Recent advances in the high performance liquid chromatography of lipids

Regio- and Stereospecific Analysis of Triacylglycerols—A Brief Overview of the Challenges and the Achievements

The efforts to reveal, in detail, the molecular and intramolecular structures of one of the main lipid classes, namely, triacyl-sn-glycerols, which are now known to affect their specific and important role in all living organisms, are briefly overviewed. Some milestones of significance in the gradual but continuous development and improvement of the analytical methodology to identify the triacylglycerol regio- and stereoisomers in complex lipid samples are traced throughout the years: the use of chromatography based on different separation principles; the improvements in the chromatographic technique; the development and use of different detection techniques; the attempts to simplify and automatize the analysis without losing the accuracy of identification. The spectacular recent achievements of two- and multidimensional methods used as tools in lipidomics are presented.

Quantitation of residual valproic acid in flu vaccine drug substance

Quantitative measurement of process-related impurities is a critical safety requirement for the production of drug substances of vaccine and therapeutic biologics. A simple and sensitive HPLC method has been developed for separation and quantitation of residual valproic acid (VPA) used in the cell transfection procedure for the manufacturing of an influenza vaccine. The method is comprised of a modified Dole liquid phase extraction followed by a quick pre-column derivatization using 2-bromoacetophenone. Nonanoic acid (NNA) is used as the internal standard (IS) and the quantification is performed by reversed-phase liquid chromatography. This new method can accurately measure as low as 6.8 μg/mL (LOQ) residual VPA in the vaccine drug substance.

Analysis of Nitraria Tangutourum Bobr-Derived Fatty Acids with HPLC-FLD-Coupled Online Mass Spectrometry

Fatty acids (FAs) are basic components in plants. The pharmacological significance of FAs has attracted attentions of nutritionists and pharmaceutists. Sensitive and accurate detection of FAs is of great importance. In the present study, a pre-column derivatization and online mass spectrometry-based qualitative and quantitative analysis of FAs was developed. Nineteen main FAs were derivatized by 2-(7-methyl-1H-pyrazolo-[3,4-b]quinoline-1-yl)ethyl-4-methyl benzenesulfonate (NMP) and separated on reversed-phase Hypersil BDS C8 column with gradient elution. All FAs showed excellent linear responses with correlation coefficients more than 0.9996. The method obtained LOQs between 0.93 ng/mL and 5.64 ng/mL. FA derivatives were identified by both retention time and protonated molecular ion corresponding to m/z [M + H]⁺. A comparative study based on FA contents in peel and pulp, seeds and leaves of Nitraria tangutourum Bobr (NTB) from different geographical origins was performed with the established method. Results indicated that NTB were rich in FAs, and the types and contents of FAs varied among tissues. On the other hand, the same tissue of NTB from different geographical areas differed in the content, but not in type, of FAs.

Authentication of vegetable oils by chromatographic techniques

Food authentication has been evolving continually to situations that were basically governed by a global market trend. Analytical techniques have been developed or modified to give plausible solutions to the devious adulterations at each moment. Classical tests have largely been replaced with newer technical procedures, most of which are based on gas chromatography, with some being based on high-performance liquid chromatography. Determination of trans-fatty acid and sterolic composition, together with sterol-dehydration products, have been used most frequently used to detect contamination and adulteration. Sophisticated new adulterations, e.g., olive oil with hazelnut oil, represent a new challenge for the next millennium, although suggestive proposals for detecting these kinds of adulterations are emerging with the contribution of databases and mathematical algorithms.

Analytical monitoring of the production of biodiesel by high-performance liquid chromatography with various detection methods

Gradient elution reversed-phase high-performance liquid chromatography (RP-HPLC) was used for the determination of compounds occurring during the production of biodiesel from rapeseed oil. Individual triacylglycerols (TGs), diacylglycerols, monoacylglycerols and methyl esters of oleic, linoleic and linolenic acids and free fatty acids were separated in 25 min using a combined linear gradient with aqueous-organic and non-aqueous mobile phase steps: 70% acetonitrile+30% water in 0 min, 100% acetonitrile in 10 min, 50% acetonitrile+50% 2-propanol-hexane (5:4, v/v) in 20 min and 5 min final hold-up. Another method with a non-aqueous linear mobile phase gradient [from 100% methanol to 50% methanol+50% 2-propanol-hexane (5:4, v/v) in 15 min] was used for fast monitoring of conversion of rapeseed oil triacylglycerols to fatty acid methyl esters and for quantitation of residual TGs in the final biodiesel product. Sensitivity and linearity of various detection modes (UV detection at 205 nm, evaporative light scattering detection and mass spectrometric detection) were compared. The individual sample compounds were identified using coupled HPLC-atmospheric pressure chemical ionization mass spectrometry in the positive-ion mode.

