Fluorometric high-performance liquid chromatography of 9-aminophenanthrene-derivatized free fatty acids

A fluorogenic reagent, 4-mercapto-7-methylthio-2,1,3-benzoxadiazole for carboxylic acids, designed by prediction of the fluorescence intensity

During the course of our studies of the development of fluorogenic reagents having a 4,7-disubstituted benzofurazan structure, we previously proposed 7-acetylamino-4-mercapto-2,1,3-benzoxadiazole (AABD-SH) as a fluorogenic reagent for carboxylic acids. Since then, progress has made it possible to estimate the fluorescence quantum yields of the 4,7-disubstituted benzofurazan compounds on the basis of the PM3 calculation of their S1-T2 energies. Subsequently, a new fluorogenic reagent, 4-mercapto-7-methylthio-2,1,3-benzoxadiazole (MTBDSH) was designed and synthesized. In the presence of condensation reagents, triphenylphosphine (TPP) and 2,2'-dipyridyl disulfide (DPDS), MTBD-SH readily reacted with n-caprylic acid within 1 min at room temperature. The derivatives of five carboxylic acids (n-caprylic acid, n-capric acid, lauric acid, myristic acid, and palmitic acid) were well-separated on a reversed-phase column and were fluorimetrically detected at 519 nm with excitation at 391 nm. The detection limits (S/N = 3) were 2.4-5.0 fmol. Thus, MTBD-SH had properties that were considered to be superior. For carboxylic acids, itwas superior not only to AABD-SH, but also to many other conventional reagents. The superiority was examined in terms of its reactivity and sensitivity and the avoidance of interfering peaks that were derived from the reagent itself or degradation products in the chromatogram.

High-performance liquid chromatographic determination of mono-, poly- and hydroxycarboxylic acids in foods and beverages as their 2-nitrophenylhydrazides

The application of direct derivatization in conjunction with high-performance liquid chromatography (HPLC) is described for the determination of both free and total carboxylic acids in foods and beverages. The method is based on the reaction of the carboxylic acids with 2-nitrophenylhydrazine hydrochloride, without complicated isolation steps, which produces their non-volatile hydrazine derivatives. The HPLC of a series of carboxylic acid groups was performed isocratically with short retention times. The analytical results showed good recovery and reproducibility using each internal standard. Due to its excellent selectivity and sensitivity, the present method can serve as a useful tool for routine determinations of mono-, poly- and hydroxycarboxylic acids in foods and beverages.

Determination of fatty acids in vegetable oil by reversed-phase liquid chromatography with fluorescence detection

The effect of temperature and organic solvent composition (acetonitrile and methanol) on the reversed-phase separation of coumarin-derivatized fatty acids according to their carbon number (C14 to C22), the degree of unsaturation, as well as cis/trans (C18:1 c/t, C18:2 cc/tt, C18:3 ccc/ttt) configuration was investigated to find out the effective separation condition. Based on the linear plots of the logarithm of the capacity factor of saturated fatty acids versus their carbon number, the equivalent chain length (ECL) of unsaturated fatty acids was calculated. The ECL values were found to be significantly altered and the differentiation between cis and trans fatty acids was increased when either the temperature or organic solvent composition was decreased. These results generally led to a better resolution at the expense of separation time. A ternary gradient composed of water, acetonitrile, and methanol was then developed to elute the solutes at 55 degrees C within a separation time of 40 min with a minimum resolution of 1.0 for the worst pair. This method was demonstrated to resolve the fatty acids in a vegetable shortening.

