Gas chromatographic method for determining free fatty acids in cheese

Summary A rapid gas chromatographic method for the analysis of individual free fatty acids (FFA) in cheese has been developed. Lipds were extract from a cheese paste acidified with diethyl ether and tetramethylammonium hydroxide (TMAM) was used for converting the FFA to TMA-soaps, which are transformed to methyl esters in the chromatographic injector. The effect of lactic acid was determined. The reproducibility of the method was studied and the coefficient of variation for the total FFA was found to be 2.2%. Recovery of individual FFA was in the range 87 to 106%.     

Gas chromatographic analysis of aromatic carboxylic acids in the presence of C1−C22 fatty acids and C2−C16 dicarboxylic acids esterified in aqueous solutions as the n-propyl esters

Summary The direct esterification and gas chromatographic analysis of aromatic carboxylic acids as n-propyl esters is described.Derivatization is performed in aqueous solution with n-propanol in the presence of sulfuric acid. The n-propyl esters of benzoic, phthalic, trimellitic and pyromellitic acids permit their gas chromatographic separation from each other and from fatty acids and aliphatic dicarboxylic acids. At mode ratios of [H₂O]/[n-PrOH]0.03 the water present does not interfere with the esterification reaction. At mole ratios above 0.03 anhydrous sodium sulfate is used for binding the water, at mole ratios of [Na₂SO₄anh.]/[H₂O]=0.51–1.52 and of [H₂SO₄]/[Na₂SO₄anh.]=0.43–1.28, respectively.     

Gas chromatographic analysis of aromatic,carboxylic acids in the presence of C1−C5 fatty acids and C2−C7 dicarboxylic acids esterified in aqueous solutions as the n-butyl esters

Summary The direct esterification and gas chromatographic analysis of aromatic carboxylic acids as n-butyl esters is described.Derivatization is performed in aqueous solution with n-butanol in the presence of sulfuric acid. The butyl esters of benzoic, phthalic, hemimellitic, trimellitic, trimesic and pyromellitic acids permit their gas chromatographic separation from each other and from fatty acids and alipatic dicarboxylic acids. At mole ratios of [H₂O]/ [n-BuOH]0.04 the water present does not interfere with the esterification reaction. At mole ratios above 0.04 anhydrous sodium sulfate is used for binding the water, at mole ratios of [Na₂SO₄ anh.]/[H₂O]=0.25–0.75.     

Simultaneous gas chromatographic determination of methanol and free glycerol in biodiesel

Summary The methanol and free glycerol content of vegetable oil methyl esters used as diesel fuel (biodiesel) is very important in describing the quality of this fuel and is therefore limited by specifications. A previously described GLC method for the determination of free glycerol in biodiesel has been further developed and also allows the simultaneous determination of methanol. Sample preparation includes dissolving in dimethylformamide, silylation with bis-trimethylsilyltrifluoracetamide (BSTFA) and separation on a methylsilicone fluid, coated-capillary column using either FID or MS-detection. Ethanol and 1,4-butanediol were used as internal standards. Both detection systems show sufficient sensitivity for concentrations relevant to biodiesel samples. The recovery was tested using a RME-sample containing known amounts of methanol and glycerol.     

Determination of cadmium in biological samples solubilized with tetramethylammonium hydroxide by electrothermal atomic absorption spectrometry, using ruthenium as permanent modifier

The feasibility of Ru as a permanent modifier for the determination of Cd in biological samples treated with tetramethylammonium hydroxide (TMAH) by ET AAS was investigated. The tube treatment with Ru was carried out only once and lasted for about 300 atomization cycles. The pyrolysis and atomization temperatures, 750 °C and 1300 °C, respectively, were chosen from the temperature curves. The sample dissolution procedure was very simple: a sample aliquot was mixed with a small volume of a 25% m/v TMAH solution, the volume was made up to 50 ml and the mixture was kept at 60 °C for 1 h. Six certified biological reference materials were analyzed and the obtained Cd concentrations are within the 95% confidence interval of the certified values, proving the accuracy of the proposed procedure for a variety of biological samples. The calibration curve, with correlation coefficient higher than 0.99, was established for a working range up to10 μg l⁻¹. The precision was good as demonstrated by relative standard deviations below 3%, except for one sample. The limit of detection (3σ) was 0.05 μg l⁻¹ and the characteristic mass was 1.30 pg, obtained in the presence of the Ru modifier.     

