Column Characterization and Selection Systems in Reversed-Phase High-Performance Liquid Chromatography

Reversed-phase high-performance liquid chromatography (RP-HPLC) is the most popular chromatographic mode, accounting for more than 90% of all separations. HPLC itself owes its immense popularity to it being relatively simple and inexpensive, with the equipment being reliable and easy to operate. Due to extensive automation, it can be run virtually unattended with multiple samples at various separation conditions, even by relatively low-skilled personnel. Currently, there are >600 RP-HPLC columns available to end users for purchase, some of which exhibit very large differences in selectivity and production quality. Often, two similar RP-HPLC columns are not equally suitable for the requisite separation, and to date, there is no universal RP-HPLC column covering a variety of analytes. This forces analytical laboratories to keep a multitude of diverse columns. Therefore, column selection is a crucial segment of RP-HPLC method development, especially since sample complexity is constantly increasing. Rationally choosing an appropriate column is complicated. In addition to the differences in the primary intermolecular interactions with analytes of the dispersive (London) type, individual columns can also exhibit a unique character owing to specific polar, hydrogen bond, and electron pair donor-acceptor interactions. They can also vary depending on the type of packing, amount and type of residual silanols, "end-capping", bonding density of ligands, and pore size, among others. Consequently, the chromatographic performance of RP-HPLC systems is often considerably altered depending on the selected column. Although a wide spectrum of knowledge is available on this important subject, there is still a lack of a comprehensive review for an objective comparison and/or selection of chromatographic columns. We aim for this review to be a comprehensive, authoritative, critical, and easily readable monograph of the most relevant publications regarding column selection and characterization in RP-HPLC covering the past four decades. Future perspectives, which involve the integration of state-of-the-art molecular simulations (molecular dynamics or Monte Carlo) with minimal experiments, aimed at nearly "experiment-free" column selection methodology, are proposed.

Cholesterol-based polymeric monolithic columns for capillary liquid chromatography. Part II

This paper presents a second part of our research devoted to cholesterol-based polymeric monolithic stationary phase. The obtained capillary columns were successfully used for separations of alkylbenzenes, steroid hormones and polycyclic aromatic hydrocarbons during isocratic or gradient elutions. The columns showed excellent thermal stability. Increasing the temperature resulted in decrease of the retention factors and peak widths, but selectivity and efficient separations have been retained in the studied temperature range of 20 to100°C. Additionally, the van't Hoff model presented a non-linear relationship of lnk versus 1/T plots, which is likely the result of liquid crystal properties of cholesterol. The studied capillary monolithic columns showed extremely fast re-equilibration after gradient elution and found to be stable under such conditions as: fast flow rate, high acetonitrile content in the mobile phase (no swelling) and high temperature.

Cholesterol-based polymeric monolithic columns for capillary liquid chromatography

A novel, cholesterol-based polymeric monolithic stationary phase for capillary liquid chromatography, was prepared by thermally initiated in-situ polymerization. Cholesteryl methacrylate (CholMA) was used as a functional monomer and trimethylolpropane trimethacrylate (TRIM) was a cross-linker, while azobisisobutyronitrile (AIBN) was an initiator. Isooctane and toluene were chosen as "poor" and "good" solvent, respectively, as constituents of the porogen solvent. Isocratic elutions of alkylbenzenes and separation of the testing mixture of o-terphenyl and triphenylene were conducted for all of the monoliths to assess their hydrophobicity and planar selectivity characteristic for cholesterol-based stationary phases. The synthesized columns demonstrated efficiency exceeding N=10,000 plates and a plate height of ca. H=30 μm. Column preparation was found to be highly reproducible; the relative standard deviation (RSD) values (n=3) for day-to-day and column-to-column were less than 4.08 and 2.02%, respectively, based on retention factor of alkylbenzenes.

Characterization of enhanced-fluidity liquid hydrophilic interaction chromatography for the separation of nucleosides and nucleotides

Hydrophilic interaction chromatography (HILIC) is a liquid chromatographic separation mechanism commonly used for polar biological molecules. The use of enhanced-fluidity liquid chromatography (EFLC) with mixtures of methanol/water/carbon dioxide is compared to acetonitrile/water mobile phases for the separation of nucleosides and nucleotides under HILIC conditions. Enhanced-fluidity liquid chromatography involves using common mobile phases with the addition of substantial proportions of a dissolved gas which provides greater mobile phase diffusivity and lower viscosity. The impact of varying several experimental parameters, including temperature, addition of base, salt, and CO₂ was studied to provide optimized HILIC separations. Each of these parameters plays a key role in the retention of the analytes, which demonstrates the complexity of the retention mechanism in HILIC. The tailing of phosphorylated compounds was overcome with the use of phosphate salts and the addition of a strong base; efficiency and peak asymmetry were compared with the addition of either triethylamine (TEA), 1,4-diazabicyclo [2.2.2] octane (DABCO) or 1,5-diazabicyclo [4.3.0] non-5-ene (DBN). DBN and DABCO both led to increased efficiency and lower peak asymmetry; DBN provided the best results. Sodium chloride and carbon dioxide were added to enhance the selectivity between the analytes, giving a successful isocratic separation of nucleosides and nucleotides within 8 min. The retention mechanism involved in EFL-HILIC was explored by varying the temperature and the mole fraction of CO₂. These studies showed that partitioning was the dominant mechanism. The thermodynamics study confirmed that the solvent strength is maintained in EFLC and that a change in entropy was mainly responsible for the improved selectivity. The selectivity using methanol/water/carbon dioxide varied greatly compared to that obtained with acetonitrile/water. Finally while this study highlights the optimization of EFL-HILIC for the separation of nucleosides and nucleotides under isocratic conditions, this is also an example of the broad range of polarities of compounds that EFL-HILIC can separate.