Simultaneous quantitative determination of benzene, toluene, and xylenes in water using mid-infrared evanescent field spectroscopy

Attenuated total reflection mid-infrared spectroscopy is applied for simultaneous detection and quantification of the environmentally relevant analytes benzene, toluene, and the three xylene isomers. The analytes are enriched into a thin polymer membrane coated onto the surface of an internal reflection waveguide, which is exposed to the aqueous sample. Direct detection of analytes permeating into the polymer coating is performed by utilizing evanescent field spectroscopy in the fingerprint range (>10 microm) of the mid-infrared (MIR) spectrum (3-20 microm) without additional sample preparation. All investigated compounds are characterized by well-separated absorption features in the evaluated wavelength regime. Hence, data evaluation was performed by integration of the respective absorption peaks. Limits of detection lower than 20 ppb (v/v) for all xylene isomers, 45 ppb (v/v) for benzene, and 80 ppb (v/v) for toluene have been achieved. The straightforward experimental setup and the achieved detection limits for these environmentally relevant volatile organic compounds in the low-ppb concentration range reveal a substantial potential of MIR evanescent field sensing devices for on-line in situ environmental analysis.     

Angular output of hollow, metal-lined, waveguide Raman sensors

Hollow, metal-lined waveguides used as gas sensors based on spontaneous Raman scattering are capable of large angular collection. The collection of light from a large solid angle implies the collection of a large number of waveguide modes. An accurate estimation of the propagation losses for these modes is required to predict the total collected Raman power. We report a theory/experimental comparison of the Raman power collected as a function of the solid angle and waveguide length. New theoretical observations are compared with previous theory appropriate only for low-order modes. A cutback experiment is demonstrated to verify the validity of either theory. The angular distribution of Raman light is measured using aluminum and silver-lined waveguides of varying lengths.     

Amperometric sensors for simultaneous superoxide and hydrogen peroxide detection

A two-channel sensor capable of almost instantaneous simultaneous detection of superoxide radical and hydrogen peroxide in the concentration range 10(-)(7)-10(-)(4) M is very important for understanding of a number of rapid kinetics processes. A glassy carbon working microelectrode covered by an electrodeposited polypyrrole/horseradish peroxidase (PPy/HRP) membrane was employed as a H(2)O(2) sensor. Another glassy carbon microelectrode covered by a composite membrane of an inside layer of PPy/HRP and an outside layer of superoxide dismutase was employed as a working electrode for superoxide detection. These two working electrodes with Pt counter and tungsten oxide (WO(3)) reference electrodes were contained in one 6 mm diameter Teflon cylinder. Simultaneous measurements were performed at a potential of -60 mV (vs WO(3) reference, pH 5.1). Additional sensor characterization was performed for pH 5.1-9.0. Superoxide sensor behavior as a function of membrane deposition conditions and coating time is reported. Sensors' mutual influence, selectivity, response times, linearity, stability, and sensitivity for hydrogen peroxide and superoxide are presented and discussed. A mathematical model of sensors' responses is proposed, with model calculation corresponding to experiment within 10%.     

Development of a hollow waveguide sampler for detection of chlorinated aromatic compounds in soils

In this paper, a Fourier transform infrared (FT-IR) spectroscopic method for detection of chlorinated aromatic compounds in soils was developed. The sensing device of this method was based on an infrared hollow waveguide, the inner surface of which was coated with a hydrophobic film. Vaporized chlorinated aromatic compounds from soils were trapped onto the hydrophobic film of the hollow waveguide sampler following detection by FT-IR spectrometry. The extraction process in this method was similar to the headspace solid-phase microextraction (HSSPME) in principle. Means of increasing the speed of transfer of the vaporized organic species to the sampler were also studied. Results indicated that, with a negative pressure on the end of the sampler, the speed of transfer increased significantly. Vapor pressures of the analytes were used as an indication to test the limitation of this method in the analysis of organic compounds in soils. Results showed that analytes with vapor pressures lower than 12 Torr could be detected quantitatively. The typical R-square of the regression on the concentration and IR signals was around 0.99 and the typical detection limits were in the range of hundreds of parts per billion.     

Tin dioxide-based gas sensors for detection of hydrogen fluoride in air

This research concentrates on the sensitivity of semiconductor tin dioxide-based gas sensors to hydrogen fluoride in air. After evaluating the characteristic detection temperature, the sensor's signals were studied for different HF concentrations. Despite the corrosive effects of hydrogen fluoride, a reproducibility of the signal was found. Likewise, we did not observe any long-term degradation to the sensor. For the experiment, the sensor was exposed to a gas mixture formed by HF, O₂, N₂ with a constant flow rate of 150 ml min⁻¹. The semiconductor gas sensor reached maximum sensitivity near 380 °C, and a minimum concentration was detected approximately 50 ppb. Moreover, the detection phenomenon appears to be reversible when considering the electrical response under a constant air flow.     

