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At least 307 records · Page 17Linked to original sources
PubMed2000

Hold-up time in gas chromatography. V. Dependence of the retention of n-alkanes on the chromatographic variables in isothermal gas chromatography.

The chromatographic behaviour of n-alkanes and other homologous series in isothermal gas chromatography has been shown to depart from the "linear" representation of the logarithms of the adjusted retention times vs. carbon number. One of the expressions proposed to describe this behaviour is tR(z)=A+exp(B+CzD). In this paper, a regression analysis shows that three of the parameters of the equation depend on different chromatographic variables such as hold-up time, average linear gas velocity, volume and polarity of the stationary phase and temperature of the column. The fourth parameter (D), responsible for the departure from the "linearity", does not depend on any chromatographic variable, and represents a gradual decrease of the contribution of the methylene groups to the general properties of n-alkanes, with no relation to the chromatographic phenomenon.

Alkanes↗
PubMed2000

Efficiency of pretreatment of aqueous samples using a macroporous strong anion-exchange resin on the determination of nerve gas hydrolysis products by gas chromatography-mass spectrometry after tert.-butyldimethylsilylation.

A pretreatment procedure, using a macroporous strong anion-exchange resin (MSA) has been established for the determination of nerve gas hydrolysis products by gas chromatography-mass spectrometry (GC-MS) after tert.-butyldimethylsilyl (TBDMS) derivatization. Aqueous solutions of methylphosphonic acid (MPA) and three alkyl methylphosphonic acids (AMPAs) (ethyl, isopropyl and pinacolyl methylphosphonic acid), were retained on the MSA column, and then quantitatively eluted with 0.1 M hydrochloric acid. The neutralized column eluate was dried, and MPA and AMPAs were derivatized with N-methyl-N-(tert.-butyldimethylsilyl)-trifluoroacetamide and analyzed by GC-MS. The column eluate was also analyzed in order to determine the exact hydrolysis product levels by capillary electrophoresis using borate and benzoate buffer (pH 6). The MSA pretreatment was examined for the clean-up of aqueous extracts of three types of soils and an aqueous solution containing 10% sucrose, which is regarded as model for a typical soft drink, after spiking with MPA and AMPAs. MPA and AMPAs were quantitatively recovered in the MSA eluate fraction from those samples, except for MPA from volcanic acid and alluvial soils. The yields of TBDMS derivatives were remarkably improved, compared with for which no pretreatment was used and also for those in which a strong cation-exchange resin was used. The achieved detection limits of MPA and AMPAs ranged from 0.12 to 0.18 microg/g of soil (S/N=3). The established MSA method was applied to the pretreatment of spiked sea water, two types of beverages, Pepsi Cola and canned coffee. Although the yields of TBDMS derivatives of MPA and AMPAs in sea water (in a range between 44 and 96%) and AMPAs in Pepsi Cola (in a range between 58 and 92%) were rather high, those for MPA in the Pepsi Cola (27%) and those for MPA and AMPAs in the canned coffee (in a range between 5 and 17%) were low.

Anion Exchange Resins↗
PubMed2000

Characterization of odorant compounds of mussels (Mytilus edulis) according to their origin using gas chromatography-olfactometry and gas chromatography-mass spectrometry.

Gas chromatography-olfactometry consists of sniffing the effluent of a gas chromatograph and leads to the direct determination of potent odorants in food. GC-olfactometry and GC-MS were applied in order to identify volatile compounds, and to characterize potent odorants of cooked wild mussels and bouchot mussels. Eighty-five volatiles were identified by GC-MS, among those the majority were identified for the first time in mussels. Using GC-olfactometry, the main contributors of cooked mussels aroma were characterized. Of the 85 volatiles identified in the flavor, only 33 were odor-active and contribute to the overall aroma of mussels. Dimethyl disulfide (sulfury odor) was the odorant the most differently perceived between the two extracts and seems to be characteristic of wild mussels. Combined GC-MS and GC-olfactometry made it possible to point out odorants which actually contribute to the aroma of cooked mussels and those which showed typical dependence on the origin of mussels.

Animals↗
PubMed2001

Gas chromatographic analysis of trace gas impurities in tungsten hexafluoride.

Highly reactive fluorinated gaseous matrices require special equipment and techniques for the gas chromatographic analysis of trace impurities in these gases. The impurities that were analysed at the low-microg/l levels included oxygen, nitrogen, carbon dioxide, carbon monoxide, sulfur hexafluoride and hydrogen. This paper describes the use of a system utilising backflush column switching to protect the columns and detectors in the analysis of trace gas impurities in tungsten hexafluoride. Two separate channels were used for the analysis of H2, O2, N2, CO, CO2 and SF6 impurities with pulsed discharge helium ionisation detection.

