Appendix A About Gas Analyzers And Laser Absorption Spectroscopy; Conventional Laser Absorption Spectroscopy - ABB GLA232 Series User Manual

ABB LGR-ICOS™ GLA232 Series Methane/Ethane Analyzer User Manual
Appendix A About Gas Analyzers and Laser
Absorption Spectroscopy

Conventional Laser Absorption Spectroscopy

For gas measurements based on conventional laser-absorption spectroscopy (Figure 34), a laser
beam is directed through a sample, and the mixing ratio (or mole fraction) of a gas is
determined from the measured absorption using Beer's Law, which may be expressed:
Where:

I
is the transmitted intensity through the sample at frequency
v

I
is the (reference) laser intensity prior to entering the cell
o

S is the absorption line strength of the probed transition

L is the optical path length of the laser beam through the sample

�� is the mole fraction

P is the gas pressure

Φ is the line-shape function of the transition at frequency ��
v
In this case,
If the laser line width is much narrower than the width of the absorption feature, high-
resolution absorption spectra may be recorded by tuning the laser wavelength over the probed
feature.
Figure 34: Typical Laser Absorption Spectroscopy Setup
ABB PROPRIETARY INFORMATION 48