Real-time monitoring of benzene, toluene, and p-xylene in a photoreaction chamber with a tunable mid-infrared laser and ultraviolet differential optical absorption spectroscopy

We describe the implementation of a mid-infrared laser-based trace gas sensor with a photoreaction chamber, used for reproducing chemical transformations of benzene, toluene, and p-xylene (BTX) gases that may occur in the atmosphere. The system performance was assessed in the presence of photoreaction products including aerosol particles. A mid-infrared external cavity quantum cascade laser (EC-QCL)-tunable from 9.41-9.88?μm (1012-1063?cm(-1))-was used to monitor gas phase concentrations of BTX simultaneously and in real time during chemical processing of these compounds with hydroxyl radicals in a photoreaction chamber. Results are compared to concurrent measurements using ultraviolet differential optical absorption spectroscopy (UV DOAS). The EC-QCL based system provides quantitation limits of approximately 200, 200, and 600 parts in 10(9) (ppb) for benzene, toluene, and p-xylene, respectively, which represents a significant improvement over our previous work with this laser system. Correspondingly, we observe the best agreement between the EC-QCL measurements and the UV DOAS measurements with benzene, followed by toluene, then p-xylene. Although BTX gas-detection limits are not as low for the EC-QCL system as for UV DOAS, an unidentified by-product of the photoreactions was observed with the EC-QCL, but not with the UV DOAS system.     

Air monitoring of a coal tar cleanup using a mobile TAGA LPCI-MS/MS

Real-time detection of air toxics is becoming increasingly important in understanding the sources and constituents of air pollution. The detection of low levels of air contaminants requires reliable sampling and calibration techniques, as well as sophisticated analytical instrumentation. Recently, a new low pressure chemical ionization (LPCI) source was developed for ambient air monitoring of benzene, toluene, xylene (BTX) and gas-phase polycyclic aromatic hydrocarbons (PAH) in real-time. This ion source in conjunction with a triple quadrupole (Q1, Q2, Q3) mass spectrometer (trace atmospheric gas analyzer (TAGA IIe)) has been proven highly useful for measuring selected air pollutants. The ion chemistry under LPCI conditions involves charge transfer reactions yielding parent ions which are selected in the first quadrupole, Q1, dissociated in the second quadrupole, Q2, and the resultant daughter ions are then identified by the third quadrupole, Q3. Monitoring of specific parent/daughter (P/D) ion pairs is used to measure concentrations of BTX and selected PAH. The response of the TAGA IIe is characterized through multi-point calibration curves. Detection limits (DL) as low as 0.5 microg/m(3) for BTX and PAH were accomplished by optimizing various TAGA IIe operating parameters. This unique method was applied in November 1999 to monitor emissions released during the cleanup of a historical coal tar site in Kingston, Ontario. This information was used by local officials for enhancing abatement activities or in some cases temporarily halting the excavation when levels of air toxics were higher than allowable provincial guidelines.     

Equilibration-based preconcentrating minicolumn sensors for trace level monitoring of radionuclides and metal ions in water without consumable reagents

A sensor technique is described that captures analyte species on a preconcentrating minicolumn containing a selective solid-phase sorbent. In this approach, the sample is pumped through the column until the sorbent phase is fully equilibrated with the sample concentration, and the exit concentration equals the inlet concentration. On-column detection of the captured analytes using radiometric and spectroscopic methods is demonstrated. In trace level detection applications, this sensor provides a steady-state signal that is proportional to sample analyte concentration and is reversible. The method is demonstrated for the detection of Tc-99 using anion-exchange beads mixed with scintillating beads and light detection, Sr-90 using SuperLig 620 beads mixed with scintillating beads and light detection; and hexavalent chromium detection using anion-exchange beads with spectroscopic detection. Theory has been developed to describe the signal at equilibration and to describe analyte uptake as a function of volume and concentration, using parameters and concepts from frontal chromatography. It is shown that experimental sensor behavior closely matches theoretical predictions and that effective sensors can be prepared using low plate number columns. This sensor modality has many desirable characteristics for in situ sensors for trace level contaminant long-term monitoring where the use of consumable reagents for sensor regeneration would be undesirable. Initial experiments in groundwater matrixes demonstrated the detection of Tc-99 at drinking water level standards (activity of 0.033 Bq/mL) and detection of hexavalent chromium to levels below drinking water standards of 50 ppb.     

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.     

