RESUMEN
In this study, we experimentally investigated the magnitude of the peroxynitrite sink: a radical-radical consumption mechanism for photochemically generated nitric oxide (NO) in surface seawater that describes NO reactions with co-generated superoxide (O2-) to yield peroxynitrite (ONOO-). Measurements of photochemically generated NO, O2- and ONOO- were conducted on seawater samples obtained from the Seto Inland Sea, Japan. Nitrite, dissolved organic carbon, chromophoric dissolved organic matter and pH were also measured in the same samples using standard analytical methods. The average photoformation rates of NO, O2- and ONOO- were: 1.78 × 10-12 M s-1, 7.19 × 10-10 M s-1 and 9.0 × 10-10 M s-1, respectively, and the average steady-state concentrations were: 67.28 × 10-12 M, 2.69 × 10-12 M and 2.26 × 10-11 M, respectively. Further evaluation of the experimental data indicated that the existence of ONOO- in seawater strongly depends on, and is limited by, photoformed NO. Seawater alkalinity favored the consumption of photoformed NO· via the peroxynitrite sink. The magnitude of average sinks (%) calculated from kinetic estimates and experimental data were: 0.17% and 0.11%, respectively. These results show that the consumption of photochemically generated NO· via the peroxynitrite sink is not significant in surface seawater. Therefore, we propose that sea-to-air efflux across the marine boundary layer is the major sink of photochemical NO· and can be regarded as a non-anthropogenic contributor to daytime atmospheric NOx concentrations.
RESUMEN
Peroxynitrite (ONOO-) is a reactive species which can degrade several classes of organic compounds via both oxidation and nitration reactions. Despite the fact that the photochemical precursors; superoxide (O2·-) and nitric oxide (NO·) radicals, have been measured in seawater under simulated solar radiation, there is no published report on actual measurements of photoformed peroxynitrite in seawater. Hence, this attempt was made to develop a fluorescence method by using coumarin boronic acid (CBA) as a chemical probe. CBA reacts with ONOO- to yield fluorescent 7-hydroxycoumarin (COH). COH was determined by reversedâphase isocratic HPLC with fluorescence detection at excitation/emission wavelengths of 332/475 nm, respectively. COH standards calibration is linear in range of (0.25-100) × 10-9 M. The detection limit, defined as 3σ of the lowest ONOO- photo-formation rates obtained within 10- min irradiation, was 1.66 × 10-12 M s-1. This method is very precise as coefficient of variation for triplicate measurements of COH photo-formation rates was a maximum of 0.052. Experimental procedures were optimized to handle potential interference by hypochlorite, and the method was applied to measure ONOO- in 13 surface seawater samples obtained from the Seto Inland Sea, Japan. ONOO- photo-formation rates, steadyâstate concentrations and lifetimes were determined to be (0.06-5.13) × 10-9 M s-1, (0.98-6.11) × 10-11 M and (0.01-0.16) s, respectively.
RESUMEN
A non-enzymatic fluorescence method for the determination of hydrogen peroxide (H2O2) was investigated. This method is based on the hydroxylation reaction of terephthalate (TP) by hydroxyl radical formed from reaction between H2O2 and Fe(II), resulting in the formation of a strongly fluorescent 2-hydroxyterephthalate (HTP). Under optimized conditions, a 3 nM detection limit and 1.0% precision at 200 nM were obtained. This was sensitive enough to determine the concentrations of H2O2 in coastal marine environments. The slopes of the calibration curve in seawater were nearly the same as those in Milli-Q water, suggesting that the fluorescent intensity was not affected by coexisting sea salts. However, the presence of nitrite at more than 10 µM interfered with the formation of HTP. The developed method was successfully applied to determine the concentrations of H2O2 in Osaka Bay on the research vessel. The results obtained in Osaka Bay demonstrated that H2O2 was photochemically produced via the photolysis of dissolved organic matter supplied from the river with fresh water at the surface layer.