High temperature capillary gas liquid chromatography of triacylglycerols and other intact lipids

Structural elucidation of triacylglycerol using online acetone Paternò-Büchi reaction coupled with reversed-phase liquid chromatography mass spectrometry

Triacylglycerol (TG) is a class of lipids that is responsible for energy storage and cell metabolism in biological systems; it is found in relatively high abundances in biological fluids such as human plasma. Due to structural complexity, analyzing TGs using shotgun lipidomic approaches is challenging because of the presence of multiple fatty acyl compositional isomers. In this work, reversed-phase liquid chromatography (RPLC) was used for separation of TG species due to the capability of separating lipids based on fatty acyl chain lengths and degrees of unsaturation. RPLC alone does not provide structurally informative information for the location of carbon-carbon double-bonds (C[double bond, length as m-dash]Cs) without using synthesized standards that correspond to each species analyzed. The Paternò-Büchi (PB) reaction was employed online to confidently characterize the location of C[double bond, length as m-dash]Cs within lipid species via photo-initiated modification of the alkene group with acetone, which was later subjected to electrospray ionization (ESI) and tandem mass spectrometry (MS/MS) to form signature fragmentation peaks. This online RPLC-PB-MS/MS system was able to distinguish fatty acyl level and C[double bond, length as m-dash]C level isomeric species. The systems allowed for the identification of 46 TG molecular species in human plasma with confident C[double bond, length as m-dash]C location assignment in fatty acyls at a limit of identification of 50 nM.

Triacylglycerols in edible oils: Determination, characterization, quantitation, chemometric approach and evaluation of adulterations

Vegetable oils are a dietary source of lipids that constitute an essential component of a healthy diet. The commonly used vegetable oils differ significantly for their triacylglycerol (TAG) profile. TAGs represent the principal components of oils and may contain different fatty acids (FA) esterified with glycerol leading to several positional isomers. To differentiate individual TAGs species in edible oils, advanced analysis systems and innovative methods are therefore required. TAGs can be considered as good fingerprints for quality control and many studies have been performed to develop rapid and low cost analytical methods to determinate the authenticity, origin and eventually evidence frauds or adulterations. The present manuscript provides a general overview on the most common vegetable oils TAGs compositions and on the related analytical methodologies recently used. Finally, the chemometric applications developed to assess the authenticity, quality and botanical origin of various edible oils are discussed.

Regioisomeric and enantiomeric analysis of triacylglycerols

A survey of useful methods for separation and identification of regioisomers and enantiomers of triacylglycerols. Gas chromatography, gas chromatography-mass spectrometry, ¹³C NMR determination of regioisomers by enzymatic methods, and supercritical fluid chromatography are briefly surveyed, whereas a detailed description is given of the analysis of triacylglycerols by liquid chromatography, especially with silver ion (Ag⁺; argentation), and nonaqueous reversed phase liquid chromatography. Special attention is paid to chiral chromatography. Details of mass spectrometry of triacylglycerols are also described, especially the identification of important triacylglycerol ions such as [M + H-RCOOH]⁺ in atmospheric pressure chemical ionization mass spectra.

Interlaboratory evaluation of injection techniques for triglyceride analysis of cocoa butter by capillary gas chromatography

As part of two international collaborative studies, in which 14 laboratories applied capillary GLC to determine the triglyceride (TG) profile of cocoa butter, the performance of different sample introduction techniques, i.e. cold on-column injection (OCI), split injection and programmed-temperature vapouriser (PTV) injection, was compared. In both studies, the participants did not apply a uniform GLC procedure. Synthetic mixtures of triglycerides were chosen to permit an accurate determination of detector response factors. No statistically significant difference was found between the mean values obtained by different injection modes. The OCI, generally recommended as best practice, did not give superior results than the PTV or the split injection techniques.

High-resolution triacylglycerol mixture analysis using high-temperature gas chromatography / mass spectrometry with a polarizable stationary phase, negative ion chemical ionization, and mass-resolved chromatography

High-temperature gas chromatography/mass spectrometry (HT-GC/MS) has been employed to study the behavior of mixtures of triacylglycerol molecular species on a polarizable stationary phase (immobilized 65% phenyl methyl silicone). The use of negative ion chemical ionization (NICI) at an ion source block temperature of 300 °C overcomes problems with interpretation of electron ionization (EI) mass spectra produced during the HT-GC/MS. The NICI spectra of triacylglycerols produced under these conditions contain abundant [RCO2](-), [RCO2 - 18](-), and [RCO2 - 19](-) ions, believed to be produced by nucleophilic gas-phase ammonolysis, that are used to identify the individual fatty acid moieties associated with peaks in triacylglycerol total ion chromatograms. The polarizable stationary phase produces significantly enhanced resolution of triacylglycerol molecular species compared to hightemperature stable apolar stationary phases, such as immobilized dimethyl polysiloxanes. The resolution of complex natural mixtures of triacylglycerols can be further improved by use of the Biller-Biemann enhancement technique to produce mass-resolved chromatograms. Investigation of the mass-resolved chromatograms provides important information with regard to the factors that affect elution orders of individual triacylglycerol molecular species. The analysis of mixtures of authentic triacylglycerols by HT-GC/MS via NICI provides data that relate to the analytical limits of the technique for the analysis of triacylglycerols that bear both saturated and polyunsaturated fatty acyl moieties.

