Characterization of a commercial gas chromatography-flame ionization detection system for the in situ determination of C5-C10 hydrocarbons in ambient air

A commercial automated gas chromatograph with preconcentration on solid adsorbents (AirmoVoc HC1010) was coupled with a mass spectrometer in parallel with the flame ionization detection (FID) system and characterized for its suitability for quasi continuous measurements of atmospheric hydrocarbons (HCs) with a time resolution of 20 min. Of the 50 identified HCs in the range C5-C10, 15 elute in isolated peaks and 20 in groups of two or more HCs. The remaining 15 HCs suffer from coelution by oxygenated and halogenated compounds. Procedures to minimize and quantify the blanks and the memory from the adsorbents are described. Calibration was based on a custom-made diffusion source. The accuracy of this calibration (+/-10%, 2sigma) was verified by analysis of a certified 70-component standard (average deviation: -4.3+/-2%). During a field experiment in Summer 1998, the HC1010 system was compared with a custom-made GC system with cryogenic preconcentration and much better separation properties but lower time resolution. In ambient air, good agreement (2sigma deviation <14% or 10 ppt) was found for HCs and groups of HCs that are free from coelution with oxygenated compounds, whereas large discrepancies (in some cases more than a factor of three) were found for those HCs that coelute with oxygenated compounds, as identified by MS. Analysis of the mass spectra from those peaks via specific target ions showed much better agreement with the FID system of the reference GC within 25%.     

Continuous flow configuration for total hydrocarbons index determination in soils by evaporative light scattering detection

A continuous flow configuration is proposed for the estimation of the hydrocarbon index in soils using for the first time evaporative light scattering detection (ELSD). The method is based on a membrane-enrichment of the hydrocarbons of interest (C10-C40), which are previously extracted from the soil matrix with a water:hexane mixture using a household microwave oven. The organic supernatant is cleaned-up through silica, evaporated and redissolved in a sodium dodecyl sulphate aqueous solution which is introduced in the loop of an injection valve. The sample, carried by an aqueous stream, passes through a continuous filtration unit fitted with a 1 microm pore size PTFE membrane, where the hydrocarbons are retained while the potential coextracted compounds are wasted. Quantitative elution of the C10-C40 fraction is accomplished by passing an acetone stream through the filter which drives the analytes to the ELSD system for analytical measurement. The detection limit was 1.8 microg ml(-1), the linear range 5-25 microg ml(-1) and the precision 4.3%. The recoveries were between 93 and 97%. The proposed method was also evaluated by attenuated total reflexion-Fourier transform (ATR-FT-IR) analysis of the extracts.     

Trichloroethylene,tetrachloroethylene and carbon tetrachloride in an urban atmosphere: mixing ratios and temporal patterns

The measurement of halogenated hydrocarbons in the atmosphere is a matter of great interest owing to their adverse effects on the human health and the environment. This work is focused on the measurement of three toxic chlorinated hydrocarbons: trichloroethylene (TCE), tetrachloroethylene (PCE), and carbon tetrachloride (CTC). Moreover, CTC is a greenhouse gas and an ozone depleting gas, restricted under the Montreal Protocol. Owing to their low reactivity, the target chlorinated hydrocarbons are considered to be persistent and, thus, many measurements only address their mean mixing ratios (a concentration measure expressed as mol/mol). Consequently, most of the reported data have low temporal resolution as daily, seasonal or yearly mean mixing ratios, obtained with few measurements. In the study reported in this paper hourly measurements were performed for a long period of time: almost two years for TCE and PCE, and one year for CTC. The main objective was to study the temporal variability of the chlorinated hydrocarbons with high temporal resolution in order to identify their main sources and to enhance the understanding of their atmospheric processes. During the measurement period, March 2007–February 2008 with N?=?3290 valid data, CTC showed a mean mixing ratio of 0.16?ppbv (SD?=?0.13) with lower temporal variability than the majority of non-methane hydrocarbons (NMHCs), being very well mixed in the urban atmosphere owing to its long lifetime. TCE and PCE mean mixing ratios for the May 2006–February 2008 period, were 0.13?ppbv (SD?=?0.42, N?=?4601) and 0.25?ppbv (SD?=?0.54, N?=?4709) respectively, with a larger temporal variability. The study of the sources of TCE and PCE reveals that both compounds have industrial and/or commercial origin, but with different main sources.     

