Comparison of separation selectivity in capillary electrokinetic chromatography using a cationic linear polymeric pseudo-stationary phase or monomeric additives of similar structure

The retention properties in electrically driven systems with monomeric additives were compared to an electrokinetic chromatographic system with a linear, charged polymer of similar chemical structure (all additives are quaternary tetraalkyl ammonium ions). The monomeric additives were tetramethylammonium (TMA), tetraethylammonium (TEA) and dimethylpyrrolidinium (DMP), respectively, the polymeric additive was poly(diallyldimethyl)ammonium (PDADMA). The additive concentration in the background electrolyte was 2 and 4% (w/w). The retention characteristics were based on the apparent mobilities of 10 non-charged analytes with different chemical functionality, which were transported by the anodic electroosmotic flow in the dynamically coated capillary, and retained by the counter-flowing cationic additives. From these data capacity factors were derived, which ranged up to 0.8. Association constants were calculated, and were found between 10 and 170. Roughly, the association constants increased for a given analyte in the sequence TMA相似文献   

Influence of neutral cyclodextrin concentration on plate numbers in capillary electrophoresis

A quantitative theory of plate number N in capillary electrophoresis was developed for buffers containing neutral cyclodextrins (CDs) capable of forming inclusion complexes. In the theory, N was modeled by longitudinal diffusion, injection extent, width of the detection window, and the detector time constant. The apparent mobility was modeled as a weighted sum of the mobilities of the free-solution analyte and the inclusion complex. The apparent diffusion coefficient was modeled as a similarly weighted sum. Both the apparent mobility and diffusion coefficient were corrected by functions that compensated for increases of buffer temperature caused by Joule heat. The experimental N's and apparent mobilities of neutral thiourea and of the anions, dansyl D- and L-leucine, dansyl D- and L-aspartic acid, benzoate, and 4-nitrophenolate, were determined in buffers containing from 0 to 15 mM beta-CD. The binding constants, and mobilities and diffusion coefficients of the free-solution analyte and inclusion complex, were calculated as regression coefficients by fitting theory to these determinations. The regression coefficients were shown to have physicochemical meaning, as assessed by literature values, independent measurements, and theoretical predictions. The assessment showed the Nernst-Einstein equation does not relate mobilities and diffusion coefficients at the electrolyte concentration used. The interdependence of mobilities, diffusion coefficients, binding constants, and other dispersion sources was interpreted to evaluate the factors affecting the variation of N with CD concentration. From the interpretation, an approximate equation for N in low-concentration CD buffers was derived. The equation depends on free-solution and inclusion-complex mobilities and diffusion coefficients, the binding constant, the potential difference over the effective capillary length, and the number of plates in a CD-free buffer.     

An extended description of the effect of detergent monomers on migration in micellar electrokinetic chromatography

Three equilibria determine the interaction of a neutral analyte with the detergent in micellar electrokinetic chromatography and therefore its migration: (i) that of the free analyte in the aqueous phase with the micelle, (ii) its association with free detergent monomers in the aqueous phase, and (iii) the partition of the associate of analyte and monomer between the aqueous solution and the micelle. For the first equilibrium, non-stoichiometric partitioning between two phases is preferred in the present work over the assumption of complex formation between one molecule of the analyte with one micelle. The second equilibrium is described by the formation of a 1:1 associate of the analyte and monomer. In this paper, thirdly an additional equilibrium is introduced, namely, the distribution of the analyte-monomer associate between the aqueous and the micelle phase; it is expressed by the according partition coefficient. The three equilibrium constants are interrelated. Mobility data for a lipophilic fluorescent compound and a series of n-alkylphenones (differing in chain length) were measured as a function of the SDS concentration below and above the critical micellar concentration. Curve fitting enabled the derivation of the equilibrium constants. It was found that the association constants of the analytes with the detergent monomers are between 2 and 75 M(-1). Interestingly, the partition coefficient of the analyte-monomer associate between the aqueous and micellar phase is by a factor of 5-200 larger than that of the free analyte.     