Application of high-performance liquid chromatography of plasma fatty acids as their phenacyl esters to evaluate splanchnic and renal fatty acid balance in vivo

Plasma fatty acids from renal and hepatic veins, and arterialized hand vein obtained in 20 subjects before and after insulin infusion were separated by reversed-phase high-performance liquid chromatography following phenacyl esterification. Separation and quantification over the range 1.0-100 nmol per injection of nine fatty acids was achieved within 60 min using [2H31]palmitic acid as internal standard. Analytical recoveries were greater than 90% and the intra- and inter-assay coefficients of variation were less than 2.5 and 4.0%, respectively. Following insulin infusion, net splanchnic uptake of total fatty acids decreased from 3.0+/-0.3 to 1.0+/-0.1 micromol/kg min (p<0.01), whereas net renal balance remained neutral (-0.04+/-0.04 vs. -0.06+/-0.03 micromol/kg min, p=N.S.). Individual fatty acid balance varied from a low of 0.012+/-0.005 (myristic acid) to a high of 0.95+/-0.08 (oleic acid) micromol/kg min across the splanchnic tissues and from 0.005+/-0.002 (stearic acid) to 0.21+/-0.1 (oleic acid) micromol/kg min across the kidney. There is a substantial diversity in changes in plasma concentration and regional balance of individual fatty acid during short-term fasting and hyperinsulinemia. This method is simple, accurate, and can be applied to assess individual fatty acid metabolism in vivo.

Ultraviolet and fluorescence derivatization reagents for carboxylic acids suitable for high performance liquid chromatography: a review

Pre- and postcolumn derivatization with a suitable chromophore or fluorophore have often been utilized to enhance the sensitivity and selectivity of detection for analytes possessing a carboxyl group. The major classes of UV and fluorescent derivatization reagents include the coumarin analogues, alkyl halides, diazoalkanes, and amines. The derivatization reaction conditions, HPLC systems for separation of the conjugates and detection limits of various analytes are presented in this review. High mass sensitivity for various carboxylic acids have been achieved with the hydrazides, anthracenes, and diazoalkanes.

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.

An improved GLC method for a rapid, simultaneous analysis of both medium chain fatty acids and medium chain triglycerides in plasma

An improved gas-liquid chromatographic (GLC) method for the direct, simultaneous analysis of both medium chain monocarboxylic acids (MCFA) and medium chain triglycerides (MCT) is reported. The calibration curve of MCFA and MCT is linear in the range from 30 ng to 1000 ng. Five nanograms for tricaproin (MCT-6), 10 ng for tricaprylin (MCT-8) and 15 ng for tricaprin (MCT-9) represent the GLC detection limits of MCT, while those of MCFA range from 40 to 15 ng depending on their chain length: the longer the chain length, the higher the detection limit. The recovery of MCFA range from 79 to 99% and that of MCT from 85 to 99%. An example of plasma concentration curves of MCT and MCFA after an intravenous bolus injection of an MCT emulsion (100 mg MCT/kgbw) in a patient with Crohn's disease is shown.

5-Bromomethyl fluorescein (5-BMF) for derivatization of carboxyl containing analytes for use with laser-induced fluorescence detection

PURPOSE

5-Bromomethyl fluorescein (5-BMF) was evaluated in this work as a pre-column, off-line derivatizing reagent for analytes containing a free carboxyl group. The reagent possessed high molar absorptivity and quantum yield and it's excitation maximum matched the intense 488.0 nm emission line of an argon ion laser. The thermal and photo-stability of the reagent were also suitable for our work.

METHODS

A mixture of 5-BMF, palmitic acid, 18-crown-6 and potassium carbonate was reacted at 76 degrees C for one hour. The reaction yield was maximized by a sequential single-factor optimization of the reaction variables. Spectral characterizations were accomplished using conventional instrumentation and detection limits were evaluated using a laboratory constructed laser-induced fluorescence (LIF) detector which was optimized for concentration detection.

RESULTS

Two apparent conjugates of palmitic acid were formed under the reaction conditions employed. The conjugate chosen for quantitative purposes was stable at room temperature over 24 hours and possessed high molar absorptivity (128,832 M-1cm-1) and quantum yield (0.82) under basic conditions. The excitation and emission maxima of the conjugate were 500 nm and 523 nm respectively which maintains suitability for argon-ion laser excitation. An argon-ion LIF detector suitable for use with conventional liquid chromatography was constructed. A detection limit of 7.56 x 10(-10)M of palmitic acid was achieved at a signal to noise ratio of three, using the strong 488.0 nm laser emission line and a commercially available flow cell component. This corresponds to 38 femtomoles of palmitic acid on-column.

