Selective and fast methylation of free fatty acids directly in plasma for their individual analysis by gas chromatography- mass spectrometry

An analytical procedure for the rapid and selective derivatization of free fatty acids into methyl esters directly in plasma without transmethylation of lipid-bound fatty acids was developed for their analysis by gas chromatography-mass spectrometry. The methyl esters of free fatty acids were obtained by reaction with methyl iodide in the solution of dipolar aprotic solvents and in the presence of solid bases. The mechanism of the methylation reaction with these reagents was investigated. Optimal conditions for the selective methylation of free fatty acids were established using different dipolar aprotic solvents and different solid bases. The possible transmethylation of covalently bonded fatty acids from plasma lipids has been investigated under different experimental conditions in order to be avoided. Total methylation of free fatty acids was achieved in 1 min at room temperature using methyl iodide and anhydrous potassium carbonate or sodium carbonate in dimethyl sulfoxide. Under these conditions, transmethylation of lipid-bound fatty acids was avoided. The methyl esters can be injected directly from the reaction solvents. A plasma volume of 50 μL was used without special purification. The detection limits were around 0.1 ng/μL. The proposed method avoids the drawbacks of the previous methods used for the one-step analysis of individual free fatty acids in human plasma.

Quantitative evaluation of perchlorethylene in groundwater before and after its oxidation by helical solid sorbent extraction and gas chromatography

An integrated method of dynamic extraction of perchloroethylene (PCE) with polydimethylsiloxane (PDMS) helical solid sorbent followed by injection into a gas chromatograph was developed for the determination of the real concentration of PCE in groundwater before and after its degradation by oxidation with KMnO(4). The main parameters (agitation, temperature, salts, pH) that affect the extraction efficiency have been evaluated and optimized. The increase of the extracted amount of PCE due to the presence of the salts could be partially compensated by the opposite effect of the insoluble MnO(2), and of the presence of HCl and the global effect of the matrix would be less important for the reproducibility of the PCE extraction. The quantitative analysis of PCE was carried out with the external standard method using the calibration-curve technique. The limit of detection for PCE was found of 0.05 mg L(-1) using flame ionization detector. The solubility of PCE in water was studied by extraction of PCE with PDMS helical solid sorbent from standard solution with different amounts of PCE added into the same volume of water and was found to decrease with settling time, because the fine droplets created during the agitation of the mixture were slowly merged in drops on the bottle walls and were not transferred into the extraction vial for GC analysis. The limit of solubility was evaluated using a graphical method and was estimated at 224.2+/-15.7 mg L(-1) of PCE in water at 25 degrees C.

Per-O-methylation reaction for structural analysis of carbohydrates by mass spectrometry

Per-O-methylation of carbohydrates is an important sample preparation step in structural analysis of complex carbohydrates, which has generated considerable interest as shown by thousands of citations in the last 10 years. This article provides a critical overview of the per-O-methylation methods applied for structural analysis of carbohydrates by mass spectrometry. The understanding of the O-methylation mechanism can help the researchers to apply the adequate O-methylation method and can generate new ideas in the effort of improving this reaction. The per-O-methylation of carbohydrates is relied upon stepwise reactions. The parameters that affect the reaction are discussed for the most important methods and are critically commented for each reaction step. The limits of each method are emphasized. The improvements of the per-O-methylation reaction are described in detail with their advantages and disadvantages and some illustrative examples are given. The methods that give complete O-methylation in non-hazardous conditions with high yields within minutes at room temperature with a very low amount of side-products are especially highlighted.

Elimination of Oxidative Degradation during the per-O-Methylation of Carbohydrates

The possible oxidative degradation mechanism occurring during the per-O-methylation of carbohydrates in dimethyl sulfoxide with methyl iodide in the presence of base is described. Evidence is presented that this process occurs only under anhydrous conditions when there is a long reaction time between the carbohydrate dissolved in dimethyl sulfoxide and methyl iodide, followed by reaction with the base. Under these specific conditions, the oxidative degradation of alditols, and cyclic carbohydrates, with and without a free hemiacetal group, is observed. The reaction between carbohydrate and methyl iodide in dimethyl sulfoxide can result in a complete oxidative degradation depending on the type of carbohydrate and the time of reaction. The oxidative degradation can be accelerated by replacing methyl iodide with dimethyl sulfate. Mass spectrometric identification of the degradation products of d-glucitol indicates simultaneous oxidation processes at all the hydroxyl groups, with site dependent rates of their reactivity. This oxidative process is not a characteristic of the methylation of carbohydrates in dimethyl sulfoxide with methyl iodide in the presence of solid sodium hydroxide and can be totally avoided by treating the carbohydrate with powdered sodium hydroxide before introduction of methyl iodide under nonanhydrous conditions, or by adding a trace of water in dimethyl sulfoxide before methyl iodide, or by using N,N-dimethylacetamide as the solvent. The degradation of the per-O-methylated carbohydrates in the liquid-liquid extraction process is also taken into account, and it is found that the degradation process can be avoid by neutralization of the base before extraction, by using benzene or tetrachloromethane as extraction solvent, and by drying the final organic extract.