Analysis of sulfobutyl ether-beta-cyclodextrin mixtures by ion-spray mass spectrometry and liquid chromatography-ion-spray mass spectrometry

A comparison is made of the retention properties of additives applied as positively charged pseudo-stationary phases for electrokinetic chromatography of neutral analytes. All additives have a quaternary ammonium as functional group. The polymeric additive [poly(N,N,N',N'-tetramethyl-N-trimethylenehexamethylenediammonium), Polybrene] has a concentration of 2% (w/w) in the background electrolyte (acetate, pH 5.2). Monomeric octyltrimethylammonium (OTMA) was used at a concentration below or above its critical micelle concentration (CMC) (140 mmol/l). At a concentration (259 mmol/l) above the CMC the system is that normally used for micellar electrokinetic chromatography with cationic micelles. However, even below the CMC, where OTMA is present as monomer, retention of the neutral analytes is observed as well. In all systems coating of the capillary wall with Polybrene establishes an electroosmotic flow directed towards the anode, counter-migrating to the electrophoretic movement of the additive. Based on the measurement of the mobility of the analytes (15 small, monofunctional aromatic compounds with different functional groups), their capacity factors, k(i), were determined in all systems. Low correlation of the k(i) values is observed between the particular systems, indicating their different selectivity at least for individual pairs of analytes. Based on the log k(i) values, a linear free energy relationship was applied to elucidate the main types of chemical interaction responsible for retention. As a result, cavity formation and n or pi electron interactions were found being significant for the micellar OTMA system, which agrees with findings described in the literature for other (cationic and anionic) micellar systems. For the polymeric system and for the monomeric OTMA system, the significant retention parameter is indicating n and pi electron interactions.     

Diffusivity of gases and main-chain cooperative motions in plasticized poly(vinyl chloride)

Permeability and time-lag measurements for H₂ and CO in poly(vinyl chloride) (PVC) plasticized with tricresyl phosphate show that the apparent diffusion coefficients at first decrease as the plas-ticizer concentration is increased. The diffusion coefficients then increase as the additive concentration is raised above 15 wt %. These changes in the apparent diffusion coefficients can be related to the behavior of a variety of mechanical properties and are attributed to antiplasticization and plasticization effects of low and high concentrations of tricresyl phosphate, respectively. The antiplasticization-plasticization effects reflect altered molecular motions of the polymer. Carbon-13 NMR rotating-frame relaxation rate measurements show directly that the cooperative main-chain molecular motions of PVC are reduced when the additive acts as an antiplasticizer and are increased when the polymer is plasticized. Both the apparent diffusion coefficient and the rotating-frame relaxation rate have a similar dependence on additive concentration. An application of the molecular theory of diffusion of Pace and Datyner accounts qualitatively for the way in which additives alter the average chain interaction energy, cooperative polymer main-chain motions, and the diffusion coefficients of gaseous penetrants.     

靛红掺杂聚吡咯膜修饰电极的电荷传输

利用交流阻抗法研究了靛红掺杂聚吡咯膜内的电荷传输，通过非线性最小二乘法拟合，得出了体系的等效电路，并计算出其电荷扩散系数和异相电子传递反应速率常数。实验结果表明，随着膜厚的增加，活性点增多，异相电子传递反应速率常数增大，同时表观扩散系数也增大。     

Separation and determination of two sesquiterpene lactones in Radix inulae and Liuwei Anxian San by microemulsion electrokinetic chromatography

