Selective differential photoacoustic method for trace gas analysis

A selective differential photoacoustic method for trace gas measurements is introduced. The method combines long path absorption spectroscopy with photoacoustic spectroscopy. A photoacoustic cell, equipped with a highly sensitive cantilever pressure sensor, was used as a selective detector for broadband IR-radiation passed through the sample gas. Therefore, an additional optical filter is not required. The detection limit at level of 10 ppb for methane was obtained (0.37 s measurement time and approximately 1 m effective absorption path). The sensitivity of the conventional photoacoustic method was enhanced by factor 2.8. The selectivity was tested by measuring the cross-interference signal between methane, carbon dioxide and butane in different conditions. The differential method enabled perfect separation of methane and carbon dioxide. Butane caused only one fifth of the cross-interference signal with methane compared to a cross-interference measured with a conventional filter system.