The effect of the temperature dependence of mobile phase viscosity on the retention time in series-coupled gas chromatographic columns

A previously established model of a system of series-coupled columns, incorporating the effects of temperature and mobile phase compressibility, is generalized to take the temperature dependence of the mobile phase viscosity coefficient into account. Expressions are reported for the linear flow velocity, the effective partition coefficient, and the retention time for both the case of constant mass flow rate and that of constant overall pressure drop. Numerical results indicate that the temperature dependence of the viscosity coefficient affects the retention time and pressure drop while the effective mass distribution coefficient remains essentially unchanged.     

Supercoiled circular DNA retention in nonequilibrium chromatography: viscosity and velocity dependence--behavior difference with proteins

This study demonstrates that the retention behavior of various circular double-stranded DNA molecules (3, 5, and 10 kb) increases over the entire flow-rate range (0.02-1.8 mL/min) at all the mobile phase viscosities (h). The transition between the two well-known nonequilibrium chromatography methods (slalom and hydrodynamic chromatography) is clearly visualized for proteins and does not appear for plasmids because of their strong compact structure. Also, the optimal conditions for F and h are determined to obtain the most efficient separation of these three plasmids in a minimum analysis time.     

Mobile phase effects on retention on a new butylimidazolium-based high-performance liquid chromatographic stationary phase

A new HPLC stationary phase based on n-butylimidazolium bromide has been characterized by a linear solvation energy relationship (LSER) approach in the binary acetonitrile/water mobile phases. The retention properties of the stationary phase were systematically evaluated in terms of intermolecular interactions between 28 test solutes and the stationary phase. The results and further comparisons with conventional reversed phase system confirm that retention properties are similar to phenyl phases in acetonitrile/water mixtures. The results obtained with acetonitrile/water mixtures are also compared with results obtained using methanol/water mixtures.     

Considerations on the temperature dependence of the gas-liquid chromatographic retention

A discussion on the temperature dependence of the partition coefficient K is developed. This discussion embraces topics such as the limitations of conventional thermodynamic approaches followed in the chromatographic literature, qualitative theoretical notions arising from molecular thermodynamics and the experimental information that is accessible through modern capillary gas chromatography. It is shown that the heat capacity difference of solute transfer for flexible molecules has at least one maximum in the chromatographic range of temperature. As a consequence, a great amount of experimental data is required for a correct thermodynamic interpretation of the chromatographic retention.     

Mobile phase effects in reversed-phase chromatography VI. Thermodynamic models for retention and its dependence on mobile phase composition and temperature

Summary The physico-chemical framework is examined by comparing the predictions of three models for the combined effects of the composition of the hydroorganic mobile phase and the column temperature on the retention ofn-alkylbenzenes on hydrocarbonaceous bonded stationary phases. The well-mixed model leads to expressions for the dependence of retention on three factors which are equivalent to those derived previously from linear extrathermodynamic relationships. The diachoric model stems from the assumption that the mobile phase is microscopically heterogeneous and the displacement model is identical to the retention model most widely used in chromatography with polar sorbents and less polar solvents. Over limited ranges of mobile phase composition and temperature, each model does describe retention behavior. However, only the wellmixed model describes retention well over the entire range of mobile phase composition and temperature studied here. The success of the well-mixed model, and its limits, give insight into the role of the organic solvent in determining the magnitude of chromatographic retention on non-polar stationary phases with hydro-organic eluents.Dedicated to Professor S. R. Lipsky on the occasion of his 60th birthday.     

Reversed phase liquid chromatographic retention behavior of polystyrene homopolymers

Summary Isocratic and gradient elution high performance liquid chromatographic measurements of the retention behavior of polystyrene homopolymers with molecular weights ranging from 2 kD to 390 kD were performed using mixed methylene chloride-methanol mobile phases of varying composition and a C-18 chemically bonded stationary phase supported on either 100 Å or 300 Å mean pore diameter silica. Isocratic measurements of the capacity factor, k, for different molecular weight homopolymers as a function of the volume fraction of methylene chloride, , permit determination of the critical composition, _c, which renders k=1 and the local slope, S=–lnk/_c of the lnk- isotherm at =_c, and also the dependence of _c and S on the degree of polymerization, M. Linear gradient elution measurements of _c and S were also performed and compared to the corresponding isocratic measurements. The general retention behavior and the dependence of _c and S on M compare favorably to the predictions of the theory of homopolymer retention and fractionation developed by Boehm, Martire, Armstrong, and Bui (BMAB). Comparison is also made between the present work and the experimental observations of other workers on related chromatographic systems involving hompolymer retention.     

Solvent viscosity dependence of the protein folding dynamics

Solvent viscosity has been frequently adopted as an adjustable parameter in various computational studies (e.g., protein folding simulations) with implicit solvent models. A common approach is to use low viscosities to expedite simulations. While using viscosities lower than that of aqueous is unphysical, such treatment is based on observations that the viscosity affects the kinetics (rates) in a well-defined manner as described by Kramers' theory. Here, we investigate the effect of viscosity on the detailed dynamics (mechanism) of protein folding. On the basis of a simple mathematical model, we first show that viscosity may indeed affect the dynamics in a complex way. By applying the model to the folding of a small protein, we demonstrate that the detailed dynamics is affected rather pronouncedly especially at unphysically low viscosities, cautioning against using such viscosities. In this regard, our model may also serve as a diagnostic tool for validating low-viscosity simulations. It is also suggested that the viscosity dependence can be further exploited to gain information about the protein folding mechanism.     

