Wavelength Modulation Spectroscopy
Wavelength Modulation Spectroscopy has distinct advantages over other spectroscopic techniques used for trace gas measurements. It provides a strong signal with very low noise. This enables a shorter optical path length with fewer reflections and reduces sensitivity to contamination of mirrors.
A multipass Herriott cell with off-axis alignment allows for a physical path of 0.5 m, with a 30 m total optical path length. Other trace gas analyzers require optical paths up to several kilometers in length to make a comparable measurement.
The 0.5 m optical cell is optimal because it enables the methane density measurement with relatively few reflections. With fewer reflections, the mirrors in the optical path are less vulnerable to contamination than mirrors used in other spectroscopic instruments. Instruments that use Cavity Ring-Down Spectroscopy (CRDS) and Integrated Cavity Output Spectroscopy (ICOS) require mirrors with 99.99% or higher reflectivity. Mirrors in WMS devices like the LI-7700 are much less sensitive to contamination than the CRDS and ICOS, making WMS a preferred choice for low-power field deployments.
The LI-7700 makes in-situ methane density measurements at ambient pressure. CRDS and ICOS require vacuum pumps to draw down pressure in the optical cell. Operating in an open path configuration at ambient pressure allows measurements without costly, high power consumption vacuum pumps, reducing power consumption by 30-150 times.
While other instruments are limited to comparatively slow measurements, WMS allows the LI-7700 to output data at high speeds, up to 40 Hz.