Development of a monitoring system for vinyl chloride gas in air by using an HCl monitoring tape and pyrolyzer

A continuous monitoring system for vinyl chloride gas in air has been developed using an HCl monitoring tape and pyrolyzer consisting of a heater around a quartz tube. It is based on the color change of the tape by reaction with HCl gas produced by decomposition of vinyl chloride gas in the heated quartz tube. The conversion efficiency of vinyl chloride into HCl depends on the temperature of the pyrolyzer. The tape impregnated with a coloring solution that includes Metanil Yellow (pH indicator; pH 1.2-2.3, red-yellow), glycerin and methanol is a highly sensitive means of detecting HCl gas. When vinyl chloride gas was passed through the heated quartz tube (910 degrees C) and the HCl gas produced was passed through the tape, the color of the tape changed from yellow to red. The degree of color change was proportional to the concentration of vinyl chloride gas with a constant sampling time and flow rate. The degree of color change could be recorded by measuring the intensity of reflecting light (555 nm). This method is scarcely affected by other gases with the exception of chlorinated hydrocarbons such as trichloroethylene and chloroform or strong acids such as HCl gas. Reproducibility tests showed that the relative standard deviation of the relative intensity (n = 10) was 4.5 for 5 ppm vinyl chloride. The detection limit was 0.4 ppm for vinyl chloride with a sampling time of 40 s and a flow rate of 300 ml min (-1).     

Ion mobility spectrometry as flow-injection detector and continuous flow monitor for aniline in hexane and water

Ion mobility spectrometry (IMS) was used as a flow-injection detector to quantitatively examine the ionization chemistry of aniline in hexane. A 5-microl sample was vaporized at 15-90-sec intervals in a flowing air stream and analyzed with an IMS equipped with acetone reactant ion chemistry, ambient temperature drift tube and membrane-based inlet. Precision was 3-11% relative standard deviation for 1-100 ppm aniline in hexane with 90-sec injection intervals and detection limits were ca. 0.5 ppm with 5-microl injections. Matrix effects with amine and organic solvent mixtures were observed and corrected for low and medium proton affinity interferences with standard addition methods. Pronounced fouling of the IMS occurred when a continuous water flow was introduced for aqueous flow injection-IMS. Continuous water monitoring without degraded IMS performance was possible by sampling air flow through a Silastic tube immersed in an aqueous sample.     

膜萃取-气相色谱/微分离子迁移谱检测水中的1,4-二恶烷

利用膜萃取-气相色谱/微分离子迁移谱（ME-GC/DMS）对水中的1,4-二恶烷污染物进行了检测。考察了射频电压、采样流速、膜渗透时间、Trap预富集时间等参数对检测二恶烷的影响规律。结果显示：在优化条件下,二恶烷的定量线性范围为2.0~20.0 μg/L,检出限为0.67 μg/L。实验证明,二恶烷与5种氯代烃的混合物在ME-GC/DMS的二维分离谱图中得到特异性响应,增加了识别的准确性。该研究为发展现场实时监测地下水中污染物的方法提供了重要参考。     

Membrane in tandem with a helical sorbent trap as continuous sampling technique of the polyvinyl chloride thermo-oxidative degradation products for their on-line gas chromatographic monitoring

A flat membrane in tandem with a helical sorbent trap has been used for continuous sampling of the volatile organic products generated in the thermal degradation process of the polyvinyl chloride (PVC) in air, followed by on-line gas chromatographic separation and mass spectrometric identification. The membrane and trap tandem makes automatic collection, concentration, and injection of PVC volatile and semivolatile degradation products, and it is simple in terms of instrumentation and operation. The poly(dimethylsiloxane) (PDMS) membrane used in this study shows a low permeation for oxygenated derivatives and a high permeation for volatile aromatic and non-aromatic hydrocarbon, and chlorinated hydrocarbons. Consequently, the final chromatogram is significantly simplified. By heating the trap at fixed intervals of time, consecutive gas chromatograms are obtained in the monitoring process. The sensitivity of the method depends on the parameters that affect the time of trapping, and the permeation through the membrane.     