A novel microemulsion electrokinetic chromatography (MEEKC) method for separating and determining two sesquoterpene lactones, alantolactone (AL) and isoalantolactone (IAL), in Radix inulae and Liuwei Anxian San has been developed. The effects of several important factors such as internal organic phases, concentration of microemulsion, concentration of acetonitrile, injection time and running voltage were systematically investigated to determine the optimum conditions. The optimum microemulsion system was composed of n-hexane (0.32% w/w), SDS (1.24% w/w), 1-butanol (2.64% w/w), acetonitrile (10% w/w) and 10 mm sodium tetraborate buffer (85.80% w/w, pH 9.2). The applied voltage was 20 kV. The analytes were detected at 214 nm. Regression equations revealed linear relationships (correlation coefficients 0.9950 for AL and 0.9946 for IAL) between the peak area of each analyte and the concentration. The limits of detection (defined as a signal-to-noise ratio of about 3) were approximately 0.45 microg/mL for AL and 0.56 microg/mL for IAL. The levels of the analytes were successfully determined with recoveries ranging from 98.2 to 104.3%. Furthermore, a simple and effective extraction method, with methanol in an ultrasonic water bath for 60 min, was used for sample preparing. Also, MEEKC was compared with micellar electrokinetic chromatography (MEKC) and shown better separation results.     

Solute-solvent interactions in micellar electrokinetic chromatography: V. Factors that produce peak splitting

The experimental conditions that produce analyte peak splitting in micellar electrokinetic capillary chromatography (MEKC) have been systematically investigated. The system studied was a neutral phosphate buffer and sodium dodecyl sulfate (SDS) micelles as pseudostationary phase. A number of analytes showing a wide variety of hydrophobicity values and several organic solvents as sample diluents have been tested. Peak splitting phenomena are mainly due to the presence of organic solvent in the sample solution. They increase with the hydrophobicity of the analyte and decrease with the increase of the surfactant concentration. When hydrophobic compounds are analyzed the suggested ways to avoid split peaks are: (i) the use of 1-propanol or 1-butanol as sample diluent instead of methanol or acetonitrile or (ii) the use of high concentration of surfactant in the separating solution when the analyte must be dissolved in pure methanol or acetonitrile.     

Effect of mobile phase anionic additives on selectivity,efficiency, and sample loading capacity of cationic drugs in reversed-phase liquid chromatography

In the present work, we study the effect of mobile phase anionic additive type and concentration on the selectivity, efficiency, and sample loading capacity of cationic drugs in reversed-phase liquid chromatography (RPLC). The type and concentration of an anionic additive are known to have a strong effect on the absolute retention of cations in RPLC; in contrast they have only a small effect on the selectivity of one cation relative to a second as seen here. This is mainly due to the similarity of the ion pair formation constants between the selected cations. The limiting retention factors of cations (i.e. the retention factor of the fully ion-paired analyte at very high additive concentration) are roughly proportional to their inherent hydrophobicities (i.e. the retention factor of the analyte in the absence of the anionic additive). With a given anion, differences in ion pairing strength between the solutes are required for effective selectivity adjustment. Based on the Wade–Lucy–Carr (W–L–C) kinetic model of overload peaks, the approach we developed in our previous work was used to study the effect of mobile phase anionic additives type and concentration on the limiting plate count (N₀) and sample loading capacity (ω_0.5) of various cationic drugs. Under linear chromatographic conditions, where the analyte exhibits its smallest peak width and thus maximum apparent plate count, the type and concentration of anionic additives have almost no effect on peak width. In comparison to neutral analytes the sorption isotherms of cationic species are very easily overloaded even when many fewer moles of cations as compared to neutrals are injected. We showed that different anionic additives profoundly affect the cations’ “overload profiles” (i.e. plots of plate count versus amount injected) by changing the sample loading capacities. The increase in sample loading capacities with different anions show the same order as the extent of ion pairing between the anions and the basic analytes. The detrimental effect of sample overloading on peak width can be greatly diminished by using either a stronger ion pairing agent or a higher concentration of a given ion pairing agent. Both effects operate by increasing the sample loading capacity, thereby allowing more solute to be injected. We believe that the increase in sample loading capacity described above is due in part to the increase in the number of ion-exchange sites as more anions sorb to the stationary phase. At the same time, the formation of a neutral ion-paired analyte also increases the amount of cation which can be loaded onto the stationary phase by allowing a greater fraction of the analyte to be present in the stationary phase as an electrically neutral (i.e. ion-paired) species.     

