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.     

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.     

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.     

Esterification of C1−C8 carboxylic acids to n-butyl esters in aqueous solutions for their gas chromatographic analysis

Summary The esterification of C₁–C₈ fatty acid in aqueous solutions and in the presence of sulfuric or hydrochloric acid and with n-butanol is described. It has been established that the esterification can be used for the quantitative determination of these fatty acids in the concentration ratio range of [H₂O]/[n-BuOH]=0.01–5.3. In the concentration ratio range of 0.01–0.8 the water present does not interfere. In the concentration range of above 0.8 anhydrous sodium sulfate is used for binding the water, in the amount of [Na₂SO₄.anh.]/[H₂O]0.2.     

Gas chromatographic analysis of C2−C16 dicarboxylic acids esterified in aqueous solutions as n-propyl esters

Summary C₂−C₁₆ dicarboxylic acids were esterified in aqueous solution in the presence of sulfuric acid. Esterification in water/n-propanol mixtures with mole ratios between 0.03 and 2.21 can be utilized for the quantitative determination of dicarboxylic acids by gas chromatography. The presence of water does not interfere at water/n-propanol mole ratios below 0.45. For mole ratios above 0.45 anhydrous sodium sulfate has been used for binding the water. The mole ratio range tested was 0.12–0.99 for anhydrous sodium sulfate/water, and 0.16–2.55 for sulfuric acid/anhydrous sodium sulfate.     

Esterification of C2−C16 dicarboxylic acids to n-butyl esters in aqueous solutions for their gas chromatographic analysis

Summary C₂–C₁₆ dicarboxylic acids were esterified in aqueous solution in the presence of sulfuric acid. Esterification in water/n-butanol mixtures with mole ratios between 0.02 and 2.53 can be utilized for the quantitative determination of the dicarboxylic acids by gas chromatography. The presence of water does not interfere at water/n-butanol mole ratios below 0.27. For mole ratios above 0.27 anhydrous sodium sulfate has been used for binding the water. The mole ratio range was 0.25–0.75 for anhydrous sodium sulfate/water, and 0.32–1.3 for sulfuric acid/anhydrous sodium sulfate.     

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.     

Gas chromatographic analysis of C1–C20 carboxylic acids esterified in aqueous solutions as isobutyl esters

Summary C₁–C₂₀ carboxylic acids have been esterified in aqueous solutions with i-butanol. Gas chromatographic analyses have been performed from a single chromatogram. It has been established that esterification in the concentration range of [H₂O]/[i-BuOH]=0.01–10.3 can be utilized for the quantitative determination of these carboxylic acids.The presence of water does not interfere in the range of [H₂O]/[i-BuOH]=0.01–1.03.In the cases of [H₂O]/[i-BuOH]>1.03, anhydrous sodium sulfate has been used for binding the water, in an amount of [Na₂SO₄]_anh./[H₂O]0.2.     

Derivatization of amines with chloroformate esters for gas chromatographic analysis

Summary The gas chromatographic analysis of amines after conversion to electron capture sensitive carbamates in two-phase systems has been studied. Hydrophilic compounds, for instance methylamine, are reacted with 2,2,2-trichloro-tert. butyl chloroformate. Quantitation below 10^–7 M can be made by thermionic or electron capture detection. A hydrophobic amine, namely N,N-dimethyl-n-octylamine, was derivatized with 2,4,6-tribromophenyl chloroformate with addition of iodide ion to the aqueous phase. The favorable effect of iodide ion as well as the choice of pH and chloroformate ester is discussed.Presented at the 14th International Symposium on Chromatography London, September, 1982     

Adsorption parameters in solutions of ethyl esters of aliphatic acids

Based on the regression analysis of electrocapillary curves (ECC) in solutions of ethyl esters of aliphatic acids (from acetic to valeric), it is shown that the adsorption of these compounds on a mercury electrode from aqueous solutions corresponds in the first approximation to the model of two parallel capacitors. Moreover, to describe experimental ECC, the knowledge of only five main parameters of this model is necessary. The dependence of the adsorption parameters found on the number of carbon atoms in an ester molecule is analyzed.__________Translated from Elektrokhimiya, Vol. 41, No. 4, 2005, pp. 406–411.Original Russian Text Copyright © 2005 by Damaskin.     

Enthalpies of solvation in cyclohexane and in water for homologous aliphatic ketones and esters

The standard enthalpies of solution at infinite dilution were determined for homologous aliphatic ketones and esters in water and in cyclohexane, using a rotating Calvet calorimeter, and solution concentrations about 5×10^–4 mole fraction. Vaporization enthalpies, obtained for each compound with an effusion calorimetric cell, were added to calculate the solvation enthalpies. Their dependence on the number of carbon atoms in the chain is discussed in terms of the Friedman and Krishnan treatment. The effect of polarization of the functional groups is evaluated, and separation from the influence of chain length and the hydrophobic interactions of the methylenes is attempted. For the aqueous solutions, the rearrangement in the structure of the solvent around solute molecules is also considered in relation to deviations from linearity. Comparisons are made with solvation enthalpies obtained for ketones and esters with branched or cyclic substitutes.     