CONCLUSION

The concentration detection limit was superior to literature reports for detection of fatty acids. The mass detection limit provided approximately an order of magnitude improvement over conventional fluorescence. The reagent is potentially useful for analysis of carboxyl containing analytes at low concentrations.

General strategies and selection of derivatization reactions for liquid chromatography and capillary electrophoresis

The general strategies, reasons and the different possibilities for the derivatization of biomedically important compounds are reviewed. Different approaches apply for small versus large analyte molecules, different advantages and disadvantages are visualized with pre- and post-column arrangements. Particular interest is focused upon solid-phase derivatization reagents.

A rapid GLC method for the direct analysis of plasma medium chain fatty acids

A new, rapid, gas-liquid-chromatographic (GLC) method for the direct determination of plasma medium-chain fatty acids (MCFA) (from C8 to C12) which does not require derivatization procedures is described. Analysis of underivatized MCFA by GLC showed a detection limit ranging from 0.50 to 0.217 microgram depending on chain length: the longer the chain, the higher the detection limit. Compared with the HPLC and GLC methods currently described in the literature, this GLC procedure appears to be easy, adequately sensitive, accurate and well reproducible for those MCFA, like n-octanoic and n-decanoic acids, present in plasma after p.o. or i.v. MCT administration and it requires a short time (about 30 min) for complete processing.

Determination of fatty acids in fish oil dietary supplements by capillary chromatography with laser-induced fluorescence detection

The 4-bromomethyl-7-methoxycoumarin derivatives of 14 saturated and unsaturated fatty acids, including the omega-3 fatty acids, were separated by reversed-phase liquid chromatography and detected by laser-induced fluorescence. Baseline resolution was obtained by using a high-efficiency packed capillary column with 240,000 theoretical plates, together with a systematic optimization of the mobile phase composition. The retention indices of the fatty acid derivatives correlated well with a predictive empirical model, showing accuracy better than 0.46% relative error and reproducibility better than +/- 0.1% relative standard deviation. The physiologically important fatty acids with 12-22 carbon atoms and 0-6 double bonds were determined at the femtomole level in fish oil dietary supplements by using this methodology.

Simple and highly sensitive determination of free fatty acids in human serum by high performance liquid chromatography with fluorescence detection

A highly sensitive and simple reversed phase high performance liquid chromatographic (HPLC) method for the quantitative determination of free fatty acids in human serum is presented. The method is based on the direct derivatization of serum fatty acids with 6,7-dimethoxy-1-methyl-2(1H)-quinoxalinone-3-propionylcarboxylic acid hydrazide. The derivatization reaction proceeds in aqueous solution in the presence of pyridine and 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide at 37 degrees C. The resulting derivatives are separated within 75 min on a reversed phase column (YMC Pack C8) with a gradient elution of aqueous acetonitrile and detected fluorimetrically. The detection limits are 2.5-5 fmol in a 10 microL injection volume. The sensitivity permits precise determination of free fatty acids in 5 microL serum. The method is simple and is without the conventional liquid-liquid extraction steps of serum fatty acids.

Effects of corn oil addition to the diet on the energy metabolism of heart, liver and kidney of female rats

Female rats (SIV-50, Sprague Dawley) given a diet enriched with corn oil (10% and 25% addition to rat chow, w/w) for four weeks showed changes in the energy metabolism of heart, liver and kidneys, that is, changes in oxygen consumption and uncoupling of oxidative phosphorylation of heart, liver and kidney mitochondria, concomitant to a decrease in heart mitochondrial creatine kinase activity and an increase in heart mitochondrial creatine content. The high fat diet also affected calcium binding and Na/K-, Mg- and Ca-ATPases of a cardiac myocyte membrane fraction. Lipid feeding also led to increase (biphasic and transient) of phospholipids, triglycerides and free fatty acids in the three organs studied, but in no case to hypertrophy.

