Abstract: A method and apparatus for in situ measurement of the concentration of a gas with a frequency modulated tunable diode laser is disclosed. The sampling cell, which is mounted in the flow of gases to be measured, is a Herriott cell. Gas enters the sampling cell through sintered metal filters that prevent entrance of particulates. Signals from a sample detector and a null detector are compared to eliminate interference patterns from the laser optics. High accuracy dynamic calibration of the apparatus is also disclosed.

Abstract: An off-line-locked laser diode species monitor system includes: reference means for including at least one known species having a first absorption wavelength; a laser source for irradiating the reference means and at least one sample species having a second absorption wavelength differing from the first absorption wavelength by a predetermined amount; means for locking the wavelength of the laser source to the first wavelength of the at least one known species in the reference means; a controller for defeating the means for locking and for displacing the laser source wavelength from said first absorption wavelength by said predetermined amount to the second absorption wavelength; and a sample detector device for determining laser radiation absorption at the second wavelength transmitted through the sample to detect the presence of the at least one sample species.