大气有机物热脱附-全二维气相色谱-火焰离子化分析方法

全二维气相色谱（GC×GC）是20世纪90年代发展起来的具有高分辨率、高灵敏度、高峰容量等优势的分离技术,在我国将其用于大气挥发性有机物（VOCs）研究方面才刚刚起步.本文将GC-GC与氢火焰离子化检测器（FID）联用,构建了用于测量大气有机物的热脱附-全二维气相色谱-氢火焰离子化分析系统（TD-GC×GC-FID）.采用HP-5MS和HP-INNOWAX色谱柱,建立了C5-C15大气有机物分析方法,实现了一次分析过程同时分离非甲烷烃（NMHCs）、含氧挥发性有机物（OVOCs）和卤代烃等多种组分.利用标准物质和四级杆质谱（qMS）进行定性,外标法结合FID质量校正因子定量.目标物在GC-GC谱图中第一和第二维保留时间变化分别小于0.6s和0.02s,峰体积平均相对标准偏差为14.3%,其中烷烃和芳香烃为4.5%.标准曲线r2均值大于0.99,平均检出限为6.04ng,平均回收率为111%.利用该方法检测到2010年1月北京市区大气中400多种有机物（信噪比大于50）,鉴定了其中的103种物质,包括烷烃、烯烃、芳香烃、卤代烃、醛、酮、酯、醇和醚等.所测定有机物平均总浓度为51.3×10-9V/V,其中OVOCs约占51%,芳香烃约占30%,烷烃约占15%,卤代烃和烯烃分别占3%和1%.平均浓度最高的前3个组分是乙醇（9.84×10-9V/V）、丙酮（6.72×10-9V/V）和甲苯（3.48×10-9V/V）.     

Problems connected with the analysis of halocarbons and hydrocarbons in the non-urban atmosphere

The problems connected with the measurement of hydrocarbons outside urban areas are considerable: The atmospheric mixing ratios of most of the hydrocarbons are very low--from a few ppb down to some ppt; the mixture of hydrocarbons is extremely complex, ranging from light n-alkanes to alkyl benzenes and terpenes; for measurements in remote areas the logistic conditions often restrict the instrumentation which can be used for sample collection or in situ measurements (such as lack of electric power supply, weight restrictions etc.). Nevertheless, sensitive and sufficiently reliable measurements of hydrocarbons in the non-urban atmosphere are important. Hydrocarbons are important factors in the tropospheric photochemistry (e.g. ozone formation) and can be used as valuable tracers for man-made atmospheric pollutants etc. Other useful tracers for anthropogenic emission are halocarbons such as dichlormethane, tri- and tetrachloroethen etc. The impact of man-made hydrocarbons on the chemistry of the troposphere can only be understood if the extent of natural (biogenic) contributions is known. From measurements of a large variety of hydrocarbons and halocarbons it is often possible to obtain information about the sources of the most important atmospheric hydrocarbon species, even for trace gases with both significant anthropogenic and biogenic sources. In this presentation some of the problems and their solutions connected with such measurements of atmospheric hydrocarbons and halocarbons are presented and discussed. Some of the results obtained by several series of measurements are described, indicating that man-made as well as biogenic hydrocarbons can be important factors for the chemistry of the atmosphere.     

