With the Yokogawa TDLS500 (ICOS) Analyzer, rapid, continuous measurements are possible for the first time in a process hardened analyzer. Traditionally trace level analysis in process conditions has required the use of process gas chromatographs. The major drawback of this approach has been the analysis time required (typically minutes), and the fact that a continuous measurement is not possible.
Off-Axis Integrated Cavity Output Spectroscopy (ICOS) utilizes a high-finesse optical cavity as an absorption cell. Unlike conventional multipass arrangements which are typically limited to path lengths less than two-hundred meters, an Off-Axis ICOS absorption cell effectively traps the laser photons so that they make thousands of passes on average before leaving the cell. As a result, the effective optical path length may be several thousands of meters using high-reflectivity mirrors and thus the measured absorption of light after it passes through the optical cavity is significantly enhanced. For example, for a cell composed of two 99.99% reflectivity mirrors spaced by 25 cm, the effective optical path length is 2500 meters.
Since the pathlength depends only on optical losses in the cavity, and not on a unique beam trajectory (like conventional multipass cells or cavity- ringdown systems), the optical alignment is very robust allowing for reliable operation in the field.
As with conventional tunable-laser absorption-spectroscopy methods, the wavelength of the laser is tuned over a selected absorption feature of the target species. The measured absorption spectra is recorded and, combined with measured gas temperature and pressure in the cell, effective path length, and known line strength, used to determine a quantitative measurement of concentration. The TruePeak measurement integrates the area of the absorbance rather than measuring peak height or peak width value alone. This means you get a true interference free analysis. As well as speed of response, the fact that the ICOS analyzer uses a first principles measurement provides the ability to"prove" the measurement using live or historical spectra.
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