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Show Table I - Candidate Molecular Measurement Techniques Infrared Absorption Tunable Diode Laser Fixed Frequency Laser FrIR Spectrometer Infrared Emission Visible and Ultraviolet Lamp/Spectrometer Absorption (DOAS) Laser Absorption Laser-Induced Fluorescence CARS Mass Spectrometry are different, and often more difficult, than those involved in quantitative absorption measurements. Infrared emission will also not be considered here, because its sensitivity, especially when relatively low temperature exhaust gases are involved, is not appropriate for trace gas measurements. Of the remaining techniques, we will focus on infrared absorption because it is applicable to the widest range of molecular species, and further concentrate on tunable diode laser absorption because of its better sensitivity and selectivity. Infrared diode laser absorption diagnostics have been investigated by a number of research groups over the past 15 years.1- 14 The great bulk of this work has focused on the CO molecule, measuring either concentration or temperature. I-IO Other molecules studied in combustion systems have included NO,12,14 NH3,13 and CO2, H20 and the only radical, OH.14 Fourier transfonn infrared (FfIR) spectrometers are now by far the most practical and sensitive non-laser infrared instruments. Current commercial models are much cheaper, more rugged, and lighter than those available only a few years ago, and offer much higher spectral resolution (and therefore infonnation content) than comparable grating or fIlter-based devices. FI1R applications to combustion systems to date have most often been in detection of emission spectra,15 although some emission-absorption, cross-stack absorption, and extractive sampling studies have been reported. 15-19 Applications of fIxed-frequency infrared lasers to combustion gas measurements have been few, although there has been a report of in-stack measurements of CO2, H20 and temperature using an HF laser.20 Such applications may increase with continued development of neutral gas lasers (such as the familiar He-Ne laser) with emission lines throughout the infrared.21 Finally, 2 |