| Abstract
| - Porous membrane tubes filled with an absorbing solutionthat change colors upon selective reactions with specificgases provide high sensitivity inexpensive gas sensors.These can be routinely used for ambient monitoring in afully automated manner. We consider both stopped andcontinuous flow operations and show the superiority ofthe stopped flow mode theoretically and experimentally.Light throughput through various membrane tubes ispresented, and superior performance of such tubes overTeflon AF is shown. Sensors for NO2 and for O3 werebased on Griess-Saltzman and indigotrisulfonate chemistries, respectively. A computer-controlled two-LED absorbance measurement system (one wavelength monitorsthe signal, the other references the system) that alsogoverns automated reagent refilling was implemented.Sub-parts-per-billion-volume detection limits are attainable within a few minutes for both gases. Comparative datawith a commercial UV-photometry-based ozone monitorshowed excellent agreement with the response pattern ofthe present instrument. Low cost, ready applicability tothe measurement of different gases by merely changingthe light source and chemistry, and high sensitivity makesthis instrument attractive for both pedagogic and practicalpurposes.
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