Stability of reactive low boiling hydrocarbons on carbon based adsorbents typically used for adsorptive enrichment and thermal desorption

In the context of evaluating air sampling methods, laboratory experiments were performed to investigate the stability of eleven selected low boiling hydrocarbons (e.g. 1,3-butadiene, isoprene) adsorbed on four different carbon based adsorbents. The carbon molecular sieves Carboxen 569, Carboxen 1003, Carbosieve SIII and the graphitized carbon black Carbotrap X were tested. The influence of storage duration on the recovery of the analytes was examined by loading the adsorbent tubes with the test compounds via a sample loop under inert gas. Furthermore, the influence of sampling the hydrocarbons continuously from air on the recovery of the analytes was investigated using a simple experimental set-up providing a flow of synthetic air spiked with the hydrocarbons. Analysis of the loaded adsorbent tubes was performed by thermal desorption and capillary gas chromatography. Losses up to 80% of 1,3-butadiene and isoprene were observed for the carbon molecular sieves, occurring even immediately after the sampling step. In contrast, no significant losses of these reactive constituents were detected for Carbotrap X over a storage time of seven days. The laboratory investigations were confirmed in a field experiment.     

Stability of reactive low boiling hydrocarbons on carbon based adsorbents typically used for adsorptive enrichment and thermal desorption

In the context of evaluating air sampling methods, laboratory experiments were performed to investigate the stability of eleven selected low boiling hydrocarbons (e.g. 1,3-butadiene, isoprene) adsorbed on four different carbon based adsorbents. The carbon molecular sieves Carboxen 569, Carboxen 1003, Carbosieve SIII and the graphitized carbon black Carbotrap X were tested. The influence of storage duration on the recovery of the analytes was examined by loading the adsorbent tubes with the test compounds via a sample loop under inert gas. Furthermore, the influence of sampling the hydrocarbons continuously from air on the recovery of the analytes was investigated using a simple experimental set-up providing a flow of synthetic air spiked with the hydrocarbons. Analysis of the loaded adsorbent tubes was performed by thermal desorption and capillary gas chromatography. Losses up to 80% of 1,3-butadiene and isoprene were observed for the carbon molecular sieves, occurring even immediately after the sampling step. In contrast, no significant losses of these reactive constituents were detected for Carbotrap X over a storage time of seven days. The laboratory investigations were confirmed in a field experiment. Received: 28 June 1999 / Revised: 18 August 1999 / Accepted: 23 August 1999     

Evaluation of solid adsorbent materials for cryogen-free trapping-gas chromatographic analysis of atmospheric C2-C6 non-methane hydrocarbons

Nine commercial solid adsorbent materials (in order of decreasing surface area: Carboxen 1000, Carbosieve S III, molecular sieve 5A, molecular sieve 4A, silica gel, Carboxen 563, activated alumina, Carbotrap and Carboxen 1016) were investigated for their ability to trap and release C2-C6 non-methane hydrocarbons (NMHCs) in atmospheric samples for subsequent thermal desorption gas chromatography-flame ionization detection analysis (GC-FID). Recovery rates for 23 NMHCs and methyl chloride (CH3Cl) were determined. A microtrap filled with the three adsorbents Carbosieve S III, Carboxen 563 and Carboxen 1016 was found to allow for the analysis of the widest range of target analytes. A detection limit of approximately 3pptC [parts per trillion (carbon)] in a 1l air sample and a linear response over a wide range of volatilities and sample volumes was determined for this configuration. Water vapor in the sample air was found to causes interference in trapping and subsequent chromatographic analysis of light NMHCs. A Peltier-cooled, regenerable water trap inserted into the sample flow path was found to mitigate these problems and to allow quantitative and reproducible results for all analytes at all tested humidity conditions.     

