Laser Absorption Spectrometry - Direct Laser Absorption Spectrometry

Direct Laser Absorption Spectrometry

The most appealing advantages of LAS is its ability to provide absolute quantitative assessments of species. Its biggest disadvantage is that it relies on a measurement of a small change in power from a high level; any noise introduced by the light source or the transmission through the optical system will deteriorate the sensitivity of the technique. Direct laser absorption spectrometric (DLAS) techniques are therefore often limited to detection of absorbance ~10−3, which is far away from the theoretical shot noise level, which for a single pass DAS technique is in the 10−7 – 10−8 range. This detection limit is insufficient for many types of applications.

There are three ways to improve the situation:

1. to reduce the noise;
2. to address transitions with larger transitions strengths and
3. to increase the interaction length.

The first can be achieved by the use of a modulation technique; the second can be obtained by using transitions in unconventional wavelength regions, whereas the third by using external cavities.