Calibration↗
PubMed1977

Use of silylating agents for the identification of hydroxylated steroids by gas chromatography and gas chromatography-mass spectrometry. Discrimination between phenolic and alcoholic hydroxyl groups.

A phenolic trimethylsilyl (TMS) group was selectively exchanged for a dimethylalkylsilyl (DMAS) group on a gas chromatographic column by use of sandwich injection technique with DMAS-imidazole, and a TMS ether derivative of a phenolic steroid was converted into a DMAS ether or a mixed TMS and DMAS ether derivative with over 95% recovery. The selective exchange reaction seemed to be caused by the difference in lability between the ethereal TMS linkages to phenolic and alcoholic hydroxyl groups. This selectivity was found to be useful for discriminating gas chromatographically between the phenolic and alcoholic hydroxyl groups in steroids.

Alcohols↗
PubMed2001

Multiresidue analysis of insecticides in soil by gas chromatography with electron-capture detection and confirmation by gas chromatography-mass spectrometry.

A rapid multiresidue method has been developed for the analysis of nine insecticides (organochlorines, pyrethroids and organophosphorus) in soil. The method is based on the sonication extraction of residues from a certain amount of soil placed in a small column, using ethyl acetate. The effect of the residence time of insecticides in soil, the material of the columns used (glass or plastic columns) and the soil moisture content on the recovery of these compounds was also studied. Residues were determined by gas chromatography with electron-capture detection. The average recovery through the method obtained for these compounds varied from 90 to 108% with a relative standard deviation between 1 and 11%. The results of this study pointed out that the recoveries of insecticide residues obtained with plastic or glass columns at different soil moisture content were similar and that the residence of these compounds in soil during several days did not affect their recovery from soil. Confirmation of residue identity was performed by gas chromatography coupled with mass spectrometry.

Chromatography, Gas↗
PubMed2002

Speciation analysis for mercury in gas condensates by capillary gas chromatography with inductively coupled plasma mass spectrometric detection.

A method allowing species-selective determination of atomic mercury, non-polar dialkylated mercury compounds,polar monoalkylated species and inorganic mercury complexes in natural gas condensates was developed. Inductively coupled plasma mass spectrometry was employed as a detection method for capillary gas chromatography and compared with microwave induced plasma atomic emission detection for the analysis of hydrocarbon-rich matrices. The method was based on two consecutive injections allowing comprehensive speciation analysis. First a sample aliquot was diluted with toluene and analysed for Hg0 and individual dialkylmercury compounds. Then, another aliquot was butylated with a Grignard reagent for the species specific determination of Hg(II) and monoalkylated mercury species. The detection limits were down to 0.08 pg level.

Chromatography, Gas↗
PubMed2002

Enthalpy of fuel gas odorants on surrogate soil surfaces by gas chromatography.

Heats of adsorption and heats of interaction for natural gas odorants on clay and organo-clay, respectively, were determined by means of wall-coated open-tubular (WCOT) column gas chromatography. The odorants studied are organic thiol and sulfide compounds. Clay stationary phases were created from the synthetic clay Laponite-RD. Subsequent coatings with octadecane created an organo-clay stationary phase. Experimental results show that, as a class, sulfide odorants have larger enthalpies on clay and organo-clay surfaces than thiol odorants. Therefore, we conclude that thiols are less likely to be sequestered on soil surfaces. The effect of hydrated clay surfaces on odorant enthalpies is also presented. Further, we demonstrate that Lewis acid-base chemistry on clay surfaces explains the significant difference in enthalpy magnitudes between the sulfide and thiol classes.

Chromatography, Gas↗
PubMed2003

Theoretical and experimental study of recycle capillary gas chromatography with carrier gas propelled by a peristaltic pump.

A new technique of recycle capillary gas chromatography (RCGC) characterized by a very high separation efficiency of more than 10(6) theoretical plates has been developed to solve the problem of separation of isomers with similar physico-chemical properties. The technique replaces the recycle valve by a peristaltic pump that propels the carrier gas. A general model has been developed for description of RCGC characteristics and experimentally verified on the retention behaviour of methane and the separation of a test pair of 3-methyl-1-butene and 2-methylbutane.

Chromatography, Gas↗
PubMed2003

Reduction of analysis time in gas chromatography. Application of low-pressure gas chromatography-tandem mass spectrometry to the determination of pesticide residues in vegetables.

