Effect of pressure on solute capacity factor in HPLC using a non-porous stationary phase

Complex Protein Retention Shifts with a Pressure Increase: An Indication of a Standard Partial Molar Volume Increase during Adsorption?

Studies of protein adsorption on reversed-phase and ion exchange stationary phases demonstrated an increase in retention with increasing pressure, which is interpreted as a standard partial molar volume decrease during the transition of the protein from a mobile to a stationary phase. Investigation of the pressure effect on the retention of lysozyme and IgG on a cation exchange column surprisingly revealed a negative retention trend with the increase of pressure. Further investigation of this phenomenon was performed with β-lactoglobulin, which enabled adsorption to be studied on both cation and anion exchange columns using the same mobile phase with a pH of 5.2. The same surface charge and standard partial molar volume in the mobile phase allowed us to examine only the effect of adsorption. Interestingly, a negative retention trend with a pressure increase occurred on an anion exchange column while a positive trend was present on a cation exchange column. This indicates that the interaction type governs the change in the standard partial molar volume during adsorption, which is independent of the applied pressure. Increasing the protein charge by decreasing the pH of the mobile phase to 4 reversed the retention trend (into a negative) with a pressure increase on the cation exchange column. A further decrease of the pH value resulted in an even more pronounced negative trend. This counterintuitive behavior indicates an increase in the standard partial molar volume during adsorption with the protein charge, possibly due to intermolecular repulsion of adsorbed protein molecules. While a detailed mechanism remains to be elucidated, presented results demonstrate the complexity of ion exchange interactions that can be investigated simply by changing the column pressure.

Methods to determine the kinetic performance limit of contemporary chromatographic techniques

By combining separation efficiency data as a function of flow rate with the column permeability, the kinetic plot method allows to determine the limits of separation power (time vs. efficiency) of different chromatographic techniques and methods. The technique can be applied for all different types of chromatography (liquid, gas, or supercritical fluid), for different types of column morphologies (packed beds, monoliths, open tubular, micromachined columns), for pressure and electro-driven separations and in both isocratic and gradient elution mode. The present contribution gives an overview of the methods and calculations required to correctly determine these kinetic performance limits and their underlying limitations.

Effect of pressure on the retention of macromolecules in ion exchange chromatography

Shorter analysis times and greater resolving power are contributing factors for transfer of separation methods from an HPLC to a UHPLC system when performing analysis in biopharmaceutical or clinical research. The effect of pressure on separations in reversed phase chromatography is well described, however such investigations on ion exchange columns were previously not conducted. In this study we describe the effect of pressure on retention properties of proteins, oligonucleotides and plasmid DNA in ion exchange chromatography. Different column inlet pressures were obtained by coupling restriction capillaries with column outlet and performing separations at a constant temperature and mobile phase flow rate. Macromolecules were separated in isocratic mode as well as with various linear gradients of salt concentration at a constant pH value. The measured retention time increase was up to 80% for isocratic and 20% for gradient separations for a 500 bar increase in pressure. The effect of pressure was validated on a separate instrument after few months from initial experiments. The influence of pressure on retention properties seems to be dependent on the size, shape and flexibility of the macromolecule and causes different retention shifts when separating a sample with diverse analytes. Such changes in retention time can sometimes exceed the criteria set by European Pharmacopoeia (Ph. Eur.) for the allowable method adjustment and are thus considered to be a result of a different separation method. Therefore, the pressure effect that follows method transfer from HPLC to UHPLC conditions should not be neglected even for gradient separations in ion exchange chromatography, as the resulting retention change may cause revalidation of the separation method.

New trends in reversed-phase liquid chromatographic separations of therapeutic peptides and proteins: theory and applications

In the pharmaceutical field, there is considerable interest in the use of peptides and proteins for therapeutic purposes. There are various ways to characterize such complex samples, but during the last few years, a significant number of technological developments have been brought to the field of RPLC and RPLC-MS. Thus, the present review focuses first on the basics of RPLC for peptides and proteins, including the inherent problems, some possible solutions and some directions for developing a new RPLC method that is dedicated to biomolecules. Then the latest advances in RPLC, such as wide-pore core-shell particles, fully porous sub-2 μm particles, organic monoliths, porous layer open tubular columns and elevated temperature, are described and critically discussed in terms of both kinetic efficiency and selectivity. Numerous applications with real samples are presented that confirm the relevance of these different strategies. Finally, one of the key advantages of RPLC for peptides and proteins over other historical approaches is its inherent compatibility with MS using both MALDI and ESI sources.

Investigation of the effect of pressure on retention of small molecules using reversed-phase ultra-high-pressure liquid chromatography

The effect of inlet pressure on the retention of a series of low molecular weight acids, bases and neutrals, was investigated at constant temperature in reversed-phase liquid chromatography using a commercial ultra-high-pressure system (Waters UPLC instrument). For neutral compounds, relatively small increases in retention factor of up to approximately 12% for a pressure increase of 500bar were noted; the largest values were obtained for polar solutes, or solutes of higher molecular weight. Ionisable acids and bases gave much larger increases in retention with pressure, in some cases as high as 50% for a pressure increase of 500bar. Thus, such compounds could show increases in retention factor approaching 100% over the pressure range available in the commercial UPLC instrument. Due to these differential increases, significant selectivity effects can be obtained for mixtures of different types of solute merely by changing the pressure.

Very high pressure HPLC with 1 mm id columns

Theoretical calculations and experimental data indicate that very high pressure HPLC can be performed using 1 and 1.5 mm id columns, and contrary to previous beliefs, the frictional heating generated does not appear to be detrimental to the separation.