Migration behavior and separation of benzenediamines, aminophenols and benzenediols by capillary zone electrophoresis

Highly Sensitive Measurement of Horseradish Peroxidase Using Surface-Enhanced Raman Scattering of 2,3-Diaminophenazine

The development of various enzyme-linked immunosorbent assays (ELISAs) coupled with surface-enhanced Raman scattering (SERS) detection is a growing area in analytical chemistry due to their potentially high sensitivity. A SERS-based ELISA with horseradish peroxidase (HRP) as an enzymatic label, an o-phenylenediamine (oPD) substrate, and a 2,3-diaminophenazine (DAP) enzymatic product was one of the first examples of such a system. However, the full capabilities of this long-known approach have yet to be revealed. The current study addresses a previously unrecognized problem of SERS detection stage performance. Using silver nanoparticles and model mixtures of oPD and DAP, the effects of the pH, the concentration of the aggregating agent, and the particle surface chloride stabilizer were extensively evaluated. At the optimal mildly acidic pH of 3, a 0.93 to 1 M citrate buffer, and AgNPs stabilized with 20 mM chloride, a two orders of magnitude advantage in the limits of detection (LODs) for SERS compared to colorimetry was demonstrated for both DAP and HRP. The resulting LOD for HRP of 0.067 pmol/L (1.3 amol per assay) underscores that the developed approach is a highly sensitive technique. We suppose that this improved detection system could become a useful tool for the development of SERS-based ELISA protocols.

Differentiation of isomeric metabolites of carbamazepine based on acid-base properties; Experimental vs theoretical approach

Metabolism of carbamazepine is complex and leads to the three isomeric derivatives whose occurrence is dependent on the type of sample material. Their unambiguous differentiation is overall important. In this work, the qualitative analysis of 2-hydroxycarbamazepine, 3-hydroxycrbamazepine and carbamazepine-10,11-epoxide was attempted for the first time using capillary zone electrophoresis, based on the models linking electrophoretic mobility with pK_a value determining the acidity of the hydroxyl groups. For this purpose, pK_a values ​​were determined using electrophoretic and theoretical methods, and then the compliance of the obtained mobility models with the measured values ​​was analyzed. Despite the slight difference in acidity ​​(0.3-0.4 pH unit), the obtained results prove that the correct identification of the metabolites under consideration, and reliable prediction of the selectivity of their separation, are possible on the basis of experimentally determined pK_a values, even with highly simplified methods assuming the lack of certain data. However, it is important to choose the optimal pH value, which should be close to pK_a. On the other hand, worse results were obtained for the theoretically determined mobilities, which differed significantly from the experimental values. An attempt was also made to explain the acidity of hydroxycarbamazepines and the associated thermodynamic parameters - deprotonation enthalpy and entropy, with respect to their structure. The lack of intramolecular hydrogen bonds and the significant contribution of entropic effects stabilizing the protonated form seems to be significant. The higher pK_a value for CBZ-2-OH probably results from the stronger effect of the energetically unfavorable organization of solvent dipoles due to ionization.

Fast and versatile fabrication of PMMA microchip electrophoretic devices by laser engraving

This paper describes the effects of different modes and engraving parameters on the dimensions of microfluidic structures produced in PMMA using laser engraving. The engraving modes included raster and vector, while the explored engraving parameters included power, speed, frequency, resolution, line-width, and number of passes. Under the optimum conditions, the technique was applied to produce channels suitable for CE separations. Taking advantage of the possibility to cut-through the substrates, the laser was also used to define solution reservoirs (buffer, sample, and waste) and a PDMS-based decoupler. The final device was used to perform the analysis of a model mixture of phenolic compounds within 200 s with baseline resolution.

Monitoring of aminophenol isomers in surface water samples using a new HPLC method

A new HPLC method was developed for the simultaneous determination of aminophenol isomers by means of a mixed-mode stationary phase containing both SCX and C18 moieties. All factors influencing the separation were discussed and optimized. The chromatographic conditions for the separation of aminophenols are the stationary phase duet SCX/C18, the mobile phase of aqueous phosphate buffer (pH 4.85):methanol = 85:15 (v/v) delivered with a flow rate of 1 mL/min and a detection at 285 nm. The method proposed was validated in terms of linearity, limits of detection and quantification, accuracy and precision. The HPLC method elaborated here was applied with good results on river water samples. In order to survey the quality of surface rivers entered in treatment plants which deliver water for Bucharest, two major rivers were included in a monitoring program which last more than 1 year.