An alternative to conventional capillary gas chromatography (GC) is evaluated as a new approach to determine pesticide residues in vegetables. Low-pressure gas chromatography-tandem mass spectrometry (LP-GC-MS-MS) is proposed after a fast and simple extraction of the vegetable samples with dichloromethane and without clean up. The use of the above-mentioned GC technique reduced the total time required to determine 72 pesticides to less than half the present time (31 min), increasing the capability of a monitoring routine laboratory. The use of guard column and plug of carbofrit into the glass liner in combination with LP-GC was evaluated. The method was validated with limits of quantitation low enough to determine the pesticide residues at concentrations below the maximum residue levels stated by legislation. In order to assess its applicability to the analysis of real samples, 25 vegetable samples previously determined using conventional-capillary GC-MS-MS were analysed by LP-GC-MS-MS. The results obtained with the compared techniques showed differences lower than 0.01 mg kg(-1).

Gas Chromatography-Mass Spectrometry↗
PubMed1998

Determination of very volatile organic compounds in environmental water by injection of a large amount of headspace gas into a gas chromatograph.

An injection method for a large amount of headspace gas which enables determination of trace amounts of very volatile organic compounds (VVOCs), dichlorodifluoromethane, chloromethane, vinyl chloride, bromomethane, chloroethane and trichlorofluoromethane in all kinds of environmental water was developed. A gas phase equilibrated with the water phase in a vial was purged with helium for a short time. The VVOCs were then introduced into a trapping tube packed with Tenax TA, which had been cooled using carbon dioxide. After trapping, the VVOCs were thermally desorbed and put into a GC-MS system for subsequent analysis. This method is applicable to various types of samples.

Chromatography, Gas↗
PubMed1999

Study on volatile components in salami by reverse carrier gas headspace gas chromatography-mass spectrometry.

Salami are a typical seasoned sausage of Italy; a number of types are produced, according to local traditional recipes. As industrial production has taken place, a number of problems rise in obtaining products similar to the traditional ones. The use of selected microbial starters is permitted by Italian law for some years and at present, microbiological research is engaged in selecting starters similar to the ones isolated from traditional products, with the aim of obtaining organoleptic characteristics close to the ones of traditional recipes. A study was carried out concerning the characterisation of volatile components of salami by headspace capillary gas chromatography-mass spectrometry. As during the sampling step, analytes could reach the analytical column, the carrier gas rate was back flushed in the latter, while a pre column was used as cold trap. Then GC-MS analysis follows. By these techniques, we were able to highlight typical profiles of different salami, as well as monitoring the ripening of a traditional and a starter added salami. Main peaks are of fermentative origin, while also peaks from spices were detected. Ethyl propionate was used as internal standard to be able to normalise the peaks amounts.

Gas Chromatography-Mass Spectrometry↗
PubMed1999

Carbohydrate profiling of bacteria by gas chromatography-mass spectrometry and their trace detection in complex matrices by gas chromatography-tandem mass spectrometry.

Bacterial cellular polysaccharides are composed of a variety of sugar monomers. These sugars serve as chemical markers to identify specific species or genera or to determine their physiological status. Some of these markers can also be used for trace detection of bacteria or their constituents in complex clinical or environmental matrices. Analyses are performed, in our hands, employing hydrolysis followed by the alditol acetate derivatization procedure. Substantial improvements have been made to sample preparation including simplification and computer-controlled automation. For characterization of whole cell bacterial hydrolysates, sugars are analyzed by gas chromatography-mass spectrometry (GC-MS). Simple chromatograms are generated using selected ion monitoring (SIM). Using total ion GC-MS, sugars can be readily identified. In more complex clinical and environmental samples, markers for bacteria are present at sufficiently low concentrations that more advanced instrumentation, gas chromatography-tandem mass spectrometry (GC-MS-MS), is preferred for optimal analysis. Using multiple reaction monitoring, MS-MS is used (replacing more conventional SIM) to ignore extraneous chromatographic peaks. Triple quadrupole and ion trap GC-MS-MS instruments have both been used successfully. Absolute chemical identification of sugar markers at trace levels is achieved, using MS-MS, by the product spectrum.

Acetates↗
PubMed1999

Gas chromatographic-mass spectrometric determination of sulfolane in wetland vegetation exposed to sour gas-contaminated groundwater.

Described is a GC-MS method for the determination of the levels of sulfolane (tetrahydrothiophene 1,1-dioxide, C4H8O2S; a water miscible chemical used in the sweetening of sour gas) in wetland vegetation (roots, shoots, berries, seeds, grasses, and leaves). The technique was developed to provide positive detection of sulfolane in a variety of wetland vegetation and to determine the extent to which sulfolane may translocate within the plants. Vegetation samples collected at a sour gas processing facility were extracted using a two-stage process which utilized a back extraction of a water extract with toluene. The main advantages of this procedure were: good extraction efficiency (recovery of 80+/-12%), exclusion of most of the highly polar co-extractives during the toluene back extraction step, and a final extract well suited to routine GC-MS selected ion monitoring of sulfolane with a detection limit of 90 ng g(-1) (wet mass). In general, the method was rugged, based on a study period of 18 months in which over 175 runs were conducted.