The permeation of binary gas mixtures through support structures of composite membranes

The dusty gas model (DGM) is used to describe transport of binary gas mixtures through porous membrane supports to quantify the resistance towards permeation. The model equations account for three different transport mechanisms for the permeating components: conventional viscous pore flow, Knudsen diffusion, and binary diffusion. Experimental data obtained with the uncoated membrane supports are used to determine the morphological parameters needed in the DGM equations. Flat sheet and hollow fiber membrane supports are characterized by the permeation of a TCE/nitrogen vapor. The DGM shows an excellent fit to experimental data when the asymmetric structure of the membrane supports is taken into account, but the morphological parameters cannot necessarily be related to precise physical structure parameters such as pore size, porosity, and tortuosity. The DGM works well even when the membrane supports are modeled as a single homogenous structure. The membrane supports exhibit different resistances towards the various transport mechanisms that occur within the porous support and the resistances vary with process conditions so that support optimization is not straightforward. With the analysis presented in this paper and transport equations specific to the dense coating and module geometries, the influence of the support layer on gas or vapor separation can be quantified.     

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.     

A simulation model study of the coupled field in the IMS drift tube

The performance of the IMS was influenced by many parameters, like temperature, gas flow rate, etc. in the drift tube. An exact and comprehensive simulation model was very useful for the IMS design and optimization. A combined simulation model was build up for the parameters simulation in the drift tube. Based on this simulation model, the heat transfer, velocity distribution, humidity and ion transportation inside the drift tube in bidirectional flow stream was simulated, and the impact on the IMS was studied. And the simulation was also validated using an IMS constructed in our laboratory. The experiment showed that the RIP intensity weakened as the humidity increasing, but the signal intensity of NO was enhanced first, and then decreased with the humidity increasing sequentially. This can be explained from the simulation results. The simulation results showed that the distribution of the velocity and temperature was not uniformed in the drift tube. And this phenomenon was more clearly when the gas flow velocity increased. It can be seen from the simulation that the humidity in the drift tube region was smaller than the sample moisture, and the resolution of the ion mobility spectrometry will be reduced by the humidity. But in the region rich in water molecules, ultraviolet photons re-acting with acetone would be obviously decreased and fewer re-agent ions were produced owing to the strong absorption of photons by water neutrals. The results showed that the coupled field simulation model can be used to study parameters effects on the IMS.     

Impact of membrane immobilization on particle formation and trichloroethylene dechlorination for bimetallic Fe/Ni nanoparticles in cellulose acetate membranes

The use of membrane immobilization to carry out the batch dechlorination of trichloroethylene (TCE) using bimetallic Fe/Ni (4:1, Fe to Ni) nanoparticles in cellulose acetate membranes is examined using modeling of transport phenomenon based on experimental results. Membranes are synthesized using both gelation and solvent evaporation techniques for phase inversion. The reduction of metal ions within cellulose acetate phase-inversion membranes was accomplished using sodium borohydride reduction to obtain up to 2 wt % total metals. Characterization of the mixed-matrix structure reveals a bimodal particle distribution ranging between 18 and 80 nm within the membrane cross section. The distribution is the result of changes in the morphology of the cellulose acetate support. The diffusivity and linear partitioning coefficient for the chlorinated organic were measured and are 2.0 x 10(-8) cm2.s-1 and 3.5 x 10(-2) L.g-1, respectively. An unsteady-state model for diffusion through a membrane with reaction was developed to predict experimental results with an error of only 7.2%. The error can be attributed to the lack of the model to account for loss of reactivity through pH effects, alloy effects (bimetallic ratio), and oxidation of nanoparticles. Simulations were run to vary the major transport variables, partitioning and diffusivity, and determine their impact on reaction kinetics. Of the two, diffusivity was less significant because it really only influences the time required for maximum TCE partitioning to the membrane to be achieved and has no effect on the limiting capacity of the membrane for TCE. Therefore, selection of an appropriate support material is crucial for development of highly reactive mixed-matrix membrane systems.     