Intercomparison of infrared cavity leak-out spectroscopy and gas chromatography-flame ionization for trace analysis of ethane

Comparison of two different methods for the measurement of ethane at the parts-per-billion (ppb) level is reported. We used cavity leak-out spectroscopy (CALOS) in the 3 microm wavelength region and gas chromatography-flame ionization detection (GC-FID) for the analysis of various gas samples containing ethane fractions in synthetic air. Intraday and interday reproducibilities were studied. Intercomparing the results of two series involving seven samples with ethane mixing ratios ranging from 0.5 to 100 ppb, we found a reasonable agreement between both methods. The scatter plot of GC-FID data versus CALOS data yields a linear regression slope of 1.07 +/- 0.03. Furthermore, some of the ethane mixtures were checked over the course of 1 year, which proved the long-term stability of the ethane mixing ratio. We conclude that CALOS shows equivalent ethane analysis precision compared to GC-FID, with the significant advantage of a much higher time resolution (<1 s) since there is no requirement for sample preconcentration. This opens new analytical possibilities, e.g., for real-time monitoring of ethane traces in exhaled human breath.     

Combination of sorption tube sampling and thermal desorption with hollow waveguide FT-IR spectroscopy for atmospheric trace gas analysis: determination of atmospheric ethene at the lower ppb level

The determination of organic trace gases in the ambient environment at the lower ppb level is demonstrated based on a novel technique combining sorption tube sampling on Molsieve and Carbosieve S-III, thermal desorption, and detection of the trace analyte by hollow waveguide Fourier transform infrared (HWG-FT-IR) spectroscopy. While ethene concentrations of approximately 5 ppm can be directly observed using HWG-FT-IR, enrichment factors of up to 5000 were achieved by sorption tube sampling and thermal desorption. Detection limits of approximately 1 ppb are reported. Efficient enrichment by the sampling tube is achieved due to the favorable internal volume ( approximately 0.4 cm(3) at a length of 470 mm) of the hollow waveguide serving as a miniaturized gas cell. This new method was validated for ethene by thermodesorption-cryofocusing-GC-FID as the reference method. Analytical performance has been compared for standard gas mixtures and for ethene measurements in urban air. Finally, ethene data from a sampling campaign at two alpine sites in Tyrol/Austria are presented.     

Nafion film/K(+)-exchanged glass optical waveguide sensor for BTX detection

An optical waveguide (OWG) sensor for the detection of BTX gases is reported. The highly sensitive element of this sensor was made by coating the copper Nafion film over a single-mode potassium ion exchanged glass OWG. We used the OWG sensor to detect toluene gas as a typical example BTX gas. The sensor exhibits a linear response to toluene in the range of 0.25-4250 ppm with response and recovery times less than 25 s. The sensor has a short response time, high sensitivity, and good reversibility.     

Periodically porous top electrodes on vertical nanowire arrays for highly sensitive gas detection

Nanowires of various materials and configurations have been shown to be highly effective in the detection of chemical and biological species. In this paper, we report a novel, nanosphere-enabled approach to fabricating highly sensitive gas sensors based on ordered arrays of vertically aligned silicon nanowires topped with a periodically porous top electrode. The vertical array configuration helps to greatly increase the sensitivity of the sensor while the pores in the top electrode layer significantly improve sensing response times by allowing analyte gases to pass through freely. Herein, we show highly sensitive detection to both nitrogen dioxide (NO(2)) and ammonia (NH(3)) in humidified air. NO(2) detection down to 10 parts per billion (ppb) is demonstrated and an order-of-magnitude improvement in sensor response time is shown in the detection of NH(3).     

Chemical warfare agent detection using MEMS-compatible microsensor arrays

Microsensors have been fabricated consisting of TiO/sub 2/ and SnO/sub 2/ sensing films prepared by chemical vapor deposition (CVD) on microelectromechanical systems array platforms. Response measurements from these devices to the chemical warfare (CW) agents GA (tabun), GB (sarin), and HD (sulfur mustard) at concentrations between 5 nmol/mol (ppb) and 200 ppb in dry air, as well as to CW agent simulants CEES (chloroethyl ethyl sulfide) and DFP (diisopropyl fluorophosphate) between 250 and 3000 ppb, are reported. The microsensors exhibit excellent signal-to-noise and reproducibility. The temperature of each sensor element is independently controlled by embedded microheaters that drive both the CVD process (375/spl deg/C) and sensor operation at elevated temperatures (325/spl deg/C-475/spl deg/C). The concentration-dependent analyte response magnitude is sensitive to conditions under which the sensing films are grown. Sensor stability studies confirm little signal degradation during 14 h of operation. Use of pulsed (200 ms) temperature-programmed sensing over a broad temperature range (20/spl deg/C-480/spl deg/C) enhances analyte selectivity, since the resulting signal trace patterns contain primarily kinetic information that is unique for each agent tested.     

