Fast high-throughput method for the determination of acidity constants by capillary electrophoresis: I. Monoprotic weak acids and bases

A new and fast method to determine acidity constants of monoprotic weak acids and bases by capillary zone electrophoresis based on the use of an internal standard (compound of similar nature and acidity constant as the analyte) has been developed. This method requires only two electrophoretic runs for the determination of an acidity constant: a first one at a pH where both analyte and internal standard are totally ionized, and a second one at another pH where both are partially ionized. Furthermore, the method is not pH dependent, so an accurate measure of the pH of the buffer solutions is not needed. The acidity constants of several phenols and amines have been measured using internal standards of known pK_a, obtaining a mean deviation of 0.05 pH units compared to the literature values.     

Determination of pKa values of quinolones from mobility and spectroscopic data obtained by capillary electrophoresis and a diode array detector

This work compares the values of dissociation constants obtained from electrophoretic mobilities of a series of quinolones at different pH values and those obtained using absorbance spectra at the maximum of the eletrophoretic peaks. The results obtained show that the two methods are complementary and constitute a valuable means of obtaining better precision. The two methods proposed can be used simultaneously without an increase in the experimental time and allow confirmation of the results obtained.     

Standard systems for measurement of pKs and ionic mobilities. 1. Univalent weak acids

Determination of pK values of weak bases and acids by CZE has already attracted big attention in current practice and proved to offer the advantage of being applicable for mixtures of analytes. The method is based on the measurement of mobility curves plotting the effective mobility vs. the pH of the background electrolyte, and following computer-assisted regression involving corrections for ionic strength and temperature. To cover the necessary range of pH for a given case, both buffering weak acids and bases are used in one set of measurements, which requires implementing computations of individual ionic strength corrections for each pH value. It is also well known that some components of frequently used background electrolytes may interact with the analytes measured, on forming associates or complexes. This obviously deteriorates the reliability of the resulting data. This contribution brings a rational approach to this problem and establishes a standard system of anionic buffers for measurements of pKs and mobilities of weak acids, where the only counter cation present (besides H(+)) is Na(+). In this way, the risk of formation of complexes or associates of analytes with counter ions is strongly reduced. Moreover, the standard system of anionic buffers is selected in such a way that it provides, for an entire set of measurements, constant and accurately known ionic strength and the operational conditions are selected so that they provide constant Joule heating. Due to these precautions only one correction for ionic strength and temperature is needed for the obtained set of experimental data. This considerably facilitates their evaluation and regression analysis as the corrections need not be implemented in the computation software. The reliability and the advantages of the proposed system are well documented by experiments, where the known problematic group of phenol derivatives was measured with high accuracy and without any notice of anomalous behaviour.     

Metal carbonyl-trifluorophosphine systems : X. Mono-and triruthenium carbonyls

Triruthenium dodecacarbonyl reacts with high pressure PF₃ or with PF₃/CO mixtures to yield two series of compounds. At high PF₃ pressures, the mononuclear species Ru(PF₃)_x(CO)_5−x (x = 5, 4 and 3) are the main products with only traces of Ru(PF₃)₂(CO)₃ and Ru(PF₃)(CO)₄ being found. With high carbon monoxide pressure and less PF₃, the species Ru₃(PF₃)_y(CO)_12−y (y = 0–6) are the main products. It is seen that Ru₃(CO)₁₂ can have up to six CO groups replaced sequentially before the cluster is broken followed by the formation of the mononuclear species. The evidence shows that the Ru(PF₃)_x(CO)_5−x species are stereochemically non-rigid.     

Combination of flow injection with capillary electrophoresis: 8. Miniaturized capillary electrophoresis system with flow injection sample introduction and electrogenerated chemiluminescence detection

The combined flow injection (FI)-capillary electrophoresis (CE) system was further exploited by coupling to an electrogenerated chemiluminescence (ECL) detection system. A low-cost miniaturized CE system was developed on a chip platform to provide easy interface both with FI sample introduction and with ECL detection. A falling-drop interface was employed to perform FI split-flow sample introduction while achieving electrical isolation from the CE high voltage. A plexiglas reservoir at the capillary outlet served as both the reaction and detection cell for the ECL reaction, with Ru(bpy)₃²⁺ reagent continuously flowing through the cell. An optical fiber was positioned within the reservoir close to the capillary outlet for transferring the ECL emission to the PMT. The relative positions of the capillary outlet, working electrode and optical fiber as well as reagent renewal flow-rate were optimized to achieve both good sensitivity and separation efficiency under non-interrupted sampling conditions, involving large numbers of samples. An on-column joint often used in other works for isolating the ECL detection system from the CE separation voltage was not found necessary. The performance of the system was illustrated by the baseline separation of proline, valine and phenylalanine with a high throughput of 50 h⁻¹ and plate height of 14 μm for proline under 147 V cm⁻¹ field strength. Detection limits (3σ) were 1.2, 50 and 25 μM and peak height precisions were 1.4, 5.4 and 4.3% R.S.D. (n=9) for proline, valine and phenylalanine, respectively.     

