Sub-part-per-billion detection of nitric oxide in air using a thermoelectrically cooled mid-infrared quantum cascade laser spectrometer

Non-cryogenic, laser-absorption spectroscopy in the mid-infrared has wide applications for practical detec ion of trace gases in the atmosphere. We report measurements of nitric oxide in air with a detection limit less than I nmole/mole (< 1 ppbv) using a thermoelectrically cooled quantum cascade laser operated in pulsed mode at 5.26 mum and coupled to a 210-m path length multiple-pass absorption cell at reduced pressure (50 Tort). The sensitivity of the system is enhanced by operating under pulsing conditions which reduce the laser line width to 0.010 cm(-1) (300 MHz) HWHM, and by normalizing pulse-to-pulse intensity variations with temporal gating on a single HgCdTe detector. The system is demonstrated by detecting nitric oxide in outside air and comparing results to a conventional tunable diode laser spectrometer sampling from a common inlet. A detection precision of 0.12 ppb Hz(-1/2) is achieved with a liquid-nitrogen-cooled detector. This detection precision corresponds to an absorbance precision of I X 10(-5) Hz(-1/2) or an absorbance precision per unit path length of 5 x 10(-10) cm(-1) Hz(-1/2). A precision of 0.3 ppb Hz(-1/2) is obtained using a thermoelectrically cooled detector, which allows continuous unattended operation over extended time periods with a totally cryogen-free instrument.