Georges Guiochon

Complete temperature profiles in ultra-high-pressure liquid chromatography columns.

The temperature profiles were calculated along and across seven packed columns (lengths 30, 50, 100, and 150 mm, i.d., 1 and 2.1 mm, all packed with Acquity UPLC, BEH-C 18 particles, average d(p) approximately 1.7 microm) and their stainless steel tubes (o.d. 4.53 and 6.35 mm). These columns were kept horizontal and sheltered from forced air convection (i.e., under still air conditions), at room temperature. They were all percolated with pure acetonitrile, either under the maximum pressure drop (1034 bar) or at the maximum flow rate (2 mL/min) permitted by the chromatograph. The heat balance equation of chromatographic columns was discretized and solved numerically with minimum approximation. Both the compressibility and the thermal expansion of the eluent were taken into account. The boundary conditions were determined from the experimental measurements of the column inlet pressure and of the temperature profile along the column wall, which were made with a precision better than +/-0.1 K. These calculation results provide the 3-D temperature profiles along and across the columns. The axial and radial temperature gradients are discussed in relationship with the experimental conditions used. The temperature map obtained permits a prediction of the chromatographic data obtained under a very high pressure gradient.

Theoretical study of the accuracy and precision of the measurement of single-component isotherms by the elution by characteristic point method

Abstract Using the equilibrium-dispersive model of chromatography, band profiles corresponding to a known Langmuir isotherm are calculated. A noise sequence is added to the calculated profile to simulate an actual chromatogram. The elution by characteristic point (ECP) method of isotherm calculation is then applied to this profile, and the derived isotherm is compared with the initial one. Significant differences between the “true” and the “measured” isotherms are observed at low or moderate column efficiencies. The direct method of determination of the isotherm from the band profile based on the numerical solution of the inverse mathematical problem gives more accurate results than ECP, especially at low column efficiency. It is recommended touse the ECP method only when the column efficiency exceeds markedly 2000 theoretical plates.

Heat Exchanges in Fast, High-Performance Liquid Chromatography. A Complete Thermodynamic Study

The successive physical transformations of the mobile phase that take place in very high pressure liquid chromatography were studied based on the formalism of classical thermodynamics. The eluent is initially under atmospheric pressure ( P (0)) and at ambient temperature ( T ext). In a first step, it is compressed to a high pressure ( P max of the order of 1 kbar) in the pump heads of the chromatograph. In a second step, the pressurized eluent is transferred to the inlet of the chromatographic column, along which, in a third step, it is decompressed to atmospheric pressure. Both the compression and the decompression of the fluid were considered to take place under conditions that can be either adiabatic or nonadiabatic and either reversible or irreversible. Applications of the first and second principles of thermodynamics allow the determination of the heat and energy exchanged between the eluent and the external surroundings during each transformation. Experimental data were acquired using acetonitrile as the mobile phase. The true state equation, rho( P, T), of liquid acetonitrile was used in the theoretical calculations. A series of four different flow rates (0.55, 0.85, 1.15, and 1.45 mL/min, corresponding to inlet pressures of 357.2, 559.5, 765.1, and 972.9 bar, respectively), were applied to a 2.1 x 100 mm column packed with 1.7-mum bridged ethane-silicon hybrid particles. Thermocouples were used to measure the eluent temperature before and after its passage through the column. These data provide estimates of the variation of the internal energy of the eluent. The heat lost through the external wall of the column during the eluent decompression was estimated by measuring the surface temperature of the column tube under steady state. Both the compression and the decompression of acetonitrile were found to be nonadiabatic and irreversible transformations. The results showed that, during the eluent decompression, the heat released by the friction forces serves four different purposes: (1) it increases the eluent entropy at constant temperature (for approximately 35%); (2) it increases the temperature of the eluent (for approximately 5%); (3) it provides heat to the laboratory atmosphere (for approximately 5%); and (4) it provides some work inside the column (for approximately 5%). This quantitative heat balance description accounts well for the actual performance of the new, very high pressure liquid chromatographic technique.

