Showing papers on "Vapours published in 2003"

Thick film organic vapour sensors based on binary mixtures of metal oxides

Abstract: Previously, composite sensors incorporating differing proportions of tin dioxide and zinc oxide were reported which exhibited high sensitivity to a range of organic vapours under dry conditions (0% relative humidity, RH). Most importantly, the composite sensors exhibited significantly higher sensitivity than sensors constructed solely from tin dioxide or zinc oxide when tested under identical experimental conditions. Work is reported which was carried out on an extended range of composite materials, and involved testing these composites to a range of vapours that have been identified as being associated with the microbial spoilage of cereal grains in storage. Both previously and in this investigation, tests at 0% RH showed that the tin dioxide/zinc oxide sensor elucidated the highest sensitivity to alcohol vapours. However, for the purpose of a practical device it would be necessary to operate the sensors under a flow of high humidity in order to nullify the effects of variations in grain moisture levels. The sensitivity of the composite sensors to known concentrations of volatile organic compounds was measured over a range of relative humidities (0–100%) at a constant temperature of 20 °C. A composite sensor comprising 50:50 (w/w) tin dioxide/indium oxide was found to give the highest sensitivity when tested to alcohol vapours at 100% relative humidity. The work also highlighted a difference in the response of sensors to classes of organic compounds when operated at differing humidities. For example, the response to alcohols was reduced significantly when operated at high humidity, whereas the response to carbonyl containing compounds particularly ketones was relatively unaffected.

Cholesteric liquid crystals for detection of organic vapours

Abstract: Studies on the use of cholesteric liquid crystals (CLCs) as sensing phase for detection of organic vapours in air are described. Stock solutions of 1.0% (w/v) cholesteryl nonanoate (CN) and cholesteryl chloride (CC) were prepared in tetrahydrofuran. Binary mixtures, with compositions ranging from 0.18 to 0.25% of CC and 0.82–0.75% of CN, respectively, were prepared by appropriate mixing of the stock solutions. Films were cast by pipetting three 10 μl aliquots of the CLC solution mixture onto a glass disk, whose reverse side was made black to absorb unscattered light. The glass disk was adapted to the common end of a bifurcated optical fibre bundle and placed in a glass vial, which provided a headspace of organic vapours. Measurements were carried out at 27±1 °C, a temperature in which the CLC mixtures maintain their liquid crystalline properties. The responses of the CLC mixtures to vapours of ethanol, acetone, benzene, pyridine and hexane were investigated. The colour of the sensing phases depended on their compositions and exposure to organic vapours gives rise to a change in the optical characteristics of liquid crystals. It was found that the CLC layers containing 0.23–0.25% of CC had no significant change in optical properties when exposed to organic vapours and that ethanol did not cause any optical changes in the liquid crystal layers. Benzene as well as hexane always turned all the coloured liquid crystalline layers to colourless. The CLC layers exhibited different behaviours to vapours of acetone and pyridine. For example, the wavelengths of maximum scattering for the 0.19% CC layer were 530 nm in air, 545 nm in pyridine and 580 nm in acetone. The CLC layers showed reversibility. The lifetimes of these layers (interval of time in which the liquid crystalline phase exists, before crystallisation) were investigated by employing acetone and n -hexane vapours. Average lifetimes of 14–15 min were found for films in contact with these vapours, while a lifetime of 205 min was possible when the CLC film was exposed to air.

Modelling the formation of organic particles in the atmosphere

Abstract: . Particle formation resulting from activation of inorganic stable clusters by a supersaturated organic vapour was investigated using a numerical model. The applied aerosol dynamic model included a detailed description of the activation process along with a treatment of the appropriate aerosol and gas-phase processes. The obtained results suggest that both gaseous sulphuric acid and organic vapours contribute to organic particle formation in continental background areas. The initial growth of freshly-nucleated clusters is driven mainly by condensation of gaseous sulphuric acid and by a lesser extent self-coagulation. After the clusters have reached sizes of around 2 nm in diameter, low-volatile organic vapours start to condense spontaneously into the clusters, thereby accelerating their growth to detectable sizes. A shortage of gaseous sulphuric acid or organic vapours limit, or suppress altogether, the particle formation, since freshly-nucleated clusters are rapidly coagulated away by pre-existing particles. The obtained modelling results were applied to explaining the observed seasonal cycle in the number of aerosol formation events in a continental forest site.