Monte Carlo simulation of ion transport in ion mobility spectrometry

A program for simulation of ion trajectories in ion mobility spectrometry (IMS) instruments has been developed and incorporated into SIMION 7.0 [Int. J. Mass Spectrom. 200 (2000) 3–25]. Simulations were based on elastic collisions between ions and gas particles and conducted for an IMS drift tube. The program was validated by comparing the reduced mobility of helium ions derived from the simulation with the experimental data for helium ions in neon drift gas in low electric fields. Typical IMS parameters, including pressure, temperature, and flow rate of the drift gas were taken into account in the simulations. The program demonstrates capabilities of generating IMS spectra and predicting ion transport efficiency and separating ions. For the IMS drift tube studied, a correlation between imperfection of the electric field distribution and low resolution has been observed.     

Mass spectrometry on-line monitoring and MS2 product characterization of TiO2/UV photocatalytic degradation of chlorinated volatile organic compounds

On-line Mass Spectrometry and MS² are applied to monitor and identify the by-products and total mineralization products of TiO₂/UV photocatalytic degradation of four chlorinated volatile organic compounds (VOCs): trichloroethylene (TCE), tetrachloroethylene (TeCE), chloroform, and dichloromethane. Selected multiple ion mass spectrometry monitoring using characteristic 70 eV electron ionization ionic fragments monitors in real time the destruction of the starting VOC and the formation of by-products, i.e., the degrees of VOC mineralization, as a function of the flow and relative humidity of the carrier gas (synthetic air). Several by-products were detected: phosgene for TCE, TeCE, and chloroform; dichloroacetyl chloride for TCE; and trichloroacetyl chloride for TeCE. Cl₂ and CO₂ were also detected as final mineralization products of the four chlorinated VOCs. Structural characterization of by-products was accomplished via MS² collision-induced dissociation of molecular ions or characteristic ionic fragments.     

Electron beam treatment of chloroethylenes/air mixture in a flow reactor

Decomposition of chloroethylenes under electron beam irradiation in a flow reactor has been studied with different reaction environments, various initial concentrations and in the presence and absence of vaporized water. Three chlorinated ethylenes—dichloroethylene (DCE), trichloroethylene (TCE), perchloroethylene (PCE)—were used as model chlorocarbons. The degree of decomposition was 48% for DCE, 98% for TCE and 90% for PCE in air reaction environment at an initial concentration of 2000 ppm and a dose of 18–20 kGy irradiation. In the presence of water vapor (5600 ppm) decomposition of TCE was about 10% higher than in dry air. The main products were found to be CO, CO₂, HCl, dichloroacetic acid (DCAA), dichloroacetyl chloride (DCAC) and dichloroethyl ester acetic acid (DEAA). DCAA, DCAC and DEAA were identified as chloro-oxygenated hydrocarbons, which could be decomposed with CO and CO₂ production. Concentration profiles show that intermediate products and yields of CO and CO₂ decrease with decreasing number of chlorine substitutions in the initial hydrocarbons.     

苯-N-甲酰吗啉体系膜基吸收传质过程全微分模型研究

根据双膜理论建立了全微分传质动力学模型, 以苯-N-甲酰吗啉(NFM)水溶液体系为代表, 研究了聚丙烯PP疏水性微孔膜接触器的传质过程, 并通过理论模拟及实验考察了气液相流速、气液相进口浓度、液相N-甲酰吗啉浓度、气液流动方式及膜接触器形态对苯传质通量及去除效率的影响. 结果表明, 模拟值与实验值吻合良好, 误差控制在20%以内. 当气相流量或气相进口浓度较低时, 气相传质为控制步骤, 而随着气相流量和气相进口浓度升高, 液相流量对传质过程的影响显著增加. 传质通量随气液相流量和气相进口浓度的增大而增大. 液相进口浓度及膜丝内径的增大显著降低传质通量. 另外, 较薄的膜丝壁厚有利于传质的进行, 气液逆向流方式较同向流方式可获得更高的传质通量.     