Gas Chromatography-Mass Spectrometry↗
PubMed1978

Determination of cytosine arabinoside in human plasma by gas chromatography with a nitrogen-sensitive detector and by gas chromatography--mass spectrometry.

A method for the determination of cytosine arabinoside in the plasma of leukemic patients being treated with this drug is described using either gas--liquid chromatography with a nitrogen-sensitive flame ionization detector or gas chromatography--mass spectrometry (GC-MS). To increase volatility, a double derivative of cytosine arabinoside was used, prepared by acetylation and subsequent methylation. Cytidine was used as internal standard for the GC procedure. GC--MS was performed with either cytidine as internal standard and detection by single-ion monitoring or by the use of [2H3] acetate-methyl derivative of cytosine arabinoside as internal standard and subsequent multiple-ion monitoring. Attempted extraction of cytosine arabinoside from plasma with various organic solvents was unsuccessful, but protein precipitation with ethanol or trichloroacetic acid followed by washing of the aqueous residue with organic solvents to remove as many of the interfering substances as possible gave satisfactory results. The minimum detectable quantity of pure cytosine arabinoside was similar for both techniques (approximately 500 pg). However, with GC using a nitrogen-sensitive detector, the lower limit of detection from plasma was found to be approximately 40--70 ng per ml plasma whilst GC--MS showed greater analytical selectivity with a detection limit in some cases as low as 1 ng per ml plasma.

Chromatography, Gas↗
PubMed1997

Detection of microbial-derived fatty acids in carious dentin by gas chromatography and gas chromatography-mass spectrometry.

The aim of the present study was to analyze the fatty acid content of carious and sound human dentin. Gas chromatography and gas chromatography-mass spectrometry revealed the presence of fatty acids of C10-C18 size in the carious dentin, whereas fatty acids of C16 size were present in minute amounts in three samples of the corresponding sound dentine controls. No fatty acids were detected in the other sound dentin control samples. The source of fatty acids was considered to be microorganisms invading the dentin during the progression of the caries lesion. The presence of bacterial fatty acids in carious dentin may serve as a marker for the pathological process and thus contribute to the understanding of the mechanisms involved.

Dental Caries↗
PubMed1998

Application of gas chromatography-mass spectrometry and gas chromatography-tandem mass spectrometry to assess in vivo synthesis of prostaglandins, thromboxane, leukotrienes, isoprostanes and related compounds in humans.

Prostaglandins, thromboxane, leukotrienes, isoprostanes and other arachidonic acid metabolites are structurally closely related, potent, biologically active compounds. One of the most challenging tasks in eicosanoids research has been to define the role of the various eicosanoids in human health and disease, and to monitor the effects of drugs on the in vivo synthesis of these lipid mediators in man. Great advances in instrumentation and ionization techniques, in particular the development of tandem mass spectrometry and negative-ion chemical ionization (NICI), in gas chromatography and also advances in methodologies for solid-phase extraction and sample purification by thin-layer chromatography and high-performance liquid chromatography have been made. Now gas chromatography-mass spectrometry (GC-MS) and GC-tandem MS in the NICI mode are currently indispensable analytical tools for reliable routine quantitation of eicosanoid formation in vivo in humans. In this article analytical methods for eicosanoids based on GC-MS and GC-tandem MS are reviewed emphasizing the quantitative measurement of specific index metabolites in human urine and its importance in clinical studies in man. Aspects of method validation and quality control are also discussed.

Dinoprost↗
PubMed2004

Comprehensive two-dimensional supercritical fluid and gas chromatography with independent fast programmed heating of the gas chromatographic column.

With comprehensive two-dimensional supercritical fluid and fast, independent temperature-programmed gas chromatography (SFCxGC), a polar column was used in the first dimension to achieve group-type analysis. The eluent of this separation was repetitively sampled and transferred to a fast, resistively heated gas chromatograph to obtain the boiling point distribution over the entire polarity separation. The SFC was operated isothermally with stopped flow to provide a sufficient time span for the GC analysis. The GC analysis had a typical cycle time of 1 min for the system demonstrated here. During this time, the GC column was independently heated at a rate of 450 degrees C/min to 250 degrees C and actively cooled again to -50 degrees C before the next GC injection took place. The analysis of petrochemical samples is presented to illustrate the technique.

Chromatography, Gas↗