Instruments - DOAS

 

Differential Optical Absorption Spectroscopy or DOAS, is based on the principle of measuring the differential absorption of the incoming signal in the wavelength region of interest after it has passed through a gas medium.  It uses a broadband light source and the measurement is made over a large wavelength range (like FTIR and NDIR, and unlike TDL that uses a very narrow wavelength range).  In general, overlap of absorption features of several gases is observed.  This requires the use of multiple regression methods of analysis. The concentrations of a gases are calculated by comparing the absorption signals with their stored reference signals.  As in any broadband measurement technique, interference from other gases must be taken into account and corrected for to improve accuracy of measurement.  This requires a prior knowledge of the gas composition before the final measurements can be made. 

DOAS is capable of measuring more than one species which either absorb in the same wavelength region (continuous monitoring of the species; e.g., NO, NH3, SO2 etc.) or in different wavelength regions (time-shared monitoring of the species; e.g., NO, NO2, HCHO etc.). 

The absorption of light by the molecules either in the near ultraviolet or in the visible region of the electromagnetic spectrum arises due the excitation of the molecules from the vibrational-rotational levels of the ground electronic state to the vibrational-rotational levels of a higher electronic state.  The observed spectrum consists of series of absorption lines whose widths and line shapes are usually represented by Voigt profile.  In order to completely resolve the individual vibration-rotation absorption lines, a monochromator of the order of few meters in length and a high resolution grating has to be incorporated into the spectrometer. This is, however, not practical in the development of a portable instrument.  With the advent of high resolution CCD arrays, compact sensitive instruments (e.g., ppbv levels in 1 m paths) can be made. 

The dynamic wavelength range spanned by the DOAS spectrometer (typically 50-60 nm to get a decent resolution, but can be up to 1000 nm depending on the grating) does not necessarily encompass the background “zero absorption” level.  In such cases, the measurement of the absolute absorption cross-section becomes difficult, and hence, the differential absorption cross-sections (difference between absorption peaks and troughs) are measured.  For simplicity, from the software point of view, it is customary that the spectrum be fitted to a polynomial (of a suitable degree) and the result divided for normalization. This gives the differential absorption spectrum of the sample under study. 

Unisearch currently provides DOAS instruments in two configurations.  An in-situ stack monitoring system for use where dust levels in the flue gas are low and path length is no greater than a few meters.  The second configuration is an extractive system which may be designed to have an internal or external flow through gas cell, depending on the sensitivity required.  In both cases the analyzer is located close to, or incorporated into the optical heads.  Fiber optic coupled systems are possible, depending on the wavelength, which allow separation of the analyzer from the optics of a few meters.
 
Various organic and inorganic gases can be measured with DOAS with sensitivities down to ppbv levels.  Many of these gases can also be measured at very high levels (several 10s of percent) with no dilution requirements. For example, SO2 can be measured down to a few ppbv as well as from 0 to 100%.  This makes the Unisearch DOAS system very flexible to a client’s needs.