On-line monitoring trihalomethanes in chlorinated water by membrane introduction-fast gas chromatography mass-spectrometry

An analytical method based on membrane introduction and fast gas chromatography-mass spectrometry (GC-MS) has been developed for the on-line monitoring of trihatomethanes (THMs) in chlorinated drinking water. The coupling of membrane introduction with fast GC-MS offers the advantage of membrane introduction as an on-line sampling device and fast GC-MS as a separation and identification method. While maintaining the on-line monitoring characteristic of traditional membrane introduction mass spectrometry (MIMS), the difficulty of distinguishing CHCl3 and CHBrCl2 in MIMS was overcome by rapid GC separation and MS analysis. Water permeated across the membrane affected the analysis of CHBr2Cl and CHBr3. A method based on controlling the injection temperature and injection time has been developed to overcome the moisture problem. This method is simple and less time consuming than the conventional moisture removing method. Under typical operating conditions, the sampling rate was about 20 samples h(-1) capable of on-line monitoring THMs in chlorinated drinking water. The detection limits of this system were found to be about 2 ppt, 4 ppt, 4 ppt, and 8 ppt for CHCl3 CHBrCl2, CHBr2Cl, and CHBr3, respectively.     

Sample decomposition in an electrically heated cold-trap inlet system for high speed gas chromatography

Thermal decomposition of some hydrocarbon and chlorinated hydrocarbon compounds in metal capillary tubes used in an inlet system for high speed gas chromatography has been investigated. The metal tube is cooled to about ?75°C by a flow of cold nitrogen gas in order to focus a vapor sample cryogenically. A capacitive discharge power supply is then used to heat the metal tube resistively in order to revaporize the sample and introduce it to the separation column as a plug 5-10 ms wide. The effects of tube temperature, tube material, sample vapor residence time, and type of carrier gas on thermal cracking are described. Use of a copper-nickel alloy tube resulted in less cracking than either pure platinum or pure nickel. Cracking is more significant with hydrogen as carrier gas than with helium. Cracking also increases with increasing sample residence time in the hot tube. Quantitative sample injection with minimum decomposition can be obtained for a variety of aliphatic and aromatic hydrocarbons and chlorinated hydrocarbon compounds.     

Decomposition of halogenated hydrocarbons using intense, penetrating bremsstrahlung

Radiolytic decomposition of chlorinated hydrocarbons and other toxic compounds has been experimentally measured using ionizing radiation produced by electron accelerator and nuclear isotope sources. Decomposition products have been identified. A portable, commercially available electron accelerator was set up at a Superfund site where vapor extraction wells were removing trichloroethylene (TCE) from a spill into the unsaturated soil. The extraction vapor was passed through the accelerator beam to decompose the TCE. On site radiolytic decomposition of TCE vapor using an accelerator is shown to be significantly less expensive than filtration of TCE vapor using activated charcoal.     

Trace analysis of BTEX compounds in water with a membrane interfaced ion mobility spectrometer

A tubular silicone membrane interface has been developed for trace detection of benzene, toluene, ethyl benzene, and xylene (BTEX) compounds in water with a portable ion mobility spectrometer. Effects of flow rate, membrane length and stirring conditions on the IMS signals have been systematically investigated. Besides conventional dynamic mode operation, static mode sampling has been demonstrated for the first time and high sensitivities were achieved by sampling of BTEX contaminated water with static mode operation. A toluene concentration of 0.101 mg l(-1) in purified water, corresponding to a headspace concentration of 2.75 (mug m(-3)), was determined by static mode sampling. Headspace sampling without the membrane interface could not detect toluene at this concentration. This method has high sensitivity for trace concentrations of gasoline components in river water with a response time of several seconds. The apparatus developed is portable and can be used for sensitive detection of organic contaminants in water, with improved performance compared to conventional modes of IMS sampling.     

