Spectroscopy method and apparatus for detecting low concentration gases

A method of detecting constituents of a gas comprising:

• a. providing a medium that is absorbent of at least a first particular gas under a first environmental condition and desorbent of said particular gas under a second environmental condition;
• b. exposing said medium to a sample gas for a first period of time under the first environmental condition;
• c. during a second period of time after said first period of time, exposing said medium to said second environmental condition to cause said medium to desorb gas into an optical cavity of a cavity ring down spectrometer and providing electromagnetic radiation into said cavity;
• d. during a third period of time after said second period of time, ceasing provision of said electromagnetic radiation into said cavity and detecting the decay of said electromagnetic radiation in said cavity;
• and
• e. analyzing the decay of said light in said cavity to obtain a spectral analysis of said sample gas.

A method of detecting a constituent of the gas comprising:

• a. providing a resonant optical cavity having at least one coupler for permitting electromagnetic radiation from a source into said cavity and permitting electromagnetic radiation to escape from said cavity;
• b. providing an absorbent medium in fluid communication with said cavity, said medium being absorbent of at least a first particular gas under a first environmental condition and being desorbent of said particular gas under a second environmental condition;
• c. introducing a sample gas into said cavity, sufficient to contact and absorb onto the medium under the first environmental condition;
• d. exposing said medium to the second environmental condition to cause said medium to desorb at least a portion of the sample gas;
• e. pumping said cavity with electromagnetic radiation for a first period of time;
• f. detecting an intensity of said electromagnetic radiation pumped into said cavity;
• g. simultaneously with step d f, detecting an intensity of electromagnetic radiation escaping said cavity;
• and
• h. determining a difference between said intensity of radiation pumped into the cavity and said intensity of electromagnetic radiation escaping from the cavity to derive an absorption characteristic of said sample gas.

A cavity ring-down spectrometer comprising:

• a resonant cavity comprising at least two mirrors;
• a source for providing electromagnetic radiation into said optical cavity;
• a detector for monitoring the amount of radiation in said cavity and generating a first detection signal based thereon;
• a path for admitting a sample gas to flow over said medium and in said cavity;
• a processor adapted to control said cavity ring-down spectrometer to expose said medium to the sample gas while under the first environmental condition for a first period of time and, at the end of the first period of time, to switch to said second environmental condition to cause said medium to desorb during a second period of time after said first period of time, turn on said source during said second period of time, turning said source off at the beginning of a third period of time following said second period of time, and measuring the decay of the electromagnetic radiation in the cavity during the third period of time;
• and
• a heater coupled to at least one of said mirrors to locally heat at the least a reflecting surface thereof.