The DOAS Principle

A common ground

DOAS is an abbreviation of Differential Optical Absorption Spectroscopy. It is the basic principle for monitoring gas concentrations in most types of OPSIS gas analysers. The principle is based on the Beer-Lambert absorption law. This law states the relationship between the amount of absorbed light and the number of molecules that the light passes through.


The words 'differential optical absorption spectroscopy' (DOAS) give a good guidance to what it is. ‘Spectroscopy’ means experimental studies of how light behaves. ‘Absorption’ suggests that we are looking at how certain wavelengths of light are attenuated. ‘Optical’ indicates that the studies are limited to the optical wavelengths, from ultraviolet through visible to infrared light. ‘Differential’ indicates the mathematical treatment of the monitored absorption spectra.

For DOAS, a broadband light source is needed, OPSIS instruments utilize a xenon lamp. The light forms a narrow and sharp beam that is sent through the region where the gas, which concentration is to be monitored, is present. The beam is also called the 'monitoring path'. To our eyes, the light beam appears to be white, but it also contains invisible infrared and ultraviolet wavelengths.


Each type of gaseous molecule (and in some cases atom) usually has a characteristic ability to absorb light at certain wavelengths. It is a kind of fingerprint that is unique to the type of molecule. A small fraction of the light emitted by a light source will therefore be absorbed by the gas molecules present in the monitoring path. The higher the concentration of the gas and the longer the measurement path, the more absorption. The light remaining at the far end of the monitoring path is sent to a spectral analyser, the key component of the gas analyser. The analyser can then detect the characteristic absorption that has occurred. By dividing the measured spectrum by a reference (a ‘clean’ spectrum without absorption, the division being the D in DOAS) and comparing this with known absorption spectra, the mean value of the gas concentration sought along the monitoring path can be calculated.

Since each type of gas molecule has its own specific fingerprint, it is possible to distinguish different types of molecules optically, and to use the same monitoring system to monitor the concentrations of several different molecule types along the monitoring path. It is all about knowing in which wavelength range each type of molecule has its unique fingerprint, and then looking for it.

Today, DOAS is a well-established method for monitoring gas concentrations. This is confirmed by numerous approvals and thousands of DOAS instruments in operation around the world.

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