Rapid communication Detection of formaldehyde using mid-infrared difference-frequency generation

Real-time detection and measurement of subppm levels of formaldehyde in ambient air using diodepumped 3.5 μm difference-frequency generation (DFG) in periodically poled LiNbO3 (PPLN) is reported. Minimum detectable concentration of 30 ppb was achieved with a compact, portable room-temperature gas sensor configured for formaldehyde (H2CO) detection. This sensitivity, coupled with high selectivity and long term stability, is sufficient for various environmental applications. PACS: 7:65; 33:00; 42.60; 42.65; 42.80 Formaldehyde is an important reactive intermediate product in tropospheric hydrocarbon oxidation initiated by the OH radical. The H2CO concentration in the atmosphere is in the 1 to 10 ppb range [1–3]. It is also a well-known pollutant that is emitted due to incomplete combustion processes [4]. Formaldehyde is a chemical widely used in many industrial manfacturing processes due to its high chemical reactivity and good thermal stability. Widely used building materials like foams and numerous consumer paint and polymer products contain formaldehyde. Studies indicate that H2CO can cause a variety of health effects ranging from irritation of eyes, nose, and throat to nausea, dizziness, and headaches at concentrations above 100 ppb. At 100 ppm it is dangerous to life and health. To protect workers from exposure to formaldehyde, an 8-hour time weighted average concentration of 750 ppb as the permissible exposure limit is set by the U.S. Occupational Safety and Health Administration (OSHA) [5]. The National Aeronautics and Space Administration (NASA) provides an even more conservative value, the Spacecraft Maximum Allowable Concentration (SMAC) of 40 ppb for crew exposure over 7 to 180 days [6]. In order to monitor the concentration and to locate emission sources, the development of an in situ, real-time, portable gas sensor capable of detecting formaldehyde of sub-ppm levels in air is of considerable interest. Previously reported formaldehyde detection in the infrared was performed using FTIR monitoring [7] and tunable diode laser absorption spectroscopy (TDLAS) with cryogenically cooled lead salt diode lasers. Harris et al. [8] demonstrated a minimum detectable concentration of 0.25 ppb using a 33.5 m optical pathlength with a typical acquisition time of 3 minutes in the 5.7 μm absorption region. More recently, Fried [9] reported an improved detection sensitivity of 0.05 ppb by monitoring H2CO absorption at 2831.5 cm−1 using a 100 m pathlength Herriott multipass cell and a data averaging time of 100 s. The gas sensor described here uses difference frequency mixing in PPLN as a convenient mid-infrared room temperature laser based source tuned to a formaldehyde absorption line at 2861.72 cm−1 (∼3.5 μm). This compact and portable instrument appears to be suitable for various applications involving air quality measurements. To date, we have demonstrated a minimum detectable H2CO concentration (limited by interference fringes produced by optical surfaces in the beam path) of better than 30 ppb (corresponding to an absorption of 0.02%) by means of direct laser absorption spectroscopy. 1 Experimental details The mid-infrared absorption spectrum of formaldehyde was first investigated in detail by Pine [10] and Brown et al. [11]. Pine reported that Q-branches of the ν1 and ν5 carbon-hydrogen stretch modes at ∼ 3.60 μm are particularly promising for atmospheric monitoring, because of their high intensity and relative freedom from interference by water and methane. Our present mid-infrared laser source was tuned to one of the P-branch absorption lines of the ν5 band at 2861.72 cm−1 (∼3.5 μm). In order to reduce interference by weak formaldehyde, methane, and water lines close by, H2CO detection was performed in air at reduced pres-sure (∼ 50 to 100 Torr) in a multipass cell. A schematic diagram of the formaldehyde sensor is shown in Fig. 1, which is similar in design to that reported previously for the detection of CH4 and CO [12–15]. The 3.5 μm spectroscopic DFG source is pumped by two compact lasers, a 100 mW single-frequency GaAlAs diode laser