Henri Colin

The role of the temperature in reversed-phase high-performance liquid chromatography using pyrocarbon-containing adsorbents

Abstract The investigation of the effect of temperature in liquid chromatography on pyrocarbon-containing adsorbents is considered in terms of heats of transfer, solvent eluotropic strength, column linear capacities, pressre drops and efficiencies. The selectivity of the system is scarcely altered on changing the temperature; it decreases by less than 10% when the temperature increases from ambient to 65°C. The heat of transfer, which is determined by the nature of the mobile phase, increases directly with increasing capacity factor of the solute. The eluotropic strength of the solvent is hardly affected by the temperature. The linear capacity of the column increases two-fold with a 40°C increase in temperature. The greater effect of temperature on the column efficiency and pressure drop is due to the decrease in the solvent viscosity, resulting in larger diffusion coefficients of the solutes.

Use of light scattering as a detector principle in liquid chromatography

Abstract The detector described is based on the large difference between the vapour pressure of solvents commonly used in liquid chromatography and that of most analytes, which thus permits phase separation between them. The column eluate is nebulized in a stream of warm gas in which the solvent vaporizes. Downstream, the particles of non-volatile analytes scatter the light of a laser beam. Part of the scattered light is collected and fed to a photomultiplier and the resulting signal is recorded. The detector responds to the mass flow of solutes, which makes it especially suitable for use with narrow-bore columns. The contribution of the detector to band broadening is between 0.1 and 0.2 μl. The baseline is very stable even with a steep concentration gradient, as illustrated by the analysis of triglycerides. The response is not linear but proportional to the power 1.8 of the mass flow of analyte. The detection limit is around 150 ng/sec for non-volatile compounds which are liquid at detector temperature. Possible improvements are discussed.

Introduction to reversed-phase high-performance liquid chromatography

Abstract Reversed-phase liquid chromatography (RPC) is being increasingly used. The different types of bonded phases (BP) for RPC applications and their preparation are reviewed. The extent of the bonding reaction, which is an important parameter affecting the quality of the BP, is discussed. The surface coverage is closely related to the structure of the starting silica. Activation conditions are reviewed and the structure of the final adsorbent is discussed, particularly the role of unshielded silanols. The main characteristics of bonded reversed-phase chromatography (BRPC) are described and some of the advantages of this technique and a few typical applications are reported.

Comparison of some packings for reversed-phase high-performance liquid—solid chromatography: II. Some theoretical considerations

Summary A comparison is made between non-polar chemically bonded phases (CBP), pyrocarbon-modified silica gel (PMS) and pyrocarbon-modified carbon black (PMCB). The retention mechanism is probably an adsorption process in all instances but the contribution of the solute-solvent interactions to the retention is larger on CBP than on PMS or PMCB, resulting in smaller retentions and a greater selectivity for polar compounds. Retentions in homologous series show that the group selectivity is better for PMS (or PMCB) than for CBP. The selectivity for geometrical isomers is also better for PMS. However, if important differences in the polarities of isomers appear, CBP can also be very selective phases. Changing the water content of the solvent mixture generally does not provide important variations in selectivity. Such variations are better achieved by changing the modifier and are more important for CBP than for PMS. A comparison of the loadability of columns is very difficult when single compounds are injected. If separation problems are to be dealt with, the most important parameter is the selectivity of the chromatographic system. The choice of the best packing therefore depends to a great extent on the separations in question.

Retention behavior of alkylbenzenes as a function of temperature and mobile phase composition in reversed-phase chromatography

The reversed-phase chromatographic behaviour of a homologous series has been investigated as a function of the mobile phase composition and temperature. In general, it was found that the methylene selectivity decreased with increasing temperature and decreasing amount of water in the mobile phase. The dependence of the calculated data on the correct determination of the hold-up time is discussed. The methylene group contribution to ΔH of transfer was calculated using two different methods and the agreement was excellent. In addition, the agreement with one other reported value was excellent. Using the homologous series data, the solvent strength of the water-methanol mobile phases was calculated at different temperatures. The results were compared with the data obtained on pyrocarbon supports and the agreement was very good. Linearization methods for calculating to are discussed and the difficulties in using such procedures are described.

