FTIR Spectroscopy
4Pages

LASER SPECTROSCOPY Using active FTIR to distinguish between multiple gases Christopher G. Leburn, Oguzhan Kara, and Derryck T. Reid An active long-range FTIR spectroscopy system can acquire high-resolution gas absorption spectra such as hydrocarbon emissions. Fugitive hydrocarbon emissions are estimated to cost the energy sector $5 billion per year and account for 12% of greenhouse gas emissions—they are also thought to jeopardize safety and public health, as well as a key factor in climate change. Whether it's to enable industrial sites close to communities to conduct 24-hour continuous monitoring, or to assess efficiencies in combustion engines,markets including oil and gas, landfill, and agriculture are driving a growing requirement for high-resolution gas-detection solutions that are compact, portable, and affordable. Analysing different gases can be achieved in a number of different ways. Differential absorption lidar (DIAL) is considered to be one of the most advanced techniques. With a range of 500 m, this technique directs high-energy laser light into the atmosphere, which is returned to a ground-based detector by weak scattering from airborne particles. Unfortunately, DIAL systems are complex, costly to run, with a large footprint and some systems are often housed on a 18 wheeler truck. By contrast, Fourier-transform infrared (FTIR) spectroscopy is naturally broadband and offers far wider coverage than DIAL. FIGURE 1. These idler spectra are produced by fan-out-grating tuning of the Chromacity OPO. The spectral shapes are determined by the phase-matching characteristics of the OPO crystal and by the OPO pump-laser spectrum. Water absorption lines are visible at shorter wavelengths. Discover More chromacitylasers.com Fugitive hydrocarbon emissions are estimated to cost the energy sector $5 billion per year and account for 12% of greenhouse gas emissions. In an open-path setup, FTIR has the capability to detect hundreds of atmospheric gases and, with a small system footprint comparable to a briefcase, is much more portable than a DIAL system. Open-path FTIR typically uses thermal sources to quantify hydrocarbon emissions, but with typical commercial solutions having resolutions of 0.5 cm-1, it is difficult to separate out multiple species when they are spectrally overlapped. In addition, in-the-field FTIR spectroscopy using thermal sources typically requires highquality retroreflecting targets to direct the source light back towards the detector. Laser-based active FTIR spectroscopy offers higher resolution, providing the capability to distinguish similar gases such as methane and ethane.

FIGURE 2. The OPO, telescope, and scanning Michelson interferometer are seen on a 60 x 90 cm breadboard (a); the layout of the Fourier-transform spectrometer is seen in (b). THE GAS-ANALYSING SYSTEM The experiment is broken into three main elements: the broadband light source, the spectrometer (consisting of interferometer and detection system), and a computer algorithm to extract the data. The light source was the Chromacity broadband, ultrafast optical parametric oscillator (OPO). This particular model was optimized to provide tunability from 2800 to 3900 nm, so that To achieve this in...

The algorithm retrieves the effective illumination spectrum (see Fig. 3a, dashed line), which represents the OPO output spectrum prior to undergoing atmospheric absorption. These experiments showed that the system was able to obtain environmental concentration values consistent with independent humidity measurements (water), established ambient levels (methane), or known control concentrations (ethane). Figure 3 gives an example of a single measured spectrum (no averaging) exhibiting densely packed absorption lines from water, methane, and ethane, as well as continuum absorption from...

under these experimental conditions (20°C, 101,800 Pa), so the inferred illumination spectrum closely follows the envelope of the measured spectrum. Figure 4b presents the measured water and methane concentrations over 400 s, showing the methane concentration rising from background levels (around 1900 ppb) to a peak of around 13,000 ppb before returning to near the original value as the gas disperses. Prior to the methane release, the root mean squared (RMS) variation of the measured concentration of background methaneatthisrange was <100 ppb. Water showed more variability, which is assumed...