Optimization of the experimental conditions in preparative liquid chromatography with touching bands

Abstract Relationships are derived between the sample size permitting the elution of the components of a binary mixture as two touching bands and the parameters characterizing the separation and the column performance. These equations take into account the competitive interactions of the mixture components. They permit the determination of the optimum experimental conditions for maximum production rate with touching band elution. The production rate increases monotonically with increasing inlet pressure available, provided that the column of optimum characteristics is used. For a given column, there is an optimum flow velocity and hence an optimum inlet pressure. Because of the competition between the mixture components for interaction with the stationary phase, the production rate under touching band conditions can be larger (under conditions of a predominant displacement effect) or lower (under conditions of a predominant tag-along effect) than the value derived when this competition is ignored.

Experimental utilization of displacement effect for the optimization of the separation of a two-component mixture

Abstract Using a mixture of two substituted cyclohexanone epimers, sample loading, binary mixture composition and column efficiency were investigated for their influence on the displacement effect observed between the first and the second eluted components (“sample self-displacement effect”). This effect occurs and can enhance production for loads as high as 1 g on a 250 × 21.4 mm I.D. column. Separations over a broad range of epimer mixture compositions, from 10:90 to 75:25, show a displacement effect on the first-eluting component by the second. In loading and composition studies, interference between the tailing portion of the first component and the front of the second component is the predominant factor influencing recovery loss for both components. A high column efficiency is essential for sharpening band zone boundaries and maximizing yields. There is a very good correlation between experimental results and reported computer simulations of the band profiles of binary mixture components.

Overloaded elution band profiles of ionizable compounds in reversed-phase liquid chromatography: Influence of the competition between the neutral and the ionic species

The parameters that affect the shape of the band profiles of acido-basic compounds under moderately overloaded conditions (sample size less than 500 nmol for a conventional column) in RPLC are discussed. Only analytes that have a single pK(a) are considered. In the buffer mobile phase used for their elution, their dissociation may, under certain conditions, cause a significant pH perturbation during the passage of the band. Two consecutive injections (3.3 and 10 microL) of each one of three sample solutions (0.5, 5, and 50 mM) of ten compounds were injected on five C(18)-bonded packing materials, including the 5 microm Xterra-C(18) (121 A), 5 microm Gemini-C(18 )(110 A), 5 microm Luna-C(18)(2) (93 A), 3.5 microm Extend-C(18 )(80 A), and 2.7 microm Halo-C(18) (90 A). The mobile phase was an aqueous solution of methanol buffered at a constant (W)(W)pH of 6, with a phosphate buffer. The total concentration of the phosphate groups was constant at 50 mM. The methanol concentration was adjusted to keep all the retention factors between 1 and 10. The compounds injected were phenol, caffeine, 3-phenyl 1-propanol, 2-phenyl butyric acid, amphetamine, aniline, benzylamine, p-toluidine, procainamidium chloride, and propranololium chloride. Depending on the relative values of the analyte pK(a) and the buffer solution pH, these analytes elute as the neutral, the cationic, or the anionic species. The influence of structural parameters such as the charge, the size, and the hydrophobicity of the analytes on the shape of its overloaded band profile is discussed. Simple but general rules predict these shapes. An original adsorption model is proposed that accounts for the unusual peak shapes observed when the analyte is partially dissociated in the buffer solution during its elution.

Analytical solution of the ideal model of elution chromatography in the case of a binary mixture with competitive Langmuir isotherms : II. Solution using the h-transform

