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L A Todd

Publications and source records attributed to L A Todd.

8 recordsLinked to original sources
PubMed2001

Experimental evaluation of an environmental CAT scanning system for mapping chemicals in air in real-time.

An innovative method is being developed which creates real-time, two-dimensional maps of chemical concentrations in air for environmental and occupational applications. This method, we call environmental CAT scanning, combines the real-time measuring technique of open-path Fourier transform infrared spectroscopy with the mapping capabilities of computed tomography to produce accurate spatial and temporal information about contaminant concentrations and dispersion patterns. With this system, a network of open-path measurements is obtained over an area; measurements are then processed using a tomographic algorithm to reconstruct the concentrations. This article describes a thorough experimental evaluation of an environmental CAT scanning system using a field-ready prototype system deployed in a room-size exposure chamber; point sample measurements were taken simultaneously in the chamber along with the CAT measurements. Twenty-eight experiments were performed using single or multiple plumes of a tracer gas in the chamber. Tomographic maps were compared with the point sample reference maps to evaluate the CAT scanning system for accuracy of concentration measurement and plume location. Quantitative agreement was very good; concentrations reconstructed with the tomographic maps agreed to within 17 percent of the point sample maps, and plume locations were reconstructed to within six inches of the plumes in the point sample maps. This technique shows real promise as a rapid and accurate method for mapping chemicals over large areas.

Air Pollutants↗
PubMed2000

Mapping the air in real-time to visualize the flow of gases and vapors: occupational and environmental applications.

This article describes a new method for measuring and mapping pollutants in air in real-time which can be used for visualizing the flow of gases and vapors in both indoor industrial and outdoor environmental applications. This method uses open-path Fourier Transform Infrared (OP-FTIR) spectrometry and computed tomography for real-time mapping of concentrations of chemicals in air. These maps may be used to evaluate human exposures, source emissions and air dispersion models; thus, this method can be used for both industrial and environmental sampling. It is being developed using computer simulations, and chamber and field studies. Computer simulations used simulated test concentration data to create maps; the original maps of concentrations were compared with the tomographic reconstructed maps. In the chamber studies, tracer gas was released into the chamber and measurements from a tomographic system were compared with point sample measurements taken at the same time. When sulfur hexafluoride was injected in a stable flow field position in the chamber, the concentrations reconstructed by the concentration maps were within +/- 15.9 percent of the measured point samples; overall, they were within +/- 27 percent of the measured point samples. On a 12-foot by 14-foot grid of cells used to model the chamber, the average peak location error was within one foot. The peak location error refers to the error involved in locating the point of highest concentration in the plume. For the field study, field-generated tomographic maps were compared with concentrations estimated using the Industrial Source Complex-Short Term (ISCST) model. Fairly good correlation (R2 = 0.67) was found between the five-minute overall-average cell concentrations in the tomographic and ISCST model maps. Overall, the tomographic map concentrations over-predicted the ISCST model concentrations by 24 percent. Optical remote sensing and computed tomography shows promise as a method to produce spatially and temporally resolved two-dimensional concentration maps indoors and outdoors. These maps would provide near real-time visualization of contaminant generation, movement, concentrations, and emission rates for multiple chemicals simultaneously at low limits of detection.

Air Pollution↗
PubMed2000

Assessment of occupational exposure patterns by frequency-domain analysis of time series data.

Laboratory evidence increasingly points to exposure pattern characteristics, including the duration, frequency, and timing of the exposure during the day, as important factors influencing the biological response to extremely low-frequency magnetic fields. An exploratory analysis of exposure patterns was conducted in 113 electric utility workers employed as electricians, cable splicers, line workers, and power plant operators. The purpose of the study was to describe extremely low-frequency magnetic field exposure pattern characteristics of electric utility workers and evaluate grouping strategies for classifying occupational exposures based on their exposure pattern characteristics. Exposure patterns describe the cyclic fluctuation in exposures over time, and were evaluated by partitioning the variation of the time series into frequency components using frequency-domain analysis of the transformed and processed time series. The study samples were classified using traditional grouping strategies based on occupation and time-weighted average (TWA), and non-traditional grouping strategies based on cluster analysis of the standardized, low-frequency exposure pattern components. Rules for classifying samples into each group were developed using linear discriminant analysis, with the performance of each grouping strategy evaluated using a crossvalidation study design to estimate the rate of misclassification. Exposure patterns appeared unrelated to grouping strategies based on quartiles of the workday TWA, but were related to pattern clusters and occupation. The linear discriminant function produced very low misclassification error rates for the cluster grouping strategy (10%) compared to occupation (50%) and TWA quartile (69%) grouping strategies. Significant differences in the exposure patterns occurring between clusters and between occupational groups were observed, indicating that at least one of the spectral estimates in two of the groups were significantly different. However, patterns clusters produced the greatest contrast in exposure patterns of all grouping strategies, explaining 99 percent of the total variation compared to 58 percent of the total variation by occupation.