A fast derivatization procedure for gas chromatographic analysis of perfluorinated organic acids

A rapid and simple derivatization procedure has been developed for gas chromatographic determination of perfluorinated organic acids (PFCAs, C₆–C₁₂), using isobutyl chloroformate (IBCF) to convert the acids into the more volatile isobutyl esters, under catalysis by pyridine. The procedure was optimized in an acetonitrile medium and applied to GC techniques with electron-capture detection (GC-ECD) and mass spectrometry with electron-impact ionization (GC-EI-MS); for the sake of comparison, HPLC with electrospray-ionization MS (HPLC-ESI(−)-MS) was also tested. The LOD and LOQ values obtained for these three techniques were compared, and the lowest LODs were obtained with GC-ECD (0.06–1.80 μg mL⁻¹). The procedure was further optimized in an aqueous medium, obtaining the best results in a phosphate buffer (pH 2.5, 50 mmol L⁻¹), in which the LOD and LOQ values were measured for GC-ECD a GC-EI-MS. The lowest LODs were found for GC-EI-MS (0.030–0.314 μg mL⁻¹). The practical applicability was tested on Vltava river water samples.     

Sorption of aliphatic alcohols from aqueous solutions by starch cryotextures

Capillary GLC was applied to study the sorption ofn-butanol,n-hexanol,n-octanol, and linalood from aqueous solutions by com starch cryotextures. The concentrations of alcohols in aqueous solutions were varied from 0.5 to 15 mmol L⁻¹. The sorption of alcohols by crytotextures formed of sols containing these alcohols and the coefficients of their. distribution depend on the initial concentration and structure of the alcohols.n-Butanol is not sorbed by the corn starch cryotextures over the range of 0.5–80 mmol L⁻¹. The sorption of other alcohols increases with increasing length of the alkyl substituent and the concentration of the alcohol. The highest sorption capacity of the cryotexture (88%) was observed forn-octanol. The sorption of linalool decreases due to the double bonds and branching in its molecules. Translated fromIzestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 734–737, April, 1999.     

Determination of bile acids in serum and bile by a modified gas chromatographic glass capillary technique

Summary A modified gas chromatographic glass capillary technique for determination of five major bile acids (Cholic acid: CA, Chenodeoxycholic acid: CDCA, Deoxycholic acid: DCA, Lithocholic acid: LCA and Ursodeoxycholic acid: UDCA) has been developed after preliminary extraction with XAD-2 resin. Enzymatic hydrolysis prevents the formation of interferring degradation products. Ether extraction with centrifugation eliminates water soluble interferring substances. Derivatization to hexafluorpropyl- instead of methyl esters results in better separation, shortens analysis time and prolongs the life span of the column.     

Sorption of aliphatic ketones from aqueous solutions by starch cryotextures

Capillary gas chromatography was applied to study the sorption of aliphatic ketones (C₆—C₁₁), including metamers, from aqueous solutions by corn starch cryotextures. The amount of ketones sorbed by cryotextures depends linearly on their concentrations in the initial sol. Equations describing the concentration dependence of sorption were proposed. The shape of sorption isotherms reflects the strength of sorption. The binding constants and the number of binding sites were determined for weakly sorbed ketones. The length of alkyl substituent and the position of the functional group are the crucial factors governing the sorption of ketones under conditions of excess binding sites. It was found that the degree of sorption increases with an increase in the carbon chain length from 6 to 9 carbon atoms. The presence of cooperation of binding sites for ketone sorption by cryotextures was demonstrated. The major part of ketones is sorbed irreversibly. This fact points to the formation of supramolecular complexes. Ketones with lower molecular masses are better sorbed by cryotextures than by native starch grains.     

Confirmatory and quantitative analysis of fatty acid esters of hydroxy fatty acids in serum by solid phase extraction coupled to liquid chromatography tandem mass spectrometry

A novel class of endogenous mammalian lipids endowed with antidiabetic and anti-inflammatory properties has been recently discovered. These are fatty acid esters of hydroxy fatty acids (FAHFAs) formed by condensation between a hydroxy fatty acid and a fatty acid. FAHFAs are present in human serum and tissues at low nanomolar concentrations. Therefore, high sensitivity and selectivity profiling analysis of these compounds in clinical samples is demanded. An automated qualitative and quantitative method based on on-line coupling between solid phase extraction and liquid chromatography–tandem mass spectrometry has been here developed for determination of FAHFAs in serum with the required sensitivity and selectivity. Matrix effects were evaluated by preparation of calibration models in serum and methanol. Recovery factors ranged between 73.8 and 100% in serum. The within-day variability ranged from 7.1 to 13.8%, and the between-days variability varied from 9.3 to 21.6%, which are quite acceptable values taking into account the low concentration levels at which the target analytes are found. The method has been applied to a cohort of human serum samples to estimate the concentrations profiles as a function of the glycaemic state and obesity. Statistical analysis revealed three FAHFAs with levels significantly different depending on the glycaemic state or the body mass index. This automated method could be implemented in high-throughput analysis with minimum user assistance.     

Multi-walled carbon nanotubes as the gas chromatographic stationary phase: role of their functionalization in the analysis of aliphatic alcohols and esters

Presented hereafter is the novel application of differently functionalized multi-walled carbon nanotubes (f-MWCNTs) as stationary phases for the GC separation of C(1)-C(5) alcohols and esters. Low-cost packed columns, easily prepared in laboratory, were used with satisfactory results. Depending on the functional groups introduced on the pristine MWCNTs, remarkably different behaviours have been observed, thus indicating that derivatization is a key factor to achieve optimal resolution. The best performance was shown by the 2,2'-(ethylenedioxy)diethylamine derivatized MWCNTs, allowing the separation of alcohol isomers, for this reason chosen for the investigation of fermentation by-products in distilled spirits. The degree of derivatization has been assessed for each packing material by thermogravimetric analysis (TGA). A computational study has been performed to correlate the physico-chemical properties of alcohol probes with the retention behaviour.