Simulation studies on the effects of mobile-phase modification on partitioning in liquid chromatography

Various driving forces have been suggested to explain retention and selectivity in reversed-phase liquid chromatography (RPLC). To provide molecular-level information on the retention mechanism in RPLC, configurational-bias Monte Carlo simulations in the Gibbs ensemble were carried out for model systems consisting of three phases: an n-hexadecane retentive phase, a mobile phase with varying water-methanol composition, and a helium vapor phase as reference state. Liquid n-hexadecane functions as a model of a hydrophobic stationary phase, and a wealth of experimental data exists for this system. Gibbs free energies for solute transfers from gas to retentive phase, from gas to mobile phase, and from mobile to retentive phase were determined for a series of short linear alkanes and primary alcohols. Although the magnitude of the incremental Gibbs free energy of transfer for a methylene segment is always larger for the gas- to retentive-phase transfer than the gas- to mobile-phase transfer, it is found that the partitioning of alkanes and alkyl tail groups is mostly affected by the changes in the aqueous mobile phase that occur when methanol modifiers are added. In contrast, the partitioning of the alcohol headgroup is sensitive to changes in both the n-hexadecane and the mobile phases. In particular, it is found that hydrogen-bonded aggregates of methanol are present in the n-hexadecane phase for higher methanol concentrations in the mobile phase. These aggregates strongly increase alcohol partitioning into the retentive phase. The simulation data clearly demonstrate that due to modification of the retentive-phase hydrocarbons by solvent components, neither the solvophobic theory of RPLC, advocated by Horvath and co-workers, nor the lipophilic theory of RPLC, advocated by Carr and co-workers, can adequately describe the separation mechanism of the hexadecane model system of a retentive phase studied here nor the more complex situation present in actual RPLC systems.     

Effect of acetonitrile/water mobile-phase composition on adsorption characteristics of reversed-phase liquid chromatography

The influence of volumetric composition of acetonitrile in a mobile phase on adsorption characteristics of reversed-phase liquid chromatography using octadecylsilyl (ODS)-modified silica gel was studied by the pulse response method and the moment analysis. The results were compared with those obtained for the reversed-phase system consisting of ODS-silica gel and methanol/water mixtures. In both systems, surface diffusion was dominant for intraparticle diffusion in ODS-silica gel particles. The contributions of three mass transfer steps in a column to peak broadening were of about the same order of magnitude. The activation energy of surface diffusion, E(s), was found to be larger than the isosteric heat of adsorption, Q(st). Because similar tendencies were observed for these adsorption characteristics, adsorption mechanisms may be analogous in both chromatographic systems. However, absolute values of the adsorption equilibrium constant, K, the decreasing ratio of hydrophobic surface area, ΔA/A, Q(st), and E(s) for acetonitrile/water systems were smaller than the corresponding values for methanol/water systems. Oppositely, greater values of D(s) were obtained for acetonitrile/water systems. It was concluded that the interaction between ODS ligands and adsorbate molecules was weaker compared with that in methanol/water mobile-phase systems when acetonitrile was used as an organic modifier in a mobile phase of reversed-phase liquid chromatography.     

Measurement of mobile-phase volume in reversed-phase liquid chromatography and evaluation of the composition of liquid layer formed by solvation of packing materials

The mobile-phase volumes (Vm) in reversed-phase liquid chromatography (RPLC) with alkyl-bonded silica, defined as the difference between the total volume of eluent in the column (V0) and the volume of the eluent solvent layer formed by solvation of the bonded phase (VL), are determined by the method derived from the eluent electrolyte effect on the retention of ionic analytes. The validity of the Vm values obtained is evaluated by comparing them with the retention volumes of various organic compounds and inorganic ions, which have been suggested as unretained markers, and those obtained from a linear dependence of the logarithmic retention factor on the carbon numbers of homologous series. From the results obtained, it has been concluded that the solvated liquid phase on a column packing material should be assigned to a part of the stationary phase and the method developed for determination of the Vm value based on the ion partition model gives the most reasonable value as the mobile-phase volume in RPLC. The volume and the solvent composition of the solvated liquid phase on C1, C8, and C18 bonded silica are estimated, and the effects of organic modifiers and the physicochemical structures of the packing materials on these values are discussed.     