Rapid analysis of porphyrins at low ng/l and microg/l levels in human urine by a gradient liquid chromatography method using octadecylsilica monolithic columns

Rapid gradient RP-HPLC method with fluorimetric detection for trace analysis of diagnostically significant porphyrins in human urine was developed for clinical and diagnostic purposes. Results show that optimized high-pressure gradient elution and monolithic column Chromolith SpeedRod RP18e enabled separation of seven urine porphyrins including baseline separation of I and III positional isomers of uro- and coproporphyrins within 3.2 min. Problems associated with high metal cation complexing ability of the analytes and common stainless steel based instrumentation were substantially reduced by use of 0.1 mol/l ammonium citrate buffer (pH 5.47) and methanol as a mobile phase components. Good reproducibilities of retention times (within +/- 0.36% RSD) and peak areas (from +/- 0.6 to +/- 2.5% RSD) at 5-20 microg/l level of the analytes were achieved. Determined LOQ (10 x S/N) values of diagnostically important porphyrins using fluorimetric detection (ex.405 nm/em.620 nm) were 82 pmol/l (65 ng/l, 1.30 pg/injection) for uroporphyrin I, 44 pmol/l (33 ng/l, 0.66 pg/injection) for uroporphyrin III, 50 pmol/l (40 ng/l, 0.80 pg/injection) for coproporphyrin I and 47 pmol/l (39 ng/l, 0.78 pg/injection) for coproporphyrin III. Attained LOQ concentration level is approximately 20-120 times lower than concentration of porphyrins in a urine of healthy person. Calculated LOD's (3 x S/N) were at a low ng/l levels, what enabled quantification of carry-over effect to be from 2.0% to 0.2% in each of three consecutive blank runs and from 2.5% to 7% in total after injection of mixed standard of porphyrins with 5-20 microg/l concentrations. Recovery of porphyrins at low microg/l concentration levels was from 93% to 97.5%. Devised method increases productivity of clinical laboratory from 2 to 10 times in dependence of duration of currently used method.     

Formation of artefacts during air analysis of volatile amines by solid-phase micro extraction.

Solid-phase micro extraction (SPME) is a promising technique for fast and low cost trace analysis. However, some limitations of the technique were encountered when using a PDMS (polydimethylsiloxane)/Carboxen fibre for sampling a mixture of volatile aliphatic amines in air. On the GC chromatogram, two supplementary peaks were noticed in addition to the analyte peaks, thus limiting qualitative and quantitative analysis in this particular case. This paper presents the investigations to identify the artefacts and determine the origin of their formation. First, GC-MS identification, by both electron impact and chemical ionisation modes, demonstrated that the two artefacts were unsaturated amines assumed to be formed by a dehydrogenation reaction of the target amines. This reaction was found to occur during thermal desorption of analytes in the GC injection port and to be catalysed by temperature and by metals consisting of the inox (stainless-steel) needle of the SPME device. It was also demonstrated that artefact formation was not significant when using PDMS or PDMS/divinylbenzene fibres. This difference with PDMS/Carboxen fibre can be explained by the high desorption temperature required for this fibre. Moreover, the microporosity of Carboxen induces a longer desorption time which increases the contact between analytes and inox and thereby enhances artefact formation.     

Investigation of Artefact Formation During Analysis of Volatile Sulphur Compounds Using Solid Phase Microextraction (SPME)

The occurrence and origin of artefact formation during the analysis of volatile sulphur compounds in air by PDMS/Carboxen fibre were investigated. Among the studied compounds (hydrogen sulphide, carbon disulphide, diethyl sulphide, methyl ethyl sulphide, isopropanethiol, methanethiol, dimethyl sulphide, dimethyl disulphide, carbonyl sulphide), essentially mercaptans were shown to react to form the corresponding dimers. However, in the presence of several oxygenated and amines compounds which are common components of industrial effluents, no further artefact formation or reaction was noticed. Artefact formation was therefore considered to be independent of the sample matrix. Thermal oxidation occurring during the desorption step in the GC injection port was assessed. As metallic elements were previously suspected to catalyse the reaction, a specially deactivated SPME needle was tested, but no significant difference was noticed compared to the original needle. It was therefore assumed that metallic elements naturally present on Carboxen may act as catalysts. Similar results were obtained by using Carboxen in adsorbent tubes, as artefacts were increased by comparison with Tenax TA and molecular sieve 5A.     