掺杂电活性离子聚吡咯膜内的电荷传输

用循环伏安法和计时库仑法研究了掺杂亚铁氰化钾离子的聚哟咯膜内的电荷传输问题。实验结果表明，聚吡咯膜内的电荷传递可以处理成电子在膜内的扩散模型，电荷传递速度可以 用电子表观扩散系数（Ｄａｐｐ）来表征。Ｄａｐｐ的大小由聚合物膜的结构、电活怀离子之间的过及对离了在膜中的运动决定。     

Using perdeuterated surfactant micelles to resolve mixture components in diffusion‐ordered NMR spectroscopy

Diffusion‐ordered NMR spectroscopy resolves mixture components on the basis of differences in their respective diffusion coefficients or molecular sizes. However, when components have near‐identical diffusion coefficients, they are not resolved in the diffusion dimension of a diffusion‐ordered spectroscopy (DOSY) spectrum. Adding surfactant micelles to these mixtures has been shown to enhance resolution when the component molecules interact differentially with the micelles. This approach is similar to that used in electrokinetic chromatography (EKC) where modifiers like micelles or polymers are used to enhance the separation of mixture components. In this study, perdeuterated surfactants are added to analyte mixtures studied with the DOSY technique. Since no micelle resonances appear in the mixture spectra, the difficulty associated with performing biexponential analyses in spectral regions where analyte and surfactant resonances overlap is avoided. The approach is demonstrated using mixtures of peptides with near‐identical diffusion coefficients. Copyright © 2008 John Wiley & Sons, Ltd.     

Chromatographic study of four sesquiterpenoids in volatile oil of Curcumae Rhizoma on reverse phase stationary phase with methyl-β-cyclodextrin as mobile additive

Abstract

The elution behavior of four sesquiterpenoids in volatile oil of Curcumae Rhizoma on reverse-phase high-performance liquid chromatography with methyl-β-cyclodextrin as mobile phase additive was studied, including germacrone, curzerene, furanodiene, and β-elemene. Stoichiometric ratio and apparent formation constants of inclusion complex formed by methyl-β-cyclodextrin and each analyte were calculated by varying the concentration of the additive in the mobile phase composed of methanol and water (90:10, v/v), in which the association constant for inclusion complex formed by the organic modifier methanol and methyl-β-cyclodextrin was also determined. Results showed that the stoichiometric ratio of all the inclusion complex was 1:1 when 0–9?mmol L^?1 of methyl-β-cyclodextrin was added in the mobile phase. Unusual retention behavior of the analyte germacrone was found, which was further investigated by the calculation of thermodynamic parameters. Meanwhile, enthalpy and entropy of the inclusion complexes and solute-stationary phase interactions were determined by linear van’t Hoff plots.     

Sweeping of neutral analytes via complexation with borate in capillary zone electrophoresis

Summary The sweeping concept is extended to capillary zone electrophoresis (CZE) separation of neutral solutes involving complexation with borate. Analogous to the pseudostationary phase in electrokinetic chromatography (EKC), the complexing agent (borate) serves as carrier for sweeping and separation in CZE. Therefore, similar to the retention factor in EKC, the focusing effect in the present system is directly related to the association constant between the analyte and complexing agent. Theoretical and some preliminary experimental studies gerenally suggest that the electrophoretic mobility of the complex and the concentration of the complexing agent affect the resulting length of narrowed zones. Moreover, sweeping using borate is affected by pH since borate complexation is pH dependent. From around 10 to 40-fold improvement in peak heights has been observed experimentally for some neutral test analytes (monosaccharides, catechols, and nucleosides)     

(Micellar) electrokinetic chromatography: an interesting solution for the liquid phase separation dilemma

