Thermal Gradients for the Control of Elution in RP-LC: Application to the Separation of Model Drugs

The use of temperature as a variable in liquid chromatography enables the facile alteration of eluotropic strength without the need to change solvent composition. The ability to change eluotropic strength via temperature alone means that thermal gradients can be used to mimic the effects of solvent gradients but without many of the unwanted effects of changes in solvent composition. Here we illustrate the use of thermal gradients as a means of controlling chromatographic separations using either constant flow or, with the flow rate increased to maintain isobaric conditions, constant pressure, performed using columns packed with 1.7 μm particles. A model is described that can be used to used to predict flow, pressure and temperature under gradient conditions. Practical experimental factors such as the need for post column cooling and the use of frit restrictors in order to obtain optimum results are described.     

Retention behaviour of unsaturated fatty acid methyl esters on porous graphitic carbon

The eluotropic strength of binary mobile phases was calculated for three homologous series of cis, trans, and cis-cis unsaturated fatty acid methyl esters (FAMEs). Binary mobile phases with chloroform, dichloromethane, or tetrahydrofuran as strong solvent and methanol or acetonitrile as weak solvent were tested. The volume fraction of strong solvent in the binary phases was between 0.3 and 0.8. Curves of eluotropic strength versus volume fraction of strong solvents showed similar trends to previously published results for saturated homologues. Correlation coefficients of the plots of eluotropic strength values for saturated versus unsaturated FAMEs were close to 1.0. Therefore these similarities validate the model of eluotropic strength previously established with saturated FAMEs as relevant for unsaturated FAMEs. The separation factors between cis and trans homologues always showed elution of the cis before the trans homologue. The difference in retention is due primarily to the geometry of the molecule. The retention is lowered more by the addition of a first carbon double bond than by the addition of a second one, independently of the mobile phase composition.     

Temperature as a variable in liquid chromatography: development and application of a model for the separation of model drugs using water as the eluent

The use of high temperatures in liquid chromatography allows for the use of a purely aqueous mobile phase. At elevated temperatures water possesses many of the characteristics of organic solvents in terms of eluotropic strength, as well as having a lower viscosity. A model is developed, based on data obtained using a range of model drugs, which demonstrates the relationship between temperature, flow and pressure. Experimental data from different column types, at temperatures from 40 degrees C to 180 degrees C, is presented which matches well with the predicted data from the model.     

Mobile phase compensation to improve NMR spectral properties during solvent gradients

A solvent compensation method based on flow injection analysis is used to obtain high quality nuclear magnetic resonance (NMR) spectra during solvent gradients. Using a binary solvent system containing D2O and CD3OD, NMR line broadening and chemical shift changes are observed with a 10% methanol per min solvent composition gradient. However, by creating a second equal but reverse gradient and combining the two solvent gradients before the NMR detector, the composition of solvent reaching the NMR flow cell is kept constant. We demonstrate a system using flow injection analysis of combining solvent gradients and show constant NMR spectral performance as a function of time as the combined flow has a constant solvent composition irrespective of the initial solvent gradient. Using this approach, methods can be developed to measure high quality NMR spectra during on-flow gradient LC-NMR experiments. The ultimate ability of this approach depends on the ability to compensate for the disturbance of the solvent gradient and reverse gradient by a pair of LC columns (the analytical and reverse gradient columns).     

Application of the split-peak effect to study the adsorption kinetics of human serum albumin on a reversed-phase support

The adsorption step of human serum albumin on a reversed-phase support was analyzed by studying the "split-peak" effect in mass-overload conditions. This behavior is characterized by the occurrence of a first non-retained fraction and is described by an analytical expression in the case of a Langmuirian adsorption isotherm. The method was applied to determine the column loading capacity, the number of mass-transfer units and the apparent adsorption rate constant measured at a given flow-rate. The nature of the organic modifier influences the split-peak effect: it increases with the eluotropic strength of the organic solvent added to the buffer. Compared to the results with pure buffer, it is the association of two effects, the decrease of the column loading capacity and that of the apparent adsorption rate constant, which increases the split-peak effects observed when methanol and 2-propanol are added to the eluent. These results allow us to gain a better understanding of the role of the organic solvent in the elution behavior of proteins in reversed-phase high-performance liquid chromatography.     