Sol-gel derived (Sr,Ni)Al2O4 composites for benzene and toluene sensors

Strontium(II) added NiAl₂O₄ composites prepared by sol-gel technique was utilized for the detection of benzene and toluene vapors. XRD, SEM and BET measurements were employed to identify the structural phases, surface morphology and surface area. The (Sr,Ni)Al₂O₄ composites sintered at 900 °C were subjected to dc resistance measurements in the temperature range of 30-200 °C to study the benzene and toluene vapor detection characteristics. The maximum sensitivity of the composites was obtained at 150 °C. An increase in the sensitivity was observed with increase in benzene and toluene concentration from 100 to 5000 ppm at 150 °C. Among the different compositions, the (Sr,Ni)Al₂O₄ composites with higher strontium content (NiSA5) showed the best sensitivity towards benzene and toluene vapor detection.     

Real-time detection of bacterial contamination in dynamic aqueous environments using optical sensors

Here we describe on-line, real-time detection of waterborne bacteria using an optical sensor based on a starburst dendrimer film containing a lipophilic fluorophore. The sensor is constructed via covalent coupling between amine-terminated polyamidoamine dendrimer and silanized glass through an amide bond. The reporter molecule is embedded in the dendrimer layer through host-guest interaction. Real-time automated detection and quantitation of the bacteria are realized by using a charge-coupled detector camera and customized imaging and analysis software. The sensor responds to bacteria introduced to an aqueous flow system within 1 min. The limit of detection is approximately 10(4)cells/mL. The operational lifetime is more than 64 h, and the storage lifetime of the sensor is at least 7 months.     

Characterization of a mid-infrared hollow waveguide gas cell for the analysis of carbon monoxide and nitric oxide

Infrared spectroscopy is commonly applied to the analysis of small gas-phase molecules. One of the limitations of using Fourier transform infrared (FT-IR) spectroscopy for these applications is the time response of long path length gas cells. Hollow waveguides (HW) that transmit in the mid-infrared spectral range have higher optical efficiencies compared to long path length cells due to smaller cell volumes. This study characterizes a silver coated, 2 mm inner diameter HW for the analysis of carbon monoxide (CO) and nitric oxide (NO) and compares the performance to a 3 m gas cell and traditional gas analyzers. The HW was found to have a CO response time less than the NDIR analyzer and approximately one-tenth of the response time on the FT-IR system equipped with a 3 m gas cell. The utility of the increased response time was demonstrated by measuring CO concentrations in sidestream cigarette smoke at the same temporal resolution as an NDIR analyzer. A 10 to 60% increase in sensitivity using various frequencies for both CO and NO was observed using the HW compared to the 3 m multipass gas cell. However, cost savings for gas-sensing applications can be achieved on a per analyte basis by using FT-IR spectroscopy, especially in combination with a HW gas-sensing module, which is significantly less expensive than a multipass gas cell.     

气相色谱法测定室内空气中“三苯”的方法研究

针对室内空气中“三苯”特点,在居住区大气中“三苯”的测定方法——气相色谱法(GB 1737-89)的基础上,进一步优化该方法的采样时间、采样流量等条件,使优化后的测定方法更加适应室内空气监测需要的准确性、简便性、快速性的特点。     

气相色谱法测定室内空气中"三苯"的方法研究

针对室内空气中“三苯”特点,在居住区大气中“三苯”的测定方法——气相色谱法(GB1737—89)的基础上,进一步优化该方法的采样时间、采样流量等条件,使优化后的测定方法更加适应室内空气监测需要的准确性、简便性、快速性的特点。     

Indium Tin Oxide thin film gas sensors for detection of ethanol vapours

Indium Tin Oxide (ITO: In₂O₃ + 17% SnO₂) thin films grown on alumina substrate at 648 K temperatures using direct evaporation method with two gold pads deposited on the top for electrical contacts were exposed to ethanol vapours (200-2500 ppm). The operating temperature of the sensor was optimized. The sensitivity variation of films having different thickness was studied. The sensitivity of the films deposited on Si substrates was studied. The response of the film with MgO catalytic layer on sensitivity and selectivity was observed. A novel approach of depositing thin stimulating layer of various metals/oxides below the ITO film was tried and tested.     