Trans-esterification of fatty acids from microorganisms and human blood serum by trimethylsulfonium hydroxide (TMSH) for GC analysis

Summary Trimethylsulfonium hydroxide (TMSH) can convert fatty acids into the corresponding fatty acid methyl esters (FAMEs) in a single step. These fatty acids may also be bound in biomolecules such as phospholipids and/or glycerides. Complex mixtures of saturated and unsaturated FAMEs which may contain hydroxy and cylopropyl groups are obtained by trans-esterification; they can easily be separated in most cases by capillary GC. When FAMEs are generated from different microorganisms e.g. bacteria the patterns of the chromatograms are characteristic. Examples of characteristic patterns of bacteria with different cell wall structures are shown. The described method of transesterification can also be applied directly to blood serum without sophisticated sample pretreatment. The profiles of the chromatograms match well those described in the literature obtained by other methods of trans-esterification or sample preparation.     

Determination of trace elements in biological materials using tetramethylammonium hydroxide for sample preparation

A method to prepare milk powder, bovine liver and bovine muscle samples for analysis by electrothermal atomic absorption spectrometry (ETAAS) is proposed. Samples are mixed with a small amount of tetramethylammonium hydroxide (TMAH) and a stable and homogeneous slurry is produced in ca. 2 h with heating at 60–70 °C. After such sample preparation and dilution with water, trace elements are determined in certified reference materials. Pyrolysis and atomisation temperatures are optimised for each element, and several modifiers are investigated. External calibration is used for every analyte. Limits of detection (LODs), precision and accuracy are reported for Cd, Pb, Ni, Cr, Cu and Ag and compared with those obtained after conventional acid digestion. The main advantages of the proposed method are the simplicity of sample preparation and the longer lifetime of the graphite tube.     

Tetramethylammonium hydroxide thermochemolysis for the analysis of cellulose and free carbohydrates in a peat bog

We have compared TMAH thermochemolysis with the classical method using acid hydrolysis for carbohydrates analysis in a peat core. Even if TMAH thermochemolysis does not analyse hemicellulosic carbohydrates and discriminate each individual carbohydrate sensu stricto, it allows the analysis of a cellulose pool hidden to acid hydrolysis and the specific analysis of free and terminal carbohydrates. Simple direct comparisons of thermochemolysis data with data generated from acid hydrolysis cannot be done because of the different mechanisms involved in each process. TMAH thermochemolysis must be viewed and used as a pertinent and complementary method for the analysis of carbohydrates protected and trapped by the organic matter in complex environmental systems.     

Esterification of amino acids to n-butyl and isobutyl esters in aqueous solutions for their gas chromatographic analysis

Summary Amino acids have been esterified with n-butanol and isobutanol in the presence of perchloric acid. It has been established that in the concentration range of [H₂O]/[n-BuOH]=0.10–0.40 esterification can be utilized for the determination of alanine, glycine, valine, leucine, proline, methionine, aspartic acid, phenylalanine, tyrosine, glutamic acid, lysine, arginine, histidine and cystine. In the case of [H₂O]/[i-BuOH] =0.05–0.20 the esterification yield decreases from 100% to 70%, in proportion to the water content.     

Esterification of aliphatic hydroxy acids to n-butyl esters in aqueous solutions for their gas chromatographic analysis

Summary Aliphatic hydroxy acids were esterified in aqueous solution in the presence of sulfuric acid. Esterification of lactic, tartronic malic and citric acids in water/n-butanol mixtures with mole ratios between 0.02 and 2.53 can be utilized for the determination of these aliphatic hydroxy acids by gas chromatography. Water does not interfere at water/n-butanol mole ratios below 0.02. For mole ratios above 0.02 anhydrous sodium sulfate is used for the binding of water. The mole ratio range was 0.25–2.53 for anhydrous sodium sulfate/water, and 0.32–1.27 for sulfuric acid/anhydrous sodium sulfate.     