Separation and determination of basic dyes formulated in hair care products by capillary electrophoresis

A capillary electrophoretic (CE) method for analyzing five basic dyes (Basic Red 76, Basic Brown 16, Basic Yellow 57, Basic Brown 17 and Basic Blue 99) sold under the trade name Arianor, which are commonly used in hair care products, has been established. A buffer of 100 mM acetic acid-ammonium acetate (50:50) containing 90% (v/v) methanol was employed in a fused-silica capillary of 40.0 cm x 50 microm I.D. with a bubble cell arrangement. Washing the capillary end immediately after injection was effective in preventing peak tailing of the basic dyes, which was due to their adsorption onto the outer wall of the capillary during the injection. Under these optimized conditions, acceptable results for reproducibility, limit of detection and quantitation, and linearity were obtained for the five authentic dyes tested. The recoveries of five authentic basic dyes spiked to three commercial hair care products also provided with acceptable results. This optimized CE method is useful for the analysis of mixed basic dyes in hair care products.

Quantitative structure property relationship study of the electrophoretic mobilities of some benzoic acids derivatives in different carrier electrolyte compositions

Quantitative structure-properties relationship (QSPR) has been applied to modeling and predicting the electrophoretic mobilities of a series of benzoic acid derivatives in different carrier electrolyte composition. Descriptors that were selected by stepwise multiple linear regression (MLR) technique are radial distribution function-lag8 (RDF-8), unweighted R-maximal autocorrelation geometry, topology and atomic weight assembly-lag4 (R-GETAWAY-4), geometrical descriptor lag-26 (GEO-26), and the overall dielectric constant of the carrier electrolyte. These descriptors were used as inputs for generated 4-7-1 artificial neural network (ANN). The results obtained using ANN and MLR were compared as well as with the experimental values and showed the superiority of ANN over MLR model. Also the appearance of these descriptors in QSPR models reveals the role of electronic and steric interactions in solutes mobility in capillary electrophoresis due to the dielectric and hydrodynamic friction forces.

Prediction of the electrophoretic mobilities of some carboxylic acids from theoretically derived descriptors

A 4-4-1 artificial neural network was constructed and trained for the prediction of the electrophoretic mobilities of some aliphatic and aromatic carboxylic acids based on quantitative structure-property relationships. The inputs of this network are theoretically derived descriptors that were chosen by the stepwise variables selection techniques. These descriptors are: shape factor, molecular surface area, the maximum value of electron density on atom in molecule, and the sum of atomic polarizability. In order to assess the accuracy and predictability of the proposed model, the cross-validation test was employed. The results obtained showed the ability of developed artificial neural network to prediction of electrophoretic mobilities of aliphatic and carboxylic acids. Also result reveals the superiority of the artificial neural network over the multiple linear regression models.

Semi-empirical relationships between effective mobility, charge, and molecular weight of pharmaceuticals by pressure-assisted capillary electrophoresis: applications in drug discovery

Relationships between effective mobility (m(eff)), calculated charge (Z(c)), and molecular weight (MW) are semi-empirically derived for pharmaceuticals using pressure-assisted capillary electrophoresis (PACE). We determined the m(eff) at 12 different pH points (2.0-11.4) of 66 pharmaceutical-like compounds ranging in MW from 79 to 825 g/mol. Plots of the observed m(eff) values versus Z(c)/MW(x ) (where x is a fractional coefficient) gave linear relationships. For anions, it was found that the best correlation (R(2) = 0.9666) exists when the fractional coefficient is equal to 0.4920, resulting in the equation m(eff) = 0.1853 (Z(c)/MW (0.4920)). For cations, the best linear relationship (R(2) = 0.9861) gave the equation m(eff) = 0.3888 (Z(c)/MW (0.6330)). The m(eff), Z(c)/MW(x) relationships were then applied to: (i) developing a technique for selecting an appropriate pH to achieve optimal separation of pharmaceuticals and (ii) determining the maximum charge of a molecule in the pH range of determination of negative log of the dissociation constants (pK(a)) by PACE, thus enabling the correct choice of model equation to be automated without structure analysis.

Performance of metal complex substituted polysiloxanes in capillary electrophoresis and capillary electrochromatography

Two novel polysiloxanes containing the metal complex, Co(TACN)(3+)2 (TACN= 1,4,7-triazacyclononane) were used as coatings for capillary electrophoresis (CE) and capillary electrochromatography (CEC). Through crosslinking and covalent bonding, the positively charged polymers were bonded to silica supports. In both CE and CEC, these coatings exhibited strong, pH-independent, and anodic electroosmotic flow (EOF), and had excellent long-term stability. Successful separations of aromatic acids were achieved in CE. In CEC, separation of alkylbenzenes (7 min) and basic compounds (20 min) was achieved with higher resolving power than conventional octadecyl silica packings. These polymers represent a new class of coatings for CE and CEC that generate pH-independent EOF.