Highly selective GaN-nanowire/TiO2-nanocluster hybrid sensors for detection of benzene and related environment pollutants

Nanowire-nanocluster hybrid chemical sensors were realized by functionalizing gallium nitride (GaN) nanowires (NWs) with titanium dioxide (TiO(2)) nanoclusters for selectively sensing benzene and other related aromatic compounds. Hybrid sensor devices were developed by fabricating two-terminal devices using individual GaN NWs followed by the deposition of TiO(2) nanoclusters using RF magnetron sputtering. The sensor fabrication process employed standard microfabrication techniques. X-ray diffraction and high-resolution analytical transmission electron microscopy using energy-dispersive x-ray and electron energy-loss spectroscopies confirmed the presence of the anatase phase in TiO(2) clusters after post-deposition anneal at 700?°C. A change of current was observed for these hybrid sensors when exposed to the vapors of aromatic compounds (benzene, toluene, ethylbenzene, xylene and chlorobenzene mixed with air) under UV excitation, while they had no response to non-aromatic organic compounds such as methanol, ethanol, isopropanol, chloroform, acetone and 1,3-hexadiene. The sensitivity range for the noted aromatic compounds except chlorobenzene were from 1% down to 50 parts per billion (ppb) at room temperature. By combining the enhanced catalytic properties of the TiO(2) nanoclusters with the sensitive transduction capability of the nanowires, an ultra-sensitive and selective chemical sensing architecture is demonstrated. We have proposed a mechanism that could qualitatively explain the observed sensing behavior.     

Trace detection and discrimination of explosives using electrochemical potentiometric gas sensors

In this article, selective and sensitive detection of trace amounts of pentaerythritol tetranitrate (PETN), 2,4,6-trinitrotoluene (TNT) and cyclotrimethylenetrinitramine (RDX) is demonstrated. The screening system is based on a sampling/concentrator front end and electrochemical potentiometric gas sensors as the detector. Preferential hydrocarbon and nitrogen oxide(s) mixed potential sensors based on lanthanum strontium chromite and Pt electrodes with yttria stabilized zirconia (YSZ) solid electrolyte were used to capture the signature of the explosives. Quantitative measurements based on hydrocarbon and nitrogen oxide sensor responses indicated that the detector sensitivity scaled proportionally with the mass of the explosives (1-3 μg). Moreover, the results showed that PETN, TNT, and RDX samples could be discriminated from each other by calculating the ratio of nitrogen oxides to hydrocarbon integrated area under the peak. Further, the use of front-end technology to collect and concentrate the high explosive (HE) vapors make intrinsically low vapor pressure of the HE less of an obstacle for detection while ensuring higher sensitivity levels. In addition, the ability to use multiple sensors each tuned to basic chemical structures (e.g., nitro, amino, peroxide, and hydrocarbon groups) in HE materials will permit the construction of low-cost detector systems for screening a wide spectrum of explosives with lower false positives than present-day technologies.     

An improved approach for accurate quantitation of benzene, toluene, ethylbenzene, xylene, and styrene in blood

Widespread exposure to benzene, toluene, ethylbenzene, xylene, and styrene (BTEXS) and the potential for this exposure to cause health effects drives the need to develop improved methods for measuring exposure. In this work, we demonstrate our latest assay for quantifying BTEXS in blood and characterize sources of both positive and negative biases. This method involves blood sample collection using common techniques followed by static headspace sampling using solid-phase microextraction and gas chromatography/mass spectrometry analysis. We found that the greatest and unexpected source of positive bias was from contamination of butyl rubber materials used in sample preparation consumables such as Vacutainer stoppers, syringe plungers, and sample vial septa. Conversely, the primary cause of negative bias observed was from the diffusion loss of BTEXS from blood during transfer into sample vials. By minimizing or eliminating these and other sources of bias, we improved method accuracy and precision to within 10% while maintaining low-picogram per milliliter detection. Furthermore, upon comparison of these results with those from other laboratories, we observe substantially lower blood BTEXS levels reported to date for nonoccupationally exposed nonsmokers. A relatively unbiased method, as such, will help elucidate any potential associations between adverse health effects and human exposure to low levels of BTEXS.     

Development of an infrared hollow waveguide as a sensing device for detection of organic compounds in aqueous solutions

In this paper, a new detection method based on an infrared hollow waveguide is developed to detect semivolatile to nonvolatile organic compounds in aqueous solutions. The hollow waveguide is produced by chemical deposition of silver on the inner surface of a polyethylene tube. The surface of the silver layer is further coated with a hydrophobic film to attract organic compounds in aqueous solution. Samples were pumped through this hollow waveguide sampler and organic compounds were attracted onto the hydrophobic film. After removal of the residual water molecules in the hollow waveguide sampler, organic compounds can be sensed by conventional Fourier transform infrared (FT-IR) spectrometry. Theoretical aspects of this type of sampler are also presented. The derived analytical equations for this type of sampler were consistent with experimental data. Under the condition of constant hydrophobic film volume, high linearity (R(2) equal to 0.9993) between the concentration of analyte and the detected signal was obtained for concentrations in the range from 2.5 ppm to 50 ppb. By co-adding 100 scans with 4 cm(-)(1) resolution, the typical detection limit in this type of sensing method can be lower than 10 ppb. Several factors such as sampling flow rate, sampling time, and hydrophobic film volume were also investigated in this work.     