Preconcentration and determination of trace elements with 2,6-diacetylpyridine functionalized Amberlite XAD-4 by flow injection and atomic spectroscopy

An on-line flow injection method for the direct determination of trace elements in environmental samples is described. A mini-column packed with 2,6-diacetylpyridine functionalized Amberlite XAD-4 was used to preconcentrate and separate 8 trace metals (Cd, Co, Cu, Mn, Ni, Pb, U and Zn) from water and extracts from solid samples. The metals were eluted with 0.1 M HNO(3) directly to the detection system (either inductively coupled plasma-mass spectrometry (ICP-MS) or flame atomic absorption spectrometry (FAAS)). As well as demonstrating that the resin could be used to preconcentrate ultra-trace analytes from natural waters, it was also shown to work well at a pH of 5.5. Therefore, after treatment of sample digests with sodium fluoride, samples that contain extremely large concentrations of iron may be analysed for trace analytes without the excess iron overloading the capacity of the resin. To this end, the analytes Cd, Co, Cu and Ni were preconcentrated from acid extracts of certified soil/sediment samples and then eluted with nitric acid to be determined on-line. Limits of detection (3sigma) of Cd = 0.33 microg l(-1), Co = 0.094 microg l(-1), Cu = 0.34 microg l(-1), Mn = 0.32 microg l(-1), Ni = 0.30 microg l(-1), Pb = 0.43 microg l(-1), U = 0.067 microg l(-1) and Zn = 0.20 microg l(-1) for the FI-ICP-MS system and Cd = 22 microg l(-1), Co = 60 microg l(-1), Cu = 10 microg l(-1) and Ni = 4.8 microg l(-1) for the FI-FAAS system were obtained. Analysis of certified reference materials showed good agreement with the certified values using the two methods.     

Use of a Drying Cartridge in On-Line Solid-Phase Extraction–Gas Chromatography–Mass Spectrometry

A drying cartridge was used and optimized for the in-line elimination of water from the desorption eluent in on-line solid phase extraction–gas chromatography (SPE–GC). The cartridge is essentially a small stainless-steel precolumn packed with a drying agent which can be regenerated by simultaneous heating and purging with a moisture-free gas. The drying cartridge was mounted on an additional valve instead of between the SPE–GC transfer valve and the on-column injector to enable regeneration of the cartridge during the GC run and, thus, to increase sample throughput. Three drying agents were tested, viz. sodium sulfate, silica, and molecular sieves. Although molecular sieves have the highest capacity, silica was preferred because of practical considerations. Large-volume injections were performed through the in-line drying cartridge using a mixture of 23 microcontaminants ranging widely in polarity and volatility. Four solvents were tested. With pentane and hexane, the more polar analytes were retained by the drying cartridge. Ethyl acetate and methyl acetate gave much better (and closely similar) recoveries for all analytes. Because water elimination on the silica cartridge proved to be less critical than with ethyl acetate, this solvent was finally selected. The entire SPE–drying cartridge–GC set-up was combined with mass spectrometric (MS) detection for the determination of a mixture of micropollutants in real-life water samples. With 10-ml tap water samples spiked at the 0.5 μg/l level, for the majority of the test compounds the analyte recoveries generally were 60–106%, and (full-scan) detection limits typically were 0.01–0.03 μg/l. Some very polar analytes such as, e.g. dimethoate, were (partially) sorbed onto the silica packing of the drying cartridge.     