Quantification of fatty acids as methyl esters and phospholipids in cheese samples after separation of triacylglycerides and phospholipids

Determination of the individual fatty acid composition of neutral- and phospholipids as well as the phospholipid content of dairy food and other foodstuffs are important tasks in life sciences. For these purposes, a method was developed for the separation of lipids (standards of triolein and diacylphosphatidylcholines as well as three cheese samples) by solid-phase extraction using a self-packed column filled with partly deactivated silica. Non-halogenated solvents were used for the elution of the lipid classes. Cyclohexane/ethyl acetate (1:1, v/v) served for the elution of neutral lipids, while polar lipids were eluted with three solvents (ethyl acetate/methanol, methanol, and methanol/water) into one fraction. The separated lipid fractions were transesterified and the individual fatty acids were quantified by using gas chromatography coupled to electron ionization mass spectrometry (GC/EI-MS) in the selected ion monitoring (SIM) mode. The recovery rate for standard phosphatidylcholines was ∼90% and cross-contamination from neutral lipids was negligible. The method was applied to cheese samples. Quantitative amounts of individual fatty acids in the phospholipid fraction were <0.002-0.29% of total lipids from camembert, <0.002-0.12% of total lipids from mozzarella, and <0.002-0.18% of total lipids in a goat cream cheese. Differences in the fatty acid pattern of neutral and polar lipids were detected. The quantity of the fatty acids determined in the phospholipid fraction was divided by the factor 0.7 in order to convert the fatty acid content into the phospholipid content of the cheese samples. This factor is based on the contribution of 16:0 to dipalmitoylphosphatidylcholine (DPPC). The resulting DPPC equivalents (DPPC_eq) were found to be representative for the average contribution of fatty acids to all classes of phospholipids in dairy products. Using this approach, the phospholipid content of lipids from mozzarella, camembert, and goat cream cheese was 0.60%, 1.42% and 0.79%, respectively.     

Esterification of aliphatic hydroxy acids to n-propyl esters in aqueous solutions for their gas chromatographic analysis

Summary Aliphatic hydroxy acids were esterified in aqueous solution in the presence of different amounts of sulfuric acid. Esterification of lactic, tartronic, malic and citric acids in water/n-propanol mixtures with mole ratios between 0.03 and 2.15 can be utilized for the determination of these acids by gas chromatography. Water does not interfere at water/n-propanol mole ratios below 0.03. For mole ratios above 0.03 anhydrous sodium sulfate is used for the binding of water. The mole ratio range was 0.24–2.13 for anhydrous sodium sulfate/water, and 0.16–1.28 for sulfuric acid/anhydrous sodium sulfate.     

Gas chromatographic analysis of ethylbenzene hydroperoxide

Summary A gas chromatographic method, based on the time normalization technique, is described for the analysis of ethylbenzene hydroperoxide in mixtures containing ethylbenzene, acetophenone, 1-phenylethanol, benzaldehyde and phenol. Different liquid phases were tested and the best results were obtained with tricresyl phosphate and neopentyl glycol sebacate. The influence of the support, the liquid loading and the gas velocity are discussed.     

A new derivatization method for the determination of valproic acid in serum using tetramethylammonium hydroxide as a catalyst

Valproic acid (VPA) pharmacokinetics is highly variable and monitoring of blood levels is necessary to determine its appropriate dosage. This study aimed to establish and validate a novel derivatization method for the determination of VPA. The method was based on the catalytic effect of tetramethylammonium hydroxide using 2,4′‐dibromoacetophenone as a derivatization reagent. After derivatization, samples were injected into the HPLC system for analysis. The method showed a good linearity in the range of 1.0–200.7 μg mL^?1, and the limit of quantification was 1 μg mL^?1. All values of the accuracy and relative standard deviations were acceptable for the analyses of biological samples. The recoveries were in the range from 91.6 to 97.4% for VPA with RSD <3.9%. A novel and high conversion‐rate derivatization method has been developed and validated for the determination of VPA in human serum. It can be applied to the analysis of VPA in clinic serum samples.     