High-performance liquid chromatography (HPLC) is a well-established method in modern analysis. The method is simple, very robust and is applicable to the majority of components to be analyzed in contrast to gas chromatography. Low efficiency and small peak capacity are sore points of HPLC when complex mixtures have to be separated. The reason for this dilemma is the small diffusion coefficient of the analytes in the liquid mobile phase compared to a gaseous phase. This review, complemented by exemplary calculated data and some latest results of our own research, illustrates the dilemma of liquid phase chromatography to achieve high efficiencies under reasonable conditions. It is shown that (micellar) electrokinetic chromatography, offering fast and efficient separations, is a very promising solution for this dilemma. Additional features of this method are possibilities of on-line analyte concentration, coupling to mass spectrometry and the easy change of selectivities by applying various separation additives. The pros and cons of electrokinetic chromatography are pointed out and some application examples are given.     

Measuring the transverse concentration gradient between adjacent laminar flows in a microfluidic device by a laser-based refractive index gradient detector

A detection scheme that probes the refractive index gradient (RIG) between adjacent laminar flows in microfluidic devices has been developed and evaluated. The behavior of low Reynolds number flows has been well documented and shows that molecular transport (mixing) between adjacent laminar flows occurs by molecular diffusion between the flow boundaries. A diode laser has been used to probe the transverse concentration gradient at a selected position along a microchannel. The concentration gradient is affected by the transverse diffusion from a flow with analyte into a flow initially without analyte. To optimize sensitivity, the RIG is probed at a position in which molecular diffusion across the boundary of the two flows has been minimal, i.e. just after the flow initially without analyte merges with the flow initially containing the analyte at a given concentration. The RIG formed causes the laser beam, impinging orthogonal to the RIG through the microchannel, to be deflected. The angle of deflection is then monitored on a position sensitive detector (PSD). Currently, this detection scheme is demonstrated to provide quantitative detection of sucrose, as a test analyte, with a concentration limit of detection (LOD) of 96 ppm (w/v) or 280 muM, corresponding to 1.3x10(-5) DeltaRI units using 3sigma baseline noise. A dynamic range of 96 ppm to 50% sucrose is obtained. This detection method provides universal detection selectivity for microfluidic analysis systems that are becoming increasingly useful in monitoring chemical systems, particularly for the polymer, pharmaceutical and life sciences fields. For a larger molecular weight analyte with a smaller diffusion coefficient, lower concentration and RI LODs were achieved since detection sensitivity is a function of analyte diffusion. For example, for the polymer poly (ethylene glycol) with a molar mass of 11 840 g mol(-1), the LOD was experimentally determined to be 56 ppm (4.7 muM), equivalent to a RI LOD of 4.5x10(-6) DeltaRI (3sigma). The detection limit for proteins was also found to be favorable. For example, with the current configuration, ribonuclease A (RNAse) had a LOD of 46 ppm (3.4 muM), and bovine serum albumin (BSA) had a LOD of 54 ppm (780 nM).     

Use of NMR binding interaction mapping techniques to examine interactions of chiral molecules with molecular micelles

NMR spectroscopy was used to investigate the association of four chiral molecules with the molecular micelle poly(sodium N-undecanoyl-l-leucylvalinate) (poly(SULV)). Adding poly(SULV) to the background electrolyte in electrokinetic chromatography (EKC) allows enantiomeric resolution to be achieved because enantiomers interact differentially with the chiral centers on the micelle headgroups as they both move in the electric field. Pulsed field gradient diffusion experiments were used to measure molecular micelle association constants for enantiomers of each analyte. These association constants were consistent with EKC elution order for the compounds 1,1'-binaphthyl-2,2'-diyl hydrogen phosphate (BNP), 1,1'-bi-2-naphthol (BOH), and Troger's base. In addition, nuclear Overhauser enhancement spectroscopy, nuclear Overhauser effect difference, and intermolecular cross relaxation diffusion experiments were used to generate binding interaction maps for each chiral analyte. These maps showed that BNP and BOH inserted into the surfactant headgroup's major chiral groove and interacted predominately with the leucine chiral center. (+)-Troger's base was also found to insert into the major chiral groove. However, this compound instead interacted with the valine chiral atom. In diffusion experiments with long diffusion times, the linearized diffusion plots for each analyte-molecular micelle mixture showed curvature characteristic of intermolecular cross relaxation. The magnitude of this effect scaled linearly with the analytes' free energies of binding.     

