Patent ID: 6493086
Filing Date: 2002-12-10
Classification: C23C,G01J,G01N

Abstract:
A method for detecting gas phase molecular species in a sample by harmonic detection spectroscopy, comprising:providing a harmonic detection spectroscopy measurement cell, the cell having at least one light entry/exit port therein, each entry/exit port containing a light transmissive window having a surface facing the sample region wherein a sample gas flows through the sample region, the cell operating at about atmospheric pressure or less; providing a frequency and/or amplitude modulated light source for directing a light beam through one of the at least one light transmissive windows into the cell and setting the light source modulation amplitude to a value which approximately maximizes the value of a harmonic signal at the center of the absorption feature due to the detected gas phase molecular species inside the sample region; adjusting the center frequency of the light source so that it is either locked to the center of the absorption feature or repetitively scanned over the frequency range which includes the feature, generating spectra which may be recorded individually or averaged as desired; providing a detector for measuring the light beam exiting the cell through one of the at least one light transmissive windows; wherein the light source and detector are contained within a chamber which is external to the cell and isolated from the sample region, the chamber and the sample region being placed in optical communication with each other through at least one of the at least one light transmissive windows, and controlling the pressure inside the chamber to a value which is positive relative to atmospheric pressure.