Monitoring and evaluation of dechlorination processes using compound-specific chlorine isotope analysis

A simple, quick and sensitive method for the compound-specific stable chlorine isotope analysis of chlorinated solvents by conventional quadrupole gas chromatography/mass spectrometry (GC/MS) is presented. With this method, compound-specific stable chlorine isotope ratios of typical chlorinated solvents like tetrachloroethene (PCE) and trichloroethene (TCE) can be determined quantitatively within 30 min by direct injection. The chlorine isotope ratios of target substances are calculated from the peak areas of several selected molecular ions and fragment ions of the substances, using a set of unique mathematical equations. The precision of the method was demonstrated through reproducibility tests. An internal precision of +/-0.4 per thousand to +/-1.1 per thousand was obtained when analyzing PCE and TCE in the 10-1000 pmol range. The validity of the method was further demonstrated by determining the chlorine isotopic fractionation factor during the reductive dechlorination of TCE in a batch experiment using zero-valent iron. The chlorine isotopic fractionation factor was calculated as 0.9976 +/- 0.0011 with a correlation coefficient of 0.9469 (n = 38). The high correlation coefficient indicates that compound-specific stable chlorine isotope analysis can be performed with sufficient accuracy using conventional quadrupole GC/MS when significant fractionation takes place during a reaction. For the first time, the chlorine isotope fractionation factor of TCE during an abiotic anaerobic dechlorination process was determined using quadrupole GC/MS, without offline sample preparation.     

Confirmation of patulin and 5-hydroxymethylfurfural in apple juice by gas chromatography/mass spectrometry

A gas chromatographic/mass spectrometric (GC/MS) method was developed for the confirmation of patulin and 5-hydroxymethylfurfural (HMF) extracted from apple juice. The extraction is based on the official AOAC method for liquid chromatographic analysis. Juice extracts are quickly and easily derivatized with bis(trimethylsilyl)trifluoracetamide under mild conditions, and the trimethylsilyl ethers of the analytes are stable for at least several hours. The analytes are determined by GC/MS using an electron-impact source and selected ion monitoring of characteristic ions. For both analytes, the interassay differences between base-peak ratios for samples and standards were all <7.1% (absolute). The presence of patulin was confirmed at fortification levels of about 30-400 microg/L and naturally occurring levels of about 80-400 microg/L. The presence of HMF was also confirmed at levels < or = 2 mg/L. The proposed mass spectral fragmentation pathways of the analytes are presented.     

Quantification of Transformation Products of Unsymmetrical Dimethylhydrazine in Water Using SPME and GC-MS

Quantification of trace concentrations of transformation products of rocket fuel unsymmetrical dimethylhydrazine (UDMH) in water requires complex analytical instrumentation and tedious sample preparation. The goal of this research was to develop a simple and automated method for sensitive quantification of UDMH transformation products in water using headspace (HS) solid-phase microextraction (SPME) in combination with GC-MS and GC-MS/MS. HS SPME is based on extraction of analytes from a gas phase above samples by a micro polymer coating followed by a thermal desorption of analytes in a GC inlet. Extraction by 85 µm Carboxen/polydimethylsiloxane fiber at 50 °C during 60 min provides the best combination of sensitivity and precision. Tandem mass spectrometric detection with positive chemical ionization improves method accuracy and selectivity. Detection limits of twelve analytes by GC-MS/MS with chemical ionization are about 10 ng L^?1. GC-MS provides similar detection limits for five studied analytes; however, the list of analytes detected by this method can be further expanded. Accuracies determined by GC-MS were in the range of 75–125% for six analytes. Compared to other available methods based on non-SPME sample preparation approaches (e.g., liquid–liquid and solid-phase extraction), the developed method is simpler, automated and provides lower detection limits. It covers more UDMH transformation products than available SPME-based methods. The list of analytes could be further expanded if new standards become available. The developed method is recommended for assessing water quality in the territories affected by space activities and other related studies.     