Anomalous temperature dependence of gas chromatographic retention indices of polar compounds on nonpolar phases

The character of the temperature dependences of the retention indices RI(Т) of polar sorbates on nonpolar stationary phases was found to depend on the dosed amounts of sorbates, but not on column overloading. A physicochemical model was suggested to explain the observed anomalies in RI(Т).     

Liquid chromatographic retention and separation of phenols and related aromatic compounds on reversed phase columns

Summary Isocratic column liquid chromatographic systems with UV absorbance detection at 280 nm have been developed for the separation of 29 phenolics and related compounds.The selectivity was investigated on silica-, carbon- and polymer-based separation columns for the separation of phenolic type of components. The effects of various acetonitrile/buffer mixtures, and pH of the mobile phase, and their impact on the retention of the phenols was assessed. Tables of retention times on the four columns for the 29 phenols with two different acetonitrile/buffer mixtures, together with the retention times at three pHs from 6.5 to 2.3 with varying levels of organic modifier on the LiChrospher RP 18 column are presented.As an application, the analysis of real river water samples from the Ebro river is described using a solid phase extraction step prior to injection into the chromatographic system.     

Solute retention in the stationary phase of a liquid-solid chromatographic system

Summary Utilizing the UNIFAC group model of activity coefficients the retention behaviour of a solute in the stationary phase of a liquid-solid chromatographic system is studied. By comparison of experimentally observed capacity ratios and calculated activity coefficients of solutes in the mobile phase, varying the concentration of a polar moderator, it is shown that the calculated activity coefficients in the stationary phase fit very well the equation formally identical with the Langmuir function. Comparison of activity coefficients in the mobile and the stationary phase proves equivalence between the solvent interaction and the competition theory.     

Trace analysis of surfactants using chromatographic and electrophoretic techniques

Because of the widespread use, increased application of new formulations and immense impact on organisms and ecology surfactants are still in the focus of analytical chemistry. The development of methods with higher selectivity and lower detection limits is important to meet the requirements of greater responsibility for health of people and environment. Efficient separation methods, like HPLC, GC and CE, in combination with sensitive detection, like MS, are to be preferred over collective techniques which can suffer from interfering components. A review on trace analysis of ionic and neutral surfactants including sample preparation steps is presented, considering especially those methods which provide information about homologous and isomeric distribution of surfactant mixtures. Examples for the determination of linear alkylbenzene sulfonates in river water by HPLC and CE are discussed to show the capability of these methods for environmental analyses. As future trends increased applications of LC/MS (very high sensitivity) and also of CE (robustness and possibility for rapid method development) can be predicted. Received: 30 July 1998 / Revised: 28 October 1998 / Accetped: 1 November 1998     

Trace analysis of surfactants using chromatographic and electrophoretic techniques

Because of the widespread use, increased application of new formulations and immense impact on organisms and ecology surfactants are still in the focus of analytical chemistry. The development of methods with higher selectivity and lower detection limits is important to meet the requirements of greater responsibility for health of people and environment. Efficient separation methods, like HPLC, GC and CE, in combination with sensitive detection, like MS, are to be preferred over collective techniques which can suffer from interfering components. A review on trace analysis of ionic and neutral surfactants including sample preparation steps is presented, considering especially those methods which provide information about homologous and isomeric distribution of surfactant mixtures. Examples for the determination of linear alkylbenzene sulfonates in river water by HPLC and CE are discussed to show the capability of these methods for environmental analyses. As future trends increased applications of LC/MS (very high sensitivity) and also of CE (robustness and possibility for rapid method development) can be predicted.     

Influences of support,stationary phase polarity and loading in gas-liquid chromatographic retention

Summary Support effects for a range of diatomite supports have been determined from variations in both retention volume and retention index data for four solute probes as a function of stationary phase loading. The phases examined were Silicone 702, Silicone QF 1 and Carbowax 600. It was found that when polar phases were used, the main cause of retention data variation was gasliquid interfacial adsorption, support activity being eliminated if the surface of the support was effectively covered by such phases. For non-polar phases, support effects may still be apparent with loadings over 5%. With liquid loadings less than this value, unreal retention volumes may result from solute-support adsorption. Retention data, as a function of stationary phase loading, for highly fluorinated and non-fluorinated compounds reveals that the Retention Indices of the former are as significant as the latter class. In both cases high stationary phase loadings are recommended for the reporting of retention data.     

