M.A. García

Analysis of Origanum vulgare volatiles by direct thermal desorption coupled to gas chromatography–mass spectrometry

Volatile components of samples of a population of Spanish Origanum vulgare have been analyzed by direct thermal desorption coupled to GC-MS. The method is fast and reliable and requires a low amount of sample, allowing analysis of leaves and flowers from a single individual plant. Volatile yield is highly variable among individual plants and concentration also presents a high variation for most Origanum volatile compounds, linalool being the main component in most samples. Statistical analyses are applied in order to find patterns in composition data.

Correlation between the logarithm of capacity factors for aromatic compounds in micellar electrokinetic chromatography and their octanol-water partition coefficients

The correlation between the logarithm of the capacity factors (log k′) and the logarithm of the octanol-water partition coefficients (log Pow) for a group of 20 organic compounds (ten benzene derivatives and ten polycyclic aromatic hydrocarbons) was studied in micellar electrokinetic chromatography. Sodium dodecyl sulphate micelles in different aqueous and hydro-organic (n-propanol and n-butanol) buffers were used as electrolyte solutions. A linear relationship between log k′ and log Pow was found. This correlation improved when the percentage of alcohol in the buffer system was increased and when the polarity of the alcohol was decreased.

Determination of solute-micelle association constants for a group of benzene derivatives and polycyclic aromatic hydrocarbons with sodium dodecyl sulphate by micellar electrokinetic chromatography

Abstract Micellar electrokinetic chromatography was applied to determine solute-micelle association constants for a group of benzene derivatives and polycyclic aromatic hydrocarbons using sodium dodecyl sulphate as surfactant. Among the different buffers studied, only those of pH⩾9 [2-(N-cyclohexylamino)ethanesulphonic acid and ammonium acetate] gave an electroosmotic flow high enough to allow the elution of all compounds studied. Determination of association constants was achieved under different experimental conditions and the resulting errors were evaluated. The effect of the nature and concentration of the buffer and the alcohol (n-propanol and n-butanol) on the values obtained by this technique for solute-micelle association constants was studied. It was observed that these factors do not affect the association constants under the experimental conditions used. The values of the solute-micelle association constants and the errors in their determination were also compared with those obtained previously by micellar liquid chromatography.

Enantiomeric separation of bupropion enantiomers by electrokinetic chromatography: quantitative analysis in pharmaceutical formulations.

The first CE method enabling the quantitation of the two enantiomers of bupropion was developed in this work. Electrokinetic chromatography (EKC) mode using cyclodextrins as chiral selectors was employed. A study on the enantiomeric separation ability of different neutral and anionic CDs was carried out. Sulfated-beta-CD was shown to provide the highest values for the enantiomeric resolution. The influence of some experimental conditions, such as pH, chiral selector concentration, temperature, and separation voltage on the enantiomeric separation of bupropion was also studied. The use of 10 mM sulfated-beta-CD in 50 mM borate buffer (pH 9.0) with an applied voltage of 30 kV and a temperature of 30 degrees C enabled the separation of the enantiomers of bupropion with high resolution (Rs > 7) and short analysis time (approximately 3.5 min). Finally, the method was successfully applied to the quantitation of bupropion in two pharmaceutical formulations.

Evaluation of distribution coefficients in micellar liquid chromatography

The possibilities of micellar liquid chromatography for evaluating distribution coefficients are discussed. Determination of solute-micelle association constants and distribution coefficients of solutes between stationary-aqueous, stationary-micellar and aqueous-micellar phases is described. Application of the calculation of distribution coefficients to the study of the retention mechanism of solutes in the chromatographic system and prediction of separation selectivity is also presented.

Study of retention in micellar liquid chromatography on a C8 column by the use of linear solvation energy relationships

Linear solvation energy relationships (LSERs) are used to investigate the fundamental chemical interactions governing the micellar liquid chromatographic retention of 22 aromatic compounds (11 benzene derivatives and 11 aromatic polycyclic hydrocarbons) in 80 mobile phases on a C8 column. The systems studied involve combinations of 0.050 to 0.140 M sodium dodecyl sulfate or cetyltrimethylammonium bromide, with 0 to 10% methanol, n-propanol, and n-butanol as mobile phase modifiers. The ability of the LSERs to account for the chemical interactions underlying solute retention is shown. A comparison of predicted and experimental retention factors suggests that LSER formalism is able to reproduce adequately the experimental retention factors of the solutes studied in the different experimental conditions investigated.

A model describing the effect on retention of the addition of alcohols to the mobile phase in micellar liquid chromatography

A physico-chemical model relating the capacity factor of a solute to micellized surfactant and organic modifier concentrations in micellar liquid chromatography with hybrid eluents is proposed. The equation derived from this model and some simplified equations were tested by using retention data in micellar mobile phases containing hexadecyltrimethylammonium bromide and sodium dodecyl sulphate as surfactants and n-propanol and n-butanol as organic modifiers. Good agreement between calculated and experimental capacity factors was found and the use of this model to predict the retention behaviour of solutes in micellar liquid chromatography is proposed.

Optimization of the separation selectivity of a group of benzene and naphthalene derivatives in micellar high-performance liquid chromatography using a C18 column and alcohols as modifiers in the mobile phase

The separation selectivity of fifteen benzene and naphthalene derivatives in micellar high-performance liquid chromatography, using a C18 column, was studied as a function of the parameters on which it depends. A multiple linear regression programme was used to find the dependence of the selectivity coefficient on the following parameters: nature of the surfactant in the mobile phase (sodium dodecyl sulphate or hexadecyltrimethylammonium bromide), surfactant concentration (0.02–0.1 M), nature of the additive in the mobile phase (methanol, n-propanol, n-butanol and sodium chloride) and percentage of the alcohol (0, 5 or 10%). Selectivity optimization corresponds to the use of sodium dodecyl sulphate at low concentrations and the addition of an alcohol of medium chain length.

LINEAR SOLVATION ENERGY RELATIONSHIP STUDY OF RETENTION IN MICELLAR LIQUID CHROMATOGRAPHY ON A C18 COLUMN USING SODIUM DODECYL SULFATE AND CETYLTRIMETHYLAMMONIUM BROMIDE MOBILE PHASES WITH ALCOHOL MODIFIERS

The fundamental chemical interactions governing the retention of 15 solutes in 40 micellar reversed-phase liquid chromatographic systems using sodium dodecyl sulfate (SDS), cetyltrimethylammonium bromide (CTAB), methanol, n-propanol, and n-butanol as mobile phase additives are studied using linear solvation energy relationships (LSERs). The influence of solute properties on retention in MLC and the trends in the coefficients as a function of SDS and CTAB concentrations are investigated. The ability of the LSERs to account for the chemical interactions underlying solute retention is shown. A comparison of predicted and experimental retention factors suggests that LSER formalism may not completely model the energetics of retention in MLC, but that the discrepancies, although systematic, are generally small. Finally, the effects of the addition of 0.035 M SDS to 10% methanol/90% water mobile phases on solute retention are discussed.

Patterns in the precision of quantitative data from multicomponent gas chromatographic or gas chromatographic–mass spectrometric analysis

The dispersion of the quantitative results in the analysis of volatile compounds from multicomponent mixtures by different fractionation techniques (solid-phase microextraction and direct thermal desorption) followed by GC or GC-MS presents nonrandom patterns related to the existence of different factors in the fractionation process or in the chromatographic separation which affect, to a different extent, the recovery of the sample components. Statistical techniques have been used to show the relative importance of these factors. The improvement in data precision achieved by using volatile compound concentration ratios is discussed.