Progress 05/01/08 to 12/31/08
OUTPUTS: The technical approach is based on absorption spectroscopy using near-infrared diode lasers of the type developed for fiber-optic communications. Specifically the approach is a variation of integrated cavity output spectroscopy (ICOS) combined with a variation of wavelength modulation spectroscopy. A long optical path is achieved by allowing the laser light to bounce thousands of times between a pair of mirrors (the optical cavity) before it is transmitted through one mirror onto the detector. Electronic signal processing of the measured spectrum then provides the concentration of hydrogen sulfide. A laboratory prototype spectrometer was assembled. The light source was a commercial diode laser that operates in the 1570 nm spectral region, emitting about 10 mW of light through a standard single mode optical fiber. This laser was used to measure spectra of hydrogen sulfide and carbon monoxide by tuning it wavelength by means of tuning the laser current and temperature. The laser was directed to an optical cavity constructed from custom high-reflectivity mirrors. The mirrors were positioned so that the laser beam makes hundreds of bounces between the mirrors before it begins to retrace its path through the cavity. Photons on average bounce about 2000 times between the mirrors. Some of the light leaks out the back mirror of the cavity, and this light is focused onto a detector equipped with a custom pre-amplifier built for this project. When measuring photocurrents of between 500 and 5000 nA, the pre-amplifier achieved near shot-noise limited sensitivity and a bandwidth of over 1 MHz, despite the high capacitance of the detector. The pre-amplified photosignal was directed to a custom circuit board that includes laser modulation as well as data acquisition and processing. The apparatus was used to measure concentrations of various gases, including hydrogen sulfide and carbon monoxide. A mathematical model was developed and used to build a numerical simulation of the signals. The numerical model and the laboratory spectrometer were used to show how the calibration of the instrument depends on mirror reflectivity and laser power. PARTICIPANTS: The project was conducted at Southwest Sciences, with significant effort from the following employees three employees: Dr. David Bomse, Dr. Steve Massick, and the PI, Dr. David C. Hovde. Contact was made with Professor Lingying Zhao of Ohio State Dept. of Food, Agriculture and Biological Engineering regarding future field testing at a farm site. TARGET AUDIENCES: Safety managers at CAFOs and researchers at universities will want to be aware of this technology as it matures. PROJECT MODIFICATIONS: Nothing significant to report during this reporting period.
The research showed the feasibility of measuring hydrogen sulfide using the modified ICOS approach. Laser-based absorption spectroscopy is highly selective owing to the excellent frequency resolution of the laser that matches the inherent spectral structure of the hydrogen sulfide gas. Calibration depends on fundamental properties of the hydrogen sulfide molecule and on the optical path length. High sensitivity is achieved by employing a long optical path length and modulation techniques to suppress laser noise. For reasonable choices of the instrument parameters, sensitivity in the range of 10 ppb can be achieved. This sensitivity easily meets that needed for monitoring worker health and safety and is comparable to the sensitivity of the best electrochemical sensors on the market. Electrical power requirements should be low, less than 10 W. The physical size of the instrument should be slightly longer than 30 cm, with a mass less than 4 kg. The parts cost of the spectrometer should support a market price below the cost of a comparable electrochemical sensor. Instrumentation based on our approach should penetrate markets for monitoring equipment at concentrated animal feed operations, natural gas production, and oil refineries.
- No publications reported this period