Fasting increases tissue uptake and interconversion of plasma unesterified linoleic acid in guinea pigs

A large part of the arachidonic acid (20:4 n-6) pools in some extrahepatic tissues can be formed by local interconversion of linoleic acid (18:2 n-6) taken up as free fatty acid (FFA) from blood in both rats and guinea pigs. This study investigates the rate of uptake and interconversion of unesterified 14C-18:2 by different tissues in fasted guinea pigs. The initial half-life of 14C-18:2 in plasma was 5.8 s. The average concentration of plasma FFA was 551.3 nmol ml-1 and of plasma FFA-18:2 was 67.3 nmol ml-1. The total amount of 20:4 formed in the liver was 1.8 +/- 0.3 nmol min-1, which was lower than that in the gastrointestinal tract (3.1 nmol min-1), bone marrow (6.0 nmol min-1) and lung (2.1 nmol min-1). Due to the fast turnover and higher concentration of plasma FFA-18:2 in the fasting state, the retained 18:2 in tissue lipids was 5.8-25.6-fold higher than that in fed guinea pigs [L. Zhou et al. Biochim. Biophys. Acta 1349 (1997) 197-210]. The total delta 6-desaturase products both in liver and in extrahepatic tissues were also increased, 3.8-fold in liver, 7.2-fold in upper small intestine, 6.0-fold in colon, and 6.5-fold in bone marrow. The increased rate of tissue uptake of FFA during fasting is thus linked to an increased local interconversion of plasma FFA-18:2, which is an important source of 20:4 in some extrahepatic tissue in guinea pigs.

Rapid quantitation of free fatty acids in human plasma by high-performance liquid chromatography

We report a rapid and sensitive method for separation and quantitation of free fatty acids (FFAs) in human plasma using high-performance liquid chromatography (HPLC). Two established techniques of lipid extraction were investigated and modified to achieve maximal FFA recovery in a reasonably short time period. A modified Dole extraction method exhibited greater recovery (approximately 90%) and short processing times (30 min) compared to the method of Miles et al. Reversed-phase HPLC using UV detection was used for plasma FFA separation and quantitation. Two phenacyl ester derivatives, phenacyl bromide and p-bromophenacyl bromide, were investigated in order to achieve optimal separation of individual plasma FFAs (saturated and unsaturated) with desirable detection limits. Different chromatographic parameters including column temperature, column type and elution profiles (isocratic and gradient) were tested to achieve optimal separation and recovery of fatty acids. Phenacyl bromide esters of plasma fatty acids were best resolved using an octadecylsilyl column with endcapped silanol groups. An isocratic elution method using acetonitrile-water (83:17) at 2 ml/min with UV detection at 242 nm and a column temperature of 45 degrees C was found to optimally resolve the six major free fatty acids present in human plasma (myristic [14:0], palmitic [16:0], palmitoleic [16:1], stearic [18:0], oleic [18:1] and linoleic [18:2]), with a run time of less than 35 min and detection limits in the nmol range. The entire process including plasma extraction, pre-column derivatization, and HPLC quantitation can be completed in approximately 90 min with plasma samples as small as 50 microl. Over a wide physiological range, plasma FFA concentrations determined using our HPLC method agree closely with measurements using established TLC-GC methods (r2 < or = 0.95). In addition, by measuring [14C] or [3H] radioactivity in eluent fractions following HPLC separation of plasma FFA, this method can also quantitate rates of FFA turnover in vivo in human metabolic studies employing isotopic tracers of one or more fatty acids.

Downstream processing of algal polyunsaturated fatty acids

Little information exists on recovering polyunsaturated fatty acids from microalgae; however, methods for concentration and purification of PUFAs from fish oil have been extensively reported. This review examines recovery and purification of microalgae derived PUFAs, but techniques developed for use with fish oil are also reviewed as being potentially useful for concentration and purification from microalgae. The two main techniques for concentrating and purifying-urea fractionation and high performance liquid chromatography-are discussed in depth and attention is focused on the process developed by the authors for obtaining highly pure PUFA. Other potentially useful techniques, such as supercritical fluid extraction and lipase-catalyzed processing are detailed.

