Probing selectivity of mixed-mode reversed-phase/weak-anion-exchange liquid chromatography to advance method development

Molecular Dynamics Simulation for the Acidic Compounds Retention Mechanism Study on Octyl-Quaternary Ammonium Mixed-Mode Stationary Phase

With the widespread application of mixed-mode chromatography in separation analysis, it is becoming increasingly important to study its retention mechanism. The retention behavior of acidic compounds on mixed-mode octyl-quaternary ammonium (Sil-C8-QA) columns was investigated by computer simulation. Firstly, the benzoic acid homologues were used as the analytes, and the simulation model was constructed by the Materials Studio. Geometric optimization, annealing and molecular dynamics (MD) simulation of these complexes resulted in optimized conformations. The binding energy, mean square displacement (MSD) and torsion angle distribution generated by MD simulation were then analyzed. The results showed that the more negative binding energy, the greater the MSD and the narrower the torsion angle distribution, indicating that the stationary phase behaves with stronger interaction and retention. The retention behavior of five acidic drugs on the Sil-C8-QA column was then successfully explained by simulation. Acidic drugs are more retentive on the mixed-mode column due to the more substantial interaction brought by the reversed-phase/ion-exchange mixed-mode mechanism compared to other single-mode columns. This simulation method is expected to provide ideas for studying the separation mechanism and predicting the retention behavior of more complex samples.

Understanding the retention mechanisms of a reversed-phase/anion exchange/cation-exchange column for the separation of epinephrine and norepinephrine

In this paper, we investigated the retention mechanisms of a reversed-phase/anion-exchange/cation-exchange column (Acclaim trinity P1, Thermo Fisher Scientific) for the separation of epinephrine (EPI) from norepinephrine (NOE). The impact of the acetonitrile (ACN) content, pH, and salt concentration on the retention of these two catecholamines was studied under an isocratic mode with a mobile phase mixture of ACN and ammonium formate or acetate (pH 3 to pH 5). To better understand the retention mechanisms, several retention models were explored, including linear solvent strength, adsorption, quadratic, and mixed-mode models, using various chemical compounds in addition to EPI and NOE. The quadratic and mixed-mode models were the most appropriate to explain the column retention mechanisms according to the Akaike information criterion (AIC). The research showed the importance of the ACN content on the retention of compounds according to the quadratic model, and satisfactory resolution between EPI and NOE (>1.4) was achieved with 50% ACN content. The most important retention parameters were integrated in the mixed-mode model, namely ACN content, pH, and salt concentration. Using a three-factor Box-Behnken design (BBD), other optimal conditions were obtained to separate EPI and NOE with a resolution Rs > 1.5. The ACN content and salt concentrations of the aqueous part of the mobile phase were the parameters with the greatest impact on the separation performance of the stationary phase for both catecholamines. Finally, a rapid and simple separation of a mixture of EPI, NOE, and tetracaine was obtained using a mobile phase composed of ACN/ammonium formate (pH 4; 10 mM) (60:40, v/v), with a satisfactory resolution (>1.5) between the analyte peaks.

Separation of phosphorothioate oligonucleotide impurities by WAX HPLC under high organic content elution conditions

The separation of impurities in phosphorothioate diester (PS) oligonucleotides is complicated by (1) the presence of a very large number of diastereoisomers, e.g., 2¹⁹ for a 20-mer oligonucleotide, (2) peak broadening due to the hydrophobic character of the sulfur atom, and (3) the chemical similarity of the impurities to the parent oligonucleotide and each other. Further difficulties arise due to the chemical nature of oligonucleotides, which display a complex mixture of ionic, hydrophobic, H-bonding, and other functionalities. To minimize hydrophobic interactions and peak broadening due to the PS modification, we have developed a novel method that combines a weak anion exchange (WAX) column with a mobile phase elution system designed to maximize separation by a single ionic/electrostatic interaction. We found that although chaotropes are helpful, the most significant beneficial effect of the hydrophilic WAX column is that high-organic, low-salt mobile phase is required for product elution. Separations are also benefitted by pH gradient effects on stationary phase electrostatic potential and analyte ionization. An extraordinary degree of separation is achieved by the new WAX method in comparison to SAX (strong anion exchange) chromatography. For the first time, the extent of deamination of PS oligonucleotides is directly determined by a chromatography-only method. The approach, representative results, and the mechanisms of separation are discussed.

Simulation of Adsorption Process of l-Tryptophan on Mixed-Mode Resin HD-1 with Combined Physical Adsorption and Ion Exchange

The mass-transfer process of l-tryptophan (l-Trp) in the hydrophobic interaction/ion-exchange mixed-mode resin HD-1 particles and fixed bed was studied experimentally and theoretically. The adsorption kinetics of l-Trp in single-component and multicomponent adsorption systems was investigated under different pH conditions. The co-adsorption of sodium ions (Na⁺) and l-Trp anions was found to be negligible. A modified liquid-film linear driving force model considering the physical adsorption of l-Trp zwitterions and anions as well as ion exchange of l-Trp cations was proposed. The dissociation equilibria of l-Trp molecules and functional groups on the resin were introduced in the model. The model could well fit the kinetic adsorption curves of l-Trp at different pH values. The presence of Na⁺ and the impurity amino acid l-glutamic acid (l-Glu) did not significantly affect the mass-transfer rate of l-Trp. The dynamic adsorption processes of l-Trp under different pH and concentration conditions were studied. A modified transport-dispersive model considering axial diffusion, liquid-film mass transfer, and a combined physical adsorption and ion-exchange equilibrium was established, which could predict the adsorption breakthrough curves of l-Trp well. During the dynamic adsorption process, the pH of mobile phase in the fixed bed changed with changing the l-Trp concentration in the mobile phase. l-Trp was well separated from Na⁺ and l-Glu with the purity of l-Trp higher than 99%, the recovery rate higher than 95%, and a concentration of 4.69 × 10^-3 mol/L. The elution chromatographic peaks of l-Trp, l-Glu, and Na⁺ and the pH of the outlet solution were predicted satisfactorily.

Adsorption separation of guanosine 5'-Monophosphate and cytidine 5'-Monophosphate by mixed-mode Resin HD-1: Experimental study and mathematical modeling

The preparative separation of guanosine 5'-monophosphate (GMP) and cytidine 5'-monophosphate (CMP) on mixed-mode resin HD-1 was experimentally and theoretically investigated. The adsorption mechanisms of the two nucleotides were elucidated by analyzing adsorption equilibria and kinetics at different pH values. The adsorption dynamics of GMP and CMP in a fixed bed packed with resin HD-1 were studied. All nucleotide monovalent cations, zwitterions, and monovalent anions were adsorbed by the resin. Further, a general adsorption isotherm model was developed by considering the adsorption of different nucleotide species and the dissociation equilibrium behaviors of resin ligands. The model fit the adsorption isotherm data of GMP and CMP well. A modified liquid-film linear driving force model with the combined physical adsorption of nucleotides in different dissociation states and ion exchange of Na⁺ was established. The dissociation equilibria of resin ligands and nucleotides were considered. The model satisfactorily predicted the adsorption kinetic data at different pH values. The values of the efficient diffusion coefficients for GMP and CMP were not significantly influenced by the solution pH. A modified transport-diffusion model with pH gradient elution was proposed. The model accurately predicted the elution chromatographic peaks of GMP and CMP, as well as the pH at the outlet of the fixed bed packed with resin HD-1.