Real-time, ultralow concentration detection of analytes in solution by infrared intracavity laser absorption

Two intracavity laser absorption techniques for ultralow concentration detection of chemicals in solution are compared. The first consists of a laser diode in a grating extended cavity, which produces a linear calibration curve for parts in 10(9) (ppb) concentrations corresponding to 17 nM. By replacing the grating with a highly reflective mirror, parts in 10(12) (ppt) concentration detection is achieved, which corresponds to 340 pM. We report, to our knowledge for the first time, ppt detection of analyte concentration in liquid solution demonstrating good agreement between theory and experiment.     

Characterization of a pressurized C5-C16 hydrocarbon gas calibration standard for air analysis

A compressed gas standard containing parts-per-billion (ppb) amounts of the volatile hydrocarbons methylpentadiene (isoprene, 540 ppb) and isooctane (259 ppb) and a series of less volatile C12-C16 n-alkanes (n-dodecane, 349 ppb; n-tridecane, 340 ppb; n-tetradecane, 202 ppb; n-pentadecane, 271 ppb; and n-hexadecane, 308 ppb) was prepared by a one-step (no further gas dilution) microgravimetric method. The gravimetric mixing ratios were confirmed by referencing to a capillary diffusion method. The cylinder was heated to 75 degrees C to minimize condensation losses of analytes to the cylinder walls. Mixing ratios were monitored over a 2.5-year period. Some initial analyte losses (approximately 3 to 20%) were observed for the heavier C14-C16 n-alkanes. Subsequently, analyte loss rates were found to be in the range of <1.0% per year for n-dodecane to n-hexadecane. The developed guidelines for preparation, storage, and retrieval of these semivolatile analytes enable the use of compressed gas standards for calibration and method development purposes in the environmental gas-phase analysis of these and related compounds.     

Room-temperature mid-infrared laser sensor for trace gas detection

Design and operation of a compact, portable, room-temperature mid-infrared gas sensor is reported. The sensor is based on continuous-wave difference-frequency generation (DFG) in bulk periodically poled lithium niobate at 4.6 mum, pumped by a solitary GaAlAs diode laser at 865 nm and a diode-pumped monolithic ring Nd:YAG laser at 1064.5 nm. The instrument was used for detection of CO in air at atmospheric pressure with 1 ppb precision (parts in 10(9), by mole fraction) and 0.6% accuracy for a signal averaging time of 10 s. It employed a compact multipass absorption cell with a 18-m path length and a thermoelectrically cooled HgCdTe detector. Precision was limited by residual interference fringes arising from scattering in the multipass cell. This is the first demonstration of a portable high-precision gas sensor based on diode-pumped DFG at room temperature. The use of an external-cavity diode laser can provide a tuning range of 700 cm(-1) and allow the detection of several trace gases, including N(2) O, CO(2), SO(2), H(2) CO, and CH(4).     

New capabilities for optimizing SAW gas sensors

It is shown how the performances of SAW gas sensors can be optimized based on pure acoustic peculiarities of SAW propagation in anisotropic single crystals. For a given gas and sensitive membrane, the calibration curve (dependence of the response R versus gas concentration n), the sensitivity S (slope of the calibration curve: S=dR/dn), the detection limit n_thr, (cut-off of the curve at the threshold R_thr), and the resolution Δn of the sensor (recognition of two close concentrations) can be controlled by a proper choice of the substrate material and its crystallographic orientation (cut and direction of the SAW propagation). An experimental test of this property is performed on SAW devices implemented on different substrate materials and crystallographic orientations, both uncoated or coated, with a sorbent membrane of polycrystalline Pd or Pd:Ni film, upon exposure to humid air as a test analyte     

Thermal desorption solid-phase microextraction inlet for differential mobility spectrometry

A splitless thermal desorber unit that interfaces a differential mobility spectrometry (DMS) sensor has been devised. This device was characterized by the detection of benzene, toluene, and xylene (BTX) in water. The detection of BTX in water is important for environmental monitoring, and ion mobility measurements are traditionally difficult for hydrocarbons in water because water competes for charge and quenches the hydrocarbon signals. This paper reports the use of a DMS with a photoionization source that is directly coupled to a solid-phase microextraction (SPME) desorber. The separation and detection capabilities of the DMS were demonstrated using BTX components. Detection limits for benzene, toluene, and m-xylene were 75, 50, and 5 microg mL(-1), respectively.     

Using thermogravimetry for weight loss monitoring of permeation tubes used for generation of trace concentration gas standards.

Permeation tubes are convenient analyte sources for generating standard gas mixtures (containing, in particular, volatile organic compounds) used in the calibration of analytical instruments. For small permeation rates, corresponding to trace levels of analytes, the calibration of permeation tubes is time-consuming. The use of thermogravimetry as a means to measure rapidly the weight loss at constant temperature was investigated. An attempt to apply this technique to calibrating a permeation tube filled with benzene is described. In the 20 ng/min range of permeation rate, day-to-day variations of <5% were observed. The continuous weight loss monitoring of permeation tubes by thermogravimetry allows their rapid characterization.     

