Photoacoustic spectroscopy of formaldehyde with tunable laser radiation at the parts per billion level

Selective and sensitive detection of formaldehyde (CH₂O) was achieved using a grazing-incidence optical parametric oscillator (GIOPO), pumped at high repetition rate, as a light source for photoacoustic spectroscopy. The photoacoustic spectrum of formaldehyde was measured in the range from 2785 to 2840 cm^-1 and an absorption line at 2805.0 cm^-1 was selected for detection. Concentrations down to 20 ppbv (parts per billion by volume) formaldehyde in nitrogen were recorded. The detection limit determined by background fluctuations was 3 ppbv (S/N=1) for 3 s lock-in time constant and 3 min acquisition time. The 2805.0 cm^-1 absorption line of the ν₁ vibrational mode was chosen because of the absence of interference with water and carbon-dioxide bands. This allowed the direct detection of formaldehyde in laboratory air without filtering. PACS 42.62.Fi; 42.68.Ca; 42.65.Yj; 07.07.Df