Abstract Using the results published by Helfferich and Klein, an exact solution of the ideal model of chromatography (infinite column efficiency) is derived, giving the band profiles for the two-component elution problem in the case when the equilibrium isotherms are given by the classical competitive Langmuir equations. The variations of the band profile of each component during elution is analyzed and the interactions between the two profiles are investigated. Two concentration shocks appear, one at the front of each component elution profile. The chromatogram is separated into three zones. The first zone, between the two concentration shocks, contains only the first component. The second zone, immediately after the second shock, contains a mixture of the two components. The third zone, at the rear, contains only the second component. The profiles of the two component in the three zones and their concentrations on both sides of the second shock are given by simple analytical equations. If the sample is injected as a rectangular pulse, it takes some time to erode the corresponding concentration plateaux of each component. On both sides of the mixed zone, a second concentration plateau appears for each component. The heights of these plateaux remain constant as long as they are present. The first component plateau disappears rapidly, but the second component plateau, whose formation explains the “tag-along” effect, remains stable as long as the second zone has not vanished and decreases progressively after the two bands are resolved. Comparison between the profiles obtained as solutions of the ideal model and those calculated using the program of the semi-ideal model, which accounts for the finite efficiency of actual columns, shows very good agreement when the column efficiency exceeds a few hundreds to 1000 plates. The extent of the agreement depends on both the sample size and the column efficiency. The concentration shocks are replaced by shock layers whose thickness is proportional to the column plate height, but depends also on the shock height. The thickness of the second shock, which separates the first and second zones, seems to depend much more than the thickness of the first shock on the actual column efficiency.

Simulated moving bed chromatography. Comparison between the behaviors under linear and nonlinear conditions

The influence of the feed concentration on the progressive changes in concentration profiles along the simulated moving bed (SMB) columns and in the concentration histories at the SMB outlets during the transition of the SMB operation toward steady state is studied. This investigation is concerned with what happens when the isotherms of the two components of the feed in the phase system have competitive Langmuir behavior and the feed concentration is raised from the linear to the nonlinear range. In order to achieve a clear comparison between the SMB behaviours in the nonlinear and the linear case, the flow rates in all the sections of the SMB are kept constant, at the same value of these operation parameters as selected in the linear case. Transition concentration profiles to the steady state were examined as functions of the degree of nonlinear behavior, i.e. for different values of the reduced concentrations of the feed components, b′i = biCi. The profiles obtained in the linear and nonlinear case are compared to illustrate the nonlinear effects. Square-wave oscillations of the plateau concentrations of the extract in column II and the raffinate in column IV, due to the cyclical nature of the SMB operation, have been reported previously in the linear case. This phenomenon is discussed in the nonlinear case and the influence of the switching time is illustrated.

Radial distribution of the contributions to band broadening of a silica-based semi-preparative monolithic column.

Using an on-column local electrochemical microdetector operated in the amperometric mode, band elution profiles were recorded at different radial locations at the exit of a 10 mm id, 100 mm long silica-based monolithic column. HETP plots were then acquired at each of these locations, and all these results were fitted to the Knox equation. This provided a spatial distribution of the values of the eddy diffusion (A), the molecular diffusion (B), and the resistance to the kinetics of mass transfer (C) terms. Results obtained indicate that the wall region yields higher A values and smaller C values than the central core region. Significant radial fluctuations of these contributions to band broadening occur throughout the exit column cross-section. This phenomenon is due to the structural radial heterogeneity of the column.

Shock layer thickness and optimum linear velocity in displacement chromatography

The width of the mixed zones between two successive bands in the isotachic train represents the loss in recovery yield achieved in displacement chromatography. Intuitively, this width depends on the mobile phase flow velocity, but no systematic study of this effect has yet been performed. On the other hand, constant pattern behavior and the theory of shock layer are well known in chemical engineering. Using this approach, and assuming competitive Langmuir isotherm behavior, an analytical equation is derived which relates the shock layer thickness (SLT) in displacement chromatography and the column design and operating parameters. Using this equation, it is possible to investigate the dependence of the SLT between two consecutive bands in the isotachic train on the mobile phase velocity, the concentration and the retention factor of the displacer and the separation factor of the two components. In displacement chromatography, the optimum mobile phase linear velocity (uopts) for minimum shock layer thickness, or maximum recovery yield depends not only on the coefficients of axial dispersion and mass transfer resistance of the two components, as does the optimum mobile phase velocity (uoptL) in linear chromatography, but also on the retention factor and the concentration of the displacer. The results of the study of this analytical equation are in excellent agreement with those of numerical calculations.