Development of an automated monitoring system for various gas-phase organic carbonyls in ambient air

An automated monitoring system for various C₁ to C₅ gas-phase organic carbonyls in ambient air is described. The system consists of a parallel plate diffusion scrubber (PPDS), which is coupled with a high-performance liquid chromatography–ultraviolet (HPLC–UV) system using an automated injection valve. Compared with an annular diffusion scrubber (DS) employed so far for gas-phase carbonyl monitoring, PPDS shows an improved collection efficiency for formaldehyde, acetaldehyde, propionaldehyde, and acetone with >97% at an airflow rate of 0.5?L/min. High gas–liquid concentration ratios of PPDS and an optimised HPLC–UV system allow limits of detection (LOD) in a range of 80–500?pptv. A low liquid hold-up volume of the PPDS results in a short response time of about 10?min. Additionally, the optimised analysis time for 13 carbonyl compounds containing calibration standard enables brief measurement intervals of 25?min. The developed PPDS–HPLC system shows its reliability from urban site monitoring in Seoul, South Korea.     

Determination of complex mixtures of volatile organic compounds in ambient air: an overview

This article reviews developments in the sampling and analysis of volatile organic compounds (VOCs) in ambient air since the 1970s, particularly in the field of environmental monitoring. Global monitoring of biogenic and anthropogenic VOC emissions is briefly described. Approaches used for environmental monitoring of VOCs and industrial hygiene VOC exposure assessments are compared. The historical development of the sampling and analytical methods used is discussed, and the relative advantages and disadvantages of sorbent and canister methods are identified. Overall, there is considerable variability in the reliability of VOC estimates and inventories. In general, canister methods provide superior precision and accuracy and are particulary useful for the analysis of complex mixtures of VOCs. Details of canister methods are reviewed in a companion paper. C. C. Austin is an Invited Scientist of the National Research Council of Canada.     

Determination of complex mixtures of volatile organic compounds in ambient air: canister methodology

Canister methodology is applicable to 150 polar and nonpolar VOCs found in ambient air from parts-per-billion by volume (ppbv) to parts-per-million (ppmv) levels, and has been validated at parts-per-trillion (pptv) levels for a subset of these analytes. This article is a detailed review of techniques related to the collection of volatile organic compounds (VOCs) in evacuated Summa and fused-silica-lined canisters, and their analysis by gas chromatography/mass spectrometry (GC/MS). Emphasis is placed on canister cleaning, VOC stability in canisters, sample dilution, water management, and VOC cryogenic and sorbent preconcentration methods. A wide range of VOC preconcentration and water management methods are identified from the literature, and their relative merits and disadvantages are discussed. Examples of difficulties that commonly arise when processing canister samples are illustrated, and solutions to these problems are provided.     

The automated determination of volatile organic contaminants in ambient air and/or soil gas by gas chromatography with selective detectors

Two systems have been developed which are suitable for the determination of sub parts per billion levels of organic contaminants in ambient air and soil gas. Gas samples are passed through an adsorbent where the contaminants are trapped. Following thermal desorption the contaminants are quantitated by capillary GC employing photoionizations electrolytic conductivity, and electron capture detection. The performance of the systems is described in detail.     

Determination of oxygenated volatile organic compounds in ambient air using canister collection-gas chromatography/mass spectrometry.

This research attempts to establish a method to measure 11 kinds of oxygenated volatile organic compound (OVOC) in ambient air by using the canister collection-gas chromatography/mass spectrometry (GC/MS) method. Since several compounds such as acetone exhibited high blank concentrations due to their laboratory use, stringent quality control was conducted for the VOC-free added water and the VOC-free nitrogen gas. In order to prevent the decline of recovery rates due to lack of sufficient relative humidity, it is necessary to add VOC-free water when pressurizing and diluting the air samples. Thus, all the target compounds in ambient air were obtained from the canisters at high recovery rates without significant contamination. Furthermore, the canister collection-GC/MS method makes it possible to apply simultaneous air monitoring of OVOCs as well as volatile hazardous air pollutants without additional sampling.     