Recent advances in capillary electrophoresis and capillary electrochromatography of pollutants

An overview of major developments in capillary electrophoresis and capillary electrochromatography systems in the environmental field is presented, covering relevant publications between the second half of 1999 and early 2001. Contributions are reviewed in relation to developments in detection, sample preparation/preconcentration, precision and applications. Many interesting examples are shown and the influence of important parameters on the performance of developed methods is discussed.

Determination of 5-aminosalicylic acid related impurities by micellar electrokinetic chromatography with an ion-pair reagent

A micellar electrokinetic chromatographic (MEKC) method was developed for the quantification of mesalazine or 5-aminosalicylic acid (5-ASA) and its major impurities 3-aminosalicylic acid, salicylic acid, sulfanilic acid and 4-aminophenol. The optimisation of the experimental conditions was carried out considering some important requirements: resolution, reproducibility, detection limits of 0.1% (m/m) or less, short total analysis time. Preliminary investigations employing sodium dodecyl sulfate (SDS) as surfactant did not lead to the necessary resolution of the studied compounds; the addition of tetrabutylammonium bromide (TBAB) to the SDS micellar system resulted in the complete separation of all the compounds. The effects on the separation by several parameters such as TBAB concentration, SDS concentration, background electrolyte pH and concentration, were evaluated. Using a fused-silica capillary (8.5 cm effective length) complete analysis was obtained in a very short time. Under the optimised final conditions [120 mM 3-(cyclohexylamino)-2-hydroxy-1-propanesulfonic acid buffer, pH 10.20, 65 mM SDS in the presence of 55 mM TBAB and 5% methanol] the method was validated for specificity, precision, linearity, limits of detection and quantitation, and robustness: the 5-ASA related impurities can be quantified at least at the 0.1% (m/m) level.

Optimization of separation and migration behavior of chloropyridines in micellar electrokinetic chromatography

The separation and migration behavior of pyridine and eight chloropyridines, including three monochloropyridines, four dichloropyridines, and 2,3,5-trichloropyridine were investigated by micellar electrokinetic chromatography using either sodium dodecyl sulfate (SDS) as an anionic surfactant or SDS-Brij 35 mixed micelles. Various parameters such as buffer pH, SDS concentration, Brij 35 concentration and methanol content that affect the separation were optimized. Complete separation of these chloropyridines was optimally achieved with a phosphate buffer containing SDS (30 mM) and methanol (10%, v/v) at pH 7.0. The resolution and selectivity of analytes could be considerably affected by the addition of methanol and/or Brij 35 to the background electrolyte. The migration order of these chloropyridines depends primarily on their hydrophobicity. However, electrostatic interactions may also play a significant role in the determination of the migration order of the positional isomers of chloropyridines.

Optimization of separation and migration behavior of cephalosporins in capillary zone electrophoresis

The influences of buffer pH, buffer concentration and buffer electrolyte on the migration behavior and separation of 12 cephalosporin antibiotics in capillary zone electrophoresis using three different types of buffer electrolyte, including phosphate, citrate, and 2-(N-morpholino)ethanesulfonate (MES), were investigated. The results indicate that, although buffer pH is a crucial parameter, buffer concentration also plays an important role in the separation of cephalosporins, particularly when cefuroxime and cefazolin, cephalexin and cefaclor, or cefotaxime and cephapirin are present as analytes at the same time. The electrophoretic mobility of cephalosporins and electroosmotic mobility measured in citrate and MES buffers are remarkably different from those measured in phosphate buffer. With citrate buffer, optimum buffer concentration is confined to a small range (35-40 mM), whereas buffer concentrations up to 300 mM can be used with MES buffer. Complete separations of 12 cephalosporins could be satisfactorily achieved with these three buffers under various optimum conditions. However, the separability of 12 cephalosporins with citrate or MES buffer is better than that with phosphate buffer. As a consequence of a greater electrophoretic mobility of cephalosporins than the electroosmotic mobility with citrate buffer at pH below about 5, some cephalosporins are not detectable. The cloudiness of the peak identification and of the magnitudes of the electrophoretic mobility of cefotaxime and cefuroxime reported previously are clarified. In addition, the pKa values of cephradine, cephalexin, cefaclor, and cephapirin attributed to the deprotonation of either an amino group or a pyridinium group are reported, and the migration behavior of these cephalosporins in the pH range studied is quantitatively described.