Sensitive Detection of Nitric Oxide Using a Quantum Cascade Laser Based QEPAS Sensor

The development and performance of a continuous wave (CW), thermoelectrically cooled (TEC) quantum cascade laser (EC-QCL) based sensor for quantitative measurements of nitric oxide (NO) concentrations in exhaled breath will be reported. Human breath contains ~400 different chemical species, usually at ultra low concentration levels, which can serve as biomarkers for the identification and monitoring of human diseases or wellness states. By monitoring exhaled NO concentration levels, a fast non-invasive diagnostic method for treatment of patients with asthma and chronic obstructive pulmonary disease (COPD) is feasible. The NO concentration measurements are performed with a 2f wavelength modulation based quartz enhanced photoacoustic spectroscopy (QEPAS) technique, which is very suitable for real time breath measurements, due to the fast gas exchange inside a compact QEPAS gas cell (<;5 mm³ typical volume). In order to target the optimal interference free NO R (6.5) absorption doublet at 1900.08 cm^-1 (λ~5.263 μ m) a Daylight Solutions Inc. widely tunable, mode-hop free 100 mW EC-QCL was used. The sensor reference channel includes a 10 cm long reference cell, filled with a 0.5% NO in N₂ at 150 Torr, which is used for line-locking purpose. A minimum detection limit (1σ) for the EC-QCL based line locked NO sensor is ~5 ppbv with a 1 sec update time by a custom built control QCL compatible electronics unit.