Fundamental equation of dual-mode gradient elution in liquid chromatography involving simultaneous changes in flow rate and mobile-phase composition

The rigorous derivation of the fundamental equation of the dual-mode gradient elution in liquid chromatography involving any type of simultaneous changes in flow rate and mobile-phase composition is developed following Drake's approach. The equation is a generalization of the already known fundamental equations of single gradient when either the mobile-phase composition or the flow rate is constant. The theory was tested in the retention prediction from isocratic data of 18 o-phthalaldehyde derivatives of amino acids in eluting systems modified by acetonitrile or methanol. The retention prediction obtained for all solutes under all dual-mode gradient conditions was excellent. The average percentage error between experimental and predicted retention times ranged from 0.9 to 2.5%. Two approximations that simplify the calculations considerably without increasing the above error were also proposed.     

Linear velocity surge caused by mobile-phase compression as a source of band broadening in isocratic ultrahigh-pressure liquid chromatography

A linear velocity surge caused by mobile-phase compression was investigated as a source of band broadening for ultrahigh-pressure liquid chromatography (UHPLC). To measure the effect of compression on mobile-phase velocity, ionic sample fronts were monitored using a contactless conductivity detector, as they migrated through long packed capillaries. Mobile-phase compression was found to cause an abnormally high linear velocity surge as the pressure was applied to the inlet of the column. An empirical equation was developed to describe the mobile-phase flow velocity during and after compression. Data fits to this equation were then used to determine the amount of additional variance caused by mobile-phase compression. For a 10/90 v/v acetonitrile/water mobile phase, the velocity surge occurred over roughly 10-15% of the column length. In addition, the velocity surge caused by mobile-phase compression was found to be capable of causing a 50% increase in the measured van Deemter C-terms for reversed-phase UHPLC.     

Theory and application of the two-mode gradient elution in liquid chromatography involving simultaneous changes in temperature and mobile-phase composition

The theory of the dual-mode gradient elution in liquid chromatography involving any type of simultaneous changes in column temperature and mobile-phase composition is developed following Drake's approach. The theory was tested in the retention prediction of six alkylbenzenes in aqueous eluting systems modified by acetonitrile. Significant delay phenomena, i.e., a lag between the programmed gradient temperature and the temperature in the oven, and a lag between the oven temperature and the effective temperature the analyte feels in the column, were detected. When these phenomena were taken into account, the retention prediction obtained for all solutes under all dual-mode gradient conditions was excellent. The average percentage error between experimental and predicted retention times is below to 2%.     

Analytical solutions of the ideal model for gradient liquid chromatography

The analytical solutions of the ideal model for gradient elution that ignores the influence of the solute concentration on the retention factor (k) were studied by using the method of characteristics for solving partial differential equations. It is found for any gradient profiles and solvent strength models used that the concentration of the solute will be discontinuous where the mobile-phase composition is. On a given characteristic curve, the product of the concentration and the retention factor is kept constant at the point where the concentration is continuous. At the point where the concentration is discontinuous, the product on the left side of this point is equal to that on the right side. We also discussed the basic equations to predict the retention time in gradient elution and introduced the injection time into them. For linear solvent strength stepwise and linear gradient elution, general expressions were proposed for the prediction and they can be used as the basis to derive others for specific gradient modes such as single linear, stepwise, and ladderlike gradients. For these modes, simple expressions to account for the band compression and the concentration change during the elution were also given.     

