Robert Tijssen

Use of the solubility parameter for predicting selectivity and retention in chromatography

Abstract The theoretical and practical aspects of the solubility parameter (“polarity”) model for predicting retention behaviour and partition coefficients have been investigated. A critical discussion of the theoretical basis shows that sufficiently accurate data for the total solubility parameter can be obtained from generalized thermodynamic functions, rather than from the usual vapour pressure data. Partial polarities are obtained from theoretical and semi-empirical relations with simple physical parameters such as refractive index, dielectric constant and dipole moment. Internal pressure, calorimetric and spectroscopic experimental data are used for estimating the contributions of charge and proton transfer (acid-base interaction). Tables of partial and total polarities are presented which replace older, less accurate data. The multicomponent model is tested with chromatographic and partition experiments and is proposed as an alternative method for selecting a proper phase system (in both chromatography and partition methods where, for example the Hansch method is frequently used).

Modelling retention of ionogenic solutes in liquid chromatography as a function of pH for optimization purposes

In liquid chromatography, the retention of ionogenic solutes is a strong function of the pH of the mobile phase, with different solutes showing different behaviour, both qualitatively (e.g., acids vs. bases) and quantitatively (values of dissociation constants). Thus, pH will also affect selectivity and it can be used as a parameter for optimizing separations. In many instances, such optimization studies will require an accurate description of retention as a function of pH. In this paper, attention is focused on basic models describing retention as a function of pH and their use in practice. The theoretical, sigmoidal curve is discussed and a number of possible causes of deviations are considered. The inaccuracies introduced by linearly or quadratically interpolating part of a sigmoidal curve are addressed, in addition to the sensitivity of sigmoidal interpolation to experimental errors.

Microcapillary liquid chromatography in open tubular columns with diameters of 10–50 μm : Potential application to chemical ionization mass spectrometric detection

Abstract The theoretical separation efficiency of open microcapillary liquid chromatography (LC) columns, including peak-broadening effects resulting from interphase resistance to mass transfer, has been considered and an expression is derived for the plate height caused by interphase resistance. The use of such columns with internal diameters down to 10 μm is explored for LC separations. The columns were prepared from soft glass tubing and coated with polar and non-polar stationary phases. Several applications in straight phase as well as reversed-phase systems demonstrate the high separation speed (up to 50 effective plates per second). Relatively wide (30–50 μm) and short (1–5 m) columns allow rapid analyses within minutes. Smaller (10–30 μm) and longer (5–25 m) columns yield extremely high plate numbers (up to 5·10 6 ), permitting very difficult separations in a reasonable time (2–5 h). The required pressure never exceeds the generally accepted value of 400 bar. Preliminary results obtained with fused silica columns are discussed. Split-injection and addition of make-up mobile phase through the (UV) detector have been applied. In order to avoid undesirable dilution of the sample zones by the make-up liquid, the microcapillaries were directly coupled to a mass spectrometer (in the chemical ionization mode). This technique has yielded promising results.

Lattice models for the description of partitioning/ adsorption and retention in reversed-phase liquid chromatography, including surface and shape effects

Abstract The retention mechanism in reversed-phase liquid chromatography (RPLC) with silica particles modified with surface-grafted alkyl chains cannot be fully understood unless the specific properties of the surface layers, such as the configurational constraints of terminally attached chains, are taken into account. The commonly accepted view that the main factor governing RPLC retention behaviour is constituted by solute—solvent interactions in the bulk mobile phase is supported by useful but simplified theories based on solvation as in bulk liquids. Solvation in bulk liquids depends on the free energy to create “cavities” for solute molecules in mobile and stationary phases. This paper first reviews possibilities and shortcomings of regular solution theories, where the partition coefficient is expressed in terms of the Flory—Huggins (FH) interaction parameters for the solute. Where enthalpic effects dominate, these parameters can be obtained from experimental data or from generalized thermodynamic functions expressed as Hildebrands solubility parameter, δ, representing the square root of the cohesive energy density. In RPLC with terminally attached chains on the support, entropy effects arising from the molecular organization of chains are also important, and entropic expulsion of solute molecules from the stationary phase is expected to take place. RPLC practice indicates that the nature of the grafted layer [e.g., flexibility of grafted chains and “phase transitions”, geometrical effects, chain length effects, chain branching and surface effects (coverage and hydroxyls)] indeed influences the “adsorptive” and retentive capacity of the bonded stationary layer. Theories specially designed for grafted layers are reviewed starting with (oversimplified) rod-like chain models, followed by several, more recent, lattice theories, which are based on extensions of the Flory—Huggins lattice theory for polymers in solution. These theories, when applied to the RPLC retention mechanism, take into account some aspects of the molecular organization in the grafted layer, but are still subject to simplifying assumptions. A more general approach is based on the self-consistent field theory for adsorption (SCFA) originally developed by Scheutjens and Fleer to describe the polymer adsorption, where in essence the segment density distribution is found resulting from minimization of free energy. Extending the SCFA theory to allow for RPLC conditions provides insight into the effects of the solvent quality (modifier content), collapse of the chain phase, the grafted and the solutes chain lengths and the grafting density (surface coverage) on the segment density profile. Both aliphatic and amphiphilic solute molecules appear to be distributed non-uniformly in the grafted layer and are accumulated in the boundary region near the interface between chain phase and bulk solvent. Using the related theory by Leermakers and Scheutjens [self-consistent anisotropic field (SCAF) theory], shape selectivity is shown for flexible chain, star- and rod-like solutes, chain length effects and alignment are also being found. In the presence of a specific affinity for the silica surface, due to residual hydroxyls, for both polar solvent molecules and solute molecules for polar groupa, both the SCFA and the SCAF theories predict an accumulation of polar segments near the silica surface with is fairly pronounced, displacing most of the (unattached) non-polar segments more towards the chain phase surface.