Analytical-scale high-performance liquid chromatography of omega-3 fatty acid esters derived from fish oils

Fish oil triglycerides were transesterified to their corresponding methyl and ethyl esters. The esters were separated by reversed-phase high-performance liquid chromatography on a 10-cm column of 5-microns octadecylsilyl silica, using a ternary mobile phase of acetonitrile-tetrahydrofuran-water (9:5:11). The separation of principal saturated and unsaturated C14-C22 fatty acid esters was accomplished in 60 min. For the polyenoic acid esters, detection limits of ca. 26 micrograms were observed, and baseline resolution of the important critical pair of esters of the omega 3 fatty acids, eicosapentaenoic acid (C20:5 omega 3) and docosahexaenoic acid (C22:6 omega 3) was achieved. However, to obtain baseline resolution of the esters from minor fatty acid esters, a urea crystallization procedure was required prior to chromatographic analysis. The influence on the reversed-phase chromatographic retention of four different ester moieties on the fatty acids were also studied. The n-alkyl esters exhibited an increase in ln k' with increasing chain length and with increasing lipophilic character of the alkyl ester moieties; however, with the aromatic benzyl derivatives there was a decrease in ln k' compared with the alkyl esters.

Determination of fatty acids as phenacyl esters in rat adipose tissue and blood vessel walls by high-performance liquid chromatography

Twenty-two biologically relevant (6:0-22:6) saturated, monounsaturated and polyunsaturated fatty acids were separated by reversed-phase high-performance liquid chromatography after derivatization with phenacyl bromide. An optimal resolution of the critical combinations linolenic-myristic, docosahexaenoic-palmitoleic-arachidonic and palmitic-oleic acids and cis and trans isomers of octadecenoic (n9) and octadecadienoic (n9, 12) acids was achieved by continuous gradient elution with methanol-acetonitrile-water. Elution of mixtures of 6:0-22:1 fatty acids was completed within 80 min at a flow-rate of 1 ml/min. By the use of UV detection at 242 nm the detection limits for short- and long-chain fatty acids were found to be about 0.8 and 12 ng per injection, respectively. Linearity was tested up to 100 ng. The method was applied to the determination of fatty acids in rat adipose tissue and blood vessel walls of animals fed hydrogenated fat diets. The results are comparable to those obtained by gas chromatography and surpass the latter for the resolution of oleic and elaidic acids.

Method for evaluating the bioconversion of radioactive polyunsaturated fatty acids by use of reversed-phase liquid chromatography

Reversed-phase high-performance liquid chromatography on a thermostatted octadecylsilyl column was used to separate mixtures of labelled polyunsaturated fatty acids (as their methyl esters) formed by successive desaturations and elongations of labelled linoleic (18:2 n - 6) or linolenic (18:3 n - 3) acid by rat liver microsomes. Acetonitrile-water mixtures were used for elution of the esters. Unsaturated and saturated esters were detected by their refractive indices. The order of elution of fatty acid methyl esters in complex mixtures varies as a function of the chain length and unsaturation, analysis temperature, water concentration and solvent flow-rate. The peak areas vary as a function of the unsaturation. Specific radioactivities of 14C-labelled fatty acids and the percentage distribution of radioactivity among fatty acids from complex mixtures can be efficiently determined by collection and direct measurement of the radioactivity in the solvent by liquid scintillation counting. The method can be applied to complete compositional analysis, but is especially useful for determination of specific radioactivities during studies on the metabolic conversion of labelled polyunsaturated fatty acids.

