Peter W. Carr

Chemistry of zirconia and its use in chromatography.

The purpose of this review is to shed some light on the complex properties of zirconias surface chemistry in order to better understand its behaviour under chromatographic conditions. We emphasize the great differences between the much better known chemistry of a silica surface and the chemistry of zirconias surface. The review describes both the physical and chemical properties of zirconium dioxide from a chromatographic point of view. The chemistry of monoclinic zirconia surface is developed from its underlying crystalline structure. The paper describes the dependence of the specific surface area, pore volume, porosity and mechanical strength on thermal treatment. Methods of synthesis of chromatographically useful zirconia are outlined. The review also covers the adsorption properties of zirconia at both gas-solid and liquid-solid interfaces. Adsorption of water, carbon dioxide, carbon monoxide and ammonia are described and the controversies concerning the surface concentration of adsorption sites are presented. The complex chemistry of a zirconia surface is pointed out and the importance of ligand exchange reactions is emphasized. In contrast to a silica surface, ligand exchange plays an important role in liquid chromatographic applications of zirconia. Strong, hard Lewis acid sites, present on a zirconia surface, can interact with hard Lewis bases and these interactions, sometimes troublesome, can be successfully exploited even for protein separations. Zirconias surface can be modified in many ways: dynamically, by addition of competing Lewis bases to the mobile phase, or permanently, by covering its surface with polymers or by depositing carbon. The review also shows that the main difficulty in achieving a wider variety of applications is probably our lack of knowledge and poor understanding of zirconias surface chemistry.

Study of retention in reversed-phase liquid chromatography using linear solvation energy relationships. I. The stationary phase

Abstract The applicability of linear solvation energy relationships (LSERs) to reversed-phase liquid chromatography (RPLC) was studied by examining the retention of a wide variety of aliphatic and aromatic compounds over the range of 20–50% (v/v) acetonitrile, methanol and tetrahydrofuran. The role of cavity formation, dispersion interaction, polarity/polarizability, hydrogen bond acidity, and hydrogen bond basicity in determining the retention behavior as the mobile phase composition was changed has been investigated. The LSER coefficients were then examined in terms of the corresponding properties of the mobile phase (cohesive energy density, surface tension, the Abraham solvophobic parameter, polarity/polarizability, hydrogen bond basicity, and hydrogen bond acidity) and from these the influence of mobile phase and stationary phase on the retention behavior was explored. In order to chemically interpret the RPLC retention results we compared them to alkane–water and octanol–water partition coefficients.

Solvatochromically based solvent-selectivity triangle

Abstract A classification of common solvents according to their dipolarity and hydrogen-bonding acidity and basicity has been developed, based on the Kamlet—Taft solvatochromic parameter scheme. This approach has been compared with the Snyder—Rohrschneider solvent-selectivity triangle (SST). The two solvent-classification schemes are found to be generally similar. Both SST-based schemes are also compared to an analysis of solvent selectivity based on linear salvation energy relationships. While there are considerable similarities, important practical differences, especially in the case of reversed-phase liquid chromatography, are evident.

Revisionist look at solvophobic driving forces in reversed-phase liquid chromatography

Abstract Based on the use of alkylbenzenes as test solutes, most of the free energy of retention in reversed-phase liquid chromatography (RPLC) is shown to arise from net attractive (exoergic) processes in the stationary phase, and not from net repulsive (endoergic) processes in the mobile phase. The classical view of the “passive” role of bonded phase ligands is challenged. However, it is also shown that variations in retention upon changing the mobile phase are dominated by alterations in the net processes in the mobile phase. Furthermore, it is shown that the free energy of transfer of a methylene group from the mobile phase to a bonded reversed phase over a wide range in mobile phase composition, is similar but not equal, to the free energy of transfer of a methylene group from the same mobile phase to pure bulk hexadecane. This observation is in accord with the partition model view of the mechanism of RPLC. Finally, by comparison of measured and computed activity coefficients, the regular solution theory is shown to be a grossly inadequate model of interactions in water and hydro—organic mixtures. It should not be used to model retention in aqueous mobile phases.

Non-linear waves in chromatography. I: Waves, shocks, and shapes

Abstract Cause and effect in non-linear chromatography are examined from the point of view of wave theory. This first of four instalments is restricted to single-component systems and examines monotonic concentration variations and chromatographic peaks and bands. It uses the wave equation, which states the velocity at which a given concentration advances, to establish the properties of “waves,” that is, monotonic concentration variations. Depending on the sense of curvature of the isotherm, a wave may be self-sharpening or nonsharpening. A self-sharpening wave remains, or sharpens to become, a shock layer; a nonsharpening wave spreads indefinitely, eventually in proportion to traveled distance. The concentration profile of a shock layer depends on the shape of the isotherm and on the dispersive effect of non-idealities, of which resistance to mass transfer usually is the most important. Mass-transfer resistance in the moving phase causes “fronting:” mass-transfer resistance within the stationary sorbent causes “tailing.” It is therefore in general not possible to model shock layers with only a single, lumped mass-transfer parameter. The concentration profile of a nonsharpening wave depends almost exclusively on the shape of the isotherm. The knowledge of wave behavior is used to examine peak shapes in elution under overload conditions and bands in displacement. The peak shape in elution is almost entirely determined by the degree of overload and the shape of the isotherm. Wave theory confirms a rule previously stated by Knox that, in columns exceeding a certain length, samples containing the same amount of solute give peaks of essentially the same shape under conditions of predominant concentration overload, predominant volume overload, or any combination of the two. In displacement development, the final pattern can be established by determination of the lengths of the bands of the individual components according to Tiselius and separate calculation of the shock-layer profiles.

