Spin trapping EPR method for simultaneous monitoring of hydroxyl radicals and hydrogen atoms in γ-irradiation process

We have examined the generation of 'OH and' H under low-dose γ-radiolysis in aqueous solution a through a spin trapping electron paramagnetic resonance (EPR) using 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) as a trapping reagent. Here we discus separately the reaction schemes of DMPO-involved radiolysis for the cases during and after radiolysis. A method based on kinetic calculations has been developed to quantify exactly the concentrations of 'OH and' H in situ. In the case when a batch monitoring method is employed, the time-resolved instability of DMPO adducts after sampling can be the sources of errors in estimating the in situ concentration of those radical species if it is not corrected with the due instability rates. The main causes of the instability of the DMPO adducts after the cessation of γ-irradaition were the homo-dimerization (adduct/adduct) reaction (DMPO-OH/DMPO-OH or DMPO-H/DMPO-H) and the hetero-(cross)-dimerization of DMPO-adducts (DMPO-OH and DMPO-H). The corresponding instability rate constants for the DMPO-OH/DMPO-OH and DMPO-OH/DMPO-H processes were 80 M ^-1s^-1 and 2.8 × 10³ M^-1s ^-1, respectively. The 'OH trap yield in the 0.05 M DMPO solution at 10 Gy was low (9.3%), which implies that 'OH might attack another position (ring position) of DMPO to form intermediate organic radicals. The intermediate organic radicals might react with the DMPO adducts, causing a slight deviation from linearity for the plot of the DMPO adducts versus the γ-dosage.