Analysis of fatty acids as their anthrylmethyl esters by high-performance liquid chromatography with fluorescence detection

A reversed-phase high-performance liquid chromatographic method for the analysis of free fatty acids in plasma has been developed which allows the resolution of the major plasma fatty acids in man. A mixture of thirteen fatty acid anthrylmethyl esters is resolved on a Spherisorb 3-micron C8 column at a flow-rate of 1 ml/min by gradient elution. The solvent system consists of acetonitrile-water (93:7) for 12 min, followed by 5 min at acetonitrile-water (86:14) and 23 min at 100% acetonitrile. The eluent is monitored with a fluorescence detector (excitation 360 nm, emission 420 nm). Three different C18 columns were evaluated, and none were able to resolve all the esters. To optimize the separation on the C8 columns it was necessary to construct plots of log capacity factor vs. percent water in the mobile phase. The slope of the line for arachidonic acid (C20:4) was markedly different from that for the saturated acid derivatives. The method was applied to the analysis of plasma free fatty acids in normal and diabetic subjects. As expected, elevated levels of free fatty acids were found in diabetic subjects. The results were shown to correlate well with an established gas chromatographic assay.

High-performance liquid chromatographic analysis of serum long-chain fatty acids by direct derivatization method

A new visible-ultraviolet labelling method for the high-performance liquid chromatographic analysis in serum of individual free fatty acids, including polyunsaturated fatty acids, is described. Without commonly used isolation steps, fatty acids in serum were directly derivatized by treatment with acidic 2-nitrophenylhydrazine hydrochloride. The derivatized fatty acids were extracted into n-hexane and separated isocratically on a reversed-phase C8 column within 15 min. The detection limits ranged from 400 fmol to 1 pmol and from 100 to 200 fmol per injection with visible and ultraviolet detection, respectively. Visible detection had better selectivity, and free fatty acid levels were determined in sera obtained from healthy controls and patients with diabetes mellitus. In all the subjects studied, the precise quantitation could be performed with 25 microliters of serum. Analytical recoveries ranged from 98.3 to 103.4%. The intra- and inter-assay coefficients of variation were less than 2.7 and 3.5%, respectively. The present method is superior to the previously published methods for routine analyses: it is cheaper, the procedure is simpler, the analysis time is shorter and both resolution and sensitivity are better.

Microanalysis of free fatty acids in plasma of experimental animals and humans by high-performance liquid chromatography

A high-performance liquid chromatographic (HPLC) method was developed for microanalysis of thirteen free fatty acids using 200 microliter of plasma. Fatty acids were derivatized with 9-anthryldiazomethane for HPLC analysis. Use of an ODS minicolumn for pretreatment of plasma gave a more accurate determination of free fatty acids in plasma than by chloroform extraction. Using this method, thirteen free fatty acids in the plasma of normal human, dog, rabbit, guinea pig and rat were determined.

Separation and quantitation of fatty acids by high-performance liquid chromatography

To facilitate the determination of the fatty acid composition of tissues and the investigation of fatty acid metabolism, we developed a method for the rapid separation by high-performance liquid chromatography and quantitation (by ultraviolet light absorption) of p-bromophenyl esters of fatty acids which vary in chain length from 10 to 22 carbon atoms. The utility of the method was demonstrated by evaluating the fatty acid composition of human uterine decidua vera tissue and human endometrial stromal cells that are maintained in monolayer culture.

A comparison of methods for the high-performance liquid chromatographic and capillary gas-liquid chromatographic analysis of fatty acid esters

