Micellar liquid chromatography: suitable technique for screening analysis

The screening capability of micellar liquid chromatography (MLC) is discussed using the reported chromatographic data of several sets of compounds (amino acids, beta-blockers, diuretics, phenethylamines, phenols, polynuclear aromatic hydrocarbons, steroids and sulfonamides) and new results (sulfonamides and steroids). The chromatographic data are treated with an interpretive optimisation resolution procedure to obtain the best separation conditions. Usually, the pH and the concentration of surfactant (sodium dodecyl sulfate, SDS, or cetyltrimethylammonium bromide) for the optimal mobile phase were 2.5-3 and < 0.12 M, respectively. The nature and concentration of organic solvent depended on the polarity of the eluted compounds: a low volume fraction of propanol (approximately 1%, v/v) was useful to separate the amino acids, with log P(o/w) < -1 (where P(o/w) is the octanol-water partition coefficient). A greater concentration of this solvent (approximately 5-7%) was needed for compounds in the range -1 < log P(o/w) < 2, as with the studied diuretics and sulfonamides, and a high concentration of propanol (approximately 15%) or a low concentration of butanol (< 10%) had to be used for less polar compounds with 1 < log P(o/w) < 3, such as the beta-blockers. Pentanol (< 6%) was more suitable for the even less polar compounds with log P(o/w) > 3, such as the steroids. For basic drugs such as the phenethylamines (0 < log P(o/w) < 1.7), eluted with a micellar eluent of anionic SDS, propanol was too weak. A study is also shown for mixtures of sulfonamides (log P(o/w) = -1.2 to 1.7) and steroids (log P(o/w) = 3.0-8.1) eluted from conventional C18 columns with SDS mobile phases containing acetonitrile and 1-pentanol, respectively, which are compared with classical acetonitrile-water and methanol-water mixtures. The results complement a previous study on beta-blockers (log P(o/w) = -0.03 to 2.8) and reveal that MLC is a very competitive technique for the screening of compounds against conventional RPLC, due to its peculiar behaviour with regard to the selectivity and elution strength. The concentration of organic solvent needed to obtain sufficiently low retention times (even for highly hydrophobic steroids with log P(o/w) = 7-8) is also appreciably smaller for MLC, which reduces the environmental impact of the mobile phases.     

Technological quality determination of pharmaceutical disintegrant by DSC cooling and DSC photovisual

A number of disintegrants are available on the market. They improve tablets’ disintegration. The objective of this work is the comparison of the technological quality parameters of disintegrants using different analytical techniques. Three batches of disintegrants and their binary mixtures (water:disintegrants) were investigated. Cooling experiments were used from –30 up to 200°C. The data obtained showed calorimetric differences between the samples. In the binary mixtures water showed different crystallization behaviour from the one found in the literature. According to the results DSC technique helped the quality control of different disintegrants.     

Characterization and quantitation of mixtures of alkyl ether sulfates and carboxylic acids by capillary electrophoresis with indirect photometric detection

The separation, characterization, and determination of mixtures of alkyl ether sulfates (AES) and fatty acids (C10-C16) in background electrolytes (BGEs) containing acetonitrile (ACN)-water mixtures is addressed. Due to inhibition of the ionization of the carboxylate groups, the migration time and the resolution between the fatty acids decreased when the water content of the BGE was reduced, but efficiency and resolution between the AES oligomers improved. The migration times increased and resolution improved by substituting 5% ACN by an equivalent amount of dioxane. A complete separation of the two surfactant classes, up to the AES oligomers with 8 ethylene oxide units (EOs) with respect to C10, with excellent resolution between the AES oligomers, while preserving a satisfactory resolution between the fatty acids, was achieved with a BGE containing 5 mM trimethoxybenzoic acid, 7 mM dipentylamine, 85% ACN, 5% dioxane, and 10% water. The two surfactant classes were increasingly resolved by further reducing the water content of the BGE. Thus, C2 (acetate) was resolved from the AES oligomers up to 7 EOs using 90% ACN and 5% dioxane, but the resolution between the heavier fatty acids was poor with this BGE. Identification of the AES oligomers was eased by the excellent regularity of the successive migration times; thus, within each AES subclass or series of oligomers with the same number of carbon atoms in the alkyl chain, the migration times decreased following a mild curve as the number of EOs increased. The way how the data obtained by indirect photometry (corrected peak areas that are proportional to the molar concentrations) should be managed to avoid systematic error when the calibration curve is constructed using an AES standard with an oligomer distribution different from that of the samples is discussed and equations are given. Decyl sulfate was successfully used as internal standard. The detection limits (S/N = 3) were of ca. 2 microM for individual AES oligomers.     

