Measurements of NH3 and CO2 with distributed-feedback diode lasers near 2.0 mu m in bioreactor vent gases

Measurements of NH3 and CO2 were made in bioreactor vent gases with distrib uted-feedback diode-laser sensors operating near 2 tm. Calculated spectra o f NH, and CO2 were used to determine the optimum transitions for interrogat ing with an absorption sensor. For ammonia, a strong and isolated absorptio n transition at 5016.977 cm(-1) was selected for trace gas monitoring. For CO2, an isolated transition at 5007.787 cm(-1) was selected to measure wide ly varying concentrations [500 parts per million (ppm) to 10%], with suffic ient signal for low mole fractions and without being optically thick for hi gh mole fractions. Using direct absorption and a 36-m total path-length mul tipass flow-through cell, we achieved a minimum detectivity of 0.25 ppm for NH3 and 40 ppm for CO2. We report on the quasi-continuous field measuremen ts of NH3 and CO, concentration in bioreactor vent gases that were recorded at NASA Johnson Space Center with a portable and automated sensor system o ver a 45-h data collection window. (C) 2001 Optical Society of America.