Multifunctional organic nitrates as constituents in European and US urban photo-smog

Air samples of the atmospheric ground layer in the cities of Ulm in Germany, Las Vegas, Nevada, and Salt Lake City, Utah, in the US were analyzed for organic nitrates. The air samples were taken around noon in summer at sunny weather conditions. 43 (C6-C13) alkyl mononitrates, 24 (C3-C6) alkyl dinitrates, and 19 (C2-C6) hydroxy alkyl nitrates have been identified. The analytical protocol included high-volume-sampling, NP-HPLC group separation, high resolution capillary gas chromatography, and detection by the highly selective mass spectrometer detector (SIM mode, m/z = 46). The levels of the sum of 15 (C6-C10) alkyl mononitrates ranged from 2.9 to 11.0 parts per trillion (ppt(v)). The levels of the sum of 21 (C3-C6) alkyl dinitrates ranged from 2.3 to 10.5 ppt(v), and the levels of the sum of 7 (C2-C4) hydroxy alkyl nitrates ranged from 7.3 to 28 ppt(v), respectively, in the urban air samples. These results emphazise the contribution of the alkyl dinitrates and hydroxy alkyl nitrates besides the alkyl mononitrates to the budget of NO(Y) compounds. No major differences in levels and pattern of the organic nitrates are present in air of the German and the US cities.     

Rapid and simple determination of aflatoxin M1 in milk in the low parts per 10(12) range

A method for extracting aflatoxin M1 from milk is proposed in which the use of disposable Extrelut clean-up columns simplifies the analysis considerably in comparison with existing methods. The quantitative determination is based on one-dimensional thin-layer chromatography and fluorescence densitometric measurement. The detection limit is 5 ppt (parts per 10(12)) in milk and the recovery is 78 +/- 4% at a level of 50 ppt.     

Qualitative and quantitative analysis of the n-alkanes C9-C17 and pristane in clean air masses

Summary An analytical method was developed for measuring n-alkanes (C₉ to C₁₇) and other hydrocarbons in tropospheric air with mixing ratios of a few ppt (10^–12) and higher. The hydrocarbons are collected in situ in absorption tubes, carefully protected against contamination and analysed later in the laboratory by gas chromatography. First data are reported for Atlantic air masses at the west coast of Ireland.

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Qualitative und quantitative Analyse der n-Alkane C₉-C₁₇ und von Pristan in reiner Luft

Zusammenfassung Es wurde eine analytische Methode entwickelt zur Messung der n-Alkane (C₉ bis C₁₇) und anderer Kohlenwasserstoffe in reiner troposphärischer Luft mit Mischungsverhältnissen von einigen ppt (10^–12) und aufwärts. Die Kohlenwasserstoffe wurden am Beobachtungsort angereichert, sorgfältig gegen Verunreinigung geschützt und später im Laboratorium gas-chromatographisch analysiert. Erste Daten für atlantische Luftmassen an der Westküste Irlands werden mitgeteilt.

     

Sampling, storage, and analysis of C2-C7 non-methane hydrocarbons from the US National Oceanic and Atmospheric Administration Cooperative Air Sampling Network glass flasks

An analytical technique was developed to analyze light non-methane hydrocarbons (NMHC), including ethane, propane, iso-butane, n-butane, iso-pentane, n-pentane, n-hexane, isoprene, benzene and toluene from whole air samples collected in 2.5l-glass flasks used by the National Oceanic and Atmospheric Administration, Earth System Research Laboratory, Global Monitoring Division (NOAA ESRL GMD, Boulder, CO, USA) Cooperative Air Sampling Network. This method relies on utilizing the remaining air in these flasks (which is at below-ambient pressure at this stage) after the completion of all routine greenhouse gas measurements from these samples. NMHC in sample aliquots extracted from the flasks were preconcentrated with a custom-made, cryogen-free inlet system and analyzed by gas chromatography (GC) with flame ionization detection (FID). C2-C7 NMHC, depending on their ambient air mixing ratios, could be measured with accuracy and repeatability errors of generally < or =10-20%. Larger deviations were found for ethene and propene. Hexane was systematically overestimated due to a chromatographic co-elution problem. Saturated NMHC showed less than 5% changes in their mixing ratios in glass flask samples that were stored for up to 1 year. In the same experiment ethene and propene increased at approximately 30% yr(-1). A series of blank experiments showed negligible contamination from the sampling process and from storage (<10 pptv yr(-1)) of samples in these glass flasks. Results from flask NMHC analyses were compared to in-situ NMHC measurements at the Global Atmospheric Watch station in Hohenpeissenberg, Germany. This 9-months side-by-side comparison showed good agreement between both methods. More than 94% of all data comparisons for C2-C5 alkanes, isoprene, benzene and toluene fell within the combined accuracy and precision objectives of the World Meteorological Organization Global Atmosphere Watch (WMO-GAW) for NMHC measurements.     