Normal coordinate analysis of malononitriles: I. G Matrix elements

The elements of the kinetic energy matrix G of CX₂(CN)₂ using several sets of internal symmetry coordinates have been calculated and discussed.     

Recent developments in electrochemical flow detections—A review: Part I. Flow analysis and capillary electrophoresis

Recent years have provided numerous new examples of applying flow-through electrochemical detectors in chemical analysis. This review, based on about 250 original research papers cited from the current analytical literature, presents their application in flow analysis and capillary electrophoretic methods. Example applications are also given for arrays of electrochemical sensors in flow analysis and electrochemical detection in microfluidic systems. Potentiometric detection with ion-selective electrodes predominates in flow analysis carried out mostly in a flow-injection system, while amperometric and conductivity detections are most commonly employed in capillary electrophoresis.     

Study on solid–liquid phase equilibria in ionic liquid: 1. The solubility of alkali chloride (MCl) in ionic liquid EMISE

This paper reports the solubility of alkali chloride MCl (M is Li, Na and K) in IL EMISE was measured in the temperature range of 293.15 K to 343.15 K. The relationship between solubility, m and temperature T may be expressed in an empiric formula: ln(m/m⁰) = A₁ + A₂T₀/T + A₃T/T⁰, where m⁰ is 1 mol/kg, T⁰ is 1 K. The observed sequence of solubility is LiCl > NaCl > KCl. The fact implies that the less the radius of alkali ion, the greater is its solubility because little ion is easy to get into the interstices of IL EMISE.     

Determination of competitive adsorption isotherms applying the nonlinear frequency response method: Part I. Theoretical analysis

In this work adsorption equilibria of binary mixtures are quantified analyzing the nonlinear frequency response of a chromatographic column. Local partial derivatives of an isotherm model can be estimated for certain steady-states from the low frequency asymptotes of the corresponding frequency response functions (FRFs). The required FRFs correspond to two different compounds and the type of the imposed inlet concentration changes, e.g. periodical inlet concentration changes of only one compound or of both of them. For an accurate determination of isotherm parameters, it is necessary to approach as close as possible the low frequency asymptotic behaviour of these functions. Based on principles valid for the FRFs corresponding to the adsorption of a single solute, frequencies needed to reach the low frequency asymptotes of the functions of interest for estimating competitive isotherms are defined in this paper. The relation between the accuracy of the isotherm parameters determined and numbers and types of periodical inlet concentration changes and steady-states analyzed is also evaluated.     

Reactivity of Vinca alkaloids in superacid: An access to vinflunine, a novel anticancer agent

This is a summary of a lecture presented at the 100th Anniversary, Moissan Symposium in Paris on Friday 10th November 2006.In HF/SbF₅, Vinca alkaloids react selectively at the D’ring of the molecule. In the presence of CHCl₃ (or CCl₄), vinorelbine yields 20′,20′-difluoro-3′,4′-dihydrovinorelbine (vinflunine), presently in phase III experimentation for treatment of bladder cancer and non small cell lung cancer.     

Ionic liquids as additives in high performance liquid chromatography: Analysis of amines and the interaction mechanism of ionic liquids

As novel solvents, ionic liquids have many applications in synthesis, catalysis and analytical separation, i.e. extraction and chromatography separation. In this paper, some amines including benzidine, benzylamine, N-ethylaniline and N,N′-dimethylaniline are separated using ionic liquids as additives for the mobile phase in high performance liquid chromatography (HPLC). The effects of the length of alkyl chain or counterions on different ionic liquids and their concentrations on the separation of these analytes are performed. The differences between ionic liquids and tetrabutylammonium bromide (TBA) on the separation of o-, m-, p-phthalic acids are compared and the results show that ionic liquids are ion-pair reagents in essence, although their hydrophobicity and hydrogen bonding also play important roles.     

Through-bond vs through-space interactions : The stabilization of carbonium ions

The solvolysis technique is suggested as an investigational tool for the possible opposing effects of through-bond and through-space orbital interactions. Existing data indicate, however, that such effects are negligible. A rationalization of the results is presented.     

Direct synthesis of polymacrocyclic boron compounds.: A convenient method for the synthesis of hemicarcerands

Six new hemicarcerand-like compounds have been synthesized via condensation reaction of 3-aminophenylboronic acid with 5,5′-methylene-bis(2-hydroxybenzencarbonyl) derivatives. The one-pot reaction constitutes an interesting approach to design molecular containers by direct synthesis in moderate yields. The strategy involves a self-assembly process through the formation of N-B coordinative bonds. The X-ray structural analysis for one derivative illustrates the inclusion of two benzene molecules within the cavity, showing in a first approximation the capability of the polymacrocyclic compounds to function as molecular receptors.     

