Photophysics and photochemistry of natural waters with emphasis on radical probe development and application

Abstract

The work presented here consists of a literature review and calculations to estimate the importance of photochemistry to carbon cycling in the oceans, followed by a photophysical study of a series of stable nitroxide radical probes that have been used for the quantitative detection of individual carbon-centered radicals and reducing species in natural waters. Two appendices follow. The first contains preliminary experiments utilizing one of the nitroxide probes in an investigation of hydroxyl radical production rates and steadystate concentrations in seawater. The second consists of an investigation of the singlet lifetimes of humic acids (HA), in order to aid in understanding their photochemical cycling and influence on other compounds.The impact of photochemical reactions on global oceanic carbon cycling was calculated from literature values. The results indicate that between 1 and 13% of all dissolved organic carbon in the oceans is oxidized photochemically. This is a significant flux term, much larger than that of riverine input for example.A photophysical study of nitroxide radical probes was undertaken. For all of the compounds studied, steady-state absorption and fluorescence spectra were identical to those of the parent fluorophores. A decrease in fluorescence lifetime and quantum yield of tens-to hundreds-fold was observed for the paramagnetic compounds relative to their diamagnetic counterparts. Very rapid fluorescence quenching rates (3 to 80 x 1010 s-1 ) were calculated for the fluorescamine moiety of the paramagnetic nitroxide compounds in a variety of solvents. Calculated energy minimized geometries were very similar for all compounds which implies that geometric differences are not responsible for the variations found in fluorescence lifetimes and quantum yields between compounds. Calculated Firster and Dexter overlap integrals do not support deexcitation by these mechanisms. Time-resolved absorption measurements resulted in no evidence for transient species due to either intersystem crossing to the triplet state or charge transfer. Of the mechanisms considered, direct internal conversion to the ground state, is most likely given our results.An investigation of the utility of 3-(aminomethyl)-2,2,5,5-tetramethyl-1-pyrrolidinyloxy free radical (3-amp) for detection and quantification of hydroxyl radicals in natural waters found that the addition of primary probe compounds resulted in the generation of secondary carbon-centered radicals that were successfully trapped by 3-amp. Competition kinetics experiments with dimethyl sulfoxide resulted in a natural scavenger rate constant that matched previous literature results for coastal seawater. As expected, the addition of formate resulted in decreases, and the addition of nitrite in increases, in the hydroxyl radical trapping rate by this method. The resulting quantum yield values were about an order of magnitude higher than previous literature results. However, probablyI IYIIIIYYI Wilkl 4 due to the use of different latitudes at which to estimat...

Optical Properties of Humic Substances and CDOM: Relation to Structure

“…Much smaller contributions to the total decay amplitude were observed for the intermediate lifetime component (τ int between 0.3 and 1.3 ns, R int between 5 and 30%), whereas the smallest contribution was observed for the longest lifetime component (τ long between 2 and 5.5 ns, R long between 2 and 18%). Importantly, although the shortest-lived components dominated the total population of emitting species (largest R), their contribution to the steadystate fluorescence intensity (f i ) was the least (SI Figures S4-S21), illustrating that steady-state emission spectra grossly misrepresent the abundance and importance of these short-lived species to the photophysical properties of these materials (eq 3) (4,33,35).…”