Biomedical subjects

Alain Tchapla

Publications and source records attributed to Alain Tchapla.

11 recordsLinked to original sources

PubMed2006

Characterization and comparison of the chromatographic performance of different types of reversed-phase stationary phases.

The chromatographic performance of several base-deactivated stationary phases was evaluated with a specific chromatographic test. Seven basic test compounds, possessing different physico-chemical properties were injected on different supports with two mobile phases: one at pH 7.0 (acetonitrile-phosphate buffer, 40:60, v/v), and the other at pH 3.0 (acetonitrile-phosphate buffer, 15:85, v/v). Chromatographic parameters obtained under these conditions were treated by principal component analysis (PCA) to separate base deactivated supports according to their silanol activity (pH 7.0 mobile phase) and hydrophobic properties (pH 3.0 mobile phase). The information given by the specific test column evaluation was improved with complementary chemometric tools such as hierarchical cluster analysis. The same base deactivated supports were also tested following a general test procedure issued from the literature and obtained fundamental properties (in particular silanol activity and hydrophobicity) were compared with column evaluation obtained with the specific test: results were in good agreement, although the use of the specific test offered a better differentiation between numerous base-deactivated supports.

Chemical Phenomena↗

PubMed2006

Secondary isotope effects in liquid chromatography behaviour of 2H and 3H labelled solutes and solvents.

The separation of solutes that differ only in the extent of isotopic substitution of their hydrogen atoms, using either mixtures of isotopically non-modified or perdeuterated solvents as mobile phases, is described. The occurrence of a secondary isotope effect is demonstrated in reversed-phase liquid chromatography, which is independent of the nature of the stationary phase (different octadecyl-bonded silicas, an embedded alkylamide-bonded silica, as well as one polymeric stationary phase were tested), and the water content and the nature of organic modifier of the mobile phase. The separation of 24 structurally different isotopologue pairs (apolar compounds and polar compounds with exchangeable or non-exchangeable hydrogen atoms) is examined using reversed-phase liquid chromatography. It is found that the greater the number of isotopically substituted hydrogen atoms in a given organic solute, the better is the separation of a particular isotopologue pair. The single secondary isotope effect is shown to be dependent on the number of isotopic substitutions. The greater the number of these substitutions, the smaller is the single isotope effect. The single secondary isotope effect is higher for aromatic hydrocarbons than for aliphatic hydrocarbons. A secondary isotope effect is also observed in chiral chromatography and normal-phase liquid chromatography, as well as on changing the nature of the substituting isotope, i.e.: tritium instead of deuterium. Thus, we have demonstrated that the total secondary isotopic effect for hydrogen/tritium is higher than for hydrogen/deuterium. This isotope effect involves only the consequences of changes in interactions due to nuclear motions. Overall this study confirms the predominance of hydrophobic effects in retention processes in reversed-phase liquid chromatography. In reversed-phase liquid chromatography, a secondary isotope effect related to mobile phase composition is also observed. The behaviour of deuterium oxide and water in mobile phases of the same composition (%, w/w) is compared. Independent of the nature of the organic modifier (methanol, acetonitrile or ethanol), the effect of replacing H2O with 2H2O in the mobile phase, is an increase in the retention factors and an improvement in the chromatographic resolution of isotopologue pairs. This increase in the resolution is not accompanied by a change in the chromatographic selectivity. The measurement of liquid-liquid extraction coefficients proves that the effect is mainly due to the modification of the phase ratio. In general the effect of 2H-labelled solvents (2H2O and C2H3CN) as mobile phase components, compared to their isotopically non-modified isomers, can be rationalized on the basis of their lower polarisabilities. Overall the use of perdeuterated rather than isotopically non-modified solvents as mobile phase components leads to the most efficient separation systems.

Chromatography, High Pressure Liquid↗

PubMed2005

A simple subcritical chromatographic test for an extended ODS high performance liquid chromatography column classification.

This paper describes a new test designed in subcritical fluid chromatography (SFC) to compare the commercial C18 stationary phase properties. This test provides, from a single analysis of carotenoid pigments, the absolute hydrophobicity, the silanol activity and the steric separation factor of the ODS stationary phases. Both the choice of the analytical conditions and the validation of the information obtained from the chromatographic measurements are detailed. Correlations of the carotenoid test results with results obtained from other tests (Tanaka, Engelhard, Sander and Wise) performed both in SFC and HPLC are discussed. Two separation factors, calculated from the retention of carotenoid pigments used as probe, allowed to draw a first classification diagram. Columns, which present identical chromatographic behaviors are located in the same area on this diagram. This location can be related to the stationary phase properties: endcapping treatments, bonding density, linkage functionality, specific area or silica pore diameter. From the first classification, eight groups of columns are distinguished. One group of polymer coated silica, three groups of polymeric octadecyl phases, depending on the pore size and the endcapping treatment, and four groups of monomeric stationary phases. An additional classification of the four monomeric groups allows the comparison of these stationary phases inside each group by using the total hydrophobicity. One hundred and twenty-nine columns were analysed by this simple and rapid test, which allows a comparison of columns with the aim of helping along their choice in HPLC.

