The prediction of fatty acid structure from selected ions in electron impact mass spectra of fatty acid methyl esters

Parallel Operation of Electron Ionization and Chemical Ionization for GC-MS Using a Single TOF Mass Analyzer

This work describes a novel mass spectrometer coupled to gas chromatography (GC-MS) that simultaneously displays the mass spectral information of electron (EI)- and chemical ionization (CI)-generated ion populations for a single chromatographic peak. After GC separation, the eluent is equally split and supplied in parallel to an EI and a novel CI source, both operating continuously. Precise switching of the ion optics provides the exact timing to consecutively extract the respective ion population from both sources and transfer them into a time-of-flight (TOF) mass analyzer. This technique enables the acquisition of complementary information from both ion populations (EI and CI) within a single chromatographic run and with sufficient data points to retain the chromatographic fidelity. The carefully designed GC transfer setup, fast ion optical switching, and synchronized TOF data acquisition system provide an automatic and straightforward spectral alignment of two ion populations. With an eluent split ratio of about 50% between the two ion sources, instrument detection limits of <40 fg on the column (octafluoronaphthalene) for the EI and <2 pg (benzophenone) for the CI source were obtained. The system performance and the additional analytical value for compound identification are demonstrated by means of different common GC standard mixtures and a commercial perfume sample of unknown composition.

Deposition and metabolism of dietary n-3 very-long-chain PUFA in different organs of rat, mouse and Atlantic salmon

There is limited knowledge about the metabolism and function of n-3 very-long-chain PUFA (n-3 VLC-PUFA) with chain lengths ≥ 24. They are known to be produced endogenously in certain tissues from EPA and DHA and not considered to originate directly from dietary sources. The aim of this study was to investigate whether n-3 VLC-PUFA from dietary sources are bio-available and deposited in tissues of rat, fish and mouse. Rats were fed diets supplemented with a natural fish oil (FO) as a source of low dietary levels of n-3 VLC-PUFA, while Atlantic salmon and mice were fed higher dietary levels of n-3 VLC-PUFA from a FO concentrate. In all experiments, n-3 VLC-PUFA incorporation in organs was investigated. We found that natural FO, due to its high EPA content, to a limited extent increased endogenous production of n-3 VLC-PUFA in brain and eye of mice with neglectable amounts of n-3 VLC-PUFA originating from diet. When higher dietary levels were given in the form of concentrate, these fatty acids were bio-available and deposited in both phospholipids and TAG fractions of all tissues studied, including skin, eye, brain, testis, liver and heart, and their distribution appeared to be tissue-dependent, but not species-specific. When dietary EPA and DHA were balanced and n-3 VLC-PUFA increased, the major n-3 VLC-PUFA from the concentrate increased significantly in the organs studied, showing that these fatty acids can be provided through diet and thereby provide a tool for functional studies of these VLC-PUFA.

Analysis of Bio-Based Fatty Esters PCM’s Thermal Properties and Investigation of Trends in Relation to Chemical Structures

As global energy demand increases while primary sources and fossil fuels’ availability decrease, research has shifted its focus to thermal energy storage systems as alternative technologies able to cover for the mismatch between demand and supply. Among the different phase change materials available, esters possess particularly favorable properties with reported high enthalpies of fusion, low corrosivity, low toxicity, low supercooling, thermal and chemical stability as well as biodegradability and being derived from renewable feedstock. Despite such advantages, little to no data on the thermal behavior of esters is available due to low commercial availability. This study constitutes a continuation of previous works from the authors on the investigation of fatty esters as novel phase change materials. Here, methyl, pentyl and decyl esters of arachidic acid, and pentyl esters of myristic, palmitic, stearic and behenic acid are synthesized through Fischer esterification with high purities and their properties are studied. The chemical structures and purities are confirmed through Attenuated Total Reflectance Infrared Spectroscopy, Gas Chromatography coupled with Mass Spectroscopy and Nuclear Magnetic Resonance Spectroscopy, while the determination of the thermal properties is performed through Differential Scanning Calorimetry and Thermogravimetric Analysis. In conclusion, some correlations between the melting temperatures and the chemical structures are discovered, and the fatty esters are assessed based on their suitability as phase change materials for latent heat storage applications.

