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A gaseous analyte including a small amount of a multiatomic moiety incorporating a particular isotope, such as 14CO2 is subjected to a standing optical wave at a resonant wavelength of the moiety while maintaining the moiety in an excited condition, such as in a gas discharge. The standing optical w

A gaseous analyte including a small amount of a multiatomic moiety incorporating a particular isotope, such as 14CO2 is subjected to a standing optical wave at a resonant wavelength of the moiety while maintaining the moiety in an excited condition, such as in a gas discharge. The standing optical wave may be applied by a laser having a sample cell containing the analyte within the laser cavity. Monitoring an induced effect such as the optogalvanic effect yields a signal directly related to the quantity of the moiety. The test can detect quantities of the moiety of an attomole or less, and in some cases on the order of 100 molecules.

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The invention claimed is: 1. An analytical method comprising the steps of: (a) maintaining a gaseous analyte including a multi-atomic moiety in a condition wherein the moiety is present in an excited state; (b) directing coherent light at a resonance wavelength of the moiety in the excited state al

The invention claimed is: 1. An analytical method comprising the steps of: (a) maintaining a gaseous analyte including a multi-atomic moiety in a condition wherein the moiety is present in an excited state; (b) directing coherent light at a resonance wavelength of the moiety in the excited state along a path through the analyte so as to maintain a substantially constant photon density at locations along the path within the analyte; (c) monitoring an induced effect in the analyte having a magnitude at least ten times greater than a magnitude expected based on the length of the path; and (d) determining at least one the presence or amount of the moiety in the analyte based on the magnitude of the induced effect. 2. A method as claimed in claim 1 wherein said step of directing coherent light is performed so as to maintain a standing optical wave within the analyte. 3. An analytical method comprising the steps of: (a) maintaining a gaseous analyte including less than about 10-18 moles of a multi-atomic moiety in a condition wherein the moiety is present in an excited state; (b) directing coherent light at a resonance wavelength of the moiety in the excited state along a path through the analyte so as to maintain a standing optical wave within the analyte; (c) monitoring an induced effect in the analyte; and (d) determining at least one of the presence or amount of the moiety in the analyte based on the magnitude of the induced effect. 4. An analytical method comprising the steps of: (a) generating coherent light at a resonance wavelength of a multi-atomic moiety in a laser; (b) maintaining a gaseous analyte including a multi-atomic moiety within a chamber outside of the laser in a condition wherein the moiety is present in an excited state; (c) directing coherent light from the laser into the chamber so as to maintain a standing optical wave within the analyte; (d) monitoring an induced effect in the analyte; and (e) determining at least one the presence or amount of the moiety in the analyte based on the magnitude of the induced effect. 5. An analytical method comprising the steps of: (a) operating a gas laser having a cavity between a pair of mirrors and a lasing medium including a multi-atomic moiety to produce coherent light at a wavelength corresponding to a transition energy of the moiety; (b) maintaining an analyte including less than about 10-18 moles of the moiety in a condition wherein the moiety in the analyte is present in an excited state between the mirrors but separate from the lasing medium so that coherent light generated in said laser passes through the analyte; (c) monitoring an induced effect in the analyte; and (d) determining at least one of the presence or amount of the moiety in the analyte based on the magnitude of the induced effect. 6. A method as claimed in claim 1 or claim 5 wherein the step of maintaining the analyte includes maintaining the analyte in an electrical discharge. 7. A method as claimed in claim 1 or claim 5 wherein the moiety is selected from the group consisting of molecules, free radicals and ions having 5 atoms or less. 8. A method as claimed in claim 7 wherein the moiety is selected from the group consisting of molecules, free radicals and ions having 3 atoms or less. 9. A method as claimed in claim 7 wherein the moiety is carbon dioxide of a particular isotopic composition. 10. A method as claimed in claim 9 wherein the moiety is 14C16O2. 11. A method as claimed in claim 9 wherein the analyte includes nitrogen and oxygen. 12. A method as claimed in claim 9 wherein the analyte includes air. 13. A method as claimed in claim 9 wherein the analyte is essentially free of helium. 14. A method as claimed in claim 1 or claim 5 wherein the analyte includes less than about 10-19 moles of the moiety. 15. A method as claimed in claim 14 wherein the analyte includes less than about 10-20 moles of the moiety. 16. A method as claimed in claim 1 or claim 5 wherein the moiety is multiatomic chemical species including a first isotope and wherein the analyte further includes said multiatomic chemical species including a second isotope different from said first isotope. 17. A method as claimed in claim 16 wherein a concentration of said first isotope in the said multiatomic chemical species is about 1 part in 1010 or less. 18. A method as claimed in claim 1 or claim 5 wherein said monitoring step includes monitoring the magnitude of the induced effect and the determining step includes determining an amount of the moiety in the analyte based at least in part on said magnitude. 19. A method as claimed in claim 18 further comprising repeating the aforesaid steps with a blank sample of the same composition as the analyte but devoid of the moiety and monitoring the magnitude of an induced effect for the blank sample, the step of determining an amount of the moiety including subtracting the magnitude of the induced effect for the blank sample from the magnitude of the induced effect for the analyte. 20. A method as claimed in claim 1 or claim 5 wherein the step of monitoring the induced effect is performed by monitoring the optogalvanic effect. 21. A method as claimed in claim 1 or claim 5 further comprising the step of modulating the amplitude of the coherent light, the step of monitoring an induced effect being performed in synchronism with the amplitude modulation. 22. Apparatus for determining the presence or amount of a moiety in an analyte comprising: (a) a sample cell containing a gaseous analyte including less than about 10-18 moles of a multi-atomic moiety; (b) an energy application associated with the sample cell for maintaining the analyte in a condition wherein the moiety is present in an excited state; (c) a source of coherent light at a resonance wavelength of the moiety in the excited state arranged to direct the coherent light along a path through the sample cell so as to maintain a standing optical wave within the analyte; and (d) a detector for monitoring an induced effect in the analyte and providing a signal representing the induced effect. 23. Apparatus as claimed in claim 22 further comprising a modulator for modulating the coherent light at a modulation frequency. 24. Apparatus as claimed in claim 23 further comprising a signal processor for filtering the signal to isolate a component at the modulation frequency. 25. Apparatus as claimed in claim 22 wherein the detector is arranged to monitor the optogalvanic effect.