Retention behaviour of ceramides in sub-critical fluid chromatography in comparison with non-aqueous reversed-phase liquid chromatography

Application of supercritical fluid carbon dioxide to the extraction and analysis of lipids

Supercritical carbon dioxide (SCCO2) is an ecofriendly supercritical fluid that is chemically inert, nontoxic, noninflammable and nonpolluting. As a green material, SCCO2 has desirable properties such as high density, low viscosity and high diffusivity that make it suitable for use as a solvent in supercritical fluid extraction, an effective and environment-friendly analytical method, and as a mobile phase for supercritical fluid chromatography, which facilitates high-throughput, high-resolution analysis. Furthermore, the low polarity of SCCO2 is suitable for the extraction and analysis of hydrophobic compounds. The growing concern surrounding environmental pollution has triggered the development of green analysis methods based on the use of SCCO2 in various laboratories and industries. SCCO2 is becoming an effective alternative to conventional organic solvents. In this review, the usefulness of SCCO2 in supercritical fluid extraction and supercritical fluid chromatography for the extraction and analysis of lipids is described.

A novel method for analyzing solanesyl esters in tobacco leaves using atmospheric pressure chemical ionization/mass spectrometer

A direct and simple method for analyzing solanesyl esters found in tobacco leaves was developed. Sample preparation was performed by accelerated solvent extractor 200 (ASE200) using n-hexane followed by evaporating solution in vacuo and dissolving residue with acetone. The separation of analytes was conducted through high-performance liquid chromatography (HPLC) equipped with an SIL-C18/5C column and the non-aqueous reversed phase chromatography (NARP) technique using acetone and acetonitrile as the mobile phase with a linear gradient. Atmospheric pressure chemical ionization/mass spectrometer (APCI/MS) in positive mode was used to detect solanesyl esters in the following conditions: capillary voltage 4000 V, corona current 10 microA, drying gas flow 5 mL/min, fragmentor voltage 200 V, nebulizer pressure 60 psi, and vaporizer temperature 500 degrees C. Each solanesyl ester was identified by the comparison of analyte with synthesized solanesyl esters. Quantification was conducted by selected ion monitoring (SIM) mode in order to detect the specific product ion (613.6 m/z) fragmented from solanesyl ester. The calibration curve was made in the range of 0.1-40 microg/mL with a regression coefficient over 0.999 on almost all solanesyl esters. The limit of detection (LOD) and limit of quantification (LOQ) ranged from 0.01 to 0.05 microg/mL and from 0.03 to 0.15 microg/mL, respectively, on the SIM mode of MS for quantification. Recovery (%) ranged from about 80 to 120%. The direct quantification using the developed method succeeded in showing a different amount and composition of solanesyl esters among various tobacco leaves.

Microanalytical systems for separations of stratum corneum ceramides

The small amount of lipids from human skin obtained with noninvasive sampling method led us to investigate microanalytical separation techniques. The lipid class analysis was performed with a micro polyvinyl alcohol-silica (PVA-Sil) column. The gradient elution was from heptane to acetone/butanol 90:10 v/v in 4%/min at 78 microL/min. In addition an evaporative light scattering detector (ELSD) was modified for micro-LC. All solvents contained 0.1% of triethylamine and formic acid in stoichiometric amount, which increased the ELSD response. In these conditions, the cholesterol eluted before free fatty acid, and squalene and triglycerides close to the dead volume. The various ceramide classes eluted following the order of the increased number of hydroxyl groups. The LOD for ceramides was 2.2 ng. The advantages of this method are the use of a normal stationary phase more reliable due to its chemical stability, its surface homogeneity and its development in microchromatography without chlorinated solvents which offers small LOD and the whole profile of lipids present in stratum corneum (SC). A method using a narrow-bore PVA-Sil column was used to collect ceramide fraction. Then the molecular species were analysed with a porous graphitic carbon column in capillary LC using a gradient from CH3OH/CHCl3 70:30 v/v to CHCl3 at 2%/min with a flow rate at 5 microL/min. The LOD obtained for ceramide was 1 ng. Both methods were assessed with SC samples obtained by rinsing a 5.7 cm2 area of the forearm with 25 mL of ethanol.