Rapid headspace solid-phase microextraction-gas chromatographic-time-of-flight mass spectrometric method for qualitative profiling of ice wine volatile fraction. I. Method development and optimization

An analytical method for the determination of volatile and semi-volatile compounds representing various chemical groups in ice wines was developed and optimized in the presented study. A combination of the fully automated solid-phase microextraction (SPME) sample preparation technique and gas chromatographic-mass spectrometric (GC-MS) system to perform the final chromatographic separation and identification of the analytes of interest was utilized. A time-of-flight mass spectrometric (TOF-MS) analyzer provided very rapid analysis of this relatively complex matrix. Full spectral information in the range of m/z 35-450 was collected across the short GC run (less than 5 min). Divinylbenzene/Carboxen/Polydimethylsiloxane (DVB/CAR/PDMS) 50/30 microm fiber performed best during the optimization experiments and it was used in the headspace SPME mode to isolate compounds from ice wine samples, consisting of 3 mL wine with 1g salt addition. After the sample incubation and extraction (both 5 min at 45 degrees C), analytes were thermally desorbed in the GC injector for 2 min (injector maintained at 260 degrees C) and transferred into the column. The MS data acquisition rate of 50 spectra/s was selected as optimal. The optimized analytical method did not exceed 20 min per sample, including both the isolation and pre-concentration of the analytes of interest, the final GC-TOF-MS analysis and the fiber bake-out. Both a linear temperature-programmed retention index (LTPRI) method using C(8)-C(20) alkanes loaded onto the fiber and a mass spectral library search were employed to identify the target compounds. The repeatability of the developed and optimized HS-SPME-GC-TOF-MS method for ice wine analysis, expressed as relative standard deviation (RSD, %, n=7), ranged from 3.2 to 9.0%.     

Solid-phase microextraction for the determination of haloanisoles in wines and other alcoholic beverages using gas chromatography and atomic emission detection

A headspace solid-phase microextraction (HS-SPME) method for the determination of 12 haloanisoles in wine and spirit samples using gas chromatography with atomic emission detection (GC-AED) was developed. The different factors affecting the efficiency of the extraction were carefully optimized. The divinylbenzene/Carboxen/polydimethylsiloxane (DVB/CAR/PDMS) fiber was the most suitable for preconcentrating the analytes from the headspace of the sample solution. Sample:water dilutions of 3:4 and 1:6 for wines and spirits, respectively, and the use of a mixed bromochloroanisole compound as internal standard allowed sample quantification against external standards prepared in the presence of 5% (v/v) ethanol. Detection limits ranged from 1.2 to 18.5ngL(-1), depending on the compound and the sample analyzed, with a fiber time exposure of 60min at 75 degrees C. The optimized method was successfully applied to different samples, and several of the studied haloanisoles were detected at concentration levels ranging from 10.3ngL(-1) to 1.14ngmL(-1).     

Glass nebulizer interface for capillary electrophoresis-Fourier transform infrared spectrometry

Despite the continuing development of SPME (solid-phase microextraction) fibre coatings, their selection presents some difficulties for analysts in choosing the appropriate fibre for a certain application. There are two distinct types of SPME coatings available commercially. The most widely used are poly(dimethylsiloxane) (PDMS) and poly(acrylate) (PA). Supelco has developed new mixed phases consisting of porous polymer particles, either poly(divinylbenzene) (DVB) or Carboxen suspended in a matrix of PDMS or Carbowax for extracting analytes via adsorption. In addition to the nature of the extracting phase, the thickness of the polymeric film must be taken into account and, surprisingly, the construction of the fibres when apparently they bear the same coating, as it is the case of the three PDMS-DVB fibres available. Other fibre structure properties not well explored were identified and must be taken into consideration. To elucidate their extraction efficiency, three PDMS-DVB fibres, namely 60 microm for HPLC use, 65 microm for GC use and 65 microm StableFlex for GC use, were compared with regard to the extraction of 36 compounds included in four pesticide groups. The first was particularly suited for the extraction of organophosphorus pesticides and triazines whereas the StableFlex exhibited advantages in the analysis of organochlorine pesticides and pyrethroids. An explanation for the extraction differences is suggested based on the different structure of the fibres. Detection limits in the range of 1-10 ng/l for organochlorine pesticides, 1-30 ng/l for organophosphorus pesticides, 8-50 ng/l for triazines and 10-20 ng/l for pyrethroids were attained in a method using the 60 microm PDMS-DVB fibre. The fibre maintains its performance at well above 100 extractions with between-day precision below 10%.     