Effects of oil pollution and persistent organic pollutants (POPs) on glycerophospholipids in liver and brain of male Atlantic cod (Gadus morhua)

Fish in the North Sea are exposed to relatively high levels of halogenated compounds in addition to the pollutants released by oil production activities. In this study male Atlantic cod (Gadus morhua) were orally exposed to environmental realistic levels (low and high) of weathered crude oil and/or a mixture of POPs for 4weeks. Lipid composition in brain and in liver extracts were analysed in order to assess the effects of the various pollutants on membrane lipid composition and fatty acid profiles. Transcriptional effects in the liver were studied by microarray and quantitative real-time RT-PCR. Chemical analyses confirmed uptake of polychlorinated biphenyls (PCBs) and chlorinated pesticides, polybrominated diphenyl ethers (PBDEs) and perfluorooctanesulfonate (PFOS) in the liver and excretion of metabolites of polyaromatic hydrocarbons (PAHs) in the bile. Treatment with POPs and/or crude oil did not induce significant changes in lipid composition in cod liver. Only a few minor changes were observed in the fatty acid profile of the brain and the lipid classes in the liver. The hypothesis that pollution from oil or POPs at environmental realistic levels alters the lipid composition in marine fish was therefore not confirmed in this study. However, the transcriptional data suggest that the fish were affected by the treatment at the mRNA level. This study suggests that a combination of oil and POPs induce the CYP1a detoxification system and gives an increase in the metabolism and clearing rate of PAHs and POPs, but with no effects on membrane lipids in male Atlantic cod.

Gas chromatography-mass spectrometry-based profiling of serum fatty acids in acetaminophen-induced liver injured rats

In this study, we have developed and validated a simple, accurate and sensitive gas chromatography-mass spectrometry (GC-MS) method for simultaneous quantification of 18 fatty acids in rat serum, including both non-esterified (NEFA) and esterified (EFA) fatty acids, and subsequent analysis of fatty acid metabolic profiles. This novel method was used to evaluate the serum levels of fatty acids from vehicle- and acetaminophen (APAP)-treated rats. Serum levels of 7 NEFAs and 14 EFAs were significantly higher in APAP-treated rats 24 h after APAP administration at 1500 mg kg⁻¹ when compared with vehicle-treated controls. Control and APAP-treated rats could be differentiated based on their metabolic profiles using two different chemometric analysis methods: principle component analysis (PCA) and partial least squares-discriminant analysis (PLS-DA). More importantly, we identified the following NEFAs as potential biomarkers of APAP-induced liver injury: oleic acid (C18:1n9), linoleic acid (C18:2n6), docosahexaenoic acid (C22:6n3) and arachidonic acid (C20:4n6). The serum concentrations of C18:1n9, C18:2n6 and C22:6n3 were all positively correlated (r > 0.8; Pearson's correlation analysis) with the activities of alanine aminotransferase (ALT) and aspartate aminotransferase (AST). These results suggest that a novel targeted metabolomics method based on the metabolic profiling of fatty acids analyzed by GC-MS provides exact serum concentrations of fatty acids as well as a prospective methodology to evaluate chemically induced hepatotoxicity.

Trans fatty acid analyses in samples of marine origin: the risk of false positives

At conditions commonly applied for trans fatty analyses by gas chromatography, fatty acids naturally occurring in marine lipids may overlap chromatographically with C16 and C18 trans fatty acids and lead to false positives. Elution patterns were studied by tracking retention indices at shifting temperature conditions on two cyanopropyl-coated capillary columns. Most overlaps can be avoided by selecting the right chromatographic conditions, but it was not possible to find a single condition that eliminates the risk of overlap between trans fatty acids and interferents. In total, 17 compounds were identified as potential interferents, and the amounts of these compounds were quantified in various samples of marine origin. The interferents that will most likely contribute to incorrect assessments of trans fatty acids in marine lipids are probably 18:3 n-4 and 18:1 n-11.