Helical sorbent microtrap for continuous sampling by a membrane and trap interface for on-line gas chromatographic monitoring of volatile organic compounds

A helical sorbent microtrap consisting of a helical sorbent fixed inside a silicosteel capillary tube is presented. The main parameters that affect the safe sampling time of the helical sorbent microtrap in continuous sampling by a membrane and trap interface for on-line gas chromatographic monitoring of organic volatiles in gaseous samples are examined, taking into account the helical configuration of the sorbent, the presence of the membrane in system, and the properties of the analytes. Thermal desorption of analytes from the helical sorbent trap was also examined having regard to the influence of the turbulent flow generated by the helical sorbent in the heat transfer and the effect of thermal backward flow on the peak shape. The practical application of the helical sorbent microtrap in a membrane and trap interface was demonstrated by on-line gas chromatographic monitoring of four volatile organic compounds in the fume hood air and of volatile organic compounds from a diesel engine exhaust. The limit of detection was in the picogram per milliliter range, depending on the time of trapping and the parameters that affect the permeation through the membrane.

Helical sorbent for fast sorption and desorption in solid-phase microextraction-gas chromatographic analysis

A new technique for solid-phase microextraction (SPME) of analytes using a helical solid sorbent followed by thermal desorption into a gas chromatographic injector is reported. The main factors that affect the mass transport of analytes in sorption and thermal desorption process using a poly(dimethylsiloxane) (PDMS) helical sorbent are described. The sorption and thermal desorption were achieved in a few seconds, being very close by the theoretical prediction. Both processes were very fast by the reduction of the thickness of boundary layer between sorbent and gaseous sample as a result of a turbulent rotational flow of the headspace air on the surface of sorbent, which is generated by the helical configuration of the sorbent. The thermal desorption was also reduced by improving heat transfer into a thin boundary layer and by increasing the temperature of the heat transporter (carrier gas). The sorption and desorption with PDMS helical sorbent were compared with those of the PDMS silica rod. The extraction time was as much as 15 times faster with the PDMS helical sorbent than with the PDMS silica rod. The desorption with the PDMS helical sorbent was very fast, giving narrow peaks without tailing and a high efficiency of separation in comparison with PDMS silica rod.

Continuous permeation of analytes through a thin poly(dimethylsiloxane) membrane followed by sorbent trapping for their gas chromatographic monitoring

Parameters that affect the permeation of analytes across a thin (0.025 mm) flat membrane of polysiloxane in a membrane and trap interface coupled to a gas chromatograph are examined taking into account a mechanism of mass transport for porous and nonporous materials and the permeation of carrier gas in the direction opposite to that of the analytes. The best permeation rate was reached with hydrogen as carrier gas at a flow-rate between 3 and 5 ml/min, an agitation of analytes outside the membrane above 0.4 m/s and thin membrane. The parameters that affect the residual diffusion time of analytes in membrane were also examined. A practical application of the thin membrane in membrane and trap interface was demonstrated by gas chromatographic monitoring of six volatile organic compounds in air. The limit of detection is at the nmol/ml level depending on the time of trapping and the parameters that affect the permeation through the membrane.

Continuous monitoring of thermooxidative degradation products of polystyrene by membrane extraction with sorbent interface and gas chromatography

A novel method for the continuous monitoring of thermooxidative degradation products of polystyrene by membrane extraction with sorbent interface (MESI) and gas chromatography (GC) is developed. The results are compared with solid-phase microextraction-GC, which can extract gases, vapors, and aerosols. The volatile and semivolatile degradation products are identified by mass spectrometry. The membrane used in the MESI-GC analysis shows a high permeation for volatile aromatic hydrocarbons; a low permeation for corresponding volatile aldehydes; and no permeation for less volatile alcohols, acids, and degradation products with a high molecular weight, thus reducing significantly the number of compounds detected from MESI-GC. Sensitivity of the method depends on the time of trapping, which is limited by the breakthrough of the trap. By heating the trap at fixed intervals of time, consecutive gas chromatograms are obtained.

Derivatization of prostaglandins and related compounds to (methoxime) alkyl ester alkyl ether derivatives for gas chromatographic analysis

Rapid and convenient methods are described for the exhaustive derivatization of carbonyl, carboxyl and hydroxyl groups of prostaglandins and related compounds to methoxime, alkyl ester and alkyl ether compounds respectively. Optimal reaction conditions were established for each group. The reactions were carried out in polar aprotic solvents. Alkyl ester alkyl ether derivatives were obtained quantitatively and rapidly in one step with n-alkyl (C1-C4) halides in the presence of sodium hydroxide. Methyl ester methyl ether derivatives have the highest volatility, but propyl ester propyl ether derivatives improved the separation of complex mixtures. The carbonyl group sometimes induced side-products, so the carbonyl group was converted into methoxime. Methoximation was achieved quantitatively by using methoxylamine in the presence of hydrochloric acid or sodium hydroxide, followed by alkylation in same reaction medium. Methoximation gave syn- and anti-isomers, which were separated chromatographically, decreasing the resolution for complex samples.