Methyl abstraction kinetics of CpFe(CO)2Me using the benzyl radical clock

The rate constant for the methyl abstraction reaction of CpFe(CO)₂Me has been measured with the benzyl radical clock as (1.1 ± 0.2) × 10⁵ M⁻¹ s⁻¹ at room temperature. Time-resolved Fourier-transform Infrared (FTIR) absorption spectroscopy pointed towards the formation of the CpFe(CO)₂ radical upon benzyl abstraction. The main stable product has been established by a linear scan of the reaction mixture as Cp₂Fe₂(CO)₄ produced by the dimerization of the CpFe(CO)₂ radicals. The transition state structure for the abstraction process was also found at UB3LYP/6-311+G* level of theory to contain a planar CH₃ group.     

Kinetics and Mechanism of Ni(II) Chelation in Model and Real Solutions of Xylem Sap of Quercus ilex

The kinetics of formation and dissociation of Ni(II) complexes with oxalic and citric acids was studied by cyclic voltammetry in model solutions of xylem sap of Q. ilex (the dominant tree growing on serpentine soils of Northeast Portugal) using representative concentrations, pH and ionic strength. The role of magnesium on complex formation was analyzed from solutions where Mg is present at concentration levels found in the xylem sap of Q. ilex growing on both nonserpentine and serpentine soils. Kinetics studies were also done in diluted solutions of real xylem sap samples, spiked with increasing amounts of magnesium. The values obtained for the apparent rate constants were those anticipated by the proposed model. To test the validity of the methodology and mechanisms, formation rate constants, k_f (M^?1 s^?1) of Ni(II) complexes with citrate and oxalate were evaluated that compare with the values from Eigen mechanism.     

Open-tubular capillary electrochromatography/electrospray ionization-mass spectrometry using polymeric surfactant as a stationary phase coating

We describe the use of the polymeric surfactant poly(sodium undecylenic sulfate) (poly-SUS) as a stationary phase coating in open-tubular capillary electrochromatography (OT-CEC) coupled with electrospray ionization-mass spectrometry (ESI-MS) for the analysis of beta-blocker and benzodiazepine analytes. The production of a polymeric surfactant coating on the capillary inner wall involves (i) adsorption of the cationic polymer poly(diallyldimethylammonium chloride) (PDADMAC) to the inner surface of capillary, and (ii) adsorption of the negatively charged poly-SUS onto the cationic polymer layer via strong physical interaction of the two polymer layers. As compared with micellar electrokinetic chromatography (MEKC) coupled with ESI-MS, the main advantage of this proposed method is minimization of introduction of the monomeric or polymeric surfactant into the mass spectrometer, thus avoiding the interference of the nonvolatile micelle in ESI-MS. The effects of buffer pH and applied voltage on the separation of the analytes are also discussed. Under optimum conditions, four of the five beta-blockers and four benzodiazepines are separated.     

Energetics of hydroxybenzoic acids and of the corresponding carboxyphenoxyl radicals. Intramolecular hydrogen bonding in 2-hydroxybenzoic acid

