Use of the kinetic plot method to analyze commercial high-temperature liquid chromatography systems

In-Depth Performance Analysis and Comparison of Monolithic and Particulate Zwitterionic Hydrophilic Interaction Liquid Chromatography Polymer Columns

The kinetic performance of different zwitterionic hydrophilic interaction liquid chromatography polymer columns is evaluated and compared in-depth. For this purpose, two lab-made monolithic columns, synthesized with different crosslinkers, and a commercial particle packed column are considered. It is found that performance evaluation techniques, such as comparing plate height curves or fitted A-, B- and C-terms, obtained by fitting experimental plate height data to a plate height model, are complicated by the determination of a reliable characteristic length. This is due to the very different morphology of these column types, and the heterogeneity of the monolithic columns. The occurrence of a convective flow through the packed particle column further complicates the interpretation of the obtained fitting parameters, as part of the C-term is wrongfully attributed to the A-term. Therefore, the use of the kinetic plot method is suggested for the comparative evaluation of these columns, as kinetic plots do not require the determination of a characteristic length, nor rely on any fitting parameters. With the kinetic plot method, it is demonstrated that the lab-made monolithic columns outperform the packed particle column for plate counts between 10,000 and 800,000. This is attributed to the higher column efficiency of these columns, due to their small domain and skeleton sizes, and their high permeability, resulting from their high external porosity and the occasional occurrence of preferential flow paths.

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.

Graphical Method for Choosing Optimized Conditions Given a Pump Pressure and a Particle Diameter in Liquid Chromatography

The general limitations on liquid chromatographic performance in isocratic and gradient elution are now well understood. Many workers have contributed to this understanding and to developing graphical methods, or plots, to illustrate the capabilities of chromatographic systems over a wide range of values of operational parameters. These have been invaluable in getting a picture, in broad strokes, about the value of changing an operational parameter or the value of one separation approach over another. Here we present a plotting approach more appropriate for determining how to use chromatography most efficiently in one's own laboratory. The axes are linear: column length vertical and mobile phase velocity horizontal. In this coordinate system, straight lines with intercept zero correspond to different values of t₀. Hyperbolas correspond to values of pressure as the product of length and velocity is proportional to pressure. For a given relationship between theoretical plate height and velocity (e.g., van Deemter), the number of theoretical plates as a function of column length and mobile phase velocity is a surface (z direction) to the x and y of velocity and length. By representing the surface as contours, a two-dimensional plot results. Any point along a constant pressure hyperbola represents the best one can do given the particle diameter, solute diffusion coefficient, and temperature. The user can quickly see how to use the pressure for speed or for more theoretical plates. Sets of such plots allow for comparisons among particle diameters or temperatures. Analogous plots of peak capacity for gradient elution are equally revealing. The plots lead instantly to understanding liquid chromatographic optimization at a practical level. They neatly illustrate the value (or not) of changing pump pressure, particle diameter, or temperature for fast or slow separations in either isocratic or gradient elution. They are illustrated with a focus on maximizing plate count with a given analysis time (isocratic), the effect of volume overload (isocratic), and separations of a limited number of peptides with a peak capacity coming from statistical peak overlap theory (gradient).

Fast gradient separation by very high pressure liquid chromatography: reproducibility of analytical data and influence of delay between successive runs

Five methods were used to implement fast gradient separations: constant flow rate, constant column-wall temperature, constant inlet pressure at moderate and high pressures (controlled by a pressure controller),and programmed flow constant pressure. For programmed flow constant pressure, the flow rates and gradient compositions are controlled using input into the method instead of the pressure controller. Minor fluctuations in the inlet pressure do not affect the mobile phase flow rate in programmed flow. There producibilities of the retention times, the response factors, and the eluted band width of six successive separations of the same sample (9 components) were measured with different equilibration times between 0 and 15 min. The influence of the length of the equilibration time on these reproducibilities is discussed. The results show that the average column temperature may increase from one separation to the next and that this contributes to fluctuation of the results.

Comparison of columns packed with porous sub-2 microm particles and superficially porous sub-3 microm particles for peptide analysis at ambient and high temperature

The objective of this study was to evaluate various chromatographic approaches for peptide analysis. Initially, the ultra-HPLC (UHPLC) strategy, which consists of using columns packed with sub-2 microm particles at a maximal pressure of 1000 bar, was tested. To limit the backpressure generated by small particles, columns packed with superficially porous sub-3 microm particles (fused-core technology) that should theoretically improve mass transfer, particularly beneficial for large biomolecules, were investigated. To evaluate these claims, kinetic plots were constructed in both isocratic and gradient modes at ambient and elevated temperature (up to 90 degrees C). For peptide analysis, both UHPLC and fused-core technologies showed a significant gain in peak capacity when compared with conventional HPLC using 5 mum particles and monolithic supports. Additionally, it has been shown that high temperature was of utmost interest to further improve kinetic performance and peak shape due to the improvement of secondary interaction kinetics. Finally, the best conditions developed for UHPLC using the gradient kinetic plot methodology were applied to the analysis of a complex tryptic digest of various proteins. The expected and experimental peak capacity values obtained were similar. In addition, the resolving power of UHPLC at 60 degrees C was appropriate for resolving complex mixtures of peptides.

High performance liquid chromatography analysis of wine anthocyanins revisited: effect of particle size and temperature

The complex anthocyanin fraction of red wines poses a demanding analytical challenge. We have found that anthocyanins are characterised by extremely low optimal chromatographic velocities, and as a consequence generic HPLC methods suffer from limited resolving power. Slow on-column inter-conversion reactions, particularly between carbinol and flavylium species, are shown to occur on the same time scale as chromatographic separation, leading to increased plate heights at normal chromatographic velocities. In order to improve current routine HPLC separations, the use of small (1.7 microm) particles and high temperature liquid chromatography (HTLC) were investigated. 1.7 microm particles provide better efficiency and higher optimal linear velocities, although column lengths of approximately 20 cm should be used to avoid the detrimental effects of conversion reactions. More importantly, operation at temperatures up to 50 degrees C increases the kinetics of inter-conversion reactions, and implies significantly improved efficiency under relatively mild analysis conditions. It is further demonstrated using relevant kinetic data that no on-column thermal degradation of these thermally labile compounds is observed at 50 degrees C and analysis times of <2h.