Formation of derivatives of chlorophenoxy acids and some other herbicides

Summary A fuming sulphuric acid-ethanol esterification method has been applied to chlorophenoxy acids and some other herbicides. This method is compared with esterification by iodoethane and diazomethane. The chlorophenoxy acids studied were: 2,4-D, dichlorprop, MCPA, MCPB, mecoprop and 2,4,5-T. Other herbicides studied were: benazolin, bentazone, bromophenoxime, bromoxynil, chlorthal, dicamba, 3,6-dichloropicolinic acid, dinoseb, ethephon, fluroxypyr, glyphosate, haloxyfop, ioxynil, picloram, 2,3,6-TBA and triclopyr. Fuming sulphuric acid-ethanol esterification can be successfully applied to chlorophenoxy acids, benazolin, 3,6-dichloropicolinic acid, dinoseb, fluroxypyr, haloxyfop, picloram and triclopyr. The reproductibility of the method is ±5%.     

Separation of superparamagnetic magnetite nanoparticles by capillary zone electrophoresis using non‐complexing and complexing electrolyte anions and tetramethylammonium as dispersing additive

Separations of bare superparamagnetic magnetite nanoparticles (BSPMNPs, approx. 11 nm diameter) was performed using non‐complexing (nitrate) and complexing (chloride, citrate and phosphate) electrolyte ions with additions of tetramethylammonium hydroxide (TMAOH), which is commonly applied to control the synthesis of stable iron oxides. The use of TMAOH as a background electrolyte (BGE) additive for capillary electrophoresis (CE) separations provided for the first time electropherograms of BSPMNPs exhibiting symmetrical and highly reproducible peaks, free of spurious spikes characteristic of nanoparticle clusters. Consequently, accurate determination of the electrophoretic effective mobility of BSPMNPs was possible, yielding a value of −3.345E‐08 m² V⁻¹ s⁻¹ (relative standard deviation (RSD) of 0.500%). The obtained mobilities of BSPMNPs in the presence of various electrolyte ions show that the degree of complexation with the surface of BSPMNPs follows the order chloride < citrate < phosphate, correlating with the stabilities of Fe(III) complexes with the respective anions. Finally, bare and carboxylated iron oxide nanoparticles were successfully separated in only 10 min using 10 mM Tris‐nitrate containing 20 mM of TMAOH as electrolyte. Our findings show that simple and rapid CE experiments are an excellent tool to characterise and monitor properties and interactions of iron oxide nanoparticles with other molecules for surface modification purposes.     

Formation of Fatty Acid Methyl Esters During Gas Chromatography Using Trimethylanilinium Hydroxide

Abstract

Fatty acid methyl esters may be formed by dissolving the fatty acids in a 0.2 M solution of trimethylanilinium hydroxide in methanol and injecting the solution into a gas chromatograph. The reaction is rapid and quantitative, and the reagent appears to be less hazardous than diazomethane.     

Improved gas chromatographic method for the determination of the individual free fatty acids in cheese using a capillary column and a PTV injector

Summary A gas chromatographic method with a capillary column and a programmed temperature vaporizer injector has been used to analyze the individual free fatty acids in cheese. The lipids were extracted from an acidified cheese slurry with diethyl ether and treated with tetramethylamonium hydroxide (TMAH) to convert the free fatty acids to tetramethylammonium soaps (TMA-soaps), which were subsequently pyrolyzed to methyl esters in the injector. Carrying out injection at the initial column temperature resulted in lower dispersion of the results, but the solvent front prevented quantitative determination of butyric and caproic acids, and an injector temperature of 300°C was therefore employed. Under the conditions tested, trimethylamine (tma) flash-off did not affect the determinations. The accuracy of the method improved at higher free fatty acid contents (coefficient of variation of 0.53% for a total free fatty acid content of 9000 mg/kg as opposed to 7.0% for a total free fatty acid content of 1400 mg/kg). The recovery rate for individual free fatty acids ranged between 91 and 103%.