Abstract
An ultra-sensitive and selective quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor platform was demonstrated for detection of carbon monoxide (CO) and nitrous oxide (N2O). This sensor used a state-of-the art 4.61 μm high power, continuous wave (CW), distributed feedback quantum cascade laser (DFB-QCL) operating at 10°C as the excitation source. For the R(6) CO absorption line, located at 2169.2 cm(-1), a minimum detection limit (MDL) of 1.5 parts per billion by volume (ppbv) at atmospheric pressure was achieved with a 1 sec acquisition time and the addition of 2.6% water vapor concentration in the analyzed gas mixture. For the N2O detection, a MDL of 23 ppbv was obtained at an optimum gas pressure of 100 Torr and with the same water vapor content of 2.6%. In both cases the presence of water vapor increases the detected CO and N2O QEPAS signal levels as a result of enhancing the vibrational-translational relaxation rate of both target gases. Allan deviation analyses were performed to investigate the long term performance of the CO and N2O QEPAS sensor systems. For the optimum data acquisition time of 500 sec a MDL of 340 pptv and 4 ppbv was obtained for CO and N2O detection, respectively. To demonstrate reliable and robust operation of the QEPAS sensor a continuous monitoring of atmospheric CO and N2O concentration levels for a period of 5 hours were performed.
Highlights
Carbon monoxide (CO), one of the major air pollutants globally, is mainly produced and released into the atmosphere by a variety of incomplete combustion activities, including the burning of natural gas, fossil fuel, and other carbon containing fuels
Wavelength modulation spectroscopy and a 2nd harmonic detection technique were used to reduce the sensor background noise and to achieve minimum detection limit (MDL) values that are sufficient for environmental detection of CO and N2O concentration
Enhancement of the quartz-enhanced photoacoustic spectroscopy (QEPAS) signal can be realized by the addition of water vapor to improve the vibrational-translational relaxation rates of the two analyzed gas species
Summary
Carbon monoxide (CO), one of the major air pollutants globally, is mainly produced and released into the atmosphere by a variety of incomplete combustion activities, including the burning of natural gas, fossil fuel, and other carbon containing fuels. Atmospheric detection of CO and N2O concentration levels using laser absorption spectroscopy based optical gas sensor platforms have been reported by numerous research groups in recent years [5,6,7,8,9,10,11,12,13]. For ultra-high sensitive measurements these types of sensors are usually bulky due to the large size of a MPC and the increased number of optical components that are needed for laser beam alignment Another useful technique for trace gas analysis is conventional photo-acoustic spectroscopy (PAS) that employs a broadband microphone for acoustic wave detection. In this manuscript the performance of a state-of the art high power 4.61 μm CW TEC DFB-QCL in combination with ultra-compact and highly sensitive QEPAS sensor technology was demonstrated for the first time
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