A simple, fast numerical method for the solution of a wide variety of electrochemical diffusion problems

Abstract A numerical analysis technique known as orthogonal collocation, which has found considerable use in solving linear and non-linear partial differential equations in chemical engineering, is applied to a variety of boundary value problems found in chronoamperometry. Due to the properties of the collocation functions, the technique requires very little implicit mathematics and results in a large savings in computer time over the common digital simulation technique or the Crank-Nicholson finite difference scheme. Simulation of a new boundary value problem requires only a few operations of calculus and matrix algebra and alteration of very few program statements. The computer program employed is quite simple and is composed almost entirely of highly efficient library subroutines. Due to the nature of the orthogonal polynomials used to represent the spatial dependence of concentration, the trial function exactly fits the partial differential equation at the collocation points and transforms it into a set of simultaneous first order ordinary differential equations. These equations are solved numerically in about 5 s on a CDC 6600 to give accurate working curves (≈0.1% error or less) for the e.c.e. and for first and second order disproportionation mechanisms at a planar electrode.

Revisionist look at solvophobic driving forces in reversed-phase liquid chromatography. III. Comparison of the behavior of nonpolar and polar solutes

Abstract Recently we showed that in reversed-phase liquid chromatography (RRLC), solute methylene units are embedded in the bonded alkyl chains of the stationary phase. That is, the “partition mechanism”, not the “adsorption mechanism”, controls retention of methylene groups. We have also shown that the net interactions of the nonpolar groups of a solute with a nonpolar stationary phase contribute more to the overall free energy of transfer than does the solvophobic interaction in the mobile phase. The present work explores the behavior of several polar functional groups (Cl, OCH 3 , CHO, NO 2 , CN, and COOCH 3 ) in RPLC. In contrast to the behavior of methylene groups, the net interactions of more polar functional groups with the mobile phase are larger than are their net interactions with the stationary phase. Further, the data indicate that the free energy of transfer of a polar functional group in bonded phase RPLC is quite different from the free energy of transfer between the mobile phase and a bulk nonpolar liquid used to emulate the bonded phase. This implies that either sorbed mobile phase significantly influences the retention properties of polar groups in the bonded phase, or that the solute functional groups reside at the interface between the mobile and stationary phases.

High-performance anion exchange of small anions with polyethyleneimine-coated porous zirconia

The preparation and chromatographic characterization of an ion-exchange high-performance liquid chromatographic support by deposition and crosslinking of polyethyleneimine (PEI) on the surface of porous zirconia is described. Adsorption and evaporation methods were used for coating PEI onto the zirconia particles. These two stationary phases were compared by elemental analysis, ion-exchange capacity and by chromatography. High efficiency and good selectivity were observed for inorganic and organic anions. The addition of a strong, hard Lewis base to the mobile phase dramatically improved the peak shape and efficiency of para benzoic acid derivatives. PEI-coated zirconia showed a distinct elution sequence for organic anions when compared to bare zirconia or silica-based phases. The polyamine coated zirconia was stable over a pH range from 2.75 to 9. Flow studies, using nitrite as a probe solute, showed that both coating procedures produced packed columns with good mass-transfer properties.

Extra-thermodynamic relationships in chromatography—study of the relationship between the slopes and intercepts of plots of ln k′ vs. mobile phase composition in reversed-phase chromatography

Abstract The chemical validity of the frequently reported linear relationship between the slopes (S) and intercepts (ln k′w) of plots of ln k′ for various solutes against mobile phase volume fraction organic modifier in reversed-phase liquid chromatography (RPLC) has been investigated. We have shown that under certain conditions, the linear S-ln k′w relationship for a chemically variegated set of solutes, even though it is mathematically quite real, is an artifact that results from statistical considerations and does not reflect chemical reality. However, S-ln k′w correlations are chemically meaningful when the set of test solutes is restricted to a homologous series. Based on conventional extra-thermodynamic reasoning, such correlations can only be due to chemistry under two circumstances; i.e., when only a single retention-governing solute property varies within the set of solutes studied, or when various retention governing processes have the same compensation composition (ϕiso). The existence of a real correlation is also shown to be at odds with the concepts of linear salvation energy relationships and other models of retention that describe retention in RPLC as the result of a variety of independent intermolecular interactions. Finally it is shown that if linearity between S and ln k′w were universal, the well-known phenomena of changes in elution sequence with the volume fraction of organic modifier in the mobile phase would not be possible.

Extra-thermodynamic relationships in chromatography enthalpy-entropy compensation in gas chromatography

Abstract The phenomenon of enthalpy-entropy compensation in gas chromatography is examined. Using 53 probe solutes that span a wide range in size (dispersive interaction), dipolarity, hydrogen-bond-donor and hydrogen-bond-acceptor strength, enthalpy-entropy compensation is not observed, while for probe solutes within a homologous series enthalpy-entropy compensation is observed as predicted by the linear salvation energy relationship methodology.