Comparative study of the adsorption performance of a multi-sorbent bed (Carbotrap, Carbopack X, Carboxen 569) and a Tenax TA adsorbent tube for the analysis of volatile organic compounds (VOCs)

A comparison between two types of adsorbent tubes, the commonly used Tenax TA and a multi-sorbent bed (Carbotrap, Carbopack X, Carboxen 569) tube developed in our laboratory, has been done to evaluate their usefulness in the analysis of VOCs in ambient air. Duplicate indoor and outdoor samples of Tenax TA and multi-sorbent tubes of 10, 20, 40, 60 and 90 l were taken in Barcelona city (Spain) on July and October of 2009. Breakthrough values (defined as %VOCs found in the back tube) were determined for all sampling volumes connecting two sampling tubes in series. The analysis was performed by automatic thermal desorption (ATD) coupled with capillary gas chromatography (GC)/mass spectrometry detector (MSD). Significant differences between the concentrations obtained-from multi-sorbent bed and Tenax TA tubes are observed for the very volatile compounds (56 °C < boiling point < 100 °C and 4 kPa < vapour pressure (20 °C) < 47 kPa) (e.g. acetone, isopropanol, n-hexane) and for alcohols and chlorinated compounds (e.g. 1-butanol, carbon disulphide, dichloromethane, chloroform, carbon tetrachloride, trichloroethylene, tetrachloroethylene), being the concentrations found higher in multi-sorbent bed than in Tenax TA tubes. On the other hand, mainly all compounds with boiling points higher than 100 °C (except α-pinene, chlorinated and polar compounds) do not show significant differences between the obtained multi-sorbent bed and Tenax TA tube concentrations. For the concentrations obtained (5 ppt to 100 ppb), Tenax TA present high breakthrough values (from 0 to 77%) for mainly all compounds and sampling volumes studied. On the other hand, multi-sorbent bed tubes do not exhibit important breakthrough values for these compounds, except the VVOCs ethanol (for all sampled volumes), and acetone, dichloromethane and isopropanol (for sampling volumes over 40 l). The concentration differences observed between Tenax TA and multi-sorbent bed tubes are directly related to the high breakthrough values determined for Tenax TA adsorbent.     

Use of open-tubular trapping columns for on-line extraction–capillary gas chromatography of aqueous samples

The applicability of open-tubular trapping columns for on-line extraction–capillary GC analysis is evaluated. The extraction step involves sorption of the analytes from water into the stationary phase of an open-tubular column, removal of the water by purging the trap with nitrogen, and desorption of the analytes with an organic solvent. The effect of swelling of the stationary phase with organic solvents on the retention power of the trap is studied. When using pentane or hexane as swelling agent breakthrough volumes of at least 10 ml can easily be obtained for non-polar compounds. For a number of medium polarity compounds breakthrough volumes of 5 ml can be achieved when chloroform is used as the swelling agent. The required drying time is less than 1 minute. Quantitative desorption requires only 75 μl of organic solvent. Solvent elimination prior to transfer to the GC column is carried out using a PTV injector and a multidimensional GC system. The system is applied for the analyses of river water, urine, and serum samples.     

Direct thermal desorption of semivolatile organic compounds from diffusion denuders and gas chromatographic analysis for trace concentration measurement

A novel method for collection and analysis of vapor-phase semivolatile organic compounds (SOCs) in ambient air is presented. The method utilizes thermal desorption of SOCs trapped in diffusion denuders coupled with cryogenic preconcentration on Tenax-TA and analysis by high resolution gas chromatography (GC)-electron-capture detection (ECD). The sampling and analysis methods employ custom-fabricated multicapillary diffusion denuders, a hot gas spike (HGS) apparatus to load known quantities of thermally stable standards into diffusion denuders prior to sample collection, a custom-fabricated oven to thermally desorb SOCs from the diffusion denuder, and a programmable temperature vaporization (PTV) inlet containing a liner packed with Tenax-TA for effective preconcentration of the analytes and water management. High flow rates into the PTV inlet of 750mLmin(-1)during thermal desorption are ca. a factor of ten greater than typically used. To improve resolution and retention time stability, the thermal desorption and PTV inlet programming procedure includes three steps to prevent water from entering the analytic column while effectively transferring the analytes into the GC system. The instrumentation and procedures provide virtually complete and consistent transfer of analytes collected from ambient air into the GC evidenced by recovery of seven replicates of four internal standards of 90.7+/-4.0-120+/-23% (mean+/-95% confidence interval, CI). Retention time based compound identification is facilitated by low retention time variability with an average 95% CI of 0.024min for sixteen replicates of eight standards. Procedure details and performance metrics as well as ambient sampling results are presented.     

