Use of isopycnic plots to understand the role of density in SFC - I. Effect of pressure variation on retention factors.

This paper aims to demonstrate the effect of pressure variations in modifying analyte retention behavior in SFC. There is a general understanding that in SFC increasing pressure decreases the retention factor (k'), and vice versa. What is not clearly discussed or explained in any recent literature is that these variations can be very different at different operating pressures, temperatures and modifier concentrations.

Estimations of temperature deviations in chromatographic columns using isenthalpic plots. I. Theory for isocratic systems.

We propose to use constant enthalpy or isenthalpic diagrams as a tool to estimate the extent of the temperature variations caused by the mobile phase pressure drop along a chromatographic column, e.g. of its cooling in supercritical fluid and its heating in ultra-performance liquid chromatography.

A scaling rule in supercritical fluid chromatography. I. Theory for isocratic systems.

Scaling is regularly done in chromatography either to transfer a successfully designed method of analysis developed in one system to another system, or to scale-up a separation method developed in analytical scale to preparative scale. For liquid chromatography there are well-tested guidelines for scaling, which makes it a routine job. For supercritical fluid chromatography (SFC), on the other hand, neither do we have any well-understood principles behind scaling nor do we know how far the strategies applied in LC could be applicable to SFC.

Adsorption of cations onto positively charged surface mesopores.

Uwe Neue developed a theoretical treatment to account for the adsorption of ions on mesopores of packing materials the walls of which are bonded to ionic ligands but left this work unfinished. We elaborated upon this treatment and refined it, based on the equivalence that he suggested between charged surface particles and a membrane that separates two ionic solutions but is impermeable to one specification. He had written that the electro-chemical potentials in both ionic solutions are equal (Donnan equilibrium).

High-resolution peptide mapping separations with MS-friendly mobile phases and charge-surface-modified C18.

Ionic analytes, such as peptides, can be challenging to separate by reverse-phase chromatography with optimal efficiency. They tend, for instance, to exhibit poor peak shapes, particularly when eluted with mobile phases preferred for electrospray ionization mass spectrometry. We demonstrate that a novel charged-surface C18 stationary phase alleviates some of the challenges associated with reverse-phase peptide separations.

Influence of pressure on the retention of sugars in hydrophilic interaction chromatography.

Pressure can influence the retention of analytes in hydrophilic interaction chromatography as well as in RP chromatography. We demonstrate that the retention of sugars in hydrophilic interaction chromatography decreases with pressure, and interpret the observation as a gain in solvation, or more specifically hydration, as the sugar molecules enter the water-rich stationary phase.

Functionalization of divinylbenzene/N-vinylpyrrolidone copolymer particles: ion exchangers for solid phase extraction.

A series of four-mixed mode ion exchangers for SPE, consisting of either weak or strong cation or anion exchangers, have been synthesized by functionalization of spherical, porous particles made from a copolymer of N-vinylpyrrolidone and divinylbenzene. These materials are able to selectively retain and release acidic and basic solutes through the judicious choice of wash solvent pH, as shown through the use of SPE recovery tests.

Comparison of mixed-mode anion-exchange performance of N-vinylimidazole-divinylbenzene sorbent.

A newly synthesized copolymer based on N-vinylimidazole-divinylbenzene (NVIm-DVB) was evaluated as a mixed-mode anion-exchange sorbent for SPE, since the NVIm monomer apart from its hydrophilic properties can be protonated at a certain pH, and then performs as an anion-exchanger. To investigate the behavior of the NVIm-based sorbent, the SPE performance was evaluated under reversed-phase (RP), weak anion-exchange, and strong anion-exchange conditions. The results for the NVIm-DVB sorbent were also compared to commercial reference sorbents from each group: Oasis HLB, Oasis WAX, and Oasis MAX, respectively.

Mechanism of retention loss when C8 and C18 HPLC columns are used with highly aqueous mobile phases.

We describe investigations into the cause of retention losses encountered when C8 and C18 HPLC columns are used with highly aqueous (> 90% water) mobile phases. A procedure for quantifying these losses is described, involving stopping and restarting the flow. This procedure was used to study the dependence of retention loss on the pore size, surface concentration, and chemical structure of the bonded phase.

Development of an accelerated low-pH reversed-phase liquid chromatography column stability test.

The important experimental design criteria for an accelerated low-pH RPLC column stability test are discussed. The influence of method variables on the amount and rate of retention-loss and the final optimized parameters for the accelerated low-pH RPLC stability test are presented. The retention-loss curves for selected C8 and C18 stationary phases are compared.

Characterization and evaluation of C18 HPLC stationary phases based on ethyl-bridged hybrid organic/inorganic particles.

The characterization and evaluation of three novel 5-microm HPLC column packings, prepared using ethyl-bridged hybrid organic/inorganic materials, is described. These highly spherical hybrid particles, which vary in specific surface area (140, 187, and 270 m(2)/g) and average pore diameter (185, 148, and 108 A), were characterized by elemental analysis, SEM, and nitrogen sorption analysis and were chemically modified in a two-step process using octadecyltrichlorosilane and trimethylchlorosilane. The resultant bonded materials had an octadecyl surface concentration of 3.