Gas chromatography of fatty acids

Gas–liquid chromatographic method for analysing complex mixtures of fatty acids including conjugated linoleic acids (cis9trans11 and trans10cis12 isomers) and long-chain (n-3 or n-6) polyunsaturated fatty acids

The optimisation and validation of a gas-liquid chromatographic (GLC) method using direct saponification with KOH/methanol followed by a derivatization with (trimethylsilyl)diazomethane was carried out trying to overcome all the difficulties posed by the analysis of complex mixtures of fatty acids (FAs) in animal fat tissues. The presented method allowed sensitive, selective and simultaneous determination of a wide range of different FAs, including short-chain FAs, branched-chain FAs and conjugated linoleic acid isomers in the same GLC run along with other well known saturated, monounsaturated and polyunsaturated FAs. To demonstrate the feasibility of the procedure, the total FA profile of beef meat was characterised.

Analysis of the fatty acid from Bupleurum Chinense DC in China by GC-MS and GC-FID

Fatty acid of the root of the medicinal plant of Bupleurum Chinense DC in China has been investigated by gas chromatography combined with mass spectroscopy. After methyl-esterification, eight fatty acid compositions were identified by GC-MS. A simple and rapid determination of the fatty acid has been firstly developed by GC-FID. The derivatization condition was investigated in order to validate this method. Palmitic acid, palmitoleic acid, oleic acid and linoleic acid were analyzed simultaneously by internal standard method. The validity of method has been examined both experimentally with good recovery, intra-assay precisions and linearity. The quick and accurate method of capillary gas chromatography could be used for the analysis of the fatty acid from Bupleurum Chinense DC.

Chlorotrimethylsilane, a reagent for the direct quantitative analysis of fats and oils present in vegetable and meat samples

Acylglycerides present in oil seeds and meat can be transformed into volatile fatty esters using chlorotrimethylsilane (CTMS) and 1-pentanol as reagents. The volatile esters can then be analysed by GC. The method is quantitative and involves only minor sample manipulation. It often permits major recoveries of the total saponifiable lipids present in solid samples. A 40 min reaction time is enough to ensure the total conversion of saponifiable lipids to the corresponding FAPEs.

Comparison between high-performance liquid chromatography and gas chromatography methods for fatty acid identification and quantification in potato crisps

A reversed-phase high performance liquid chromatographic (RP-HLPC) method was compared with a gas chromatography-flame ionization detection (GC-FID) method for determining fatty acids in potato crisps. Different extraction procedures were used. Fatty acids were quantified by linear regression. Both methods presented good precision (R.S.D. < or = 5.88%) and recovery (> or = 82.31%). The precision using HPLC method was slightly better than for GC-FID method. There was good agreement between the fatty acid composition of potato crisps analysed by both methods. For most purposes the HPLC method would be better. However, when more fatty acids need to be analysed, GC is a more suitable method.

Analysis of FA contents in individual lipid fractions from human placental tissue

Data on FA contents in the human placenta are limited. Different methods have been used for the FA analysis, and only percentage results have been presented. We developed and evaluated a method for the determination of FA concentrations in placental tissue. Lipids were extracted from placental tissue with a chloroform/methanol mixture; and phospholipids (PL), nonesterified FA (NEFA), TG, and cholesterol esters (CE) were isolated by TLC. Individual lipid fractions were derivatized with methanolic hydrochloric acid, and the FAME were quantified by GC with FID. The CV of intra-assay (n = 8) of absolute concentrations were evaluated for FA showing a tissue content > 0.01 mg/g. CV ranges were 4.6-11.0% for PL, 6.4-9.3% for NEFA, 6.1-8.9% for TG, and 11.4-16.3% for CE. The relative FA composition across a term placenta indicated no differences between samples of central and peripheral locations of maternal and fetal site (CV 0.5-9.9%), whereas the absolute FA concentrations were only reproducible in the PL fraction (CV 7.0-12.8%). The method shows a reasonably high precision that is well suited for physiological and nutritional studies.