General view of molecular interaction mechanisms in reversed-phase liquid chromatography

Abstract The compilation of the results obtained during the last 10 years on the retention behaviour of homologous series in reversed-phase liquid chromatography makes it possible to build a coherent model of molecular interactions for this type of sample. This model takes into account the effects of the chain length and the polarity of the sample molecule. It rationalizes the influence of the bonding density and the function nature (monomeric/polymeric) of the stationary phase and the role of temperature and the nature of the organic modifier in partially aqueous and non-aqueous reversed-phase liquid chromatography. The use of homologous series to investigate retention mechanisms gives the opportunity to distinguish between the solvophobic and the partition theories. Depending on the experimental conditions, both theories can explain the results observed and the changes in retention pattern when altering one of the chromatographic parameters previously mentioned. The description of the structure of the stationary phase as a brush, a picket fence or a stack is also integrated in the model. The experimental results accumulated with homologous series also suggest that the molecular interaction mechanisms governed by molecular recognition sites differ for molecules that are members of different homologous series, polycyclic aromatic hydrocarbons or carotenes.

Reversed-phase liquid-solid chromatography on modified carbon black

Abstract The preparation of carbon adsorbents suitable for reversed-phase high-performance liquid chromatography is described. The effect of the amount of carbon deposited by benzene pyrolysis on thermal carbon blacks on the hardness and specific surface area of the particles obtained is discussed. Optimum performances are obtained with 20% of pyrocarbon. The chromatographic performance of the columns is discussed in terms of coefficients of the Knox and Giddings equations and a scale of solvent eluotropic strengths is established. Efficiencies corresponding to HETP values of 3 and 7 times the particle diameters are obtained for unretained (k′ = 0) and retained (k′ = 1.1) compounds. The polarity of the solvent has little influence on the retention but affects the solubility in the mobile phase. The weakest solvents are low-molecular-weight, polar molecules such as water, methanol and acetonitrile, and the strongest solvents are heavy and/or aromatic molecules, such as chloroform, benzene and xylene. Homologous compounds are easy to separate, as well as geometrical isomers. Some examples of separations are described and discussed.

Development and use of carbon adsorbents in high-performance liquid-solid chromatography : I. Carbon-coated silica particles

Abstract The technique for coating silica particles with a layer of pyrocarbon is described. The conditions of preparation are important as they determine both the chromatographic performance and the degree of agglomeration of particles during coating. The effect of the coating was studied in terms of both efficiency and retention data. For the silica studied (Spherosil XOB 75), moderate amounts of pyrocarbon (less than 15%) gave the best results; greater degrees of coating give a large decrease in efficiency with increasing retention, while smaller degrees of coating give very small retentions and polar adsorbents. Some simple separations are shown to illustrate the analytical characteristics of these adsorbents.

Modified carbon black in high-performance liquid chromatography

Abstract The potential use of modified carbon blacks as adsorbent for high-performance liquid chromatography is discussed. The technique of particle hardening through benzene pyrolysis is described. It is shown that the consolidation must be carefully controlled, as too little hardening gives mechanically unstable packings and too much gives a heterogeneous adsorbent of small specific surface area. The advantages of graphitization after hardening are discussed in terms of efficiency and column capacity. The problems of specific surface area, small particle size and efficiency of the adsorbent at relatively large retention are pointed out. Some separations are given in order to illustrate the performance of the adsorbent.

Gradient elution in liquid chromatography

Abstract The theory of ternary gradients in reversed-phase liquid chromatography, which makes it possible to calculate retention volumes, is presented. These calculations are also possible for isocratic elution using ternary solvent mixtures. The calculation procedure for retention volumes was verified experimentally and the agreement between the experimental and theoretical values was satisfactory. The results show that the calculation procedure may be suitable for the prediction of retention volumes in reversed-phase chromatography using ternary mobile phase gradients. The application of ternary gradients can improve the separation of certain solute mixtures with respect to the binary gradients. An example of the separation of phenols is discussed, where the optimal ternary gradient could be predicted theoretically. Conventional gradient elution equipment which makes possible the mixing of two liquids only can be used for work with ternary gradients also.