Compatibility of electron ionization and soft ionization methods in gas chromatography/orthogonal time-of-flight mass spectrometry

Orthogonal acceleration time-of-flight (oa-TOF) mass spectrometry (MS) was coupled to gas chromatography (GC) to measure ion yields (ratio of ion counts to number of neutrals entering the ion source) and signal-to-noise (S/N) in the electron ionization (EI) mode (hard ionization) as well as in the soft ionization modes of chemical ionization (CI), electron capture negative ion chemical ionization (NICI) and field ionization (FI). Mass accuracies of the EI and FI modes were also investigated. Sixteen structurally diverse volatile organic compounds were chosen for this study. The oa-TOF mass analyzer is highly suited for FI MS and provided an opportunity to compare the sensitivity of this ionization method to the more conventional ionization methods. Compared to the widely used quadrupole mass filter, the oa-TOF platform offers significantly greater mass accuracy and therefore the possibility of determining the empirical formula of analytes. The findings of this study showed that, for the instrument used, EI generated the most ions with the exception of compounds able to form negative ions readily. Lower ion yields in the FI mode were generally observed but the chromatograms displayed greater S/N and in many cases gave spectra dominated by a molecular ion. Ion counts in CI are limited by the very small apertures required to maintain sufficiently high pressures in the ionization chamber. Mass accuracy for molecular and fragment ions was attainable at close to manufacturer's specifications, thus providing useful information on molecular ions and neutral losses. The data presented also suggests a potentially useful instrumental combination would result if EI and FI spectra could be collected simultaneously or in alternate scans during GC/MS.

Comparison of three GC/MS methodologies for the analysis of fatty acids in Sinorhizobium meliloti: development of a micro-scale, one-vial method

Three protocols for fatty acid analysis in Sinorhizobium meliloti were improved by the addition of a number of standards/controls and a silylation step which allowed the determination of recoveries, extents of conversion of lipids to fatty acid methyl esters (FAMEs) and extents of side reactions. Basic hydrolysis followed by acid-catalyzed methylation and transmethylation with sodium methoxide, were the best for the analysis of 3-hydroxy- and cyclopropane fatty acids, respectively. A micro-scale, one-vial method that employed sodium methoxide/methanol was equally efficient and on a 1000-fold smaller scale than standard methods. Because this method avoids aqueous extractions, 3-hydroxybutanoic acid was detected as its trimethylsilyloxy methyl ester along with FAMEs.

Prediction of equivalent chain lengths from two-dimensional fatty acid retention indices

A recently introduced two-dimensional fatty acid retention index system (2D-FARI) was used as basis for prediction of equivalent chain lengths (ECL) of fatty acid methyl esters (FAME) on a BPX-70 stationary phase. Models for the relationship between 2D-FARI data and ECL values of a calibration sample with 30 common fatty acids were established by a simple multivariate regression. The models were thereafter applied on 2D-FARI data for other FAMEs and used to predict the ECLs for these compounds. The 2D-FARI values for the fatty acids in the calibration sample are given by definition. Thus, the only information necessary to calculate the ECL value for a compound run under identical conditions as the calibration sample is the 2D-FARI values for the compound, which can be acquired from literature data. The method was validated with test sets analysed with different temperature and flow programs. ECLs of various marine FAME and trans isomers of Eicosapentaenoic and Docosahexaenoic acid were predicted with root mean squared error of prediction from 0.002 to 0.012 ECL units.

