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

Abstract

Measurements of NH₃ and CO₂ were made in bioreactor vent gases with distributed-feedback diode-laser sensors operating near 2 mm. Calculated spectra of NH₃ and CO₂ were used to determine the optimum transitions for interrogating with an absorption sensor. For ammonia, a strong and isolated absorption 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 widely varying concentrations [500 parts per million (ppm) to 10%], with sufficient signal for low mole fractions and without being optically thick for high mole fractions. Using direct absorption and a 36-m total path-length multipass 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 measurements of NH₃ and CO₂ concentration in bioreactor vent gases that were recorded at NASA Johnson Space Center with a portable and automated sensor system over a 45-h data collection window.