A novel approach for the control of drug release rate through hydrogel membrane: II. Thermodynamic modeling of the partition control scheme

In advanced drug delivery systems, drug permeation rate is the key parameter that governs performance. Among the factors that influence the permeation rate, partition effect is presently given less attention. In the first part of this study [L. Shang, S. Zhang, H. Du, S S. Venkatraman, A novel approach for the control of drug release rate through hydrogel membrane. I. Effect of drug immobilization on drug release rate by copolymerization method. Eur. J. Pharm. Biopharm. 68 (2008) 715–723], a scheme was proposed to alter the drug release rate through controlling the partition behavior by immobilizing drug molecules in the membrane. It was hypothesized that the immobilized drug contributes to the total chemical potential of all drug molecules, resulting in a reduction in the partition coefficient. In this paper, the working mechanism of the control scheme is studied through thermodynamic modeling on the assumption that substances in the system are dependent upon one another (rather than independent as they are usually treated). Experimental results provide satisfactory verification of the model. With this model, drug permeation rate can be quantitatively tailored.     

Multivariate approach for the enantioselective analysis in micellar electrokinetic chromatography-mass spectrometry: I. Simultaneous optimization of binaphthyl derivatives in negative ion mode

A mixture of two molecular micelles polysodium N-undecenoxy carbonyl-l-leucinate, (poly-l-SUCL) and polysodium N-undecanoyl leucylvalinate, (poly-l-SULV) was utilized in micellar electrokinetic chromatography-electrospray ionization-mass spectrometry (MEKC-ESI-MS) to simultaneously separate and detect enantiomers of binaphthyl derivatives. Separation parameters such as background buffer composition, voltage, temperature, and nebulizer pressure were optimized using a multivariate central composite design (CCD). Baseline enantioseparation for both analytes was achieved. The CCD was also used in the optimization of sheath liquid and spray chamber parameters to achieve optimum ESI-MS response. The results demonstrate that CCD is a powerful tool for the optimization of MEKC-MS parameters and the response surface model analysis can provide in-depth statistical understandings of the significant factors required to achieve maximum enantioresolution and ESI-MS sensitivity.     

New integrated measurement protocol using capillary electrophoresis instrumentation for the determination of viscosity, conductivity and absorbance of ionic liquid-molecular solvent mixtures

The low vapor pressure and the versatility of the physico-chemical properties of ionic liquids make them really attractive as an alternative for conventional molecular solvents. The knowledge of their physico-chemical properties (viscosity, conductivity, miscibility with organic solvents and anion-cation interactions) has appeared mandatory for better targeting their applications, although it is generally still lacking or incomplete.This work promotes capillary electrophoresis instrumentation as an integrated apparatus for measurement of viscosity, conductivity and absorbance of pure ionic liquids and ionic liquid-molecular solvent mixtures. Compared to current conventional techniques, the assets of this instrumentation for this purpose are the combined availability of a pressure delivery system, power supply, diode array absorbance detector and thermoregulation device, allowing unattended, automatic and easy operation, involving minimum sample handling. Most importantly, the required sample volume can be reduced to about 50 μL, making this protocol very cost-effective. A protocol was optimized with respect to time, sample consumption and data reliability for the determination of these physico-chemical parameters. Ionic liquids selected for method development and validation differed in the nature of their cation (butyl- and ethyl-methylimidazolium) and anion (trifluoromethanesulfonate and bis(trifluoromethanesulfonyl)imide). Various molecular solvents were mixed with these ionic liquids (acetonitrile, methanol, dimethylformamide and trifluoroethanol) and the same physico-chemical properties were determined by optimized methods. The knowledge of these data should be of great support in various application areas, including the development of new separation media for capillary electrophoresis and chromatographic techniques.     

Determination of stability constants of chelate complexes: Part II. Applications

The proposed simplified method for calculating the stability constants of chelate complexes from pH and pM measurements (Part I) is applied to two systems. On the basis of data reported by Österberg, the stability constants of copper o-phosphorylserylglutamic acid are calculated and good agreement is achieved. In addition, the stability constants of the mononuclear, binuclear and trinuclear silver complexes of TTHA (triethylenetetraminehexaacetic acid) were calculated from potentiometric pAg data. These calculations yielded the following values of the cumulative constants: log β_AgL=8.7, log β_AgHL= 17.6, log β_AgH2L= 23.8, log β_Ag2L= 14.0, log β_Ag2HL = 20.5, log β_Ag2H2L = 25.6, log β_Ag2L =17.0.