Measurements of the 12C/13C kinetic isotope effects in the gas-phase reactions of light alkanes with chlorine atoms

The carbon kinetic isotope effects (KIEs) of the reactions of several light non-methane hydrocarbons (NMHC) with Cl atoms were determined at room temperature and ambient pressure. All measured KIEs, defined as the ratio of the Cl reaction rate constants of the light isotopologue over that of the heavy isotopologue (Clk12/Clk13) are greater than unity or normal KIEs. For simplicity, measured KIEs are reported in per mil according to Clepsilon=(Clk12/Clk13 -1)x1000 per thousand unless noted otherwise. The following average KIEs were obtained (all in per thousand): 10.73+/-0.20 (ethane), 6.44+/-0.14 (propane), 6.18+/-0.18 (methylpropane), 3.94+/-0.01 (n-butane), 1.79+/-0.42 (methylbutane), 3.22+/-0.17 (n-pentane), 2.02+/-0.40 (n-hexane), 2.06+/-0.19 (n-heptane), 1.54+/-0.15 (n-octane), 3.04+/-0.09 (cyclopentane), 2.30+/-0.09 (cyclohexane), and 2.56+/-0.25 (methylcyclopentane). Measurements of the 12C/13C KIEs for the Cl atom reactions of the C2-C8 n-alkanes were also made at 348 K, and no significant temperature dependence was observed. To our knowledge, these 12C/13C KIE measurements for alkanes+Cl reactions are the first of their kind. Simultaneous to the KIE measurement, the rate constant for the reaction of each alkane with Cl atoms was measured using a relative rate method. Our measurements agree with published values within+/-20%. The measured rate constant for methylcyclopentane, for which no literature value is available, is (2.83+/-0.11)x10-10 cm3 molecule-1 s-1, 1sigma standard error. The Clepsilon values presented here for the C2-C8 alkanes are an order of magnitude smaller than reported methane Clepsilon values (Geophys. Res. Lett., 2000, 27, 1715), in contrast to reported OHepsilon values for methane (J. Geophys. Res. (Atmos.), 2001, 106, 23, 127) and C2-C8 alkanes (J. Phys. Chem. A, 2004, 108, 11537), which are all smaller than 10 per thousand. This has important implications for atmospheric modeling of saturated NMHC stable carbon isotope ratios. 13C-structure reactivity relationship values (13C-SRR) for alkane-Cl reactions have been determined and are similar to previously reported values for alkane-OH reactions.     

Feasibility of preparing and analyzing gas standards containing heavy hydrocarbons (C10–C16)

State and federal agencies are beginning to monitor ambient air for compounds related to diesel exhaust. The National Institute of Standards and Technology (NIST) was asked to develop standards containing heavy (C10-C16) alkanes which could then be used in air monitoring and exhaust studies. Several primary gravimetric standards, containing heavy (C10-C16) alkanes in nitrogen, were developed and analyzed by gas chromatography (GC) with flame-ionization detection (FID). The results of this research indicate that accurate gas standards containing these hydrocarbons can be prepared. However, the analytical results show that the temperature of the transfer system from the gas cylinder to the GC column (including the gas-sample valve) must be heated in order to prevent adsorption of these compounds within the analytical system. The results indicate that even at elevated temperatures these compounds are being absorbed within the system. The results show that quantitative results cannot be obtained by using one compound such as hexane, as an internal standard to determine the concentration of other hydrocarbons. Quantitative and accurate results are best obtained if standards for each hydrocarbon of interest are used to determine concentrations of unknowns for the respective hydrocarbon.     

