OCR Text |
Show the Differential Optical Absorption Spectroscopy (DOAS) technique has benefitted from the availability of a commercial system for only a few years, so that reports of applications to combustion systems have not appeared, although it has been quite successful in related long-path monitoring tests.22 ~easurementCJeomenies All of the above absorption techniques can be applied either by using an in situ, cross-stack measurement geometry, or by extractively sampling into a multipass absorption cell. In the latter case, sensitivity can often be increased by operating the cell at a small fraction of atmospheric pressure. TIris is because as the pressure and so the molecular concentrations decrease, the line widths also decrease, leaving peak absorptions the same- meanwhile, neighboring interfering lines also narrow, raising the contrast of the lines of interest against the background absorption. Figure 1 shows the simplest implementations of each option for the case of tunable diode laser absorption. The cross-flow, in situ sampling measurement could be extended to multiple passes if the optical quality of the flow allowed it (40 or more passes are easily obtained across laminar flows). In the extractive sampling case, if the flow is nonuniform and its average concentrations must be well represented, the sampling 'probe can be equipped with multiple inlets. Feasibility Estimation The typical task in assessing the feasibility of a trace gas measurement is to estimate the minimum detectable concentration under conditions where the absorption line or lines used will be quite weak, and then compare this value with the expected minimum concentrations in the system to be studied. To make such an estimate, we must assume a minimum detectable absorption for the instrument and measurement procedure we plan to use. Based on our experience, we have chosen this limit for tunable diode laser absorption to be a 10-3 peak absorbance (optical depth). With additional effort, we and other groups have been able to lower this limit by several orders of magnitude, but w~ wanted the estimates presented here to use a conservative value, allowing for real-world effects and the need for relatively routine operation in many diagnostic programs. For the moment ignoring the effects of interfering lines, the detectability of a molecule using a single absorption line is directly proportional to the molecular concentration, the path 3 |