High-resolution spectral analysis of ammonia near 6.2 μm using a cw EC-QCL coupled with cavity ring-down spectroscopy

Abstract

We report on the development of a mid-infrared cavity ring-down spectrometer (CRDS) coupled with a continuous wave (cw) external cavity quantum cascade laser (EC-QCL), operating between 6.0 μm and 6.3 μm, for high-resolution spectroscopic studies of ammonia (NH3) which served as a bench-mark molecule in this spectral region. We characterized the EC-QCL based CRDS system in detail and achieved a noise-equivalent absorption (NEA) coefficient of 2.11 × 10-9 cm-1 Hz-1/2 for a 100 Hz data acquisition rate. We thereafter exploited the system for high-resolution spectroscopic analysis of interference-free 10 transition lines of the ν4 fundamental vibrational band of NH3 centred at ∼6.2 μm. We probed the strongest interference-free absorption line RQ(4,3) of ν4, centred at 1613.370 cm-1 for highly-sensitive trace detection of NH3 and subsequently achieved a minimum detection sensitivity (1σ) of 2.78 × 109 molecules per cm3 which translated into the detection limit of 740 parts-per-trillion by volume (pptv/10-12) at a pressure of 115 Torr for an integration time of ∼167 seconds. To demonstrate the efficacy of the present system in real-life applications, we finally measured the mixing ratios of NH3 present in ambient air and human exhaled breath with high sensitivity and molecular specificity.

Methanogenic pathways and δ13C values from swine manure with a cavity ring‐down spectrometer: Ammonia cross‐interference and carbon isotope labeling

“…In addition to influencing the CH 4 isotopologues, NH 3 had a significant effect on the measured H 2 O concentration, corresponding to 0.41% H 2 O per ppm NH 3 (Figure S1, supporting information). This interference is also apparent in Figure A at the H 2 O absorption lines that are located around 6028.78 cm −1 and is consistent with studies reporting closely related absorption features of NH 3 and H 2 O …”

High-precision measurements of nitrous oxide and methane in air with cavity ring-down spectroscopy at 7.6 µm

“…From these analyses we estimate the measurement sensitivity, relative accuracy, and absolute accuracy of our experimental setup for CH 4 and N 2 O detections in the air are around 10-20 pptv, 2, and 20 ppbv. The absolute accuracy could be improved to be comparable to the relative accuracy by calibrating the measurement with standard "known" sample of ppb-level concentration and controlling accurately the laser frequency during the spectral measurements Maithani et al, 2018).…”

“…In recent years, with the rapid development of advanced tunable high-power midinfrared sources, especially external-cavity quantum cascade lasers (EC-QCL) (Botez et al, 2018), the LODs of CRDS for trace-gas detections have been greatly improved. For example, Maity et al (2017) achieved an LOD of 52 pptv for CH 4 at 7.5 µm, Banik et al (2017) achieved an LOD of 5 ppbv for N 2 O at 5.2 µm, Long et al (2016) achieved an LOD of 2 pptv for N 2 O at 4.5 µm, Maithani et al (2018) achieved an LOD of 740 pptv for NH 3 at 6.3 µm and Zhou et al (2018) achieved an LOD of 410 pptv for NO at 5.3 µm.…”

Cavity Ring-Down Spectroscopy for Molecular Trace Gas Detection Using A Pulsed DFB QCL Emitting at 6.8 µm

“…Unfortunately, in this region, water absorbs strongly, making the region less explored. Only Maithani et al reported, in 2018, high resolution and high-sensitive spectroscopic analysis of NH 3 centered at 6.2 µm [18]. Therefore, further work is needed in the wavelength ranges between 5 and 7 µm where strong absorption lines of fundamental and combinational vibrational bands of ν4 and ν2 are located.…”