Bell-Vol High Volume Sampler

Abstract

Maintenance of a constant flow rate has been a long-standing problem with hi-vol samplers used to collect ambient aerosol. Numerous types of commercially available flow controllers are currently in use. A passive flow controller has now been developed that operates without external power or electrical circuits, and, when coupled with an AC inductive motor and cyclone blower, effectively controls flow rate between 100 and 1500 L/min.

The flow controller described here controls mass flow rate rather than motor speed - the method used in commercially available hot wire anemometer electronic flow controllers - by continuously applying pressure to the outlet air stream. Thus, the controller is unaffected by such problems as brush wear and line voltage variations that are common to hi-vol samplers. In operation, the pressure drop across any constant flow resistance, such as an orifice, elbow, or length of tubing in conjunction with a diaphragm, functions as a servo mechanism to maintain a constant drop across this constant resistance.     

Comparison of Different Compositions of Mixed Pentane/1,4-Dioxane Mobile Phases in SFC

Abstract

The effective plate number has been chosen for a comparison between different mixed mobile phases composed of pentane and 1,4-dioxane. A column packed with unmodified silica gel and pyrene as the analyte was used. The dependence of the effective plate number on pressure and composition for a constant temperature, or a constant reduced temperature, both at constant mass flow, are shown in three dimensional network plots. The same dependence is shown at constant temperature and constant linear velocity.

For a detailed discussion, the dependence of the effective plate number on pressure, composition, capacity ratio, and other chromatographic parameters is given in two dimensional graphs. The main result for the present chromatographic system is that a Financial support by the Arbeitsgemeinschaft Industrieller Forschungsvereinigungen (AIF) is gratefully acknowledged.     

Characteristic features of the evaporation of liquids

Abstract

The evaporation rate variation of liquids (water, ethanol) is studied versus their vapour pressure. From measurements of liquid-vapour interface displacement versus time, the state change whose rate is v_o at initial time-is seen to slow down before reaching a constant rate v₁ under stationary working conditions. The difference between both rates increases as pressure decreases. In addition, whereas the v_o(P) curves are monotonous and increasing from saturation vapour pressure to vacuum, the v₁(P) curves have a typical shape, characterised by two extremums.

From the simultaneous measurement of temperature to assess the endothermic character of state change, it is shown that the previous observations may be related to the occurrence of a thermal flow at interface level.

The new profile of the v₁(P) curves can neither be predicted nor explained from conventional expressions of evaporations rate. The present paper shows that a thermodynamic interface model involving coupling of mass and heat flows allows the features detected to be accounted for with out any ambiguity.     

“PRISMA” Model for Computer-Aided HPLC Mobile Phase Optimization Based on an Automatic Peak Identification Approach

Abstract

The tripartite “PRISMA” optimization model, as part of the “PRISMA” system, includes all possible solvent combinations between 1–4 solvents, with a possible fifth one as modifier. The solvent composition is characterized by the solvent strength (S_T) and the selectivity points (P_S).

At a constant S_T the correlation between the P_S and the retention data (horizontal function) can be described by a quadratic function. For constant P_S the solvent strengths and retention data correlate (vertical function) with a logarithmic function. These correlations are used to formulate a mathematical model for the dependence of retention times (capacity factor) on the mobile phase composition. Unknown compounds are estimated in the mathematical model from a sequence of standard chromatograms after having identified individual peaks by an automatic procedure. Only retention times, relative peak areas, and information about the mobile phase compositions are required as input for the identification approach. The approach involves a combination of statistical methods which exploit both the basic properties of retention data and the mathematical relation between retention data, selectivity points, and solvent strength as derived from the “PRISMA” model. Diagnostic information for checking the identification is generated as a by-product. The mathematical model completed by the estimated constants predicts the expected retention times for each possible mobile phase combination. Peak start and peak end times are predicted in a way similar to the retention times, once the identification has been performed. The most important aspects of a chromatogram can thus be predicted for arbitrary mobile phases.

The separation quality of predicted chromatograms is assessed by the chromatographic response function (CRF). The optimal mobile phase combination is that which theoretically generates the chromatogram with the maximal CRF value. This optimal composition is found by a simple mathematical procedure, which maximises the CRF in dependence upon the mobile phase combination. The optimum found is a local one if the starting set of chromatograms contains no variation of the solvent strength, and a global one if, in the set of starting chromatograms, the solvent strength is varied in a suitable way. Recommendations for the starting position are given.

