Static vs. Dynamic Electrostatic Repulsion Reversed Phase Liquid Chromatography: Solutions for Pharmaceutical and Biopharmaceutical Basic Compounds

Many efforts have been made to separate basic compounds, which are challenging to resolve in reversed phase liquid chromatography. In this process, they are strongly retained and the peak shape undergoes significant distortion. The principal origin of this has been identified with the non-negligible interaction with residual deprotonated silanols. Consequently, all solutions that efficiently shield silanols are being sought. This review is an upgrade on the use of the electrostatic repulsion reversed phase (ERRP) approach: retention of bases, in protonated form, can be achieved by modulating the charge repulsion caused by the presence of positive charges in the chromatographic system. This study successfully (i) introduced fixed positive charges in the structure of stationary phases, (ii) used cationic and hydrophobic additives in the mobile phase, and (iii) used the ERRP-like approach employed at the preparative level for peptide purification.

Hydrophilic Liquid Chromatography versus Reversed-Phase Liquid Chromatography in the Absence and the Presence of 1-Hexyl-3-methylimidazolium Chloride for the Analysis of Basic Compounds

In reversed-phase liquid chromatography (RPLC), positively charged basic compounds yield broad and asymmetric peaks, as a result of ionic interactions with free silanols that remain on conventional silica-based columns. Diverse solutions have been proposed to mask the silanophilic activity, which is translated to an improved peak shape. In this work, the chromatographic performance of hydrophilic interaction liquid chromatography (HILIC) was evaluated as an alternative to the addition of an ionic liquid (IL) to the aqueous-organic mobile phase used with RPLC columns, for the analysis of eight β-adrenoceptor antagonists. ILs change the behavior of RPLC stationary phases owing to adsorption on their surface. Meanwhile, in HILIC, a layer of adsorbed water is formed on the stationary phase surface. The association of cationic basic compounds with the adsorbed additive ions, hydrophilic partitioning on the HILIC columns, and other interactions, give rise to complex retention mechanisms. The chromatographic behavior was examined in terms of retention, elution strength, selectivity, peak shape and resolution, using acetonitrile-water mobile phases buffered at pH 3. Both chromatographic modes, RPLC with added IL and HILIC, proved to be a viable solution to the problem of poor peak shape for basic compounds.

Estimation of the octanol-water distribution coefficient of basic compounds by a cationic microemulsion electrokinetic chromatography system

The octanol-water partition coefficient (Po/w), or the octanol-water distribution coefficient (Do/w) for ionized compounds, is a key parameter in the drug development process. In a previous work, this parameter was estimated through the retention factor measurements in a sodium dodecyl sulfate (SDS) - microemulsion electrokinetic chromatography (MEEKC) system for acidic compounds. Nonetheless, when ionized basic compounds were analyzed, undesirable ion pairs were formed with the anionic surfactant and avoided a good estimation of log Do/w. For this reason, an alternative MEEKC system based on a cationic surfactant has been evaluated to estimate Po/w or Do/w of neutral compounds and ionized bases. To this end, it has been characterized through the solvation parameter model (SPM) and compared to the octanol-water partition system. Results pointed out that both systems show a similar partition behavior. Hence, the log Po/w of a set of neutral compounds has been successfully correlated against the logarithm of the retention factor (log k) determined in this MEEKC system. Then, the log Do/w of 6 model bases have been estimated at different pH values and they have been compared to data from the literature, determined by the reference shake-flask and potentiometric methods. Good agreement has been observed between the literature and the estimated values when the base is neutral or partially ionized (up to 99 % of ionization).

Comparison of surfactant-mediated liquid chromatographic modes with sodium dodecyl sulphate for the analysis of basic drugs

A comprehensive overview of the performance of MLC, HSLC and MELC for the analysis of basic compounds.