Condensed from the OPSIS knowledge base page on measurement methods.
Different molecules absorb in different wavelength ranges, and the technique has to match. Molecules that absorb in the ultraviolet are measured with UV-DOAS; some have their absorption features in the infrared, where an ultraviolet measurement cannot reach, and there FTIR-DOAS is used. As with UV-DOAS, light is projected across the measurement path and captured by a receiver, which passes it to the analyser over a fibre; again different molecules absorb different wavelengths, and a selected spectral window is recorded — only this time in the infrared. In fact the same light can supply a UV-DOAS analyser and an FTIR-DOAS analyser at the same time. The analyser holds a computer and control electronics, but instead of a spectrometer it contains an interferometer, which is the right tool for infrared spectra and unsuitable for the ultraviolet, just as the spectrometer is right for the ultraviolet. The interferometer splits the light at a beamsplitter onto two mirrors; changing the mirror position continuously over the measurement period produces an interferogram, and a Fourier transform — the FT in FTIR — converts it into a spectrum of the kind a spectrometer records directly. Optical filters in front of the detector optimise the interferogram for a particular infrared band.
In practice the two techniques are specified together rather than separately. A cement kiln or an incinerator usually needs NO and SO2 in the ultraviolet and CO, CO2, HCl, HF, NH3 or water vapour in the infrared, so a system with one analyser of each type, sharing a light path where possible, covers the list without duplication. The engineering consequence is two optical channels to keep aligned and two sets of reference data to maintain, with the data acquisition merging them into one record on a consistent basis. On Chinese projects the combined CEMS then follows HJ 75 and HJ 76, and the assignment of components between the two analysers should be documented, because the acceptance comparison is performed per component rather than per instrument.
FTIR-DOAS Technology — read the original at opsis.se ↗
Source: OPSIS knowledge base, monitoring methods. The summary and the A.Johnson note above are our own text; the OPSIS article remains the authoritative version. Chinese reader? See the full Chinese article.
Send the gases, ranges, duct or path dimensions and required certification — we will respond with a technical proposal.