Joseph R. Stetter

Theoretical basis for identification and measurement of air contaminants using an array of sensors having partly overlapping selectivities

Abstract The objective of this paper is to provide a theoretical basis for the selection and effective use of an array of chemical sensors for a particular application. The array is to be used in conjunction with a microprocessor in a small portable device capable of detecting, identifying and quantifying any of a large number N of hazardous compounds, either singly or in mixtures of up to A such compounds. A compact array of S different sensors is used, each of which can operate in M differently selective modes so as to yield P=MS parameters, or data channels. The minimum number of parameters required to identify the compounds solely on the basis of the presence or absence of significant signals in the various channels is determined by the inequality For example, P ⩾ 18 or P ⩾ 22 for N = 100 and A = 3 or 4, respectively. A computational approach is proposed to first eliminate all candidate compounds whose known response patterns do not overlap the channels exhibiting the strongest signals. The remaining candidate compounds are then quantified by solving simultaneous linear equations. A determinant, which is used to solve the simultaneous equations, provides a measure of the selectivity and/or non-redundancy of the set of channels exhibiting the strongest signals. This determinant is also inversely proportional to the experimental error or to the required accuracy of the measurements. By computing the values of this determinant for various mixtures, the selectivities and/or non-redundancies of alternative sets of P parameters with respect to a given set of N hazardous compounds can be evaluated and compared.

A chemical concentration modulation sensor for selective detection of airborne chemicals

Abstract A method for selective detection of airborne chemicals based on concentration-modulated signals is described. A variable energy source ( e.g. , a heated filament) causes a modulated chemical reaction in a flowing gas stream, and a chemical sensor monitors the resultant modulated concentration signal. A parameter that is specific for a particular chemical species ( i.e. , a pseudo-activation energy) can be determined. Because this parameter is unique for a particular chemical and set of operating conditions, devices based on this method offer selectivity in sensing chemical vapors. A single detector capable of selectively identifying a large number of chemicals is possible using this approach.

Monitoring of electrochemically inactive compounds by amperometric gas sensors

Abstract Electrochemical sensors have many useful applications in air and toxic gas monitoring, but they are extremely limited in the gaseous species that they can detect. To extend the applicability of amperometric gas-sensing instruments to electrochemically inactive compounds, gas samples were exposed to a heated platinum or gold filament before being introduced into the sample chambers of several different amperometric sensors. The sensors were of the three-electrode type, having platinum-black reference and counter electrodes and sensing electrodes made of platinum black, powdered gold or vapor-deposited platinum or gold on porous tetrafluoroethylene membranes. All three electrodes were in contact with a 25 – 30 wy.% sulfuric acid solution. The responses of four different sensors to various compounds at 20°C were measured at sensing electrode potentials ranging from 0.9 V to 1.4 V versus RHE (reversible hydrogen electrode in the same electrolyte), with and without a heated platinum filament at the sample inlet. Of 10 compounds tested, only two elicited significant responses without the filament. With the filament heated to about 700 °C, each of the tested compounds elicited a significant response under certain conditions. Moreover, the particular sensors and electrode potentials corresponding to the strongest responses were different for each compound. Qualitatively similar, but quantitatively more pronounced, responses were obtained with the same filament heated to 800 °C or 1050 ± 50 °C, or with a gold filament heated to 950 ± 50 °C. The responses were proportional to concentration in the 0 – 50 ppm range, and usually proportional in the 0 – 200 ppm range.

Sensitive electrochemical detector for gas chromatography

Abstract A sensitive electrochemical detector is described which uses a PTFE-bonded diffusion electrode to overcome the major limitations of other electrochemical detectors. This technique was shown to have the maximum sensitivity for H 2 S (lower detectable limit of 7·10 −2 g) where eight electrons per equivalent are transferred and lower sensitivities for NO, CO, SO 2 and NO 2 . It is rugged, easy to operate, and can be used with a wide range of carrier gases. Its use for the detection of other electrochemically active gases is discussed.