Determination of C4-C14 carboxylic acids by capillary zone electrophoresis. Application to the identification of diamide degradation products and partitioning studies

Capillary zone electrophoresis (CZE) was investigated for the determination of linear saturated carboxylic acid homologues ranging from C4 to C14. Separation conditions were optimised to overcome the problems of decreasing solubility and decreasing selectivity between successive homologues with increasing chain length. Separations were performed at 20 degrees C, using a 20 kV separation voltage and a pH 8 electrolyte containing 30% methanol. A suitable chromophore (4-aminobenzoate) was added to ensure indirect UV detection of the analytes. Calibration curves and repeatability were established. Minimum detectable concentrations of 3 x 10(-6) mol l(-1) were achieved. Resolution between successive homologues was better than 2. The electrophoretic mobility of each homologue (n=7-14) was assessed and a quasi-linear relationship between the mobility value and 1/n was observed. The quantitative analysis of a diamide degradation solution was performed and compared to potentiometric results. The CZE method was also applied to the determination of C7-C14 partitioning between an organic medium containing tributylphosphate in n-dodecane and different basic solutions. Their behaviour was established according to the chain length and the pH of the aqueous phase. For C10-C14 compounds, results were validated by comparison with gas chromatographic analysis of the organic phases.

Indirect detection of organic acids in non-aqueous capillary electrophoresis

Non-aqueous capillary electrophoresis (NACE) with indirect detection has been applied to the determination of fatty acids (FAs) and ascorbic acid (AA), respectively. C2-C18 FAs have been separated in less than 12 min using 8-hydroxy-7-iodoquinoline sulfonic acid as chromophores in NACE with indirect absorbance. The dissociation constant (pKa) values of C8-C18 FAs obtained from the slope of the linear plot -log [(mu 0/mu)-1] vs. pH, using 20% isopropanol and 40% acetonitrile as the organic modifier in NACE, are all above about two units than those obtained in aqueous solution. NACE with indirect laser-induced fluorescence, using merocyanine 540 (MC540) as fluorophores, has been performed to the analysis of AA and its stereoisomer, isoascorbic acid (IAA), and the limits of detection of AA and IAA are 0.30 microM and 0.17 microM, respectively. This method has been applied to the determination of AA in a lemon juice spiked with IAA as the internal standard in less than 3 min and its concentration is 76.7 +/- 0.4 mM.

Simultaneous determination of cyclohexene oxide and its metabolites in rat plasma and urine by gas chromatography

An assay was developed for the simultaneous measurement of cyclohexene oxide and its metabolites (cyclohexanol, trans-cyclohexane-1,2-diol, cyclohexane-1,2-diol-O-glucuronide, and N-acetyl-S-(2-hydroxycyclohexyl)-L-cysteine) in rat urine and plasma using gas chromatography. A mixture of ethyl acetate-acetonitrile (70:30) was used as the extracting solvent for both matrices. This liquid-liquid extraction procedure was followed by the separation of cyclohexene oxide and its metabolites on an HP-FFAP fused-silica capillary column. In order to determine the amount of cyclohexane-1,2-diol-O-glucuronide, samples were incubated at 37 degrees C with beta-glucuronidase and the amount of cyclohexane-1,2-diol formed from the reaction determined. The extraction efficiencies of cyclohexene oxide and cyclohexanol were greater than 90% both in urine and plasma. However, recovery from the plasma and urine for trans-cyclohexane-1,2-diol (60-68%) and N-acetyl-S-(2-hydroxycyclohexyl)-L-cysteine (approximately 76%) were considerably less. Long term stability studies showed that urine samples spiked with cyclohexene oxide and trans-cyclohexane-1,2-diol are stable at -20 degrees C for up to 9 weeks. However, plasma samples are only stable for up to 2 weeks under the same conditions. The calibration curves for all analytes were linear over the range of 12.5 to 400 micrograms/ml and correlation coefficients (r2) were greater than 0.990. The limit of detection for cyclohexene oxide, cyclohexanol, and N-acetyl-S-(2-hydroxycyclohexyl)-L-cysteine is 1.56 micrograms/ml, while the limit of detection for trans-cyclohexane-1,2-diol is 3.12 micrograms/ml. This method has been used for the determination of the disposition and metabolism of cyclohexene oxide, and may be applied in environmental monitoring, as well as in microbiological studies for other epoxide materials.