Comparative study of solvent extraction and thermal desorption methods for determining a wide range of volatile organic compounds in ambient air

This paper compares two analytical methods for determining levels of 90 volatile organic compounds (VOCs) commonly found in industrial and urban atmospheres. Both methods are based on two official methods for determining benzene levels and involve collecting samples by active adsorptive enrichment on solid sorbents. The first method involves solvent extraction and uses activated charcoal as a sorbent. After sampling, the sorbent is extracted with 1 mL of carbon disulfide and then 1 μL of the extract is analysed in a GC-MS. The second method involves thermal desorption (TD) and uses Tenax TA and Carbograph 1TD as sorbents, which allows the whole sample to be analysed. In general, the thermal desorption method showed the best repetitivity and recovery and the lowest limit of detection and quantification for all target compounds. Because of its lower sensitivity, the solvent extraction method needs the preconcentration of large sample volumes of air (720 L vs. 2.64 L for the thermal desorption method) to yield similar limits of detection.The performance of both methods in real samples was tested in a location near to a petrochemical complex. The results of the 24-h samples for the solvent extraction method were compared with the average of 12 2-h samples for the TD method. In some cases, both methods found differences in the VOC concentrations, especially in those compounds whose concentrations fluctuate significantly during the day.     

Application of a unique selective PLOT capillary column for the analysis of oxygenated compounds in ambient air

Summary A new PLOT column (CP-LOWOX) designed specifically for the analysis of oxygenated compounds has been used for the gas chromatographic determination of semi-volatile carbonyl compounds. The separation behavior of the new column was investigated by comparing it with the widely used non-polar polydimethylsiloxane and polar poly(ethylene glycol) columns. The CP-LOWOX column has unique selectivity for aldehydes and ketones enabling a selective separation of these analytes from predominating hydrocarbon matrices. Application of the CP-LOWOX column for the analysis of polar compounds in ambient air is demonstrated. Sampling was performed by adsorptive enrichment coupled with thermal desorption. The suitability of Tenax TA and a multi-bed adsorbent trap Carbotrap C and Carbotrap) was tested for the sampling of semivolatile carbonyl compounds. Presented at Balaton Symposium on High Performance Separation Methods, Siófok, Hungary, September 1–3, 1999     

Determination of volatile organic compounds in ambient air. Comparison of methods

Two analytical methods for the determination of benzene vapour in ambient air are compared in this paper. The methods differ from each other in the sampling technique, type of sorbent, method of extraction and method of detection. The investigation of the methods using various techniques for sample analysing showed a significant influence of the way in which the analysis is carried out, on the final result of the analysis. Calculation of the standard deviations, relative standard deviations and confidence intervals allowed for assessment of the precision and repeatability of the methods. Of the two examined methods, that using an automated system of contaminant sampling and thermodesorption was more precise. This method has been applied to measurements of concentrations of benzene, toluene and xylenes in ambient air.     

Construction and validation of automated purge-and-trap-gas chromatography for the determination of volatile organic compounds

An automated purge-and-trap chromatographic system for the determination of dissolved volatile organic compounds in aqueous samples was built in the laboratory with minimum cost both in the construction and routine operation. This system was built upon a commercial gas chromatograph with full automation capability using self-developed hardware and software. The use of a multi-sorbent bed quantitatively trapped a wide range of volatile organic compounds at ambient temperature, including the extremely volatile ones such as dichlorofluoromethane (CFC-12). Flash heating for rapid desorption and adequate plumbing for minimizing dead volume resulted in excellent chromatographic separation at above-ambient temperatures, which eliminated the need for cryogen for cooling at the head of the column, a second refocusing stage, or entire GC oven for refocusing. This cryogen-free system was tested with standard solutions and environmental samples for determining hydrocarbons with flame ionization detection, and halogenated compounds with electron-capture detection. An innovative method was also developed for validating the system's linearity for extremely volatile compounds. By introducing ambient air, which usually contains constant levels of anthropogenic halocarbons, e.g., CFC-12 and CFC-11 (CCl3F), the need to prepare aqueous standards containing extremely volatile compounds is avoided, hence providing a convenient method for evaluating a purge-and-trap system.     