Test to evaluate countercurrent chromatographs. Liquid stationary phase retention and chromatographic resolution

Countercurrent chromatography (CCC) is a liquid chromatography (LC) technique with a special column able to retain a liquid stationary phase while the liquid mobile phase is pumped through. The coil planet centrifuge machines are made of open tube wound on spools. A simple test is proposed. The methanol-water (90:10, v/v)-heptane biphasic system is used with heptane as the mobile phase in the ascending or tail-to-head mode. The methanol-water stationary phase retention volume is measured at different flow-rates and rotor rotation speeds. After every machine equilibration, an alkylbenzene mixture is injected and the retention factors, peak efficiencies and resolution factors are measured or calculated for each solute. The wealth of information contained in the data set obtained is demonstrated. Four coil planet centrifuge machines of very different characteristics and one hydrostatic CCC machine with channels and ducts were submitted to the test. It was shown that the Sf, stationary retention factor, obtained with these machines was linearly dependent on the square root of F, the mobile phase flow-rate [Q. Du, C. Wu, G. Qian, P. Wu, Y. Ito, J. Chromatogr. A 835 (1999) 231-235]. It is shown that the slopes of the Sf versus F(1/2) lines could be related to a minimum rotor rotation, omega(mini), necessary to obtain the hydrodynamic equilibrium. The Sf and F parameters give the mobile phase linear velocity, u. It is shown that u is proportional to the square root of omega, the rotor rotation speed. The slope and intercept of the latter relationship also result in an omega(mini) value coherent with the first one. With the peak efficiencies and chromatographic resolution factors obtained for toluene and hexylbenzene, the parameters: number of plates per tubing turn, machine volume for one plate, and tubing length for one plate, were calculated and compared for the five machines. The internal diameter of the tubing used is shown to be a critical parameter acting on the machine volume and number of tubing turns.     

三聚氰胺在双色谱柱串联模式下的色谱保留研究

研究了亲水相互作用色谱柱串联C_(18)柱上三聚氰胺的色谱保留,并应用牛奶基质中三聚氰胺的高效分离。将HILIC柱与C_(18)柱串联,研究不同色谱柱串联顺序及色谱分离条件下三聚氰胺的保留情况,结果表明:当流动相为乙腈-乙酸铵(10mmol/L)=85∶15(v/v)、柱温30℃、流速0.75mL/min、检测波长220nm、HILIC柱在前C_(18)柱在后串联时,三聚氰胺分离效果最佳。在最佳的色谱条件下,对市售牛奶进行测定,未检测出三聚氰胺,加标回收率在81.1%～119%,三聚氰胺在双柱模式下分析效果良好。     

Prediction of protein retention times in gradient hydrophobic interaction chromatographic systems

A two-step methodology has been developed for the prediction of protein retention time in linear-gradient HIC systems. Isocratic retention parameters were determined from ln(k')-salt concentration plots for a number of commercially available proteins with a range of properties. Quantitative structure property relationship (QSPR) models based on a support vector machine (SVM) approach were generated for predicting isocratic retention parameters for proteins not included in the model generation. The predicted parameters were then used to calculate protein gradient retention times and the results indicate that this approach is well suited for predicting experimental gradient retention data. The approach presented in this paper may have implications for HIC methods development at both the bench and process scales.     

Resolution of enantiomeric amides on a cellulose tribenzoate chiral stationary phase. Mobile phase modifier effects on retention and stereo-selectivity

The effect of the steric structure and concentration of the mobile phase modifier on the retention (kappa') and stereoselectivity (alpha) of a series of enantiomeric amides has been investigated. The amides were chromatographed on a commercially available cellulose tribenzoate chiral stationary phase (CSP) using mobile phases composed of hexane and two homologous series of alcohols: methanol, ethanol, 1-propanol and 2-propanol, 2-butanol, 2-pentanol, 2-hexanol. The results of the study indicate that the alcoholic mobile phase modifiers compete with the solutes for achiral and chiral binding sites and that the steric bulk around the hydroxyl moiety of the modifier plays a role in this competition. Increased steric bulk tends to result in increased kappa' and alpha. However, the results also suggest that the effect of the alcoholic mobile phase modifiers on stereoselectivity may also be due to binding to achiral sites near or at the chiral cavities of the CSP which alters the steric environment of these cavities.     

Gas chromatographic retention behaviour of dibenzothiophene derivatives on a smectic liquid crystalline polysiloxane stationary phase

The gas chromatographic retention behaviour, on a smectic liquid crystalline polysiloxane stationary phase, of methyldibenzothiophenes and eighteen C₂-dibenzothiophenes (sixteen dimethyldibenzothiophenes and two ethyldibenzothiophenes), out of twenty possible compounds, was investigated. The retention, in addition to vapour pressure and polarity, was greatly influenced by the molecular geometry of the solutes. The major factor affecting this behaviour was the length to breadth ratio (L/B). The elution order of the dimethyldibenzothiophenes was fairly well correlated with L/B values: Roughly, the lower the L/B values, the earlier the dimethyldibenzothiophenes were eluted. However, the solute molecular shape, although of less importance, was also a significant retention-affecting factor, having the contrary effect to L/B values on the elution order. Arc-like molecules (dibenzothiophenes) with groups attached to the outer curved side (i.e., in positions 3 and 4) were retained longer than predicted by L/B values. In contrast, isomers conforming to the arc-like arrangement (i.e., with substituents in positions 1 and 2) were less retained than predicted. The application of these identifications to a crude oil sample is described.