Accurately describing weak analyte-additive interactions by capillary electrophoresis

When modeling analyte-additive interactions in capillary electrophoresis (CE), it is necessary to correct for all changes in the apparent electrophoretic mobility of an analyte that are not due to specific binding. Current models based on dynamic complexation have corrected for bulk viscosity changes in the background electrolyte (BGE) when additives are used, while assuming negligible changes in the dielectric constant and other physicochemical properties of the solution. In this report, a study of weak interactions between deoxyribonucleotides and hydroxypropyl-beta-cyclodextrin (HP-beta-CD) revealed significant nonideality in binding isotherms. Changes in the dielectric properties of the solution due to the addition of high concentrations of HP-beta-CD to the BGE was observed to alter the electrophoretic mobility of analytes. A relative dielectric correction factor was required to normalize analyte mobilities to a reference state of zero additive concentration. The use of both a relative dielectric factor and a viscosity correction factor was found to increase the accuracy of the model, reflected by a higher degree of correlation between predicted and measured analyte mobilities. This type of correction is particularly relevant when studying weak analyte binding interactions or when using high concentrations of additive in the BGE. This work is vital for accurate determination of weak binding constants and mobility values, as well as providing a deeper understanding of the fundamental parameters influencing a separation in CE.     

Measurement of apparent diffusion coefficients within ultrathin nafion Langmuir-Schaefer films: comparison of a novel scanning electrochemical microscopy approach with cyclic voltammetry

The use of scanning electrochemical microscopy (SECM) to evaluate the apparent diffusion coefficient, Dapp, of redox-active species in ultrathin Nafion films is described. In this technique, an ultramicroelectrode (UME) tip, positioned close to a film on a macroscopic electrode, is used to oxidize (or reduce) a species in bulk solution, causing the tip-generated oxidant (reductant) to diffuse to the film/solution interface. The oxidation (reduction) of film-confined species regenerates the reductant (oxidant) in solution, leading to feedback to the UME. A numerical model is developed that allows Dapp to be determined. For these studies, ultrathin films of Nafion were prepared using the Langmuir-Schaefer (LS) technique and loaded with an electroactive species, either the ferrocene derivative ferrocenyltrimethylammonium cation, FA+, or tris(2,2'-bipyridyl)ruthenium(II), Ru(bpy)32+. The morphology and the thickness of the Nafion LS films (1.5 +/- 0.2 nm per layer deposited) were evaluated using atomic force microscopy (AFM). For comparison with the SECM measurements, cyclic voltammetry (CV) was employed to evaluate the concentration of electroactive species within the Nafion LS films and to determine Dapp. The latter was found to be essentially invariant with film thickness, but the value for Ru(bpy)32+ was 1 order of magnitude larger than for FA+. CV and SECM measurements yield different values of Dapp, and the underlying reasons are discussed. In general, the Dapp values for these films are considerably smaller than for recast Nafion films, which can be attributed to the compactness of Nafion LS films. Nonetheless, the ultrathin nature of the films leads to fast response times, and we thus expect that these modified electrodes could find applications in sensing, electroanalysis, and electrocatalysis.     

Recent advances in micellar electrokinetic chromatography

This review contains nearly 200 reference citations, and covers advances in electrokinetic capillary chromatography based on micelles, including stabilized micelle complexes, polymeric and mixed micelles from 2003-2004. Detection strategies, analyte determinations, and applications in micellar electrokinetic capillary chromatography (MEKC) are discussed. Information regarding methods of analyte concentration, analyte specific analyses, and nonstandard micelles has been summarized in tabular form to provide a means of rapid access to information pertinent to the reader.