Twelve measurements are necessary for a local optimum, and 15 for the global one. To increase the accuracy, six measurements at three different solvent strength levels are proposed. Generally the highest and the lowest solvent strength level differ by ±(5)% from the middle level.

This strategy is also relevant when modifiers are used in constant amounts. The chromatographic behavior of substances to be separated can be predicted with 1% accuracy from correlations of k' values and selectivity points. Based on these relationships, an automatical mobile phase optimization strategy for isocratic separations is suggested with the “PRISMA” model.     

A new version of differential flow heat capacity calorimeter; tests of heat loss corrections and heat capacities of aqueous NaCl from T = 300 K to T = 623 K

A new differential flow heat capacity calorimeter was constructed. It is designed to operate at temperatures up to 700 K and pressures up to 35 MPa and its primary use is for determining the massic heat capacities at constant pressure of dilute aqueous solutions. The instrument works in the so-called isoperibol regime, where the fluid sample flowing through the cell is heated by an electrical heater and the power necessary to provide a constant temperature rise is measured relative to that for a reference fluid (water). From the two values of power for sample and water the ratio of massic heat capacities of the sample to that of water can be calculated. A thorough investigation of calibration techniques showed that the calorimetric performance is very sensitive to the thermal conductivities of the sample and reference fluids. Measurements under turbulent flow conditions are questionable since there is no guarantee that by changing the flow rate the experiments and the calibrations would be performed at the same flow conditions. The procedure is very accurate and sensitive when measuring the difference in heat capacity between a solvent and a dilute solution of solute in the same solvent. The calorimeter was used to measure heat capacities of aqueous solutions of NaCl at eight temperatures up to 623 K and pressures to 30 MPa. The newly obtained values show consistency with previously published results and enlarge the database of experimental values aboveT =  573 K, where experimental data are rare.     

HCl-NaCl-C3H8O3-H2O体系溶液热力学性质

在恒定丙三醇质量分数x=0．1的条件下，测定了无液接电池（A）和电池（B）的电动势根据电池（A）电动势确定了丙三醇和水混合溶剂中的Ag－AgCl电极的标准电极电势，讨论了HCl的迁移性质；由电池（B）测得的电动势计算了HCl在该体系中的活度系数γA，计算的结果表明，对于所讨论的体系，在溶液中总离子强度保持恒定，HCl的活度系数服从Harned规则．在溶液组成恒定时，IgγA是温度倒数1／T的线性函数，讨论了混合物中HCl的相对偏摩尔焓及介质效应．     

The McMillan–Mayer framework and the theory of electrolyte solutions

In electrolyte thermodynamics one often speaks of two thermodynamic frameworks; the Lewis–Randall framework (characterised by temperature, pressure, and mole numbers) and the McMillan–Mayer framework (characterised by temperature, total volume, solute mole numbers, and solvent chemical potential). However, there is only one framework in thermodynamics; the apparent difference between the two ‘frameworks’ is, in electrolyte thermodynamics, due to the change in the pressure caused by the charging process at constant volume and solvent chemical potential.The so-called McMillan–Mayer framework is set in the context of the classical thermodynamics and the use of it is examplified by the Debye–Hckel theory. The so-called McMillan–Mayer framework is superfluous when the thermodynamics of the electrolyte solutions is described by the Helmholtz energy functions.     

Low-Cost Liquid Chromatography (LC-LC). IV. “Pulsed Open Tube Gradient Generation”, A New Approach for Generating Nanoliter Volume Linear and Tailored Gradients for Capillary Electrophoresis and Micro-LC

Abstract

There is a need to generate gradients of nanoliter volumes compatible with 25–100 micron i.d. capillaries used in capillary electrophoresis (CE). Balchunas and Sapaniak [1] showed that manually produced step gradients of increasing propanol and surfactant concentration with micellar electrokinetic chromatography gave an improved separation. And while SDS-PAGE has shown remarkable results in capillaries e.g. [2], gradients in gel crosslinking to extend the molecular weight range that can be resolved has not yet been shown in capillaries. Gradients in pH in polyelectrolytes for isoelectric focusing show some of the highest resolution known. These have been generated in-place by electromigration. Small gradients are also needed in micro-liquid chromatography, a field receiving renewed interest today because of the high costs of solvent disposal.

