The effect of stationary-phase pore size on retention behavior in micellar liquid chromatography

One of the limitations that has restricted the applicability of micellar liquid chromatography (MLC) is the weak eluting power of micellar mobile phases compared to conventional hydro-organic mobile phases used in reversed-phase liquid chromatography. This may be the result of Donnan or steric exclusion of the micelles from the pores of the stationary phase, within which nearly all (> or = 99%) of the stationary phase resides and the analytes spend most of their time. To determine whether wide-pore stationary phases would overcome this limitation in MLC, several C8 and C18 stationary phases ranging from 100 to 4000 A were investigated using a diverse set of test solutes and micellar solutions of anionic, neutral, and cationic surfactants as mobile phases. With the larger pore size stationary phases, the eluting power of the MLC mobile phases was enhanced with all surfactant types, the greatest effect being with the neutral surfactant. Differences in retention behavior were observed between various solute types and between the C8 and C18 stationary phases. These differences appear to be related to the relative hydrophobicity of the solutes and to differences in the surfactant-modified stationary phases. Partitioning behavior of representative solutes on the large-pore C8 and C18 columns was shown to follow the three-phase partitioning model for MLC. Methylene group selectivity data showed only minor differences in the stationary-phase characteristics between the small- and large-pore size C18 columns. The true eluting power of micellar mobile phases was revealed with wide-pore stationary phases and was demonstrated by the separation and elution of an extended series of alkylphenones on C18 columns.     

Temperature-dependent solute retention time variations, base-line drifts, and solute peak splits during the analysis of fenvalerate by a high-performance liquid chromatography/diode array system

The optical isomers of fenvalerate, (RS)-alpha-cyano(3-phenoxyphenyl)methyl (RS)-2-(4-chlorophenyl)-3-methylbutyrate, were analyzed by high-performance liquid chromatography (HPLC) on chiral columns eluted with mixtures of 2-propanol in hexane. The HPLC eluates were monitored by a photodiode-array ultraviolet absorbance detector. When the HPLC columns were maintained at ambient temperature, chromatograms with drifting base lines of four to eight peaks, instead of four, and isograms, three-dimensional chromatograms, with "negative" peak artifacts coeluted with fenvalerate isomers were usually obtained during a continuous run of the HPLC system for many days and by injecting onto the system only fenvalerate analytical standard solutions. Chromatograms and isograms without artifacts and straight base lines were obtained only when the HPLC column was thermostated and well equilibrated for at least 8-10 h.     

Geometrical model for the retention of fullerenes in high-performance liquid chromatography

In high-performance liquid chromatography (HPLC) using a poly(octadecylsiloxane) as a stationary phase, methanol as a mobile phase, C(60) and C(70) fullerenes as solutes, and water as a mobile phase modifier, a study on the surface tension effect of water on fullerene retention was carried out by varying the water concentration [W] and the column temperature T. The thermodynamic parameters for fullerene transfer from the mobile to the stationary phase were determined from linear van't Hoff plots. An enthalpy-entropy compensation revealed that the types of interactions between fullerenes and the stationary phase were independent of the fullerene structure and the mobile phase composition. An analysis of the experimental variations of the retention factor and the selectivity values with [W] was performed using a novel geometrical model. It was shown that the increase in fullerene retention accompanying the water concentration was due to the increased effects of surface tension. This brought about an increase in the interactions between fullerene and the stationary phase, explaining the observed thermodynamic parameter trends over the water concentration range. The theoretical model provided an estimation of the radius of fullerene which was found for C(60) to be equal to 3.3 ? and an activation energy during the transfer equal to 9.8 kJ/mol.     

Influence of the hofmeister series on the retention of amines in reversed-phase liquid chromatography

The chromatographic behavior of protonated amines in reversed-phase liquid chromatography (RPLC) is influenced markedly by the identity of the mobile-phase anion. For example, retention factor values, k, obtained from protonated nordoxepin, nortriptyline, and amitriptyline increase almost 1 order of magnitude across the following series of anions employed as mobile-phase modifiers: H2PO4- < HCOO- < CH3SO3- < Cl- < NO3- < CF3COO- < BF4- < ClO4- < PF6-. Early eluting primary, secondary, and tertiary benzylamines are retained and resolved using BF4-, ClO4-, and PF6- but elute in or very near the void using all other mobile-phase anions tested. In contrast, a neutral hydrophobic marker, acenaphthene, shows no significant changes in retention with mobile-phase anion identity. Such large differences in amine retention with anion identity can be rationalized via both an ion-pairing model and the Hofmeister effect. Two key findings are reported. First, the dependence of amine retention on mobile-phase anion identity is attributed unambiguously to the Hofmeister effect and is quantified using a simple equation based solely on differences in the solvation of anions. Accurate prediction of k values from the excess chemical potential of anions in water suggests that anion-solvent interactions dominate the retention of amines in RPLC. Thus, controlling amine retention depends critically on judicious selection of mobile-phase anion (in addition to the usual experimental parameters such as organic modifier, temperature, pH, and stationary phase). Second, more lipophilic molecular anions can provide retention and tailing properties comparable to those obtained from traditional amphiphilic ion-pairing reagents such as octanesulfonate, but with the benefit of a superior gradient background and solubility at high concentrations of organic modifier.     

