Using Mid-IR Spectroscopy for Environmental Monitoring

Environmental gas monitoring plays a critical role in several areas including maintaining high air quality, detection of leaks in industrial sites and regulating greenhouse gases.

Infrared molecular spectroscopy, using the Hedgehog laser from Daylight Solutions, enables gas monitoring to be performed quickly and with an exceptional signal-to-noise ratio.

Environmental monitoring of gases is essential in many areas of research and industry. A vital application is the measurement of atmospheric gases so as to regulate the concentration of greenhouse gases, such as carbon dioxide (CO₂) and methane (CH₄) that cause climate change¹. Greenhouse gases are typically released from the vehicle and industrial exhausts as well as from the livestock sector^2,3.

Gas monitoring is also crucial for sensing leaks in industrial sites where toxic or flammable gases are present, such as fracking sites, refineries, chemical plants, waste-water treatment facilities, and gas turbines. For instance, hydrogen peroxide (H₂O₂) gas is regularly monitored in industrial sites, where it is used for sterilizing medical devices and bleaching paper⁴.

The crucial need for environmental gas monitoring can be matched by gas sensing methods, which use mid-infrared (mid-IR) lasers. While symmetrical diatomic molecules cannot be seen using IR spectroscopy, many important molecules, such as CH₄, N₂O, CO, CO₂, H₂O₂, and H₂O, display unique absorption bands.

Mid-IR Spectroscopy for Selective and Sensitive Detection

For H₂O₂ monitoring, both extremely sensitive and selective detection of the gas is essential as water and other trace gases can affect the measurement⁴. Using Daylight Solutions’ Hedgehog (HHOG), an external-cavity quantum cascade laser, Sanchez et al. realized an interference-free detection of H₂O₂ at parts per billion (ppb) level.

Interference with other gases was circumvented by selecting the spectral line of 1234.055 cm^-1 for the measurement, where typical interference gases such as CO and H₂O, do not absorb. Their research reveals a linear response of the sensor system with the H₂O₂ concentration, which makes the system ideal for monitoring H₂O₂ in both medical applications and industrial sites.

Achieving a Greater Signal:Noise Ratio using Less Laser Power

Another current example of the application of the HHOG laser is the tracking of CO₂ in an infrared laser-induced fluorescence (IR-LIF) arrangement⁵.

During LIF, a molecule absorbs laser radiation and then discharges a photon when it relaxes back from the excited state to the equilibrium state. One can attain the fluorescence signal using a camera or detector and the technique enables the measurement of local gas conditions. Using the HHOG quantum cascade laser in continuous wave mode, Goldenstein et al. accomplished a 200 times larger absorbance of the CO₂ asymmetric stretch vibration band (at 4.3 m) than formerly reported using an IR-LIF method.

This permitted a detection limit of 20 ppm CO₂ in Ar. They further attained spatially resolved measurements of CO₂ concentration, pressure, and temperature. Compared to earlier studies using either high-power pulsed or low-power continuous-wave lasers, the HHOG laser realized a signal-to-noise ratio (SNR) that was 50 times larger while using approximately 10⁷ times less laser power.

The Hedgehog from Daylight Solutions

The research studies cited show the benefits of the HHOG laser and its wide application range for numerous types of gas monitoring. The high SNR that can be attained with the HHOG laser is because of the combination of reliable performance of previous generations of Daylight Solutions lasers with additional improvements. Consistent with previous generations of Daylight solutions lasers, the HHOG realizes:

• A high-quality light output (for instance, low beam pointing, high power stability, narrow linewidth option, near-diffraction-limited beam quality) that enables a high SNR in all spectroscopic applications
• Exceptional continuous wave or pulsed output
• Tuning range choices across 3-13 m as a result of the unparalleled library of the quantum cascade laser chip

Additional advancements in the HHOG — the latest generation of Daylight Solutions lasers — include a 30 times higher tuning speed and 10 times better spectral repeatability. The former enables faster data acquisition, which is vital for commercial applications and can be useful in academic research.

Moreover, the higher tuning speed allows for better signal averaging within a fixed data acquisition time frame, resulting in an improved SNR. Enhanced spectral repeatability, which denotes a lower variability in wavelength, and spectral content of the laser output, also facilitates higher SNR in spectroscopy applications.

To sum up, for applications of environmental monitoring using mid-IR spectroscopy, the HHOG laser convinces specifically with both rapid and high SNR data acquisition.

References and Further Reading

1. J. Mulrooney et al. [2007] Detection of carbon dioxide emissions from a diesel engine using a mid-infrared optical fibre based sensor, Sensor Actuat. A-Phys., 136:104–110.
2. M. Meinshausen et al. [2011] The RCP greenhouse gas concentrations and their extensions from 1765 to 2300, Climatic Change, 109: 213.
3. Food and agriculture organization of the United Nations [2010] Greenhouse Gas Emissions from The Dairy Sector, Rome, Italy.
4. N. P. Sanchez et al. [2016] Mid-IR laser-based sensor for hydrogen peroxide detection, SPIE, doi: 10.1117/2.1201601.006295.
5. C. S. Goldenstein et al. [2015] Infrared planar laser-induced fluorescence with a CW quantum cascade laser for spatially resolved CO₂ and gas properties, Appl. Phys. B, 120:185–199.

This information has been sourced, reviewed and adapted from materials provided by Daylight Solutions Inc.

For more information on this source, please visit Daylight Solutions Inc.