Chromatography, High Pressure Liquid↗

PubMed2005

A study of interlaboratory influence on column evaluation.

A liquid chromatography method for the characterization of base deactivated columns was investigated in a collaborative study involving six laboratories. This work was carried out on two chromatographic supports (Xterra RP 18 and Symmetry Shield). Different cooling systems, namely water bath and air oven, were tested and it was shown that column thermoregulation did not significantly influence chromatographic data. In order to control the mobile phase composition, the latter was prepared by weight rather than volume. Thanks to the injection of a set of selected neutral compounds, extra-column effects were evaluated in each of the participating laboratories. The results showed that chromatographic supports tested in different laboratories and following the same test protocol could be effectively compared.

Chromatography, High Pressure Liquid↗

PubMed2005

A chemical investigation by headspace SPME and GC-MS of volatile and semi-volatile terpenes in various olibanum samples.

Six different olibanum samples with certified botanical origin were analyzed by headspace SPME-GC/MS in order to define their mono-, sesqui- and diterpenic composition, as pertinent criteria of identification. Boswellia carteri and Boswellia sacra olibanum have quite similar chemical composition, with isoincensole acetate as the main diterpenic biomarker. Although Boswellia serrata olibanum also exhibits this biomarker, the presence of methylchavicol, methyleugenol and an unidentified oxygenated sesquiterpene distinguishes B. serrata olibanum from the two other species. The characteristic chemical compounds of Boswellia papyrifera are the diterpenic biomarkers incensole and its oxide and acetate derivatives, n-octanol and n-octyl acetate. Boswellia frereana olibanum is devoid of diterpenes of the incensole family but contains a high amount of many dimers of alpha-phellandrene. The chemical composition of olibanum, which is demonstrated to be different for each Boswellia species allowed the determination of the taxonomic origin of frankincense samples purchased on various markets in East Africa, in the Near East and in Yemen. Moreover, terpenic fingerprints allowed the botanical origin of olibanum used in traditional incense mixtures to be identified. Furthermore, this study gave us the opportunity to assign a botanical origin to an archaeological frankincense sample.

Boswellia↗

PubMed2004

Glycolipid class profiling by packed-column subcritical fluid chromatography.

The potential of packed-column subcritical fluid chromatography (SubFC) for the separation of lipid classes has been assessed in this study. Three polar stationary phases were checked: silica, diol, and poly(vinyl alcohol). Carbon dioxide (CO2) with methanol as modifier was used as mobile phase and detection performed by evaporative light scattering detection. The influence of methanol content, temperature, and pressure on the chromatographic behavior of sphingolipids and glycolipids were investigated. A complete separation of lipid classes from a crude wheat lipid extract was achieved using a modifier gradient from 10 to 40% methanol in carbon dioxide. Solute selectivity was improved using coupled silica and diol columns in series. Because the variation of eluotropic strength depending on the fluid density changes, a normalized separation factor product (NSP) was used to select the nature, the number and the order of the columns to reach the optimum glycolipid separation.

Chromatography, Liquid↗

PubMed2004

Post-column addition as a method of controlling triacylglycerol response coefficient of an evaporative light scattering detector in liquid chromatography-evaporative light-scattering detection.

The non-linear response is generally the main limitation to the general quantitative use of evaporative light-scattering detection (ELSD). In the particular case of triacylglycerol (TG) analysis, we present a preliminary paper dealing with the use of post-column additives as a means of monitoring the response of such a detector. As TG can form molecular association complexes (ligand-ligate associations) with either cholesterol, urea or silver nitrate, we report the influence of the concentration of each of these chemical compounds in the liquid phase directed towards the ELSD system. The results show that the response coefficient b of the calibration curve either decreases from 1.25-1.30 to 0.51 or increases from 1.25-1.30 to 1.78 according to the nature and concentration of post-column additive. The use of cholesterol as additive, at a discrete concentration, may lead to a linear response curve (b = 1), i.e. to the direct proportionality of ELSD response versus the TG concentration, making quantitative analysis of such solutes easier. On the other hand, to improve sensitivity, the addition of silver nitrate may be chosen for an increase in b value.

Cholesterol↗

PubMed2004

Characterisation of embalming materials of a mummy of the Ptolemaic era. Comparison with balms from mummies of different eras.