Prediction of gas chromatographic retention of polyunsaturated fatty acid methyl esters

Multivariate regression models were applied to predict retention indices as equivalent chain lengths (ECL) for methylene-interrupted polyunsaturated fatty acids. Simple molecular descriptors, the chain length, the number of double bonds and the position of the double bond system, were used as predictors. The merits of different variable combinations were evaluated. For general models, it was necessary to include the distance from the double bond system to both the carbonyl group (Delta-position) and the methyl end of the fatty acid (n-position). The best accuracy was found for models including higher order terms of Delta and n. For models restricted to n-3 and n-6 isomers, it was not necessary to include the n-position among the variables. The highest residuals for the most accurate models were below 0.06 ECL units, and root mean square error of prediction was below 0.030. The ECL data was achieved by three different temperature programs on a cyanopropyl column.

Properties of trans isomers of eicosapentaenoic acid and docosahexaenoic acid methyl esters on cyanopropyl stationary phases

The trans isomers of 5,8,11,14,17-eicosapentaenoic acid (EPA) and 4,7,10,13,16,19-docosahexaenoic acid (DHA) methyl esters were prepared by isomerisation with paratoluenesulfinic acid (PTSA) in dioxane. The isomers were fractionated by silver ion liquid chromatography with baseline resolution between the isomers with different number of trans double bonds. The fractions were analysed by GC-MS and the gas chromatographic properties of the EPA and DHA isomers with one and two trans double bonds were investigated on BPX-70 and SP-2560 cyanopropyl stationary phases. Different temperature and pressure programs were applied to introduce variations in retention indices of the isomers. The retention indices of all the trans isomers showed a strong linear correlation to the retention indices of the equivalent all-cis isomer, but the slopes for corresponding linear regression lines varied with the number of trans double bonds in the molecule. The regression lines were used to predict optimal conditions for the separation of trans isomers from the corresponding all-cis isomers. For DHA on BPX-70, and for EPA on both columns, it was possible to find windows where isomers with one trans double bond can be resolved from the corresponding all-cis isomers with R(s) > 1.0. In general, BPX-70 seems to have a more suitable selectivity for the analysis of these isomers than SP-2560. Two-dimensional fatty acid retention indices (2D-FARI) were found to be suitable for identification of trans geometry in polyunsaturated fatty acids (PUFA). Although there were substantial overlaps in the range of retention times between the all-cis isomers and isomers with one and two trans double bonds, 2D-FARI separated the isomers into distinct groups according to the number of trans double bonds.

Identification of fatty acids in fishes collected from the Ohio River using gas chromatography-mass spectrometry in chemical ionization and electron impact modes

Analyses of fatty acid (FA) composition in freshwater fishes promote understanding of the potential relationship between fish health or human nutrition and specific FAs. Therefore, the chemical identity of FAs in endemic fishes must be established. Paddlefish, sauger, and white bass were collected from the Ohio River. The structural identification of esterified FAs from fish-fillet lipids was conducted using gas chromatography-mass spectrometry (GC-MS). The same 13 FAs, composing more than 90% of the mass of FAs extracted by techniques used in this research, were found in all three species examined. Carbon chain length and degree and position of unsaturation were determined from the characteristic ionization and fragmentation of FA methyl esters (FAMEs) resulting from GC-MS electron impact (EI) and chemical ionization (CI) modes. Assignment of structure to the extracted FAs required complementary interpretation of both EI and CI MS. The EI spectra observed substantiate findings reported in the literature. The novelty of this research is in the thorough interpretation of CI spectra for which less data are available. The observations reported for analyses of fishes will be useful to all researchers studying FAs regardless of sample media.

Two-dimensional fatty acid retention indices

A two-dimensional retention index system for fatty acid methyl esters (2D-FARI) is proposed. The system is based on the application of different temperature and pressure programs on a single capillary column. A calibration sample is analysed and the retention data is calibrated against a set of reference 2D-FARI values. The calibration models are then applied to predict the 2D-FARI values for compounds that are not present in the calibration sample. The two dimensions in the retention index system lead to increased selectivity and a reduced risk of retention index overlap between different compounds. The 2D-FARI system is also more robust towards differences in stationary phase properties than ordinary retention indices and is therefore convenient for comparison of retention data acquired on different columns, or at different times at the same column.