Poly(alkoxy-bithiophenes) sensors for organic vapours

Abstract: This work reports electrical resistance measurements and on line UV–Vis–NIR spectra for sensors, fabricated from poly(3,3′-dipentoxy-2,2′-bithiophene) (I) or poly[3,3′-2-(2-ethoxy)ethoxy-2,2′-bithiophene] (II) doped with FeCl 3 , Fe(ClO 4 ) 3 , or CuCl 2 , and tested against vapours of o -xylene, n -butanol, benzene, methanol, n -heptane, and 3-pentanone over a 50–66,444 ppm (v/v) concentration range in argon. Each sensor was exposed repeatedly and randomly to the test vapours in a wide concentration range over a maximum of 41 times and 25 days. Neat argon sweeping was performed between test vapour injections to recover base line conditions and test sensor reversibility. The percentage changes of electrical resistance, relative to the value measured before exposure, and of the sensor absorbance at 880–1400 nm, both varied linearly according to the concentration used. The sensitivities α were calculated from the linear regression analysis of 80 sets of Δ R , C values for a total of about 400 measurements, in order to assess the influence of several parameters, i.e. the chemical nature of the sensor, the fabrication, the exposure time to the test vapour, and the ageing in air. The data analysis demonstrated that sensitivity depends on the chemical nature of both the sensor and the test vapour and on the length of exposure. Data collected with a 90 s exposure gave the narrower range of standard deviation values, amounting to 14% of the calculated α value in the worst case, whereas α values over the range of sensor and test vapour combinations varied from 3.4×10 −5 for I doped FeCl 3 versus MeOH to 231×10 −5 for I doped with Fe(ClO 4 ) 3 versus o -xylene. A significant base line drift was observed at the longest working time, but it was not proven to affect the sensitivity of the sensor. The concomitant changes of the sensors and of the electronic spectra offer intriguing scope to further investigate the nature of the sensor–test vapour interaction.

Ion-beam sputtered palladium-fluoropolymer nano-composites as active layers for organic vapours sensors

Abstract: Ion-beam sputtering deposited palladium-fluoropolymer (Pd-CFx) nano-structured composite films have been investigated as active layers for the detection of organic solvents vapours. The composites sensing properties have been studied and compared to those of homologous Au-CFx films by means of ellipsometric and quartz crystal microbalance (QCM) experiments. Differences have been shown between sensitivity, selectivity and response repeatability of Au-CFx and Pd-CFx sensors exposed to organic solvent vapours. X-ray photoelectron spectroscopy (XPS) analysis has revealed strong similarities between the two materials as to the polymer matrix structure, but higher concentration of metal oxides and fluorides in the case of palladium. The results of the analytical characterisation have been used to discuss the performance level of the sensors.

Standardisation of a Vapour Generator for Calibration of Environmental Monitoring Instruments

Abstract: Very low vapour pressure of 2,4,6 trinitrotoulene (TNT) yields extremely low vapour concentrations at different flow rates in the air, yet considerable quantity of vapours and TNT dust during handling may be present at the workplace environment which is harmful to the health of the personnel working there. The explosive vapours, such as TNT, 2,6-dinitrotoluene (DNT), etc., though harmful to the health of the personnel, are not covered either in the emission standards or in the ambient air quality standards. Presently, no instrument is available for air monitoring of TNT vapours. These vapours need to be collected on-site to estimate TNT concentration. Realising the need for real-time air monitoring of TNT, efforts have been made to understand and device an instrument for on-site determination of TNT vapours parts per billiion (ppb) range. Low-level TNT vapours and TNT buried in the soil in military operations are required to be detected. The instruments for this require careful calibration to yield accurate and reliable results. Hence, an effort has been made to develop a trace-level ppb vapour generator. The vapour generator of a spiral glass column of length 170 cm and inner diameter 4 mm 2 0.5 mm has been used. An activated charcoal glass tube has been used for sampling TNT vapours. The adsorbed TNT vapours were desorbed and analysed using high performance liquid chromatography. The solid support used has been studied. These vapours generated at different flow rates have been evaluated. The calibrated instrument can be used for in situ and on-site analysis of samples of TNT and also for samples collected.

Chromatography method and device with recovery of solvent

Abstract: A method and device for chromatographic separation producing at least two fractions are provided, in which: (i) the solvent is at least partially evaporated from the fractions produced; (ii) the solvent vapours thus produced are separated; stage (i) of the method being carried out, for at least one of the fractions, at least partially with the separated solvent vapours originating from one of the other evaporated fractions.