DOAS (Differential Optical Absorption Spectroscopy)

DOAS stands for Differential Optical Absorption Spectroscopy. It is the fundamental principle for measuring gas concentrations in most of OPSIS’ gas analysers.

The acronym DOAS gives us good clues to what it’s about: “Spectroscopy” means the experimental investigation of the nature of light. “Absorption” indicates that we look at the “disappearance” of specific wavelengths. “Optical” limits us to study the optical wavelengths, ranging from ultraviolet via visible to infrared light. Finally, “differential” is the mathematical process applied to the recorded optical absorption spectra.

DOAS utilizes a broad band light source; in OPSIS’ case a xenon lamp. The light is made to form a narrow and intense beam which is sent across the volume where the gas to measure resides; often referred to as the “measurement path”. The ray of light looks white to the human eye but also includes “invisible” infrared and ultraviolet wavelengths.

A specific type of gaseous molecule (and in some cases also atom) usually has narrow-band absorption features in certain wavelength regions. It can be thought of as a fingerprint of the molecule type. A fraction of the light emitted from the light source will be absorbed by the molecules present in the path. The higher concentration of the gas or the longer measurement path, the more absorption will occur. The remaining light that arrives at the end of the measurement path is forwarded to a spectrum analyser, the key component of the gas analyser. The analyser can then detect the features of the absorption. Dividing the recorded spectrum by a reference (i.e. a “clean” spectrum, this division is the “D” in DOAS) and comparing it with known absorption spectrum of the sought-for gas yields the average gas concentration along the measurement path.

Each type of gaseous molecule has its own characteristic absorption abilities in specific wavelengths, and thereby its own specific fingerprint. This makes it possible to optically separate different gases from each other, and to use the same measurement system to detect multiple gaseous compounds in the light path. It is a matter of knowing in what wavelength region the respective type of molecule has its unique fingerprint, and then look for that.

Today, DOAS is a well-established principle for gas concentration measurements, as evidenced by numerous approvals and thousands of DOAS systems deployed throughout the world.