Separation by flow (hydrodynamic chromatography) of macromolecules performed in open microcapillary tubes

It is shown experimentally that, in principle, separations of macromolecular species are possible by differential migration in the flow of a solvent through a microcapillary tube (diameter less than ca. 10 μm). Compared to gel permeation chromatographic separations, hydrodynamic chromatographic separations are rapid and show little axial dispersion of the sample zones. The results are in a qualitative agreement with the theory of separation-by-flow, proposed by Guttmann and DiMarzio.

Peer Reviewed: Electrokinetic Separations for Synthetic Polymers.

In terms of speed and sample throughput, CE-like methods have an enormous advantage over size-exclusion chromatography for large numbers of samples.

Experimental Aspects of Partitioning Tracer Tests for Residual Oil Saturation Determination With FIA-Based Laboratory Equipment

Partitioning tracer tests are used to determine residual oil saturation (ROS). Laboratory equipment based on the flow injection analysis (FIA) method has been constructed for rapid (within 2 hours) and accurate (better than 8%) determination of the equilibrium partition coefficient of tracers between crude oil and brine under simulated reservoir conditions. The apparatus makes possible investigation of the influence of all relevant parameters on the partition coefficient: tracer type and concentration, crude oil type, pressure (up to 34 MPa (4,930 psi)), GORs, temperature (up to 150{degrees}C (302{degrees}F)), and brine salinity. The FIA equipment has been critically evaluated with model compounds. Its versatility has been verified with experimental results obtained on dead and live crudes.

Chapter 4 Hydrodynamic chromatography of polymers

Publisher Summary This chapter discusses the hydrodynamic chromatography (HDC) of polymers. HDC is mainly used for separating colloidal particles of different sizes. Particle-size separation takes place when the finite dimensions of particles are large enough to interfere with the flow profile in a packed bed or open tubular flow system. For geometric reasons, particles are excluded from the slower moving wall regions in the flow channel, which travel at a velocity greater than the average velocity of the solvent. Hence, larger particles elute first from the flow system, followed by the smaller ones. HDC can also be used for separating large molecules such as proteins, polystyrenes, and water-soluble polymers of high molecular mass. A microcapillary is a column of circular cross section with an internal diameter of less than 10 μ m. The most important part of a microcapillary HDC separation technique is the detection of the separated zones, which is also the operation that strongly limits the practicability of the technique.

Mechanisms of the Separation and Transport of Polymer Systems in Chromatographic Media

“Chromatographie” techniques which are based on the principle of separation according to differences in the size of materials in solution only, belong to the class of SSC methods (Size Separation Chromatography). The ultimate parameter in SSC is, by definition, the aspect ratio λ = (particle size)/(channel size). Notably the size of polymer molecules near impermeable channel walls is described, including reptation of polymer chains. Next SEC/GPC (Size Exclusion/Gel Permeation Chromatography) and HDC Hydrodynamic Chromatography (or separation—by—flow) are treated. For the latter technique the so-called “Tubular Pinch” effect, stemming from “lift” forces from inertia in sheared flow, is also discussed. Finally, it is concluded that HDC and SEC are complementing SSC methods.