Gas chromatography-mass spectrometry has been used to determine the nature of organic materials used in mummification balms. A comparative analysis of samples taken from Egyptian mummies is developed. The results are given in two parts. First, it is shown that the chemical composition of the balm is practically independent of the part of the mummy from which it is taken. This study was done on a Ptolemaic mummy (circa 100 BC from the Guimet Museum in Lyon). Fats, beeswax, and diterpenic resins were the main components: they were found everywhere. Castor oil was also very often detected (in half of the samples). This particular fat is present in the balm inside the thorax but not in the skull. Moreover it is shown that a vegetable tannin was employed. Components indicative of vegetable tannin input (gallic acid and inositols) were found in seven samples out of eighteen, particularly close to the body and on the canopic pack of the heart. Secondly, some conclusions from a comparative study of the composition of balms from mummies of various social levels as well as of different Egyptian periods are reported. It is shown that beeswax was used as from very early times (XVIIIth dynasty). The mixture of beeswax, fats, and diterpenoid resins would appear to be more recent. The balms of three mummies dating from more recent Egyptian periods (XIXth to XXVth dynasty) were analysed. No evidence of a resin, gum-resin, or plant gum could be found. Some mummies would appear to have been embalmed with fats or beeswax. Finally, the entrails canopic pack said to belong to Ramses II undoubtedly shows an embalming process with a triterpenic resin of the mastic type. The adopted analytical methodology enabled us to achieve simultaneous detection of four components of the balm of the Ptolemaic mummy. Analysis of the other five mummies revealed far less complex chemical compositions for the balms. This may be an indication of different embalming processes, although we should bear in mind the question of organic matter preservation through the ages.

Castor Oil↗

PubMed2004

Headspace solid phase microextraction for screening for the presence of resins in Egyptian archaeological samples.

The aim of this study was to use headspace solid phase microextraction (SPME) to reveal the presence of resin in archaeological samples, such as mummification balms, from ancient Egypt. Experiments were first performed with fresh resins of known origin. The SPME fibre readily extracted mono- and sesquiterpenes and, to a lesser extent, diterpenes. Using mass spectra and retention indices of constitutive compounds, qualitative analysis of the volatile fraction allowed us to differentiate resins or gum-resins such as myrrh, olibanum, galbanum, labdanum, mastic, and conifer resins. SPME was then successfully applied to archaeological samples from ancient Egypt in which the presence of resins was detected. Volatile components were desorbed and trapped according to the same SPME procedure as was applied to fresh resins, after a sample preparation consisting of a fine grinding.

Journal Article↗

PubMed2003

Optimization of headspace solid phase microextraction for gas chromatography/mass spectrometry analysis of widely different volatility and polarity terpenoids in olibanum.

The aim of this study was the optimization of headspace SPME conditions for trapping diterpenes present in frankincense (olibanum). Diterpenes like cembrenes or incensole and its derivatives are characteristic of olibanum. So in order to detect by SPME the occurrence of olibanum in archeological objects, it appears essential to have the best extraction conditions for these diterpenes that will be in very small quantities. Both sampling time and extraction temperature were studied and five fiber coatings were tested: polydimethylsiloxane (PDMS), polydimethylsiloxane/divinylbenzene (PDMS/DVB), carboxen/polydimethylsiloxane (CAR/PDMS), divinylbenzene/carboxen/polydimethylsiloxane (DVB/CAR/PDMS) and carbowax/divinylbenzene (CW/DVB). The PDMS/DVB fiber was found to be the most efficient for trapping olibanum characteristic diterpenes, with a sampling time of 1 h and a sampling temperature of 80 degrees C.

Boswellia↗

PubMed2003

Isolation of ceramide fractions from skin sample by subcritical chromatography with packed silica and evaporative light scattering detection.

Separative method of lipid classes from the stratum corneum was developed with packed silica and supercritical CO2 containing 10% of methanol at 15 degrees C, 15 MPa and 3 ml min(-1). The elution order of lipid classes was first esterified cholesterol, triglycerides, squalene co-eluted in a single peak, then free fatty acids, free cholesterol, ceramides and finally glycosylceramides. The ceramides were eluted in several fractions which depended on the number of hydroxyl groups in the molecule, i.e. more hydroxyl groups were contained in ceramides, more important was the retention. Moreover, the retention was not altered by the presence of carbon double bond and variation of the alkyl chain length. The ceramide response with the evaporative light scattering detector was improved by turning the influence of the solvent nature on the response to advantage. Therefore, addition of various solvents with or without triethylamine and formic acid were tested in post-column due to the incompatibility of such modifiers with silica stationary phase. Thereby the solvent conditions for the separation and the detection can be adjusted almost independently. The response was greatly increased by post-column addition of 1% (v/v) triethylamine and its equivalent amount of formic acid in dichloromethane introduced at 0.1 ml min(-1) into the mobile phase. This device had allowed the detection of 400 ng of ceramide with a S/N = 21, whereas no peak was observed in absence of the post-column addition. Finally, the method was applied to the treatment of skin sample which led to highly enriched ceramide fraction.

Ceramides↗