Nonaqueous capillary electrophoresis in pharmaceutical analysis

This review presents different solvents and electrolytes commonly used as BGEs in NACE for the analysis of pharmaceutical compounds. Most NACE applications carried out since 1998 for the analysis of compounds of pharmaceutical interest are presented in four tables: (i) analysis of drugs and related substances, (ii) analysis of chiral substances, (iii) analysis of phytochemical extracts and (iv) analysis of drugs in biological fluids. These selected examples are used to illustrate the interest in NACE versus conventional aqueous CE.     

Intersystem crossing processes in nonplanar aromatic heterocyclic molecules

A comprehensive study of the photophysical properties of a series of monoaza[5]helicenes is presented on the basis of joint optical spectroscopy and quantum chemistry investigations. The molecules have been characterized by absorption and CW/time-resolved luminescence measurements. All quantities related to spin-orbit-coupling processes, such as intersystem crossing rates and radiative phosphorescence lifetimes, were found to depend strongly on the nitrogen position within the carbon backbone. Density functional theory and semiempirical quantum-chemical methods were used to evaluate the molecular geometries, the characteristics of the excited singlet and triplet states, and the spin-orbit coupling matrix elements. We demonstrate that the magnitude of spin-orbit coupling is directly correlated with the degree of deviation from planarity. The trends from the calculated photophysical quantities, namely, radiative fluorescence and phosphorescence decay rates and intersystem crossing rates, of the mono-aza-helicenes are fully consistent with experiment.     

Rapid determination of p<Emphasis Type="Italic">K</Emphasis><Subscript>a</Subscript> values of 20 amino acids by CZE with UV and capacitively coupled contactless conductivity detections

A rapid and universal capillary zone electrophoresis (CZE) method was developed to determine the dissociation constants (pK _a) of the 20 standard proteogenic amino acids. Since some amino acids are poorly detected by UV, capacitively coupled contactless conductivity detection (C⁴D) was used as an additional detection mode. The C⁴D coupling proved to be very successful on a conventional CE-UV instrument, neither inducing supplementary analyses nor instrument modification. In order to reduce the analysis time for pK _a determination, two strategies were applied: (i) a short-end injection to reduce the effective length, and (ii) a dynamic coating procedure to generate a large electroosmotic flow (EOF), even at pH values as low as 1.5. As a result, the analysis time per amino acid was less than 2 h, using 22 optimized buffers covering a pH range from 1.5 to 12.0 at a constant ionic strength of 50 mM. pK _a values were calculated using an appropriate mathematical model describing the relationship between effective mobility and pH. The obtained pK _a values were in accordance with the literature. Figure a UV (1) and C⁴D (2) detectors placed on-line on the CE capillary. b Curve of effective mobility as a function of pH for histidine     

On-line solid-phase extraction and multisyringe flow injection analysis of Al(III) and Fe(III) in drinking water

A new analytical method was developed for on-line monitoring of residual coagulants (aluminium and iron salts) in potable water. The determination was based on a sequential procedure coupling an extraction/enrichment step of the analytes onto a modified resin and a spectrophotometric measurement of a surfactant-sensitized binary complex formed between eluted analytes and Chrome Azurol S. The optimization of the solid phase extraction was performed using factorial design and a Doehlert matrix considering six variables: sample percolation rate, sample metal concentration, flow-through sample volume (all three directly linked to the extraction step), elution flow rate, concentration and volume of eluent (all three directly linked to the elution step). A specific reagent was elaborated for sensitive and specific spectrophotometric determination of Al(III) and Fe(III), by optimizing surfactant and ligand concentrations and buffer composition. The whole procedure was automated by a multisyringe flow injection analysis (MSFIA) system. Detection limits of 4.9 and 5.6 μg L⁻¹ were obtained for Al(III) and Fe(III) determination , respectively, and the linear calibration graph up to 300 μg L⁻¹ (both for Al(III) and Fe(III)) was well adapted to the monitoring of drinking water quality. The system was successfully applied to the on-site determination of Al(III) and Fe(III) at the outlet of two water treatment units during two periods of the year (winter and summer conditions).     

Spiroiminodihydantoin nucleoside formation from 2'-deoxyguanosine oxidation by [(18)O-labeled] singlet molecular oxygen in aqueous solution

The main singlet molecular oxygen ((1)O(2)) oxidation products of free 2'-deoxyguanosine (dGuo) in aqueous solution were identified as a pair of diastereomeric spiroiminodihydantoin 2'-deoxyribonucleosides (dSp) together with 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodGuo). In the present work, evidence is provided from (18)[(1)O(2)] and H(2) (18)O labeling experiments, using HPLC-ESI-MS/MS, that the formation of dSp is explained by the addition of water to a reactive quinonoid intermediate, and a second reaction pathway leading to dSp involves (1)O(2) oxidation of initially generated 8-oxodGuo.     

