A photothermal interferometer for gas-phase ammonia detection

Detection of gas-phase ammonia is particularly challenging because ambient ammonia concentrations may be less than 1 ppb (molecules of NH3 per 10(9) m olecules of air), ammonia sticks to many materials commonly used to sample air, and particles containing ammonium may interfere with gas-phase measure ments, We have built a new and sensitive photothermal interferometer to det ect gas-phase ammonia in situ, under typical atmospheric conditions. Ammoni a molecules in sampled air absorb infrared radiation from a CO2 laser at 9. 22 mu m,with consequent collisional heating, expansion, and refractive inde x change, This change in refractive index is detected as a phase shift in o ne arm of a homodyne interferometer. Measurements of vibrational and electr ical noise in the interferometer correlate to an instrumental lower limit o f detection of 6.6 ppt ammonia in 1 s, The CO2 laser output is modulated at 1.2 kHz, and the ac signal from the interferometer is measured with a lock -in amplifier. The detector is zeroed by sampling through a H3PO4-coated de nuder tube and is calibrated by dynamic dilution of two permeation tube out puts and by standard addition. Signal gain is insensitive to CO2 or H2O in the sample, and the signal is linear over 5 orders of magnitude. The instru ment 2 sigma precision is 31 ppt when the signal is integrated for 100 s an d 250 ppt with a 1-s integration time. The windowless sample cell and inlet is fabricated entirely of glass to minimize sample loss and hysteresis, Th e instrument response time is demonstrated to be about 1 s.