Detection of volatile compounds with mass-sensitive sensor arrays in the presence of variable ambient humidity

Mass-sensitive sensor arrays were established for the detection of isomeric or highly analogue analyte mixtures, which show similar physical and morphological properties. Supramolecular host-guest chemistry and arrays of four mass-sensitive quartz crystal microbalances have been successfully combined with multivariate calibration techniques in the presence of variable air moisture. This system enabled even the separation of xylene isomers [Formula: see text] a task that might be crucial even by gas chromatography. The data of the sensor arrays were analyzed with partial least squares and artificial neural networks. The xylene isomers could be detected with an accuracy of ～1% in the range of 0-200 ppm, nearly eliminating the residual water cross-sensitivity of the sensor coatings, which allows effective work place or environmental monitoring of toxic compounds with fast response levels.     

应用热解吸/气相色谱技术分析食品级二氧化碳中痕量苯

引用对室内空气中微量苯和TVOC的检测原理,利用Tenax TA吸附剂吸附浓缩二氧化碳中微量苯,提高进入色谱分析系统的苯的绝对量。从而大大提高了气相色谱仪的方法检测灵敏度,实现食品级二氧化碳中痕量苯的分析,其最小的检测浓度为0.37×10^-9mol/mol。实验表明,测试方法重复性较好,相对标准偏差〈5%,线性相关系数〉0.9999。     

Real-time, on-line characterization of diesel generator air toxic emissions by resonance-enhanced multiphoton ionization time-of-flight mass spectrometry

The laser-based resonance-enhanced multiphoton ionization time-of-flight mass spectrometry (REMPI-TOFMS) technique has been applied to the exhaust gas stream of a diesel generator to measure, in real time, concentration levels of aromatic air toxics. Volatile organic compounds, as well as several polycyclic aromatic hydrocarbons were detected in the concentration range of 10-200 ppb in the steady-state diesel generator exhaust. The results were verified and compared with conventional extractive sampling and analytical techniques using gas chromatography/mass spectrometry (GC/MS). The high isomer selectivity of the REMPI-TOFMS instrument provided data for individual xylene isomers that are otherwise (partially) coeluting in standard GC/MS analyses. Good agreement was observed between results for volatile and semivolatile organic compounds obtained with REMPI-TOFMS and conventional extractive sampling. Transient events, such as cold start-ups of the diesel generator, resulted in sharp (less than 15 s) peak emissions that were, for benzene, up to a factor of 90 higher than the predominately constant concentrations observed during steady-state operation; warm restarts resulted in lower peak concentrations by a factor of 2.5. These fast transient emissions are only detectable using a real-time approach (1-s resolution) as demonstrated here using REMPI-TOFMS.     

Narrow-linewidth vertical-cavity surface-emitting lasers for oxygen detection

The use of vertical-cavity surface-emitting lasers (VCSEL's) for optical detection of atmospheric oxygen is described. The VCSEL's were custom designed for single-mode emission in the 763-nm wavelength range, with low noise and narrow optical linewidth. Using standard wavelength modulation spectroscopy and a second-harmonic detection scheme with a 1-m air path, we determined an oxygen concentration resolution of 0.2%. Because of its small size, low power dissipation, and good tunability characteristics, the VCSEL promises to be an attractive light source for use in compact, low-cost optical sensor microsystems for trace gas detection.     

Adaptive K-NN for the detection of air pollutants with a sensor array

The field of air-quality monitoring is gaining increasing interest, with regard to both indoor environment and air-pollution control in open space. This work introduces a pattern recognition technique based on adaptive K-nn applied to a multisensor system, optimized for the recognition of some relevant tracers for air pollution in outdoor environment, namely benzene, toluene, and xylene (BTX), NO/sub 2/, and CO. The pattern-recognition technique employed aims at recognizing the target gases within an air sample of unknown composition and at estimating their concentrations. It is based on PCA and K-nn classification with an adaptive vote technique based on the gas concentrations of the training samples associated to the K-neighbors. The system is tested in a controlled environment composed of synthetic air with a fixed humidity rate (30%) at concentrations in the ppm range for BTX and NO/sub 2/, in the range of 10 ppm for CO. The pattern recognition technique is experimented on a knowledge base composed of a limited number of samples (130), with the adoption of a leave-one-out procedure in order to estimate the classification probability. In these conditions, the system demonstrates the capability to recognize the presence of the target gases in controlled conditions with a high hit-rate. Moreover, the concentrations of the individual components of the test samples are successfully estimated for BTX and NO/sub 2/ in more than 80% of the considered cases, while a lower hit-rate (69%) is reached for CO.