Inhibitors of arterial relaxation among components of human oxidized low-density lipoproteins. Cholesterol derivatives oxidized in position 7 are potent inhibitors of endothelium-dependent relaxation

BACKGROUND

Oxidized low-density lipoproteins (LDLs) are known to impair arterial relaxation. The aim of the present study was to identify the components of oxidized LDL that may account for inhibition of endothelium-dependent relaxation.

METHODS AND RESULTS

LDLs from 12 healthy subjects were either maintained at 4 degrees C (native LDL) or incubated at 37 degrees C in the presence of copper sulfate (oxidized LDL). Unlike pretreatment with native LDL, pretreatment with oxidized LDL reduced significantly the acetylcholine-mediated relaxation of rabbit aortic segments compared with control segments incubated in Krebs' buffer (maximal relaxation [Emax], 72.0 +/- 6.7% versus 94.1 +/- 0.8%, respectively, P < .01; negative logarithm of the concentration required to produce a half-maximal relaxing effect [pD2], 6.6 +/- 0.1 versus 7.2 +/- 0.1, respectively, P < .001). The absolute difference between Emax values obtained with oxidized and native LDL (delta Emax) correlated significantly with the formation of 7 ketocholesterol, 7 alpha-hydroxycholesterol, and 7 beta-hydroxy-cholesterol. In contrast, delta Emax did not correlate with the amount of lipoperoxides or lysophosphatidylcholine formed, and the difference of pD2 values measured with oxidized and native LDL (delta pD2) did not correlate significantly with any of the oxidation-derived LDL compounds. When added individually, 7-ketocholesterol and 7 beta-hydroxycholesterol reduced Emax values but not pD2 values in a time- and concentration-dependent manner.

CONCLUSIONS

Cholesterol derivatives in oxidized LDL can reduce maximal arterial relaxation through a specific effect on vascular endothelial cells.

In vitro oxidation of i.v. lipid emulsions in different all-in-one admixture bags assessed by an iodometric assay and gas-liquid chromatography

Polyunsaturated fatty acids (FAs) of intravenous (IV) lipid emulsions can peroxidize to potentially harmful lipid hydroperoxides. In order to assess in vitro peroxidation of IV fat emulsions in all-in-one (AIO) admixture bags, an iodometric titration to determine lipid hydroperoxide content expressed by the peroxide value (PV) and a gas-liquid chromatographic (GLC) assay to determine changes of the FA pattern were established. A long-chain triglyceride (LCT) and medium-chain triglyceride-LCT emulsion were compared for the PV and the pH during storage at room temperature and daylight in AIO bags made of ethylvinylacetate (EVA) and polypropylene:polyamide 7:3 (V90). In contrast to storage in glass bottles, significant peroxidation was detected in both emulsions with 0.5-3.4 mmol peroxides/L after 28 d (150 times the control PV). A pH drop of at least 0.3 (EVA) and 1.2 (V90) units was measured. Initial PVs and peroxidation kinetics of the emulsions were different; V90 material showed better barrier properties against oxygen. PV was increased by higher temperature and light exposure. The FA pattern of an LCT emulsion with a PV > 6 (storage: 40 degrees C in a dark room for 28 d in AIO bags) assayed by GLC remained unchanged. The iodometric peroxide and the GLC assay were reproducible and easy to handle. Only the iodometric method was sensitive enough to detect peroxidation effects (detection limit: 0.02 mmol peroxides/L). IV fat emulsions can be checked for lipid hydroperoxide content with the rapid iodometric assay to guarantee optimal quality of IV lipids used for AIO admixtures. To prevent peroxidation, lipids in AIO bags should be stored light-protected in a refrigerator an oxygen-tight overwrap is mandatory for extended periods.