Previous papers on “low cost liquid chromatography” (LC-LC) described how to eliminate the injection valve for “weak-eluent sariple-loading” [3, 41 and how a flow-through system can be used to take eluents to a fixed and reproducible pH., eliminating the time and effort to prepare eluents [5, 6].

Our previous paper on LC-LC described a simple-gradient method [7, 8, 91 in which an abrupt discontinuous interface between the weak and strong eluent can be spread into tin S-shaped gradient by passing the interface through an open tube. Volumes were in the microliter range (20 to 110 uL), determined by the open tube dimensions (0.53 m i.d.) and the flow rate through the open tube. Here we extend this simple-gradient approach to the high nanoliter range using 0.10 mm i.d. tubing.

This paper describes a new, very flexible method for producing shaped nanoliter gradients (“nano-gradients”) og a broad vo1ume range, independent of tube dimensions and flow rates. In the pulsed open tube generator, (I) the gradient can be changed in volume over a broad range, and (2) the yadient shape can be tailored. The new approach tailors the shape of the gradient by using a multiport valve to pulse in alternating slugs of weak and strong eluent; the timing determines the composition; and the open tube mixes the segments together.

The pulsed open tube generator described here should permit nanoliter and lower (picoliter) gradient generation when narrower bore open tubes are used (e.g. 50 to 5 i.d. micron open tubes).     

Modeling of thermal processes in high pressure liquid chromatography: II. Thermal heterogeneity at very high pressures

Advanced instruments for liquid chromatography enables the operation of columns packed with sub-2 μm particles at the very high inlet pressures, up to 1000 bar, that are necessary to achieve the high column efficiency and the short analysis times that can be provided by the use of these columns. However, operating rather short columns at high mobile phase velocities, under high pressure gradients causes the production of a large amount of heat due to the viscous friction of the eluent percolating through the column bed. The evacuation of this heat causes the formation of significant axial and radial temperature gradients. Due to these thermal gradients, the retention factors of analytes and the mobile phase velocity are no longer constant throughout the column. The consequence of this heat production is a loss of column efficiency. We previously developed a model combining the heat and mass balance of the column, the equations of flow through porous media, and a linear isotherm model of the analyte. This model was solved and validated for conventional columns operated under moderate pressures. We report here on the results obtained when this model is applied to columns packed with very fine particles, operated under very high pressures. These results prove that our model accounts well for all the experimental results. The same column that elutes symmetrical, nearly Gaussian peaks at low flow rates, under relatively low pressure drops, provides strongly deformed, unsymmetrical peaks when operated at high flow rates, under high pressures, and under different thermal environments. The loss in column efficiency is particularly important when the column wall is kept at constant temperature, by immersing the column in a water bath.     

L'analyse thermique a vitesse de decomposition constante

A general method of thermal analysis is presented, whose aim is to reduce at will pressure and temperature gradients inside the sample submitted to thermolysis. The basic idea is to control the sample temperature so as to keep constant a parameter related to the decomposition rate. Attention is specially called on the case when the controlled parameter is pressure, which allows to monitor at the same time two parameters (pressure and decomposition rate). As an example, one apparatus is described, working in the pressure range between 20 and 10^?3 torr. This method of Constant Rate Thermal Analysis (CRTA) appears to be specially suited for thermal analysis under controlled vacuum, for the preparation of well defined porous samples, and for the study of decomposition mechanisms.     