The continuous removal of oxygen from flowing solutions

An apparatus for continuous removal of oxygen or other dissolved gases from liquid samples is described; it is useful in continuous analyses. The gas diffuses through a semipermeable membrane into a space with a lower partial pressure of the particular gas. The separation unit consists of two concentric tubes and is practical and efficient. The mathematical model for transport under conditions of stationary gas diffusion with laminar flow of the liquid in the tube did not correspond satisfactorily to the experimental relationship, probably because of turbulence in the flow.     

The determination of chlorinated aliphatic hydrocarbons in air, natural waters, marine organisms, and sediments

A gas Chromatographic procedure is described for the determination of chlorinated aliphatic hydrocarbons in the atmosphere, natural waters, aquatic organisms and sediments. Air samples are passed through activated carbon traps and the chloro compounds are later desorbed by heating in a current of nitrogen. Chloro compounds are stripped from water samples by bubbling with nitrogen and from bio-materials and sediments by heating in a current of nitrogen. In each instance, the chlorinated compounds are trapped in copper columns packed with Chromosorb coated with silicone oil, and cooled to -78°. The chloro compounds are subsequently swept off these columns into a gas chromatographic column with a current of argon. Detection of the Chromatographic peaks is performed with an electron-capture detector. The procedure gives near quantitative recoveries of a range of chlorinated hydrocarbons from natural samples.     

Chlorinated Organic Compounds Decomposition in a Dielectric Barrier Discharge

The decomposition of chlorinated volatile organic compounds by non-thermal plasma generated in a dielectric barrier discharge was investigated. As model compounds trichloroethylene (TCE) and 1,2-dichloroethane (DCE) were chosen. It was found that TCE removal exceeds 95% for input energy densities above 0.2 eV/molecule, regardless of the initial concentration of TCE, in the range 100–750 ppm. On the other hand, DCE was more difficult to decompose, the removal rate reached a maximum of 60% at the highest input energy used. For both investigated compounds the selectivity towards carbon dioxide was significantly influenced by their initial concentration, increasing when low concentrations were used. The gas flow rate had also an effect on CO₂ selectivity, which is higher at low flow rate, due to the higher residence time of the gas in the plasma. The best values obtained in these experiments were around 80%.     

Hollow-fiber-type pore-filling membranes made by plasma-graft polymerization for the removal of chlorinated organics from water

Hollow-fiber-type pore-filling membranes were prepared to reduce the emission of toxic chlorinated organics into the environment. These membranes can remove 1,1,2-trichloroethane (TCE) or dichloromethane (DM) from water, and concentrate them in the permeate. The pore-filling membrane can efficiently remove organics from water because of the suppression of the membrane swelling by the porous substrate matrix, and the fact that it can maintain a high solute diffusivity, because of the linear graft chains that fill the substrate pores. Laurylacrylate (LA) or n-butylacrylate (BA) grafted layers were formed inside the porous hollow-fiber substrate, and the pores were filled with the grafted chains formed from plasma-initiated graft polymerization. The hollow-fiber-type LA-grafted membranes showed extremely high separation properties: a 0.09 wt.% TCE aqueous solution was condensed to 99 wt.% TCE in the permeate. The membrane can remove TCE from a water stream, and at the same time, the membrane can purify the TCE for re-use. The membrane also showed high separation performance for an aqueous DM solution. The mass transfer resistance outside the membrane was estimated by using a concentration polarization model. When the mass transfer coefficient at the membrane and feed stream boundary layer was below 10⁻⁴ m/s, the boundary layer resistance affected the membrane performance. This needs to be taken into account when designing the membrane module and operating conditions.