Influence of the derivatization procedure on the results of the gaschromatographic fatty acid analysis of human milk and infant formulae

Many different analytical procedures for fatty acid analysis of infant formulae and human milk are described. The objective was to study possible pitfalls in the use of different acid-catalyzed procedures compared to a base-catalyzed procedure based on sodium-methoxide in methanol. The influence of the different methods on the relative fatty acid composition (wt% of total fatty acids) and the total fatty acid recovery rate (expressed as % of total lipids) was studied in two experimental LCP-containing formulae and a human milk sample. MeOH/HCl-procedures were found to result in an incomplete transesterification of triglycerides, if an additional nonpolar solvent like toluene or hexane is not added and a water-free preparation is not guaranteed. In infant formulae the low transesterification of triglycerides (up to only 37%) could result in an 100%-overestimation of the relative amount of LCP, if these fatty acids primarily derive from phospholipids. This is the case in infant formulae containing egg lipids as raw materials. In formula containing fish oils and in human milk the efficacy of esterification results in incorrect absolute amounts of fatty acids, but has no remarkable effect on the relative fatty acid distribution. This is due to the fact that in these samples LCP are primarily bound to triglycerides. Furthermore, in formulae based on butterfat the derivatization procedure should be designed in such a way that losses of short-chain fatty acids due to evaporation steps can be avoided. The procedure based on sodium methoxide was found to result in a satisfactory (about 90%) conversion of formula lipids and a reliable content of all individual fatty acids. Due to a possibly high amount of free fatty acids in human milk, which are not methylated by sodium-methoxide, caution is expressed about the use of this reagent for fatty acid analysis of mothers milk. It is concluded that accurate fatty acid analysis of infant formulae and human milk requires a careful and quantitative derivatization of both polar and nonpolar lipid classes. Sodium methoxide seems to be a reliable and time-saving method for routine fatty acid analysis of infant formulae, which should be validated by interlaboratory comparison. Anhydrous procedures based on methanolic hydrogen chloride including an additional nonpolar solvent are also suitable for infant formulae but seem to be preferable for human milk samples.

Stratification and age-related differences in blubber fatty acids of the male harbour porpoise (Phocoena phocoena)

Fatty acid composition of blubber was determined at four body sites of 19 male harbour porpoises. A total of 65 fatty acids were quantified in each sample. The array of fatty acids contained in harbour porpoise blubber was similar to those found in other marine mammals. While chemical composition of total blubber was uniform over the body, with the exception of the caudal peduncle, vertical stratification was evident between the deep (inner) and superficial (outer) blubber layers. Fatty acids with chain lengths shorter than 18 carbons were present in significantly greater amounts in the outer blubber layer, while the longer-chain unsaturated fatty acids were more prevalent in the inner layer. This distribution suggests that the inner blubber layer is more active metabolically than the outer layer in terms of lipid deposition and mobilization. The degree of stratification between the two layers appears to increase with age, indicating a predictable turnover in the blubber layer of male porpoises. Harbour porpoise blubber contained high levels (2-27%) of isovaleric acid in the outer blubber layer, and these levels were positively correlated with age.