Methods for the analysis of triacylglycerols

This article discusses the methods most commonly employed in the analysis of the triacylglycerols (TAGs) in natural fats and considers the main advantages and disadvantages of each and the techniques for optimising analytical conditions. Complete analysis of the composition of a natural fat calls for a method of extracting and purifying the triglyceride fraction, normally by preparatory thin-layer and column chromatography. Determination of the individual components of triglyceride mixtures still entails certain difficulties, namely, the separation and identification of the TAGs in natural fats. High-performance liquid chromatography (HPLC) offers significant advantages over gas and thin-layer chromatography. Many workers have developed non-aqueous, reversed-phase HPLC systems capable of successfully resolving complex mixtures of TAGs, and combining reversed-phase (RP) HPLC and argentation chromatography may improve the results. Identification of the TAGs separated by HPLC becomes an extremely complex task if many different fatty acids are involved and if the sn-stereoscopic positions on the glycerol are to be determined. Enzymatic analysis and chiral-phase chromatography are capable of localising fatty acids on the TAG molecule. In closing, some of the most interesting biomedical applications of TAG analysis are summarised.

Chromatographic methods in the analysis of cholesterol and related lipids

Methods using thin-layer chromatography, solid-phase extraction, gas chromatography, high-performance liquid chromatography and supercritical fluid chromatography are described for the analysis of single cholesterol, esterified and sulfated cholesterol, and for cholesterol in the context of other lipid components, like other sterols and lipid classes. In connection with these techniques several clinical applications are mentioned.

Capillary gas chromatography of underivatized fatty acids with a free fatty acid phase column and a programmed temperature vaporizer injector

Underivatized fatty acids occurring in lipids of a number of biological specimens (blood plasma, tissues, food) were studied using a capillary column with a chemically bonded free fatty acid phase and a programmed temperature vaporizer. Non-linear calibration dependences were obtained for almost all the acids at lower concentrations because of losses in the column, non-linearity increasing with increasing carbon number and degree of unsaturation. Sample introduction at temperatures below the solvent boiling point eliminates losses in the injector. The lower recovery of long-chain polyunsaturated acids is caused by interactions occurring in the column. Some critical pairs remain unresolved in the underivatized form (18:1n9-18:1n7, 20:4n6-20:3n3), whereas the corresponding methyl esters exhibit baseline resolution.

Analysis of human blood plasma triacylglycerols using capillary gas chromatography, silver ion thin-layer chromatographic fractionation and desorption chemical ionization mass spectrometry

The identification and quantitation from pooled human plasma of over 50 molecular species of triacylglycerols, including 22 not previously reported, are reported. The triacylglycerols were first resolved by silver ion thin-layer chromatography into seven fractions, which were independently analysed by polarizable capillary gas chromatography and desorption chemical ionization mass spectrometry in the presence of an internal standard. The two methods gave similar values for the estimates of oligoenoic species, but the former method underestimated the polyenes (due mainly to significant losses caused by thermal degradation), and the latter method overestimated the saturates. The results show that an effective analysis of molecular species of plasma triacylglycerols cannot be obtained by either technique alone.

Gas-liquid chromatographic profiling of plasma lipids using high-temperature-polarizable capillary columns

We have examined the potential usefulness in plasma lipid profiling of capillary columns coated with a high-temperature-polarizable phenylmethyl silicone liquid phase, previously employed for gas-liquid chromatography (GLC) of natural triacylglycerols. The column yielded an excellent resolution for plasma triacylglycerols, cholesteryl esters and the trimethylsilyl (TMS) and tert-butyldimethylsilyl (t-BDMS) ethers of free cholesterol, as well as of the TMS and t-BDMS ethers of the diacylglycerols and ceramides released from plasma phospholipids by phospholipase C digestion. The various lipid classes were resolved into molecular species according to chain length and number of double bonds. The different molecular species were recovered in variable proportions as indicated by comparisons of peak area percentages obtained on the polar columns with those recorded on non-polar columns, which have been previously shown to give correct quantitative proportions. The cholesteryl esters are thermally degraded at high temperatures and must be eluted at as low a temperature as possible. Under the selected experimental conditions, the cholesteryl esters were found to partly interdigitate with the triacylglycerol species of carbon numbers 46-48. The TMS and t-BDMS ethers of diacylglycerols and ceramides emerged clearly ahead of the leading triacylglycerol of the 42 acyl carbon series. However, the diacylglycerol and ceramide species overlapped and interdigitated extensively with each other. Due to low polarity below 280 degrees C, the polarizable-liquid phase was not suitable for the resolution of the molecular species of monoacylglycerols and free fatty acids, which were separated on the basis of carbon number only. Nevertheless, capillary GLC on the polarizable liquid phases provides a novel and informative profiling of plasma lipids, the application of which to the assay of plasma lipid abnormalities deserves further examination.