Electromagnetic Fields↗
PubMed2000

Evaluation of open-path FTIR spectrometers for monitoring multiple chemicals in air.

There has been mounting interest in the use of open-path Fourier transform infrared (OP-FTIR) spectrometers for occupational and environmental air monitoring. Although this technology is gaining acceptance in the environmental field, there has not yet been a comprehensive assessment of instrument performance and the analytical limitations of this method have not been thoroughly delineated. Unlike extractive FTIR spectrometers, calibration of OP-FTIR spectrometer systems presents unique problems because the optical beam is exposed to the atmosphere. Therefore, it is difficult to get an adequate clean background and perform evaluation tests used by extractive instruments. One solution to the problem of evaluating an open-path system is to place a sample cell directly in the path of the infrared beam. The purpose of this study was to investigate the use of a specially designed external calibration cell as a tool for laboratory and field evaluation of the accuracy of OP-FTIR spectrometers and to investigate various commonly used instrument performance parameters such as root mean square (RMS) noise, return intensity, instrument precision, and detector saturation. These performance parameters were measured to see if they could be used to predict whether an instrument is operating correctly. Six instruments from the same manufacturer were evaluated with a prototype calibration cell using NIST traceable sulfur hexafluoride, n-hexane, and cyclohexane. Reference concentrations generated in the calibration cell were compared with OP-FTIR spectrometer measured concentrations measured through the cell. Excellent correlation and slopes were obtained for all three chemicals. The instrument performance measures could not be used to predict accuracy. The external calibration cell shows promise as a method of validating the operation of an OP-FTIR spectrometer for quality assurance and for quality control.

Air Pollution↗
PubMed1997

Spatial and temporal visualization of gases and vapours in air using computed tomography. Numerical studies.

Numerical studies were performed to evaluate a new method for human exposure assessment and source monitoring, based upon optical remote sensing (ORS) and computed tomography (CT). With an ORS-CT system, two-dimensional maps of chemical concentrations in air, with good spatial and temporal resolution, can be created over a confined space such as a workplace room. The ORS-CT system was evaluated using 15 simulated test maps that model the generation and dispersion of contaminant plumes over time. A program simulated field measurements from these maps assuming different remote sensing scan times. Using these measurements, reconstructed maps were generated and compared with original maps. Qualitative and quantitative methods were used to evaluate the effect on reconstruction quality of sample time, number of iterations used by the maximum likelihood expectation maximization reconstruction algorithm and sample density. For this study, scanning an entire room in 10 min was adequate for exposure evaluation, source monitoring and leak detection.

Air Pollutants↗
PubMed1996

Mapping air contaminants indoors using a prototype computed tomography system.

Single and multiple plumes of sulphur hexafluoride were measured and mapped using a prototype open-path Fourier transform infrared spectrometer (OP-FTIR)-computed tomography (CT) system in an indoor exposure chamber. The OP-FTIR-CT system accurately mapped the position of concentration peaks at different locations in the chamber, and estimated concentrations within a range of 4-50% when compared with point samples measured by gas chromatography with an electron capture detector. Two OP-FTIR spectrometer ray configurations, one with 100 rays and the other with 136 rays, were used to scan the chamber and reconstruct the concentration maps. Ray configuration and time to scan the entire chamber were found to have a profound effect on the quality of the reconstructed maps. The ability to obtain real-time, non-invasive measurements, and to generate spatially and temporally resolved maps of multiple chemicals, makes the OP-FTIR-CT system a promising technique for monitoring source emissions and evaluating exposures to air contaminants in a workplace.

Air Pollutants↗
PubMed1980

Differential induction of sister chromatid exchanges by indirect-acting mutagen-carcinogens at early and late stages of embryonic development.

To examine the development of drug-metabolizing enzyme systems in the early chick embryo, the procarcinogens, aflatoxin B1 (AF-B1) and 2-acetylamino-fluorene (2-AAF), and the direct-acting carcinogen, ethyl methanesulfonate (EMS) (positive control), were given to embryos; the sister-chromatid-exchange (SCE) technique was used as an indicator of conversion to active mutagenic metabolites. Chick embryos at two stages of incubation (3-day and 6-day) were exposed to the same graded series of dosages of the compounds for a period of 22 hours. All three mutagens increased the frequency of SCE above the control rate of 1,8 SCEs/cell. While a dose-dependent increase in SCE was obtained for both procarcinogens at each age, the mean SCE frequency was significantly higher in the 6-day embryos for each dosage given. In contrast, the direct-acting mutagen, EMS,. gave a reduced level of SCEs at the older age. These results suggest that the ability of early chick embryos to activate promutagens to forms capable of inducing SCE increases as development advances from three to six days of incubation (DI). In the 6-day embryo, the metabolic conversion is enhanced, resulting in a significant increase in the mutagenicity of the test chemicals.

2-Acetylaminofluorene↗