Thermodynamic interpretation of retention equilibrium in reversed-phase liquid chromatography based on enthalpy-entropy compensation

The fundamentals of the retention equilibrium in reversed-phase liquid chromatography (RPLC) are studied on the basis of enthalpy-entropy compensation (EEC). First, retention data were acquired and the influence of the nature of the compounds, organic solvent modifier, and temperature on these data was assessed. Then, the data were analyzed according to the four different methods proposed by Krug et al., and an EEC was formally established. Linear correlations were observed between the logarithm of the adsorption equilibrium constants under the different RPLC conditions, suggesting linear free energy relationships (LFERs). Finally, the variations of the retentions with the experimental conditions are shown to be quantitatively explained by a new model based on EEC. This model affords a comprehensive interpretation of the variations of retention originating from changes of either one parameter alone or several simultaneously. The slope and intercept of the LFER that relates two equilibrium systems are accounted for by the new model. The parameters of this model are the changes of enthalpy and entropy associated with the retention, the compensation temperatures, and the experimental conditions.     

Theoretical and experimental studies of microflows in silicon microchannels

The determination of fluid flows in silicon microchannels is important for the design of microfluidic systems. In this paper, experimental investigations on the characteristics of low fluid flows (few μl h^− 1) in silicon trapezoidal microchannels (21 μm in depth, length and width ranging from 200 to 440 mm and 58 to 267 μm, respectively) are presented. The test-devices have been fabricated using micromachining technologies. A double KOH etching process has been used to achieve microchannels in (100)-oriented silicon wafers as well as deep in-plane cavities used for capillary connections. Silicon has been finally anodically bonded on Pyrex substrates. The experimental set-up, based on the measurement of a differential pressure and a liquid–air interface displacement in a gauged tube, is fully detailed in terms of fluidic connections and measurement principle. The experimental results are in good agreement with the Navier–Stokes theory, solved by a simple iterative method. However, finite element modelling has been used to study complex 3D problems that were found in the devices and the experimental set-up. Finally, we propose abacuses for three different channel cross-sections that may be used to easily compute the flow in a microchannel.     

Effect of a variety of organic additives on retention and efficiency in micellar liquid chromatography

The effect of 21 organic additives (alkanols, alkane diols, dipolar aprotic solvents, alkanes) on the chromatographic behavior (retention, elution strength, efficiency) of probe solutes of widely differing hydrophobicity, such as benzene and 2-ethylanthraquinone, have been examined using a C18 stationary phase and sodium dodecyl sulfate (SDS) micellar mobile phases. The mobile-phase elution strength parallels the octanol-water partition coefficients of the additives or their ability to bind to the SDS micellar system, due to the increased solubility in the mobile phase and reduced affinity for the additive-modified surfactant-coated stationary phase. The comparison of the elution strength of micellar mobile phases with that of a reference acetonitrile-water system indicates that the elution strength is lower for micellar systems and depends on the nature of the eluted solute. The displacement of the solute-micelle and solute-stationary phase binding equilibria is quantified for several probe solutes eluted with micellar mobile phases in the presence of 1-propanol, 1-butanol, 1-pentanol, and acetonitrile. A correlation was also observed between the number of theoretical plates and the hydrophobicity of the alcohol additives: the efficiency initially increased steeply and reached a plateau. Compared to benzene, a more hydrophobic additive was needed to attain the maximum efficiency for the more hydrophobic 2-ethylanthraquinone analyte. Dipolar aprotic solvents appear to be somewhat more effective in enhancing the efficiency than alcohols. The results are rationalized in terms of the ability of the organic additives to alter the composition, structure, dynamics, and properties of the micelles and the surfactant-coated stationary phase.     