Modelling of ceramide interactions with porous graphite carbon in non-aqueous liquid chromatography

Interactions of solutes on porous graphitic carbon (PGC) with non-aqueous mobile phases are studied by the linear solvation energy relationship (LSER). Studies have been carried out with eight binary mixtures composed of a weak solvent (acetonitrile or methanol) and a strong solvent (tetrahydrofuran, n-butanol, CH2Cl2, 1,1,2-trichloro-2,2,1-trifluoroethane). The systematic analysis of a set of test compounds was performed for each solvent mixture in isocratic mode (50:50). The results were compared to those obtained on PGC with hydro-organic liquids and supercritical fluids. They were then correlated with the observed retention behaviour of lipid compounds, more particularly ceramides.

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.

Advantages of the use of monolithic stationary phases for modelling the retention in sub/supercritical chromatography

The low viscosity of supercritical fluids enables the coupling of columns, which favours both the high efficiency of separation and the ability of tuning the selectivity. However, it increases the inlet pressure then modifies the fluid density, i.e. the eluotropic strength of the mobile phase. In this case, the latter is rather different depending on the number of coupled columns. This fact prevents the calculation of the chromatographic parameters for coupled columns from the results obtained from one. In subcritical conditions, by using silica rod columns, which have bimodal porous structure, the flow resistance parameter is dramatically reduced. Consequently, the addition of monolithic columns induces only slight internal pressure changes and the fluid density does not vary with the column length. In this case, the calculation of retention factor and selectivity based on retention values obtained on each separate column provides accurate results allowing to determine the optimum column length in regard to the studied separation. After a better characterisation of the stationary phase included in the Chromolith column, this paper describes the beta-carotene isomers separation obtained by coupling up to six Chromolith columns to an octadecyl bonded particulate one. These compounds were studied because of the difficulty to separate these cis/trans isomers. No abnormal apparent dead volume change due to fluid density variation was reported, and good correlations between experimental and calculated retention factors and selectivities were observed. The optimum separation requires five highly porous columns coupled to a YMC Pack Pro. Moreover, the use of monolith packing allows to decrease both the retention factor and the analytical time by comparison to previous studies.

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.

Structure-retention diagrams of ceramides established for their identification

Molecular species analysis of ceramides was carried out using porous graphitic carbon with gradient elution: chloroform-methanol from 45:55 to 85:15 with a slope at 2.7%/min. These conditions gave a linear relationship between retention data and structure of ceramides. It was demonstrated that linearity occurred when a high slope value of linear gradient elution was used. Thereby the linear diagram was evolved by plotting the adjusted retention time against the total number of carbon atoms of ceramide molecules. Each line represents one ceramide class. Such a Structure-Retention Diagram describes ceramide retention and thus constitutes an identification method using only retention data. This Structure-Retention Diagram was assessed and compared to another obtained from octadesyl-grafted silica in terms of their reproducibility, precision and ability to provide ceramide identification. Better identification was obtained using the results from both Structure-Retention Diagrams. This approach with a two-dimensional separation system allowed to take advantage of the specificity of both identification models.

Structural analysis of commercial ceramides by gas chromatography-mass spectrometry

A simple method using gas chromatography-mass spectrometry was applied to analyse structures of ceramides. Identification of trimethylsilylated ceramides were obtained in short analysis times (derivatization of ceramides in 30 min at room temperature and 20 min gas chromatography mass spectrometry run) even for complex mixtures. For example in ceramide Type III, 18 peaks were observed which represent 27 various structures. The coeluted compounds were ceramides containing the same functional groups and the same carbon number but with a different distribution on the two alkyl chains of the molecule. They were accurately differentiated by mass spectrometry. Therefore, 83 structures of trimethylsilylated ceramides were identified in 11 different commercial mixtures. For 52 structures of these, mass spectral data were not described in the literature, neither full mass spectra nor characteristic fragments.