Chiral Stationary Phases with Grafted Macrocyclic Antibiotics

“…This technique is just beginning to gain popularity in chiral chromatography. It has been used to study the adsorption of binary eluents on chiral stationary phases (CSPs) Whelk-O1 [9], DACH-ACR [10], and Chiralcel OD-I [11]. The adsorption of aqueous organic solvents was used to study the surface properties of CSPs with grafted macrocyclic antibiotics in [12,13], and the authors of [14] investigated the effect the composition of an adsorbed layer of the mobile phase has on the mechanism of amino acid retention on antibiotic columns.…”