Selectivity improvement of semi-conducting gas sensors by selective filter for atmospheric pollutants detection

The monitoring of atmospheric pollution using chemical gas sensors is a challenge due to the lack of selectivity of most existing devices. However, their performances can be improved using filtering films achieving the separation or the removal of some gases. This study is focused on the detection of carbon monoxide and of oxidant pollutants (nitrogen dioxide and ozone) by sensors constituted of SnO₂, or phthalocyanine compounds. Two types of filters were investigated. Filters based on MnO₂ powder are successful to remove ozone while preserving nitrogen dioxide in a large temperature range from ambient to 400 °C, but they partially convert carbon monoxide. The second type of filter constituted of indigo powder is also efficient to remove ozone without modification of nitrogen concentration at ambient. Then, these filters were associated with sensing element. MnO₂ thick films were deposited by screen-printing on SnO₂. With resulting sensors, the interference of ozone for the detection of CO or of NO₂ is reduced, but some technological problems such as adhesion of MnO₂ layer have to be solved. For phthalocyanine devices, the indigo filter placed upstream to the sensitive layer makes the sensor selective to nitrogen dioxide.     

Oxidation of gas phase trichloroethylene and toluene using composite sol-gel TiO2 photocatalytic coatings

The previously developed composite sol-gel (CSG) process is proposed for the deposition of thick (10-50 microm) porous films of photocatalytic TiO2. The CSG titania was developed by binding pre-calcined TiO2 particles with TiO2 sol. It had relatively high surface area (15-35 m2/g) and good resistance against mechanical stress and abrasion. Photocatalytic activity tests were carried out on trichloroethylene (TCE) and toluene, and compared with those of standard Degussa P-25 titania. The CSG photocatalyst provided good photo-efficiency in removing both pollutants from contaminated air streams. When compared with P-25 titania, the CSG photocatalyst showed a similar photo-efficiency with first-order kinetic rate constants not significantly different from that of P-25. For both photocatalysts the rate of photocatalytic oxidation of TCE was significantly greater than that obtained for toluene. Overall, the combination of better mechanical integrity, resistance against abrasion, and comparable photocatalytic efficiency of the CSG titania versus that of P-25 titania, make the composite sol-gel (CSG) photocatalyst a viable alternative for industrial applications where long term stability, superior mechanical properties, and good photo-efficiency are of critical value.     

Effect of TiO2/adsorbent hybrid photocatalysts for toluene decomposition in gas phase

12 hybrid photocatalysts consisting of titania (TiO₂) and an adsorbent such as mordenite were investigated for the photocatalytic decomposition of toluene, a major indoor contaminant in indoor air. The highest decomposition rate was obtained with the use of mordenite and silicon dioxide (SiO₂) as additives to TiO₂. The photocatalytic activities of hybrid photocatalysts in decomposing toluene are 1.33 times as high as pure P25 at the net weight loading of 0.49 mg/cm² under the test condition. Scanning electron microscopy (SEM) images confirmed that the hybrid photocatalyst films were very porously distributed; TiO₂ was adsorbed on the surface of mordenite and SiO₂, increasing the reaction area of TiO₂. The unimolecular Langmuir–Hishelwood model and mass-transfer-based (MTB) method were used to evaluate the reaction coefficients and adsorption equilibrium coefficients of hybrid photocatalysts. It is evidenced that the reaction areas of two hybrid photocatalysts were 1.52 and 1.64 times larger than that of P25, respectively, which is the major reason to make the high removal efficiency of toluene.     

Development of sensors based on CuO-doped SnO2 hollow spheres for ppb level H2S gas sensing

An effort has been made to develop a new kind of SnO₂–CuO gas sensor which could detect an extremely small amount of H₂S gas at relatively low working temperature. The sensor nanomaterials were prepared from SnO₂ hollow spheres (synthesized by employing carbon microspheres as temples) and Cu precursor by dipping method. The composition and structural characteristics of the as-prepared CuO-doped SnO₂ hollow spheres were studied by X-ray photoelectron spectroscopy, X-ray powder diffraction, scanning electron microscopy, and transmission electron microscopy. Gas-sensing properties of CuO-doped SnO₂ hollow sphere were also investigated. It was found that the sensor showed good selectivity and high sensitivity to H₂S gas. A ppb level detection limit was obtained with the sensor at the relatively low temperature of 35 °C. Such good performances are probably attributed to the hollow sphere nanostructures. Our results imply that materials with hollow sphere nanostructures are promising candidates for high-performance gas sensors.