A hydroxyl radical detection system using gas expansion and fast gating laser-induced fluorescence techniques

An OH radical measurement instrument based on Fluorescence Assay by Gas Expansion (FAGE) has been developed in our laboratory. Ambient air is introduced into a low-pressure fluorescence cell through a pinhole aperture and irradiated by a dye laser at a high repetition rate of 8.5 kHz. The OH radical is both excited and detected at 308 nm using A-X(0,0) band. To satisfy the high efficiency needs of fluorescence collection and detection, a 4-lens optical system and a self-designed gated photomultiplier (PMT) is used, and gating is actualized by switching the voltage applied on the PMT dynodes. A micro channel photomultiplier (MCP) is also prepared for fluorescence detection and can interchange with the gated PMT. Then the weak signal is accumulated by a photon counter in a specific timing. The OH radical excitation spectrum range in the wavelength of 307.82–308.2 nm is detected and the excited line for OH detection is determined to be Q₁(2) line. The calibration of the FAGE system is researched by using simultaneous photolysis of H₂O and O₂. The minimum detection limit of the instrument using gated PMT is determined to be 9.4 × 10⁵ molecules/cm³, and the sensitivity is 9.5 × 10^− 7 cps/(OH·cm^− 3), with a signal-to-noise ratio of 2 and an integration time of 60 sec, while OH detection limit and the detection sensitivity using MCP is calculated to be 1.6 × 10⁵ molecules/cm³ and 2.3 × 10^− 6 cps/(OH·cm^− 3). The laboratory OH radical measurement is carried out and results show that the proposed system can be used for atmospheric OH radical measurement.