Two-dimensional gas chromatography for determination of volatile compounds in ambient air

A two-dimensional gas chromatograph is described for the analysis of volatile compounds. The chromatographic system consists of two separate chromatographs linked together with an interface containing an intermediate trap. The trap is cooled with nitrogen (?150°C) and a cryogradient is created inside the trap enclosure. The sample is reinjected during controlled (chromatographic) conditions, using thermostated air. The sample components are eluted from the trap as narrow symmetrical peaks; the shape and width of the peaks eluted do not noticeably affect the subsequent chromatography. The enrichment of n-butane in the trap is quantitative in the range 10^?10 – 10^?5 g.     

Novel multi-sorbent for sampling and determination of trace volatile organic sulfur compounds in ambient air

Novel adsorbent APSG-MW (average particle size 215?µm and specific surface 98 m²?g^?1) bonding multi-walled carbon nanotubes (MWCNTs) on silica gel are obtained. Then the sampling tubes filled with Tenax TA and APSG-MW are prepared and the adsorptive capacity of Tenax TA/APSG-MW for volatile organic sulfur compounds (VOSCs) is studied. The data show that the adsorption and desorption recoveries of multi-sorbent for VOSCs are satisfactory (>85%), and the breakthrough values are large (>16?L?g^?1) enough to absorb VOSCs in ambient air. The sampling precision of the sorbent tubes meets TO-17 criteria. The sampling tubes are successfully used to concentrate and analyze a sample of landfill air, and the major S compounds are identified.     

Fullerenes-extracted soot: a new adsorbent for collecting volatile organic compounds in ambient air

Fullerenes-extracted soot (FES) is the by-product of fullerenes production. Retention characteristics at different temperatures for 17 volatile organic compounds (VOCs) on FES are measured. The adsorption and desorption efficiencies for VOCs on FES adsorbent tubes range from 40.8 to 117%, most of them being 100+/-20%. The values are compared with Tenax GR, an adsorbent commonly used in environmental analysis. FES can be used as an adsorbent of low cost to collect VOCs in environmental samples.     

Construction and validation of an automated spray-and-trap gas chromatograph for the determination of volatile organic compounds in aqueous samples

An automated spray-and-trap (ST) chromatographic system was constructed for fast and efficient extraction of volatile organic compounds (VOCs) in aqueous samples with the capability to be deployed in the field for unattended continuous monitoring of surface or ground water. This system was built upon a commercial gas chromatograph with full automation capability using self-developed hardware and software. For sample analysis, fine droplets of the aqueous solution were generated in the extraction chamber by pressure expansion of a clean air stream through a spray nozzle. A portion of the VOCs distributed into the gas phase was retained by a multi-sorbent micro-trap kept at ambient temperature. Flash heating of the sorbent trap desorbed the enriched VOCs onto the gas chromatography (GC) with flame ionization detection (FID) for hydrocarbons or electron-capture detection (ECD) for halocarbons. In order to validate the performance of the ST method. it was compared with a more conventional method, i.e., a purge-and-trap (PT), by analyzing a serious of standard solutions containing benzene, toluene, ethylene. and o-, m-xylenes. Using a purge-and-trap method as a reference for complete extraction, the ST method showed less sensitivity. Extraction recoveries are in consistent with Henry's law constants. To test response time the ST-GC-ECD was periodically switched between tap and underground waters. Negligible carry-over of halogenated species and reproducibility better than 2% relative standard deviation (R.S.D.) can be achieved regardless of large concentration difference between the two sources, thus demonstrating applicability of the ST system for on-site monitoring.     

