Ultra-sensitive carbon monoxide detection by using EC-QCL based quartz-enhanced photoacoustic spectroscopy

Abstract

A quartz-enhanced photoacoustic spectroscopy (QEPAS) based sensor for carbon monoxide detection at ppbv levels was developed with a 4.65 μm external-cavity quantum cascade laser operating both in continuous wave (cw) and pulsed modes. A 23-fold enhancement of the measured CO signal amplitude was obtained when water vapor, acting as a catalyst for vibrational energy transfer, was added to the targeted analyte mixture. In the cw mode, a noise-equivalent sensitivity (NES, 1σ) of 2 ppbv was achieved at a gas pressure of 100 Torr, for 1-s lock-in amplifier (LIA) time constant (TC), which corresponds to a normalized noise equivalent absorption coefficient (NNEA) of $1.48\times 10^{-8}~\mathrm{cm}^{-1}\,\mathrm{W}/\sqrt{\mathrm{Hz}}$. In the pulsed mode, the determined NES and NNEA were 46 ppbv and $1.07\times 10^{-8}~\mathrm{cm}^{-1}\,\mathrm{W}/\sqrt{\mathrm{Hz}}$, respectively, for a 3-ms LIA TC at atmospheric pressure with a laser scan rate of 18 cm^-1/s and a 50 % duty cycle. An intercomparison between cw and pulsed QEPAS-based CO detection is also reported.