In-situ Calibration of the Hydroperoxyl Radical Using an Immobilized TiO 2 Photocatalyst in the Atmosphere

The present study is the first report of utilizing TiO2 photocatalyst to analytically calibrate the hydroperoxyl radical (HO2). An in-situ calibration method of HO2 is proposed for air monitoring by using an 2-methyl-6-(pmethoxyphenyl)-3,7-dihydroimidazo-[1,2-a]pyrazin-3-one (MCLA)-chemiluminescence (CL) technique. In this method, HO2 (pKa = 4.80) is produced by the ultraviolet (UV) photolysis of immobilized TiO2 using a constant flow rate of air equilibrated water, in which HO2 is controlled by using various lengths of knotted tubing reactor (KTR). The principle of the proposed calibration is based on the experimentally determined halflife (t1/2) of HO2 and its empirically observed pH-dependent rate constant, kobs, at a given pH. The concentration of HO2/O2 is increased as pH increases. This pH dependence is due to the different disproportionative reactivities between HO2/O2 and HO2/O2. Experimental results indicate the practical feasibility of the approach, producing very promising method.