Construction of a cryogen‐free thermal desorption gas chromatographic system with off‐the‐shelf components for monitoring ambient volatile organic compounds

An automated gas chromatographic system aimed at performing unattended measurements of ambient volatile organic compounds was configured and tested. By exploiting various off‐the‐shelf components, the thermal desorption unit was easily assembled and can be connected with any existing commercial gas chromatograph in the laboratory to minimize cost. The performance of the complete thermal desorption gas chromatographic system was assessed by analyzing a standard mixture containing 56 target nonmethane hydrocarbons from C₂–C₁₂ at sub‐ppb levels. Particular attention was given to the enrichment efficiency of the C₂ compounds, such as ethane (b.p. = –88.6°C) and ethylene (b.p. = –104.2°C), due to their extremely high volatilities. Quality assurance was performed in terms of the linearity, precision and limits of detection of the target compounds. To further validate the system, field measurements of target compounds in ambient air were compared with those of a commercial total hydrocarbon analyzer and a carbon monoxide analyzer. Highly coherent results from the three instruments were observed during a two‐month period of synchronized measurements. Moreover, the phenomenon of opposite diurnal variations between the biogenic isoprene and anthropogenic species was exploited to help support the field applicability of the thermal desorption gas chromatographic method.     

A versatile and uncomplicated method for the analysis of volatile organic compounds in ambient urban air

A method was developed for sampling and selective quantitative determination of typical volatile organic compounds (VOCs) in ambient urban air. A mobile and self-contained dual-channel air sampling tool based on solid phase adsorption was constructed. A simple calibration procedure circumventing the adsorption/desorption process was designed. The method was validated for seven “key-analytes”: n-hexane, 3-methyl-2-pentene, benzene, tetrachloroethene, styrene, 1,2,4-trimethylbenzene and acetophenone. The complete air sampling equipment is easily accommodated in a business suitcase. The lower limits of the practical working ranges are between 0.1 μg m^–3 (tetrachloroethene) and 1.2 μg m^–3 (benzene). Air samples were collected at a location in Salzburg with heavy motor vehicle traffic and measured in order to prove a satisfactory method performance under practical monitoring conditions. Received: 4 January 1998 / Revised: 14 September 1998 / Accepted: 21 October     

A versatile and uncomplicated method for the analysis of volatile organic compounds in ambient urban air

A method was developed for sampling and selective quantitative determination of typical volatile organic compounds (VOCs) in ambient urban air. A mobile and self-contained dual-channel air sampling tool based on solid phase adsorption was constructed. A simple calibration procedure circumventing the adsorption/desorption process was designed. The method was validated for seven “key-analytes”: n-hexane, 3-methyl-2-pentene, benzene, tetrachloroethene, styrene, 1,2,4-trimethylbenzene and acetophenone. The complete air sampling equipment is easily accommodated in a business suitcase. The lower limits of the practical working ranges are between 0.1 μg m^–3 (tetrachloroethene) and 1.2 μg m^–3 (benzene). Air samples were collected at a location in Salzburg with heavy motor vehicle traffic and measured in order to prove a satisfactory method performance under practical monitoring conditions.     

Construction of an automated gas chromatography/mass spectrometry system for the analysis of ambient volatile organic compounds with on-line internal standard calibration

An automated sampling and enrichment apparatus coupled with a gas chromatography/mass spectrometry (GC/MS) technique was constructed for the analysis of ambient volatile organic compounds (VOCs). A sorbent trap was built within the system to perform on-line enrichment and thermal desorption of VOCs onto GC/MS. In order to improve analytical precision, calibration accuracy, and to safe-guard the long-term stability of this system, a mechanism to allow on-line internal standard (I.S.) addition to the air sample stream was configured within the sampling and enrichment apparatus. A sub-ppm (v/v) level standard gas mixture containing 1,4-fluorobenzene, chloropentafluorobenzene, 1-bromo-4-fluorobenzene was prepared from their pure forms. A minute amount of this I.S. gas was volumetrically mixed into the sample stream at the time of on-line enrichment of the air sample to compensate for measurement uncertainties. To assess the performance of this VOC GC/MS system, a gas mixture containing numerous VOCs at sub-ppb (v/v) level served as the ambient air sample. Various internal standard methods based on total ion count (TIC) and selective ion monitoring (SIM) modes were attempted to assess the improvement in analytical precision and accuracy. Precision was improved from 7-8% RSD without I.S. to 2-3% with I.S. for the 14 target VOCs. Uncertainties in the calibration curves were also improved with the adoption of I.S. by reducing the relative standard deviation of the slope (Sm%) by an average a factor of 4, and intercept (Sb%) by a factor of 2 for the 14 target VOCs.     

