Portable, low power soil gas analyzer
Mesa Photonics successfully demonstrated two new technologies for soil gas analysis in the field. Mid-infrared absorption spectroscopy using an integrating sphere as multipass cell achieved for N2O detection sensitivity and precision within an order magnitude of a state-of-the-art cavity ringdown analyzer manufactured by Picarro Inc. 10 ppb sensitivity is useful for soil gas measurements where subsurface N2O concentrations are often larger than atmospheric ambient 330 ppb and biochemical processes drive flux into and out of the soil. The integrating sphere is particularly useful because no alignment is needed and gas samples are at ambient pressure. Performance was limited by thermal drift of the sphere changing the speckle pattern of the laser light emitted from the output port. Temperature stabilizing the sphere will likely improve performance by a factor of 2 to 5. The other technology, Raman spectroscopy of soil gas, is useful because four of five gases of interest (CO2, CH4, O2, and N2) are measured simultaneously. N2 is included because it provides mass balance when redox processes impact O2 concentrations. The Phase II prototype ended up larger, heavier, and requiring more power than we envisioned. We expected high laser power would be possible using an optical buildup cavity that can achieve intracavity circulating powers ~4 W using 532 nm light emitted at 40 mW. We also anticipated pressurizing gas samples to about 700 psia to take advantage of the linear Raman response to pressure. Both ideas were not suitable for a field instrument. The buildup cavity is too sensitive to vibration and to changes in refractive index as the sample gas is compressed and the best choice (based on size, weight, price, and power consumption) pump could pressurize samples to about 220 psia. Despite the practical demands on Raman system size, weight, and power, the instrument performs well achieving in the lab 90 ppm detection limits for CO2, O2, and N2 and 12 ppm for CH4. Field operation was limited by temperature sensitivity of the QEPro spectrometer used to record Raman spectra.
- Research Organization:
- Mesa Photonics LLC
- Sponsoring Organization:
- USDOE Office of Science (SC)
- DOE Contract Number:
- SC0017158
- OSTI ID:
- 1900165
- Type / Phase:
- SBIR (Phase II)
- Report Number(s):
- DOE_MP_0017158
- Country of Publication:
- United States
- Language:
- English
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