Tissue uptake and interconversion of plasma unesterified 14C linoleic acid in the guinea pig

Part of the arachidonic acid (20:4, n - 6) pools in the gastrointestinal tract and blood forming tissues may be formed by local interconversion of linoleic acid (18:2, n - 6) taken up as a free fatty acid from blood. This study examined the rate of uptake and interconversion of unesterified 14C-18:2 by different tissues in young guinea pigs. The clearance rate of 14C-18:2 was fast, and the initial half-life was 6.3 s. The retention of 14C in tissue lipids was 1.6-1.8% g-1 in the liver, 0.4% g-1 in stomach, 0.7% g-1 in small intestine, 0.2% g-1 in colon, 0.4% g-1 in bone marrow and 0.7% g-1 in spleen. Autoradiographic localization of 3H-18:2 under light microscope demonstrated that most of the 3H radioactivity of the gastrointestinal tract was in the mucosa, in both villus and crypt cells. In bone marrow smears, a high density of silver grains was found in megakaryocytes. The percent of 14C in delta6 desaturase products was higher in gastrointestinal tract, heart, lung, bone marrow and spleen than in liver. The ratio of 14C-20:3/14C-20:4 formation in most tissues was high, and a notable finding being a lower rate of 20:4 formation from plasma free 18:2 in the liver, (170 pmol min-1) than in the gastrointestinal tract (428 pmol min-1) and bone marrow (1203 pmol min-1). The local interconversion of 18:2 into delta6 desaturase products is thus an important source of 20:4 in these organs in guinea pigs.

Quantitative analysis of triglycerides using atmospheric pressure chemical ionization-mass spectrometry

Atmospheric pressure chemical ionization-mass spectrometry (APCI-MS) was used for quantitative analysis of triglycerides (TG) separated by reverse-phase high-performance liquid chromatography. APCI-MS was used for analysis of mono-acid TG standards containing deuterated internal standard, of a synthetic mixture of heterogeneous TG, of randomized and normal soybean oils and of randomized and normal lard samples. Quantitation of the TG by four approaches based on APCI-MS were compared, and these were compared to quantitation obtained using liquid chromatography (LC) with flame-ionization detection (FID). The APCI-MS methods were based on (i) calibration curves from data for mono-acid TG standards, (ii) response factors obtained from a synthetic mixture of TG, (iii) response factors calculated from comparison of randomized samples to their statistically expected compositions, and (iv) response factors calculated from comparison of fatty acid (FA) compositions calculated from TG compositions to FA compositions obtained by calibrated gas chromatography (GC) with FID. Response factors derived from a synthetic mixture were not widely applicable to samples of disparate composition. The TG compositions obtained using APCI-MS data without application of response factors had average relative errors very similar to those obtained using LC-FID. Numerous TG species were identified using LC/APCI-MS which were undetected using LC-FID. Two quantitation methods, based on response factors calculated from randomized samples or on response factors calculated from FA compositions, both gave similar results for all samples. The TG compositions obtained using response factors calculated from FA compositions showed less average relative error than was obtained from LC-FID data, and were in good agreement with predicted compositions for the synthetic mixture and for randomized soybean oil and lard samples.

Metabolism of orally fed [3H]-eicosapentaenoic and [14C]-arachidonic acid in essential fatty acid-deficient rats

The metabolism of individual polyunsaturated fatty acids (PUFA) may be influenced differently by nutritional status and nutritional intake. In normal rats, radioactive arachidonic acid (20:4(n-6), is preferentially retained in tissue phospholipids compared to linoleic (18:2(n-6), or eicosapentaenoic acid (20:5(n-3). This study compares the fate of 20:4(n-6) and 20:5(n-3) acids in essential fatty acid-deficient (EFAD) rats. [3H]-20:5 and [14C]-20:4 were fed in a fish oil emulsion to EFAD rats. Tissue lipids were analysed for radioactivity at 1, 2 and 4 h. The conversion of [3H]-20:5 to docosapentaenoic acid (22:5) and docosahexaenoic acid (22:6) was examined using high performance liquid chromatography (HPLC). The recovery of 3H in small intestine was lower than that of 14C (26 vs. 36% after 4 h, p < 0.001), but was higher in the liver (26 of 3H vs. 22% of 14C, p < 0.01), kidneys (1.5 vs. 1.2%, p < 0.001) and colon (0.3 vs. 0.2%, p = 0.01). The percentages of 3H and 14C in phospholipids were investigated in intestine and liver, and were higher in EFAD rats than in normal rats, particularly for phosphatidylethanolamine (PE). The proportions of [3H]-20:5 transformed to 22:5 and 22:6 did not exceed 7% in the intestine and 10% in the liver. In conclusion, the metabolism of dietary fatty acids 20:4 and 20:5 differed less than in normal rats, mainly due to the preferential retention of both fatty acids in phospholipids.

Lipid oxidation on foods

This review discusses the basic chemical reactions that affect food flavor quality. Although there are many reactions that can lead to the deterioration of quality in foods, this review focuses on lipid oxidation and how it adversely affects flavor principals. It also presents technological advances for studying the basic mechanism of lipid oxidation, for measuring its intensity, and for retaining food quality. The food commodities that provide the subject matter for this review include vegetable oils, legumes, cereal grains, eggs, beef, lamb, poultry, seafoods, and catfish. The methodologies for assessing food quality form a multidisciplinary approach that includes primarily instrumental analysis by direct gas chromatography, chemical analysis by the TBA test, and sensory analysis by quantitative descriptive determinations. The author hopes that the information presented in this review is applicable to food commodities not discussed.

