Simplifying the laser photoacoustic trace detection of ammonia by effective suppression of water vapour and of carbon dioxide as the major absorbing atmospheric constituents

Highly sensitive (2–3 ppbv) interference‐free photoacoustic and real time detection of ammonia in a matrix of absorbing gases (water vapour and carbon dioxide) at CO2 laser wavelengths was accomplished by combining the mechanical modulation of the laser (frequency f1) and the Stark modulation (frequency of the periodically varying electric field f2). Ammonia was phase sensitive detected at either the sum or the difference frequency sidebands resulting in a dramatic su pression of the interfering absorption caused by water vapour and carbon dioxide.