Analysis of class 1 residual solvents in pharmaceuticals using headspace-programmed temperature vaporization-fast gas chromatography-mass spectrometry

A sensitive method is presented for the fast screening and determination of residual class 1 solvents (1,1-dichloroethene, 1,2-dichloroethane, 1,1,1-trichloroethane, carbon tetrachloride and benzene) in pharmaceutical products. The applicability of a headspace (HS) autosampler in combination with GC equipped with a programmed temperature vaporizer (PTV) and a MS detector is explored. Different injection techniques were compared. The benefits of using solvent vent injection instead of split or splitless-hot injection for the measurement of volatile compounds are shown: better peak shapes, better signal-to-noise ratios, and hence better detection limits. The proposed method is extremely sensitive. The limits of detection ranged from 4.9 ppt (benzene) to 7.9 ppt (1,2-dichloroethane) and precision (measured as the relative standard deviation) was equal to or lower than 12% in all cases. The method was applied to the determination of residual solvents in nine different pharmaceutical products. The analytical performance of the method shows that it is appropriate for the determination of residual class 1 solvents and has much lower detection limits than the concentration limits proposed by the International Conference on Harmonization (ICH) of Technical Requirements for the Registration of Pharmaceuticals for Human Use. The proposed method achieves a clear improvement in sensitivity with respect to conventional headspace methods due to the use of the PTV.     

Study of the relative response factors of various gas chromatograph-flame ionisation detector systems for measurement of C2-C9 hydrocarbons in air

The assumption of an instrument response that is linear with carbon number is frequently used to quantify atmospheric non-methane hydrocarbons (NMHCs) when using gas chromatography (GC) and detection by flame ionisation detector (FID). In order to assess the validity of this widely used method the results of intercomparison measurements by 14 laboratories across Europe were evaluated. The intercomparison measurements were made on synthetic, gravimetrically-prepared, gas mixtures containing 30 hydrocarbons (C2-C9) in the low ppbv range, using various different GC-FID systems. The response per carbon atom of GC-FID systems to individual NMHCs, relative to that of butane, were found to differ by more than 25% across different systems. The differences were mostly caused by analytical errors within particular GC-FID systems and to a more minor degree by systematic deviations related to the molecular structure. (Correction factors due to the molecular structure would lessen the differences, e.g. by about 5% for olefin compounds.) The differences were larger than 10% even after elimination of obvious outliers. Thus, calibration of GC-FID systems with multicomponent NMHC mixtures is found to be essential whenever the accuracy of NMHC measurements is required to be better than about 10%. If calibration by multicomponent gas mixtures is not possible and effective carbon atom response factors are used to quantify the individual NMHC compounds then the particular analytical system should be carefully characterised and its responses to individual compounds be verified.     

Separation and identification of volatile compounds from liquid cultures of Trichoderma harzianum by GC-MS using three different capillary columns

A simple, fast, repeatable and less laborious sample preparation protocol was developed and applied for the analysis of biocontrol fungus Trichoderma harzianum strain FA1132 by using gas chromatography-mass spectrometry. The match factors for sample spectra with respect to the mass spectra library of fungal volatile compounds were determined and used to study the complex hydrocarbons and other volatile compounds, which were separated by using different capillary columns with nonpolar, medium polar and high polar stationary phases. To date, more than 278 volatile compounds (with spectral match factor at least 90%) such as normal saturated hydrocarbons (C7-C30), cyclohexane, cyclopentane, fatty acids, alcohols, esters, sulfur-containing compounds, simple pyrane and benzene derivatives have been identified. Most of these compounds have not previously been reported. The method described in this paper is a more convenient research tool for the detection of volatile compounds from the cultures of T. harzianum.     