The energetics of the phenolic O-H bond in the three hydroxybenzoic acid isomers and of the intramolecular hydrogen O-H- - -O-C bond in 2-hydroxybenzoic acid, 2-OHBA, were investigated by using a combination of experimental and theoretical methods. The standard molar enthalpies of formation of monoclinic 3- and 4-hydroxybenzoic acids, at 298.15 K, were determined as Delta(f)(3-OHBA, cr) = -593.9 +/- 2.0 kJ x mol(-1) and Delta(f)(4-OHBA, cr) = -597.2 +/- 1.4 kJ x mol(-1), by combustion calorimetry. Calvet drop-sublimation calorimetric measurements on monoclinic samples of 2-, 3-, and 4-OHBA, led to the following enthalpy of sublimation values at 298.15 K: Delta(sub)(2-OHBA) = 94.4 +/- 0.4 kJ x mol(-1), Delta(sub)(3-OHBA) = 118.3 +/- 1.1 kJ x mol(-1), and Delta(sub)(4-OHBA) = 117.0 +/- 0.5 kJ x mol(-1). From the obtained Delta(f)(cr) and Delta(sub) values and the previously reported enthalpy of formation of monoclinic 2-OHBA (-591.7 +/- 1.3 kJ x mol(-1)), it was possible to derive Delta(f)(2-OHBA, g) = -497.3 +/- 1.4 kJ x mol(-1), Delta(f)(3-OHBA, g) = -475.6 +/- 2.3 kJ x mol(-1), and Delta(f)(4-OHBA, cr) = -480.2 +/- 1.5 kJ x mol(-1). These values, together with the enthalpies of isodesmic and isogyric gas-phase reactions predicted by density functional theory (B3PW91/aug-cc-pVDZ, MPW1PW91/aug-cc-pVDZ, and MPW1PW91/aug-cc-pVTZ) and the CBS-QMPW1 methods, were used to derive the enthalpies of formation of the gaseous 2-, 3-, and 4-carboxyphenoxyl radicals as (2-HOOCC(6)H(4)O(*), g) = -322.5 +/- 3.0 kJ.mol(-1) Delta(f)(3-HOOCC(6)H(4)O(*), g) = -310.0 +/- 3.0 kJ x mol(-1), and Delta(f)(4-HOOCC(6)H(4)O(*), g) = -318.2 +/- 3.0 kJ x mol(-1). The O-H bond dissociation enthalpies in 2-OHBA, 3-OHBA, and 4-OHBA were 392.8 +/- 3.3, 383.6 +/- 3.8, and 380.0 +/- 3.4 kJ x mol(-1), respectively. Finally, by using the ortho-para method, it was found that the H- - -O intramolecular hydrogen bond in the 2-carboxyphenoxyl radical is 25.7 kJ x mol(-1), which is ca. 6-9 kJ x mol(-1) above the one estimated in its parent (2-OHBA), viz. 20.2 kJ x mol(-1) (theoretical) or 17.1 +/- 2.1 kJ x mol(-1) (experimental).     

Linear free-energy relationships and the density functional theory: an analog of the hammett equation

Density functional theory has been applied to describe electronic substituent effects, especially in the pursuit of linear relationships similar to those observed from physical organic chemistry experiments. In particular, analogues for the Hammett equation parameters (sigma, rho) have been developed. Theoretical calculations were performed on several series of organic molecules in order to validate our model and for comparison with experimental results. The trends obtained by Hammett-like relations predicted by the model were found to be in qualitative agreement with the experimental data. The results obtained in this study suggest the applicability of similar correlation analysis based on theoretical methodologies that do not make use of empirical fits to experimental data can be useful in the study of substituent effects in organic chemistry.     

TR-FTIR absorption spectroscopy of transition metal carbonyl radicals generated by photodissociation of metal-metal bonds, by halogen abstraction or by radical ligand substitution

Three methods of obtaining time-resolved Fourier-Transform infrared (TR-FTIR) absorption spectra of transition metal carbonyl radicals in hexane are reported here. For the first method, CpM(CO)₂L and Cp*M(CO)₂L (M = Mo, W; L = CO, PR₃) radicals have been generated by photodissociation of the corresponding metal-metal bonded dimers. Radicals of formula M(CO)₄L (M = Mn, Re; L = CO, PR₃, AsPh₃, SbPh₃) and CpM(CO)_n (M = Fe, Mo; n = 2, 3) have been produced via the second method which is halogen abstraction of the transition metal carbonyl halides using CpMo(CO)₃ radical. For the third method, fast radical ligand substitution kinetics has been exploited to generate CpMo(CO)₂PR₃ radicals from CpMo(CO)₃ in the presence of free phosphines. An assessment of the three methods with respect to TR-FTIR spectroscopic detection of radicals was also discussed.     

Determination of interferon-gamma in Chinese hamster ovary cell culture supernatant by coupled-column liquid chromatography

A robust tandem HPLC method coupling size-exclusion (Shodex Asahipak GS-320HQ) and reversed phase (Vydac 218TP54) columns with ultraviolet detection was developed for quantitative determination of interferon-gamma (IFN-gamma) in Chinese hamster ovary cell culture supernatant. The 2D-HPLC system was linked up by a 6-port 2-position low hold-up volume switch valve. Compared to a commercial ELISA kit for IFN-gamma, the coupled column LC approach was able to detect and quantify soluble IFN-gamma, regardless of the glycoprotein's molecular/conformational variability and sample background. Each LC-LC analysis took 90 minutes inclusive of column regeneration. The relative standard deviation of measurements (n = 5) was less than 3%. The limit of detection (LOD) was determined to be 0.35 microg IFN-gamma.