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.

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.

High-performance liquid chromatography of lipids for the identification of human metabolic disease

We describe a system for quantitative lipid analysis employing ternary gradient high-performance liquid chromatography with evaporative light scattering detection. This technique was applied to extracts of cultured fibroblasts, cultured lymphocytes, and leukocytes and to liver and spleen biopsy specimens. Separation of nonpolar lipids, glycolipids, phospholipids, and sphingolipids was achieved in a single run. Detection did not depend on the presence of any specific chemical reactions, uv absorption, or fluorescence. The sensitivity of the technique is well below 200 ng for individual lipids, and many individual lipid classes were detected in samples as small as 1 mg of total protein, the yield of a single flask of cultured skin fibroblasts. The characteristic stored lipids cholesterol ester and sphingomyelin were seen in excess in human fibroblast cultures from patients with Wolman's disease and Niemann-Pick disease, respectively. A biopsy spleen sample from a patient with Gaucher's disease showed a large glucosylceramide peak. This system provides a tool for detecting lipids that accumulate in tissues of patients with currently unidentified metabolic storage disorders.

Assessment of fatty acids in cardiac tissue as 9-anthryldiazomethane esters by high-performance liquid chromatography

A high-performance liquid chromatographic technique for the rapid assessment fatty acids in cardiac tissue is described. A level of 50.4 +/- 14.9 nmol fatty acids per g wet weight of rat myocardial tissue could be monitored. The content of the individual fatty acids C14:0, C16:0, C16:1, C18:0, C18:1, C18:2 and C20:4 amounted to 1.9, 13.5, 0.6, 14.4, 6.1, 6.5 and 7.2 nmol/g wet weight, respectively. A comparison of this method with a well established gas chromatographic technique yielded good agreement. In contrast with time-consuming gas chromatographic techniques, there is no need to isolate (unesterified) fatty acids from the other lipid classes with column chromatography or thin-layer chromatography, because the derivatizing reagent 9-anthryldiazomethane reacts highly specifically with fatty acids.

Automated separation and quantitation of lipid fractions by high-performance liquid chromatography and mass detection

This report describes an improved separation and quantitation of lipid fractions in a total lipid extract by high-performance liquid chromatography using a modified solvent and gradient system delivered by dual pumps and incorporating a mass detector and autosampler. The detector responses for various lipid fractions (cholesteryl esters, triacylglycerols, free cholesterol, and seven major phospholipid classes) were fitted to a quadratic equation, y = ax2 + bx + c, and quantified after detector calibration by a computer. This new system has the advantage of automation and reproducible separation. The present method was applied to rat liver analysis.

The presence of oxidative polymeric materials in encapsulated fish oils

Encapsulated health food oils such as fish oils are readily available in health food stores, pharmacies, and supermarkets. They are popular in the United States as well as in the European countries. However, such oils, because of their high degree of unsaturation, are easily oxidized and form complex mixtures of high molecular weight oxidation products. The present work reports the application of high-performance size exclusion chromatography to the determination of these materials in encapsulated fish oils. Of the six samples studied, five showed from 1-10% of dimeric triacylglycerols and one contained 6.3% trimeric triacylglycerols and 3.1% oligomeric triacylglycerols. Further investigation of this sample with silicic acid chromatography indicated that it contained a total of 36.3% polar material.

High-performance liquid chromatography separation of phospholipid classes and arachidonic acid on cyanopropyl columns

An HPLC method for the separation and analysis of arachidonic acid and eight phospholipid classes is described: phosphatidylglycerol, phosphatidylinositol, cardiolipin, phosphatidylserine, phosphatidylethanolamine, phosphatidylcholine, sphingomyelin, and 2-lysophosphatidylcholine. The separation is carried out at 60 degrees C on 2 cyanopropyl columns using a gradient of acetonitrile and 5 mM sodium acetate (pH 5.0). Cyanopropyl columns require a lower proportion of water in the mobile phase to elute the more polar phospholipids than other types of columns and are thus less prone to equilibration problems. The method is highly reproducible (average coefficient of variation for each retention time less than or equal to 3.5%) and permits analysis of peaks by phosphorus content. Data obtained by analyzing lipid extracts from rat alveolar macrophages prelabeled with [G-3H]-arachidonic acid were analyzed by this HPLC method and compared to standard analysis by TLC. There was a significant correlation between the radioactivity profiles obtained with the two chromatographic methods (HPLC versus TLC) by linear regression analysis [HPLC = 0.83 (TLC) + 3.58, n = 25, r = 0.95, P less than 0.001].