Design and performance of a microchip electrophoresis instrument with sensitive variable-wavelength fluorescence detection

A modular instrument for high-speed microchip electrophoresis (MCE) equipped with a sensitive variable-wavelength fluorescence detection system was developed and evaluated. The experimental setup consists mainly of a lamp-based epifluorescence microscope for variable-wavelength fluorescence detection and imaging and a programmable four-channel bipolar high-voltage source capable of delivering up to +/- 10 kV per channel. The optical unit was equipped with a high-sensitivity photomultiplier tube and an adjustable aperture. The system was applied to MCE separations of flurescein isothiocyanate (FITC)-labelled amines utilizing blue light (450-480 nm) for excitation as well as for the separation of rhodamines utilizing excitation light in the green spectral region (531-560 nm). At optimized conditions baseline separation of four FITC-labelled amines could be obtained in less than 50 s at a detection limit of 460 ppt (1 nM) with a signal-to-noise ratio of 3:1. Three rhodamines could be baseline-separated in less than 6 s at a detection limit of 240 ppt (500 pM). The relative standard deviations of absolute migration times determined in repetitive MCE separations of FITC-labelled amines were below 2.5% (n= 25). By the application of cyclodextrin-modified electrolytes, chiral separation of FITC-labelled amines could be performed in seconds demonstrating the potential of microchip electrophoresis for chiral high-throughput screening.     

Comparison of analytical methods for ozone precursors using adsorption tube and canister

Recently, ozone concentrations have increased dramatically as a result of vehicle usage in metropolitan areas. Ozone precursors are composed of hydrocarbons of organic compounds. Because hydrocarbons are indicative of ozone formation, they need to be monitored in ambient air. Since ozone precursor are present at very low levels (from ppb to ppt) in ambient air, they are difficult to analyze accurately. This study investigates ozone precursors in ambient air. The main purpose of this study is to compare analytical methods for the measurement of ozone precursors in atmosphere. Two measurement methods were evaluated using canister (Silco-canister) and adsorbent (300-mg Carbopack B+150-mg Carbosieve SIII) tube. Differences in measurements for total ozone precursor emissions were 54.1% between the adsorption tube and canister-GC/MS, 51.1% between adsorption tube and canister-GC/FID, and 16.3% between canister-GC/MS and canister-GC/FID.     

Determination of triazine herbicides in surface and drinking waters by off-line combination of liquid chromatography and gas chromatography-mass spectrometry

Summary Mass spectra of 12 triazines were obtained by electron impact (EI), positive-ion chemical ionization (PCI) and negative-ion chemical ionization (NCI) using methane and isobutane as reagent gases. EI mass spectrometry is more sensitive than PCI and NCI, although the chemical ionization modes increase selectivity markedly. A pre-column packed with polymer stationary phase was employed to preconcentrate surface and drinking water samples. After desorption of the analytes with ethyl acetate, an aliquot was injected directly into the GC-MS system. Atrazine and simizine were found in these samples at 10–80 ppt levels. The limits of detection for both herbicides were below 10 ppt in drinking water.     

Direct gas chromatographic analysis of halogenated hydrocarbons at the part-per-trillion level

Large sample volume on-column injection (up to 250 μL) of n-pentane solutions of halogenated hydrocarbons has been employed for the direct measurement of both low-boiling and high-boiling compounds in what is essentially a single run. Two-bonded phase, fused-silica capillary columns are joined in series, through which the low-boiling compounds are first chromatographed and detected with an electron capture detector. High-boiling compounds are then trapped in a section joining the two columns, and subsequently chromatographed in the second column, using the same detector. This procedure permits analysis at the ppt level.