热脱附-气相色谱-质谱法测定环境空气中67种挥发性有机物

采用热脱附-气相色谱-质谱法,建立了同时分析环境空气中67种挥发性有机物的分析方法。对比了5种不同填充材料不锈钢吸附管对78种挥发性有机物的吸附能力。填充材料为Tenax TA和Carbograph 1TD的混合填料吸附管对分析物的捕集效果最好,在30 mL/min高纯He气持续吹脱45 min的情况下,未发生穿透（即穿透率小于10%）的化合物达67种,分析物的种类包括芳香烃、脂肪烃、卤代烃和含氧挥发性有机物等。优化了使用该吸附管测定67种目标物时的热脱附条件。在5~100 ng范围内,目标化合物的色谱响应值与其量间具有良好的线性关系,其相关系数（r）均在1.0000~0.9977之间。方法检出限为0.3~2.4 ng,以采样体积1 L计算,检出限为0.3~2.4 μg/m³。加标量为20 ng时,7次重复实验目标化合物回收率均在81.6%~114.9%之间,目标化合物的相对标准偏差为1.2%~10.1%。采用该方法对某车厢内空气进行了检测,检出了包括酯类、卤代烷烃、卤代烯烃以及芳香族化合物在内的19种目标化合物,其范围为1.1~84.1 μg/m³。该方法准确、可靠、灵敏度高,实现了对环境空气中67种目标污染物的准确定量。     

Sorbent-based sampling methods for volatile and semi-volatile organic compounds in air. Part 2. Sorbent selection and other aspects of optimizing air monitoring methods

Sorbent tubes/traps are widely used in combination with gas chromatographic (GC) analytical methods to monitor the vapour-phase fraction of organic compounds in air. Applications range from atmospheric research and ambient air monitoring (indoor and outdoor) to occupational hygiene (personal exposure assessment) and measuring chemical emission levels. Part 1 of this paper reviewed the main sorbent-based air sampling strategies including active (pumped) tube monitoring, diffusive (passive) sampling onto sorbent tubes/cartridges plus sorbent trapping/focusing of whole air samples that are either collected in containers (such as canisters or bags) or monitored online. Options for subsequent extraction and transfer to GC(MS) analysis were also summarised and the trend to thermal desorption (TD)-based methods and away from solvent extraction was explained. As a result of this trend, demand for TD-compatible sorbents (alternatives to traditional charcoal) is growing. Part 2 of this paper therefore continues with a summary of TD-compatible sorbents, their respective advantages and limitations and considerations for sorbent selection. Other analytical considerations for optimizing sorbent-based air monitoring methods are also discussed together with recent technical developments and sampling accessories which have extended the application range of sorbent trapping technology generally.     

An automated monitoring system for VOC ozone precursors in ambient air: development,implementation and data analysis

An automated system for the monitoring of volatile organic compound (VOC) ozone precursors in ambient air is described. The measuring technique consists of subambient preconcentration on a cooled trap followed by thermal desorption and GC/FID analysis. First, the technical development, which permits detection limits below 0.05 ppbv to be reached, proceeded in two steps: (1) the determination of optimum sampling parameters (trap composition and conditioning, outlet split, desorption temperature); (2) the development of a reliable calibration method based on a highly accurate standard. Then, a 4-year field application of the hourly measuring chain was carried out at two urban sites. On the one hand, quality control procedures provided the best VOC identification (peak assignment) and quantification (reproducibility, blank system control). On the other hand, the success and performances of the routine experience (88% of the measurements covered more than 40 target compounds) indicated the high quality and suitability of the instrumentation which is actually applied in several French air quality monitoring networks. Finally, an example of data analysis is presented. Data handling identified important organic compound sources other than vehicle exhaust.