液体载冷剂法不确定度的理论与试验分析

基于液体载冷剂法测试的基本原理及数学模型,对冷水机组制冷量测试结果的不确定度进行理论分析,并通过试验数据得出引起测量结果不确定度的因素及各因素对制冷量测试结果所做的贡献量。结果表明,被测机组的进、出水温度和水流量的测量结果对制冷量不确定度贡献较大。指出提高传感器的精度可以减小制冷量测量结果的不确定度,从而提高试验室的测试能力。     

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.     

Theoretical and experimental studies on molecular spectra of the silica hollow spheres

The IR and UV spectra of the silica hollow spheres (SHSs) have been investigated using semi-empirical PM3 method. The calculated results have been compared with those from experiments. A cluster model of porous silica containing up to 36 SiO₄ has been proposed to characterize the fine structure of the SHSs. Effects of model size on the spectra have been discussed. The theoretical IR absorption peaks caused by OSiO vibration have matched the experimental response well. The UV absorption has been observed to move to larger wavelengths as the number of cluster and the number of configuration interaction (CI) increase. The PM3 calculations can assist greatly in analyzing the structure and molecular spectra of complicated materials with special morphologies.     

A framework based on the extended Wyman concept for analyzing the salt effects on the solute retention in high-performance affinity chromatography.

The analysis of binding data of a ligand to a macromolecule in the presence of an additive can be classically formulated in terms of the linked functions of Wyman. In the case of a salt, this approach has been extended by Tanford such that the contributions of both salt and water are taken into account. In this paper, the extended Wyman theory was applied to high-performance affinity chromatography (HPAC) in order to define a general model describing the effects of the mobile-phase salts on the ligand binding. Various HPAC literature data, as well as our data concerning dansyl amino acid retention on a vancomycin stationary phase, were examined in relation to this model. From the results, this theoretical approach was considered to be adequate to describe accurately the salt dependence on solute retention. This work shows the importance of taking into account the effects of both ionic species and water in the investigation of relative contributions of the interactions involved in the ligand binding to immobilized receptor.     

Prediction of chromatographic retention and protein identification in liquid chromatography/mass spectrometry

Liquid chromatography coupled on- or off-line with mass spectrometry is rapidly advancing as a tool in proteomics capable of dealing with the inherent complexity in biology and complementing conventional approaches based on two-dimensional gel electrophoresis. Proteins can be identified by proteolytic digestion and peptide mass fingerprinting or by searching databases using short-sequence tags generated by tandem mass spectrometry. This paper shows that information on the chromatographic behavior of peptides can assist protein identification by peptide mass fingerprinting in liquid chromatography/mass spectrometry. This additional information is significant and already available at no extra experimental cost.     

Theoretical and experimental studies on tightly focused vector vortex beams

The high-NA focusing properties of vector vortex beams are studied theoretically and experimentally. The vector vortex beams are generated by space-variant segmented subwavelength metallic gratings first. Then the mathematical expressions for the focused fields are derived based on the vector diffraction theory, and some numerical simulations are presented that show that the focused fields are not dark at the center and the focusing spot size of vector vortex beams with high topological charges approaches the diffraction limitation at high NA. Finally, to verify the theoretical analysis, the tightly focused fields are measured based on a confocal microscopy system when the NA of the objective lens is 0.90. The research results confirm the potential of vector vortex beams in some applications, such as optical trapping, laser printing, lithography, and material processing.     

Theoretical and experimental studies of broadband phase-controlled attosecond mirrors

We present procedures to develop and characterise chirped multilayer mirrors for attosecond pulses. The design procedure involves a simulated annealing optimisation algorithm to obtain a multilayer structure with the desired performances. The characterisation step requires the use of well-calibrated attosecond and synchrotron beam lines, allowing one to measure the amplitude and phase response of the mirror. We illustrate these approaches with a set of mirrors designed to have zero or negative dispersion. The remarkable agreement between theoretical and experimental performances validates this overall process, and demonstrates the capability of such mirrors to control the temporal profile of attosecond pulses. These tools and techniques for developing attosecond chirped mirrors will facilitate the generalisation of such components on attosecond experiments.     

Theoretical and experimental studies of a novel cone-jet sensor

Modeling of a novel cone-jet sensor using two-dimensional (2-D) finite element analysis was investigated for dimensional measurement. Theoretical and experimental studies demonstrated that a cone-jet sensor supplied with air can be used to accurately measure displacement, and its work range of 1.5 to 4.2 mm is some ten times greater than a simple back-pressure sensor. It is anticipated that this type of sensor will find wide applications in manufacturing industry due to its wider working range, high precision, and other features