A reversed-phase high-performance liquid chromatographic (RP-HPLC) method for the analysis of free fatty acids in plasma was compared with a method using capillary gas-liquid chromatography (GLC). The same extraction procedure was used for both assays. In the RP-HPLC method, the acids were separated as their anthrylmethyl esters on a C18 reversed-phase column, and detected by fluorescence. The coupling agent 2-bromo-1-methylpyridinium iodide was used with 9-(hydroxymethyl)anthracene. A mobile phase of acetonitrile-water (98:2) was used with flow programming. The derivatives of the C14:0, C16:1 and C18:2 acids could not be fully resolved. For capillary GLC, the acids were separated as their methyl esters following on-column injection into a 25-m OV-101 glass capillary column and detected using flame ionization detection. The esterifying agent used was diazomethane. The C18:2 and C18:3 esters were not fully resolved. The precision and sensitivity of both methods were similar. In an application of the methods, the free fatty acid concentrations in the plasma of a group of diabetic patients and their age-matched controls were estimated. Fatty acid concentrations tended to be higher in the diabetic group but, in the small number of patients studied, wide inter-individual variations prevented a significant difference from being detected. Estimates of individual fatty acids were higher by the RP-HPLC method. The identity of the acids in the extract was confirmed by gas chromatography-mass spectrometry of their methyl esters.

Highly sensitive determination of free fatty acids in human serum by high-performance liquid chromatography with fluorescence detection

A highly sensitive and rapid high-performance liquid chromatographic method for the determination of free fatty acids in human serum is described. The fatty acids are converted into the corresponding fluorescent derivatives by the reaction with 3-bromomethyl-6,7-dimethoxy-1-methyl-2(1H)-quinoxalinone in the presence of potassium carbonate and 18-crown-6 in acetonitrile. The derivatives are separated simultaneously within 44 min on a reversed-phase column (YMC-Pack C8) with a gradient elution of aqueous methanol and detected fluorimetrically. The detection limits are 0.5-2 fmol in a 10-microliters injection volume. This sensitivity permits precise determination of free fatty acids including lauric, myristoleic and linolenic acids, which occur in serum at very low concentrations, in 5 microliters of sera from healthy subjects and patients with diabetes.

Improved method of determination of biologically important C10:0-C22:6 fatty acids as their 2-nitrophenylhydrazides by reversed-phase high-performance liquid chromatography

Fatty acids were separated by reversed-phase high-performance liquid chromatography after derivatization with 2-nitrophenylhydrazine hydrochloride in the presence of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride. The separation of a mixture of fourteen kinds of biologically important fatty acid hydrazides (C10:0-C22:6) was achieved within 15 min. Using margaric acid (C17:0) as internal standard, each fatty acid could be quantitated over the range of 2.5-5000 pmol per injection. Analytical recoveries ranged from 98.1 to 102.6%. The intra- and inter-assay coefficients of variation were less than 2.5 and 3.2%, respectively. For the determination of esterified fatty acids in fats and oils, the saponified mixture was directly derivatized without extraction. This method was compared gave similar fatty acid profiles to those obtained with the conventional liquid-liquid extraction method. It is simple, rapid and accurate for routine analyses of esterified fatty acids in biological materials.

Determination of the amounts of free arachidonic acid in resident and activated rabbit alveolar macrophages by fluorometric high performance liquid chromatography

An improved method of high performance liquid chromatography (HPLC) has been developed for the separation and quantitation of low levels of free fatty acids as they occur in mammalian tissues. The fatty acid analysis is based on the esterification of the carboxylic group with 9-anthryldiazomethane (ADAM). HPLC separation and fluorescence measurement of fatty acid ADAM esters allow the determination of pmole amounts of fatty acids. The amounts of free fatty acids of resident and activated alveolar macrophages were determined by the fluorometric HPLC method. There were approximately 2 micrograms/10(6) cells of free fatty acids. In resident macrophages, free 20:4 was a minor component (0.8% of total free fatty acids), while significant amounts of 20:4 were found in the total glycerophospholipids, representing 16.6% of the total fatty acids. A marked increase in amounts of 20:4 (8 times) occurred in activated macrophages stimulated for 1 hr with opsonized zymosan. Small but significant increases (1.5 times) also occurred in other fatty acids. These results show that the release reaction of fatty acids was not selective for 20:4 in alveolar macrophages after the challenge with opsonized zymosan.