火箭推进剂液氢中氦的分析方法

使用气相色谱法分析氢中氦时,在常用色谱柱上氦出峰在氢后面,并且位置很近,较大的氢峰会掩盖氦峰。针对上述问题,研制了一种新的色谱柱固定相,用于色谱法分析氢中氦组分。给出了这种固定相对氢、氦组分的分离原理、制作方法和实验过程。实验结果表明,采用气相色谱热导检测器,配用金属钯做色谱柱固定相,可成功地完成对氢、氦组分的分离和检测。     

Analytical methodology for TEX86 paleothermometry by high-performance liquid chromatography/atmospheric pressure chemical ionization-mass spectrometry

The TEX86 is a recently proposed paleothermometer through which ancient seawater temperatures of up to 120 My ago can be reconstructed. It is based on the relative distribution of glycerol dibiphytanyl glycerol tetraethers as measured by high-performance liquid chromatography/atmospheric pressure chemical ionization-mass spectrometry (HPLC/APCI-MS). The aim of this study was to examine and improve several analytical aspects in the determination of this important proxy in environmental matrices. Comparison of TEX86 analysis using single ion mode (SIM) and mass scanning (m/z 950 to 1450) detection, respectively, revealed that SIM is up to 2 orders of magnitude more sensitive and that the TEX86 can be determined with a reproducibility of +/-0.004 or +/-0.3 degrees C using this method. Comparison of TEX86 values obtained with two different HPLC/APCI-MS set-ups revealed no significant differences. In addition, analysis of TEX86 of extracts obtained by Soxhlet, ultrasonic, and accelerated high-pressure extraction techniques also showed no significant differences between the methods. Our results suggest that TEX86 analysis by HPLC/APCI-MS is robust and can be determined with analytical errors comparable to those of other temperature proxies.     

High-temperature ultrafast liquid chromatography

A novel liquid chromatographic system which enables high temperature ultrafast liquid chromatography (HTU-FLC) has been designed through the careful consideration of heat transfer, band broadening, and pressure drop. Studies of the effect of linear velocity on the HETP show that column efficiency at high velocity, especially of well-retained solutes, dramatically improves at higher temperatures. At 150 degrees C, at a flow rate of 15 mL/min with a 5 cm by 4.6 mm (i.d.) column packed with 3 microns polystyrene-coated zirconia porous particles, long chain alkylphenones were completely resolved, and the analysis time could be decreased by a factor of 50 compared to that at room temperature (25 degrees C) at a conventional flow rate (4 mL/min). In addition, using pure water as the mobile phase, five phenols were separated in less than 30 s.     

Extrathermodynamic relationships in reversed-phase liquid chromatography

Enthalpy-entropy compensation (EEC) and linear free energy relationships (LFER) are extrathermodynamic correlations frequently used to discuss the mechanistic similarities of chemical equilibria and reaction kinetics. Although empirical, they are widely applied, proving the substantial effectiveness of fundamental studies based on them. Many attempts have been made to interpret theoretically the necessary conditions (or preconditions) of EEC and LFER. LFER is known to rest on the existence of EEC. However, the intimate correlations between EEC on one hand and LFER and the temperature dependence of LFER on the other hand were insufficiently discussed from the viewpoint of molecular structure contributions. We present a simple LFER model relating the slope and intercept of LFER to the compensation temperatures, themselves derived from EEC analyses, and to several parameters characterizing the molecular contributions to the changes in enthalpy and entropy associated with the passage from one phase of the chromatographic system to the other. A theoretical explanation is supplied for the intimate correlation between the two types of extrathermodynamic relationships, EEC and LFER. We demonstrate also that the characteristics of EEC and LFER depend on the structural parameters. This new model allows a proper interpretation of the temperature dependence of LFER. It should permit further progress of fundamental studies of chemical reaction mechanisms based on extrathermodynamic relationships.     

Quantification in comprehensive two-dimensional liquid chromatography

Although the use of comprehensive two-dimensional liquid chromatography (LCxLC) as a powerful separation technique is continuously increasing, its employment in quantification experiments is rather limited. The present research is focused on the quantification of a series of standards, as well as real-world sample compounds, by using dedicated laboratory-constructed LCxLC software, developed through a novel approach. Moreover, the difficulties encountered during software operation, in various elution conditions, are described and discussed. The results attained were compared with those observed in conventional LC, and no statistically significant differences were observed in the determination of aurapten in grapefruit oil. However, a loss in sensitivity was observed when using LCxLC (limit of detection = 0.10 ppm) compared to conventional LC (limit of detection = 0.05 ppm) as a consequence of the sample dilution in comprehensive two-dimensional liquid chromatography.