Charge transport parameters of the pentathienoacene crystal

Pentathienoacene, the thiophene equivalent of pentacene, is one of the latest additions to the family of organic crystal semiconductors with a great potential for use in thin film transistors. By using density functional theory and gas-phase ultraviolet photoelectron spectroscopy, we investigate the microscopic charge transport parameters of the pentathienoacene crystal. We find that the valence band exhibits a stronger dispersion than those in the pentacene and rubrene single crystals with marked uniaxial characteristics within the molecular layer due to the presence of one-dimensional pi-stacks; a small hole effective mass is also found along the direction perpendicular to the molecular layers. In the conduction band, strong intermolecular sulfur-sulfur interactions give rise to a significant interstack electronic coupling whereas the intrastack dispersion is greatly reduced. The intramolecular vibronic coupling (reorganization energy) is stronger than that in pentacene but comparable to that in sexithiophene; it is larger for holes than for electrons, as a result of low-frequency modes induced by the sulfur atoms. The polarization energy is large, but its effect on the vibronic coupling remains small. Charge transport is discussed in the framework of both band and hopping models.     

Hybrid gadolinium oxide nanoparticles: multimodal contrast agents for in vivo imaging

Luminescent hybrid nanoparticles with a paramagnetic Gd2O3 core were applied as contrast agents for both in vivo fluorescence and magnetic resonance imaging. These hybrid particles were obtained by encapsulating Gd2O3 cores within a polysiloxane shell which carries organic fluorophores and carboxylated PEG covalently tethered to the inorganic network. Longitudinal proton relaxivities of these particles are higher than the positive contrast agents like Gd-DOTA which are commonly used for clinical magnetic resonance imaging. Moreover these particles can be followed up by fluorescence imaging. This study revealed that these particles suited for dual modality imaging freely circulate in the blood vessels without undesirable accumulation in lungs and liver.     

Relation between the particle size distribution and the kinetic performance of packed columns. Application to a commercial sub-2 microm particle material

To study the influence of the particle size distribution (PSD), we measured the chromatographic performance of a series of sub-2 microm particle high performance liquid chromatography (HPLC) columns packed with four different particle mixtures having a purposely imposed different size distribution. Using the reduced kinetic plot representation by plotting the separation impedance (E(0)) versus the plate number ratio (N(opt)/N), the different columns could be classified according to their chromatographic performance without the need to specify a mean particle diameter or a molecular diffusion coefficient, as is needed in the classical reduced plate height and flow resistance analysis. The present analysis shows that it is not so much the width or span of the particle size distribution, but rather the presence of fines that greatly determines the chromatographic performance of particulate columns.     

High throughput liquid chromatography with sub-2 microm particles at high pressure and high temperature

In this study, ultra performance liquid chromatography (UPLC) using pressures up to 1,000 bar and columns packed with sub-2 microm particles has been combined with high temperature mobile phase conditions (up to 90 degrees C). By using high temperature ultra performance liquid chromatography (HT-UPLC), it is possible to drastically decrease the analysis time without loss in efficiency. The stability and chromatographic behavior of sub-2 microm particles were evaluated at high temperature and high pressure. The chromatographic support remained stable after 500 injections (equivalent to 7,500 column volumes) and plate height curves demonstrated the capability of HT-UPLC to obtain fast separations. For example, a separation of nine doping agents was performed in less than 1 min with sub-2 microm particles at 90 degrees C. Furthermore, a shorter column (30 mm length) was used and allowed a separation of eight pharmaceutical compounds in only 40s.     

Recent developments in liquid chromatography--impact on qualitative and quantitative performance

In order to reduce the analysis time and maintain good efficiency in liquid chromatography (LC), several solutions are currently being investigated. The focus of this study was to compare, both qualitatively and quantitatively, the chromatographic performance of a conventional LC with selected approaches, namely monolithic supports, high temperature LC (up to 90 degrees C), and sub-2 microm particles combined with high pressure (up to 1000 bar). This comparison was achieved from a qualitative point of view with a special attention paid to the analysis of time reduction, efficiency improvement, and pressure constraint. For this purpose, the different approaches were discussed using Knox curves and other kinetic plots. It appeared that columns packed with sub-2 microm particles under high-pressure conditions (UPLC) were well adapted and this option represents an attractive alternative to conventional LC; however, the other alternative approaches should not be neglected. The quantitative evaluation of these techniques was performed on the basis of the validation of results of a pharmaceutical formulation (Rapidoca?ne), following SFSTP 2003 guidelines. Fast-LC approaches demonstrated equivalent performance to conventional LC in terms of trueness, precision, and accuracy profile, with a significant time reduction (up to 8x) according to the selected methodology.