Studies on the aroma of cupuassu liquor by headspace solid-phase microextraction and gas chromatography

Headspace solid-phase microextraction (HS-SPME) coupled to gas chromatography with ion trap mass spectrometric detection and with atomic emission detection (GC-AED) was employed to identify possible odor-impact volatile organic compounds in cupuassu (Theobroma grandiflorum Spreng) liquor, as well as to quantify alkylpyrazines present in these samples. SPME fibers coated with 100 microm polydimethylsiloxane (PDMS), 65 microm PDMS-divinylbenzene (DVB) and 75 microm Carboxen (CAR)-PDMS were tested, the later being chosen for the optimized extraction procedure. The principal compounds found in the sample headspace were 3-methylbutanal, dimethylsulfide, dimethyldisulfide, beta-linalool and several alkylpyrazines (notably tetramethylpyrazine). The procedure for quantitation of the alkylpyrazines, using GC-AED for their separation and detection, allowed the detection of microg g(-1) levels of the analytes in the samples, with acceptable precision (R.S.D. less than 10%).     

Photochemical sources of organic acids. 1. Reaction of ozone with isoprene, propene, and 2-butenes under dry and humid conditions using SPME

Formation of C4 and smaller carboxylic acids from gas-phase ozonolysis of several alkenes under dry (relative humidity (RH) < 1%) and humid (RH = 65%) conditions have been investigated. We have developed a technique based on solid-phase microextraction (SPME) and gas chromatography/mass spectrometry (GC/MS) to quantify the acids, as well as other products, and applied it to the reactions of ozone with propene, trans-2-butene, 2,3-dimethyl-2-butene, and isoprene. Acetic acid yields from propene and trans-2-butene ozonolysis in the presence of an OH scavenger were 2.7 +/- 0.6 and 2.9 +/- 0.6%, respectively, under dry conditions and 1.8 +/- 0.4 and 2.3 +/- 0.5% at 65% RH. Isoprene ozonolysis produced methacrylic and propenoic acids with yields of 5.5 +/- 1 and 3.0 +/- 1%, under dry conditions and 4.1 +/- 1 and 1.5 +/- 0.3% under wet conditions, respectively. That water inhibits acid formation indicates that the water reaction with stabilized Criegee intermediates is at most a minor source of acids. Acids that may form as coproducts of the OH radical elimination pathway, acetic acid from 2,3-dimethylbutene and isoprene, and propenoic acid from isoprene were also observed with significant yields (up to 10%), although the production of acetic acid was not a linear function of the alkene reacted. Carbonyl products are also reported.     

Identification of gas emanated from human skin: methane, ethylene, and ethane.

We investigated whether methane, ethylene and ethane gas can be detected in gas emanating from human skin, which is called skin gas. Skin gas was collected with a homemade stainless-steel trap system, which was cooled with liquid nitrogen, and analyzed with a gas chromatograph fitted with a flame ionization detector (FID). Skin-gas samples were obtained by covering a hand for 30 min with a polyfluorovinyl bag in which pure helium gas was introduced. The bag, the trap system and GC were set up online to avoid any contamination by air. Methane, ethylene and ethane in skin gas were successfully collected at an average amount emanated for 30 min (from ten subjects) of 150 +/- 63, 20 +/- 11 and 17 +/- 8 [mean +/- SD] pg/cm2, respectively.