Gas chromatographic analysis of fatty acid methyl esters

The full process of fatty acid methyl ester (FAME) analysis consists of esterification of lipids, and of injection, separation, identification and quantitation of the FAMEs. In order for the required accuracy and precision to be attained, each of these steps has to be optimized. Esterification of lipids can be carried out with several reagents based on acid-catalysed or base-catalysed reactions. The advantages and disadvantages of these reagents are discussed. The most critical step in the gas chromatographic analysis of FAMEs is sample introduction. The classical split injection technique, which is the most widely used technique in the analysis of FAMEs, has the potential disadvantage of boiling-point-dependent sample discrimination. Cold injection of the sample, either on-column or by programmed-temperature vaporization, does not present this problem and should therefore be preferred. Modern, commercially available fused-silica capillary columns offer excellent separation of FAMEs from biological samples. Very polar stationary phases give excellent separation of all FAMEs but have relatively low thermal stability, resulting in long retention times. Non-polar phases have a much greater thermal stability but inferior selectivity. For many analyses, phases of intermediate polarity, which combine the advantages of a relatively high resolution capability with relatively high thermal stability, are the most suitable. FAMEs can be identified by comparison of their retention times with those of individual purified standards or secondary standards based on lipids that have been well characterized in literature. Relative retention times and equivalent chain-length values also provide useful information. FAMEs can be quantitated by peak areas via calibration factors, and absolute concentrations can be determined by adding an internal standard. Among numerous applications in biomedical research, the analysis of fatty acids from body tissues may contribute to the understanding of the link between the dietary intake of fatty acids and the diseases with which these acids are associated.

General strategies in chromatographic analysis of lipids

Lipid extracts of natural sources contain a large number of lipid classes and molecular species. Completely reproducible samples are obtained only with great care and skill. Analytical methods other than chromatography and/or mass spectrometry are of little use for resolution and identification of lipid molecules even in simple mixtures. The analytical information desired governs the selection of the chromatographic and mass spectrometric method, which determine the sample preparation and derivative needed. Usually a combination of chromatographic methods is necessary to identify specific species of lipids. The recent development of soft ionization techniques, that are readily interfaced with mass spectrometers, have greatly simplified the sample preparation and have largely eliminated the need for derivatization. Because these techniques require expensive equipment and dedicated operators, the methods selected must be consistent with the true analytical needs and the available resources. Although personal preference cannot be eliminated entirely, the general strategies outlined below should help to reduce the number of possibilities facing a lipid analyst to a few practical choices.

Fatty acid profiles of lipid samples

Most lipids are best characterized by their fatty acids which differ in chain length, the degree of unsaturation, configuration and position of the double bonds, and the presence of other functionalities. The fatty acid profiles are currently most frequently determined by capillary gas chromatography of the methyl esters which are prepared by a variety of methods. These are discussed with an emphasis on more recent developments, along with the stationary phases used for the separations and the methods employed for identification of the fatty acids. HPLC is applied less frequently for ascertaining fatty acid profiles than GC, but a very large number of derivatives for ultraviolet and fluorescence detection have been proposed. This method continues to evoke increasing interest, particularly in conjunction with fluorescence detection. This technique enables attainment of greater sensitivities than with standard GC methods employing flame ionization detection. Extensive applications of it to the analysis of free fatty acids in blood and other biomedical samples are clearly discernible. Other methods, including supercritical fluid chromatography, have found only limited application for fatty acid profiling.

Determination of plasma non-esterified fatty acids and triglyceride fatty acids by gas chromatography of their methyl esters after isolation by column chromatography on silica gel

Non-esterified fatty acids (NEFA) and triglycerides were isolated from human plasma by column chromatography on silica gel. Eight principal fatty acids of each of these lipid classes were determined by gas chromatography of their methyl ester derivatives and quantified relative to multipoint standard curves. Within-day relative standard deviations for plasma non-esterified fatty acid and triglyceride fatty acid determinations were 2.4 and 3.2%, respectively. Day-to-day relative standard deviations for plasma non-esterified fatty acid and triglyceride fatty acid determinations were 1.4 and 1.1%, respectively. The total plasma concentration and the relative proportions of the eight non-esterified fatty acids determined by this method were significantly different from results obtained according to two generally accepted methods for direct plasma non-esterified fatty acid determination without a specific isolation step. These comparisons suggested that considerable fatty acid ester lipid hydrolysis occurred during these direct determination procedures, and that this hydrolysis resulted in 3-fold overestimation of plasma NEFA content by those methods. Measured levels of arachidonic acid are substantially overestimated by these direct determination methods in which non-esterified fatty acids are not isolated before derivatization.