Surfactant-mediated hydrophobic interaction chromatography of proteins: gradient elution

Addition of 3-[(3-cholamidopropyl)dimethylammonio]-l-propanesulphonate (CHAPS) to mobile phases in gradient elution hydrophobic interaction chromatography (HIC) on SynChropak Propyl causes changes in observed elution times for nine globular proteins. The nine proteins showed different percentage reductions in capacity factor, k', demonstrating the ability of CHAPS to change the selectivity of the separations. Three basic types of gradient experiments have been explored for surfactant-mediated gradient elution HIC. Type I gradients are conducted with constant salt and variable surfactant concentration. Type II gradients with variable salt and constant surfactant concentration, and Type III gradients with variable salt and surfactant concentrations. By the criterion of a linear relationship between gradient time and retention time the linear solvent strength condition applies to Type II and Type III gradients. Type III gradients, with the fastest re-equilibration time, are preferable for repetitive analyses. Type I gradients are relatively ineffective in making use of the solvent strength of CHAPS, and Types I and II gradients require long equilibration times due to large changes in surface concentration of CHAPS which occur during elution. The presence of CHAPS had a negligible effect on peak shapes of the proteins examined, except for bovine serum albumin which yielded a narrower, less distorted peak in the presence of CHAPS.     

Predictive Optimization of the Separation of Phenylurea Pesticides using Ternary Mobile Phase Gradients in Reversed-Phase HPLC

Abstract

The method for optimization of ternary mobile phase gradients in HPLC developed earlier is illustrated on a practical example of reversed-phase separation of phenylurea pesticides. Predictive calculation methods were used to optimize subsequently “solvent strength”, “selectivity” and “combined solvent strength - selectivity” gradients. The merits of the three types of ternary gradients for this specific separation problem are discussed to elucidate the basic principles of the optimization strategy. The predicted retention data are compared with the experimental results and the limitations of the approach are shown. The present method makes it possible to reduce significantly the number of experiments and calculations necessary to perform the optimization in comparison to a systematic search of the optimized parametr space.     

Kinetics of Curing of Epoxy Oligomer by Diaminodiphenyl Sulfone: Rheology and Calorimetry

Diaminodiphenyl sulfone is used as a curing agent to obtain an epoxy-resin-based binder with improved thermal stability. The kinetics of curing of this composition is studied by rheokinetic and calorimetric methods. It is shown that complete conversion is attained at a temperature of 200°С or higher. The rate of viscosity increase in the course of time in curing under flow conditions does not depend on the shear rate; the viscosity changes monotonically. The curing reaction occurs in the homogeneous mode without separation of a crosslinked polymer; it is described by a second-order equation with self-deceleration at low temperatures. The rheokinetic characteristics and temperature dependences of kinetic parameters of the process are determined.     

Ion Interaction Chromatographic Separation of Amino Acids Using a Basic-Tetraalkylammonium Salt Mobile Phase

Abstract

A basic mobile phase containing a tetraalkylammonium (R₄N⁺) salt was used to enhance the retention of free amino acids (AA) in their anion form on a polystyrene divinylbenzene copolymeric (Hamilton PRP-1) nonpolar stationary phase adsorbent. Major variables, which can be readily manipulated to alter this retention and resolve complex AA mixtures, are: structure and concentration of R₄N⁺ salt, type and amount of organic modifier in the mobile phase solvent, concentration and selectivity of the counteranion present, and mobile phase pH and ionic strength. Mobile phase gradients based on a pH change, or an ionic strength change and their combination, while all other variables are constant, were evaluated for the separation of complex AA mixtures. Detection was accomplished by absorbance or fluorescence after a post-column ortho-phthalaldehyde reaction.     

Numerical investigation on cooling performance of PCM/cooling plate hybrid system for power battery with variable discharging conditions

To solve the cooling problems of power battery with variable discharging conditions, a hybrid thermal management system combined with phase change materials (PCM) and cooling plate is designed. Moreover, the ANSYS FLUENT is adopted to simulate the three-dimensional model. As a result, the effects of water flow direction and variable discharging conditions are discussed on the maximum temperature and maximum temperature difference inside the battery as well as the liquid fraction of PCM. The numerical results indicate that the maximum temperature is governed by the physical parameters of PCM, whereas the water flow direction in the cooling plate plays a dominant role on the maximum temperature difference. Moreover, the flow direction scheme of case 5 is benefit to reduce the maximum temperature and temperature difference simultaneously. Although the cooling performance of hybrid thermal management system can be deteriorated by increasing the pulse duration and heat flux, the melting of PCM dramatically suppresses the increase in maximum temperature and temperature difference. Considering the limited quality of PCM, enhancing the thermal conductivity of PCM and employing cooling scheme with staggered flow direction are recommendable ways to extend the applicability of the hybrid thermal management system for power battery with complex discharging conditions.