Johannes P.C. Vissers

Recent developments in microcolumn liquid chromatography

An overview of the most recent developments in microcolumn liquid chromatography (LC) is presented. A short theoretical discussion on chromatographic dilution and extracolumn bandbroadening is given and also the recent progress and advances in column technology and instrumentation are reviewed. However, the emphasis of this review is on miniaturized sample clean-up, sample introduction techniques and on both established and more recent detection techniques for microcolumn LC. The hyphenation of miniaturized LC columns with other techniques, specifically on multidimensional chromatography and the coupling of microcolumn LC to mass spectrometry is discussed in detail. Both the on-line and automated off-line interfacing to other separation and detection techniques will also be addressed. Finally, a number of typical microcolumn LC applications are presented in order to demonstrate the potential of microcolumn LC methods in a variety of scientific areas.

Microcolumn liquid chromatography: Instrumentation, detection and applications

This review discusses many different aspects of microcolumn liquid chromatography (LC) and reflects the areas of microcolumn LC research interest over the past decades. A brief theoretical discussion on a number of major issues, such as column characterisation, chromatographic dilution effects and extracolumn band broadening in microcolumn LC is given. Recent progress in column technology and the demands and developments of instrumentation and accessories for microcolumn LC are also reviewed. Besides that, the developments in a large number of established and also more recent detection techniques for microcolumn LC are also discussed. The potential of hyphenation of microcolumn LC with other techniques, more particularly of multidimensional chromatography and microcolumn LC coupled to mass spectrometry is reviewed. Finally, the perspective of microcolumn LC separation methods is stressed by a number of relevant applications.

Optimised injection techniques for micro and capillary liquid chromatography

Abstract Large volume injections —involving on-column focusing— were evaluated for packed micro and capillary liquid chromatography columns of 300 μm and 1.0 mm inner diameter (I.D.) respectively. It was found that the I.D. of the injection loop plays a critical role in sample dispersion, effecting peak asymmetry and injection reproductibility. The use of injection loops with too large or too small an I.D. resulted in a reduced injection performance. Ideally the I.D. should be in between 100–150 μm. Further it was investigated what influence the loop volume and the use of a low dispersion injection technique had on the column stability. The combination of large volume injections—up to 1 μl and 5 μl for 300 μm and 1.0 mm I.D. columns, respectively—and additional switching of the injection valve did not affect column lifetime. Typical decreases of only 10% in efficiency over 1500 injections were found.

Automated on-line ionic detergent removal from minute protein/peptide samples prior to liquid chromatography-electrospray mass spectrometry

An automated on-line ionic detergent removal pre-column system coupled to capillary liquid chromatography-electrospray mass spectrometry is described. The system involves two micro precolumns, composed of a specific ionic detergent trapping column and a preconcentration column, respectively, and a packed 300 microns I.D. analytical column. Sample loading to the micro precolumns and regeneration of the detergent trapping column were performed at a flow-rate of 50 microliters/min, while the flow-rate through the analytical column was set at 5.0 microliters/min. Ionic detergent-containing tryptic-digested protein samples were directly applied to the micro precolumns without sample pretreatment and were analysed by UV absorption detection and electrospray mass spectrometry. The presented system allows for the fully automated removal of SDS with virtually no loss in protein/peptides. Maximum SDS load and breakthrough have been determined. Excellent protein recovery and complete removal of SDS is found. The chromatographic separation after SDS removal was completely restored and equalled the reference chromatogram. Mass spectral data confirm these findings. Finally, this technique allows for SDS removal from minute protein samples without the need for any sample handling.

Hydrodynamic aspects of slurry packing processes in microcolumn liquid chromatography

A Stokesian dynamics computer simulation based method is presented for the estimation of the bed porosity of slurry-packed capillary liquid chromatography (LC) columns. A colloidally well-described reversed-phase stationary phase-slurry liquid suspension was used as a model system. The applied simulation method takes into account the velocity of the slurry and colloidal interaction forces, as well as inter-particle hydrodynamic interactions. The predicted bed porosities suggest that a lower slurry velocity leads to a denser packing structure due to the increased effect of colloidal repulsion effects. The results of the simulations were compared with the external porosity and chromatographic performance of capillary LC columns that were packed at different filtration and compaction pressures. However, the trends that were observed in the experimental results suggest that hydrodynamic packing parameters have no or little effect on the chromatographic performance of capillary LC columns. Within the experimental parameter window, the chromatographic performance and the column porosity were not influenced by the filtration and compaction pressure, nor by the duration of the compaction process.

Sedimentation behaviour and colloidal properties of porous, chemically modified silicas in non-aqueous solvents

The sedimentation behaviour and colloidal properties of porous, chemically modified silicas dispersed in non-aqueous solvents have been studied. The free settling behaviour of non-aggregated silica suspensions could effectively be described with a modified Stokes equation that takes into account the possible inclusion of gas in the pores of the particles. The effects of hindered settling and of the dimensions of the sedimentation vessel on the rate of sedimentation were investigated too. The colloidal properties of the silica particles were compared with predictions by the DLVO theory. The Hamaker and Lifshitz theories were utilized to describe the attraction forces between the chemically modified silica particles. The electrophoretic mobilities of the particles in the non-aqueous liquids were determined in order to quantify the electrostatic repulsion forces. The electrostatic repulsion appears to generate a large barrier against coagulation with all investigated porous silicas. However, the way in the particle suspensions are prepared determines whether the particles remain in the primary minimum or are colloidally stable. The level of gas entrainment provides information on the homogeneity of the particle coating.

The Hamaker and the Lifshitz approaches for the Van der Waals interaction between particles of composite materials dispersed in a medium

Based on the superpositional principle of the classical Hamaker–De Boer approach to the Van der Waals interaction, an equation for the interaction energy between two identical composite particles in a medium has been developed. The particles are supposed to be composed of an arbitrary number of components that are intimately mixed to avoid appreciable scattering of interacting EM waves. A discussion is presented on how intimately the components have to be mixed for the theory to be applicable. As an example, the main theoretical result is applied to porous polystyrene particles in water, while the pores are not filled with water (‘non-wetting surface’). Numerical results for composite particles are evaluated, using interaction parameters as obtained by various methods that are based on either the Hamaker–de Boer or the Lifshitz approach.