A room-temperature electrochemical sensor and instrument for monitoring methane

Abstract Electrochemical sensors at room temperature that quantitatively detect methane in air have been developed. Methane is directly oxidized using a highly anodic working-electrode potential and a non-aqueous electrolyte. Stable and durable sensors have been fabricated that incorporate sintered glass wicks and glass fibre filters as electrolyte reservoirs. The monitoring instrument has two potentiostats that regulate two sensors, one of which is used to compensate for background current variations. The instrument requires less than 250 mW to power all components. The response of the monitor is proportional to methane concentration over a wide range, from the minimum detectable concentration (0.6% methane in air) to 100% methane. It is also substantially independent of fluctuations in relative humidity and flow rate. The response time is less than one minute to 90% of signal at a flow rate of 100 cm3/min. The monitor exhibits a net sensitivity of 0.5 – 1 μA/% CH4. It also exhibits low sensitivity to parts-per-million levels of CO or NO2, but yields measurable responses to 0.3% H2, 49 ppm NO and 50% C2H6.

Characterization of airborne particles at a high-btu coal-gasification pilot plant

Airborne particles in fugitive emissions have been measured at a slagging fixed-bed coal-gasification pilot plant using lignite. Sampling was conducted during shutdown operations and opening of the gasifier following an aborted startup. Aerosol collected with a Sierra high-volume impactor was subjected to analysis by gas chromatography, mass spectrometry, and scanning electron microscopy; aerosol collected with an Andersen low-volume impactor was subjected to flameless atomic absorption analysis. The data show that the bulk of the trace organic material is associated with small particles: these data are similar to data on ambient air reported in the literature. Particle morphologies resemble those of fly ash from coal combustion, including smooth spheres, vesicular spheres, and crystalline material. Trace element size distributions are bimodal and resemble data for ambient air. Pb-containing particles are generally submicron, while particles containing Al, Fe, and other crustal species are mostly of supermicron size. Aluminum-based aerosol enrichment factors calculated from the lignite composition show that the composition of the aerosol resembles that of the coal, with the exception of modest enrichments of Mg, Na, As, and Pb in the submicron size range. Aerosol enrichment factors based on the earths crustal composition are somewhat greater than those based on coal composition for several elements, suggesting potential errors in using crustal enrichment data to investigate chemical fractionation during aerosol formation.

Cryogenic sampling for aniline vapor with subsequent liquid chromatography-electrochemical detection analysis

Abstract A cryogenic sampler for aniline vapors was evaluated by collecting samples of 10 ppm aniline in air and directly analyzing the collected aniline by liquid chromatography with electrochemical detection. The cryogenic sampler achieved nearly 100% collection and recovery efficiency, with a detection limit of ≈ 4 parts-per-billion (109) aniline in air for a sample collected at 10 cm3/min for 40 s in a 200-μl stainless-steel loop at — 10°C. About 5 min was necessary to dissolve the trapped aniline into 200 μl of a mobile phase desorbing solution at 20°C.

Characterization of airborne trace metal and trace organic species from coal gasification.

Fugitive emissions from a slagging fixed-bed coal-gasification pilot plant were analyzed by flameless atomic absorption spectrophotometry, gas chromatography, and mass spectrometry for trace metal and trace organic species. Analysis of the size distributions of airborne particulate matter inside the plant showed an abundance of large metal-containing particles; outdoor distributions in the vicinity of the plant resembled the indoor distributions, suggesting the importance of the gasifier in influencing ambient air quality. This conclusion was further supported by identification of similar organic compounds inside and outside the plant. Trace element enrichment factors based on the earths crustal composition were greater than those based on the composition of the lignite used in the gasifier, showing the importance of characterizing the proper source material when inverstigating chemical fraction during aerosol formation. Enrichments in the present study were much greater than those found in previous sampling during aborted start-up and cleaning procedures, where normal operating temperatures had not yet been reached. Both studies showed evidence of enrichment factors which decreased with increasing particle size. Although much of the airborne mass was associated with large particles having low respirability, the high concentrations of some metals indoors suggests that further assessment of potential occupational exposures is warranted.