Determination of ambient ethanol concentrations in aqueous environmental matrixes by two independent analyses.

A new method for the determination of ethanol in aqueous environmental matrixes at nanomolar concentrations is presented and compared to an existing method that has been optimized for low-level alcohol determinations. The new analysis is based upon oxidation of ethanol by the enzyme alcohol oxidase obtained from the yeast Hansenula sp. which quantitatively produces acetaldehyde after reaction for 120 min at 40 °C and pH 9.0. The acetaldehyde reacts with 2,4-dinitrophenylhydrazine forming a hydrazone that is separated from interfering substances and quantified by high-performance liquid chromatography (HPLC) with UV detection at 370 nm. Comparison of initial acetaldehyde concentration with that after enzymatic oxidation yields the ethanol concentration with a corresponding detection limit of 10 nM. Analytical results were verified by intercomparison with a completely independent technique utilizing a solid-phase microextraction (SPME) Carboxen/PDMS SPME fiber. A 12 mL aqueous phase sample was heated at 50 °C for 10 min prior to loading onto the SPME fiber. Extraction of ethanol was performed by introducing the fiber into the headspace above a pH 4.4 buffered sample containing 30% NaCl for 20 min. Samples were agitated during heating and extraction by magnetic stirring at a rate of 750 rpm. The fiber was thermally desorbed for 1 min at 230 °C in the injection port of a gas chromatograph equipped with a flame ionization detector (FID) set at 250 °C. The resulting ethanol detection limit is 19 nM. Results of an intercomparison study between the enzymatic and SPME analyses produced a trend line with a slope of unity demonstrating that methods produced statistically equivalent ethanol concentrations in several natural waters including rainwater, fresh surface waters, and sediment pore waters.

Oxidation of Fe(II) in rainwater.

Photochemically produced Fe(II) is oxidized within hours under environmentally realistic conditions in rainwater. The diurnal variation between photochemical production and reoxidation of Fe(II) observed in our laboratory accurately mimics the behavior of ferrous iron observed in field studies where the highest concentrations of dissolved Fe(ll) occur in afternoon rain during the period of maximum sunlight intensity followed by gradually decreasing concentrations eventually returning to early morning pre-light values. The experimental work presented here, along with the results of kinetics studies done by others, suggests thatthe primary process responsible for the decline in photochemically produced Fe(II) concentrations is oxidation by hydrogen peroxide. This reaction is first order with respect to both the concentrations of Fe(II) and H2O2. The second-order rate constant determined for six different authentic rain samples varied over an order of magnitude and was always less than or equal to the rate constant determined for this reaction in simple acidic solutions. Oxidation of photochemically produced ferrous iron by other oxidants including molecular oxygen, ozone, hydroxyl radical, hydroperoxyl/superoxide radical, and hexavalent chromium were found to be insignificant under the conditions present in rainwater. This study shows that Fe(II) occurs as at least two different chemical species in rain; photochemically produced Fe(II) that is oxidized over time periods of hours, and a background Fe(II) that is protected against oxidation, perhaps by organic complexation, and is stable against oxidation for days. Because the rate of oxidation of photochemically produced Fe(II) does not increase with increasing rainwater pH, the speciation of this more labile form of Fe(II) is also not controlled by simple hydrolysis reactions.

Organic complexation of Fe(II) and its impact on the redox cycling of iron in rain.

More than 80% of the iron(II) present in a dilute (pH 4.5) H2SO4 solution was oxidized by hydrogen peroxide (3 microM) in 24 h, whereas in rainwater Fe(II) remained stable for days indicating that a complexed form of Fe(II) exists in rainwater that protects it against oxidation. When a rain sample was irradiated for 2 h with simulated sunlight, there was a 57 nM increase in Fe(II) resulting from photoreduction of organic Fe(III) complexes. Once irradiation ceased, the photoproduced Fe(II) rapidly oxidized back to its initial concentration of 32 nM prior to irradiation, but not to zero. These photochemical studies demonstrate that during the daytime when sunlight is present there are dynamic interconversions between complexed and uncomplexed Fe(II) and Fe(III) species in rainwater. During the night, after the photochemically produced Fe(II) is reoxidized to Fe(III), virtually all remaining Fe(II) is complexed by ligands which resist further oxidation. Rain samples oxidized under intense UV light lost their ability to stabilize Fe(II), suggesting the ligands stabilizing Fe(II) are organic compounds destroyed by UV-irradiation. Additional UV-irradiation studies demonstrated that on average 25% of the Fe-complexing ligands in rainwater are extremely strong and cannot be detected by spectrophotometric analysis using ferrozine. The stability of organically complexed Fe(II) has important implications for the bioavailability of rainwater-derived Fe in the surface ocean.

Photochemical production of Fe(II) in rainwater.

Significant concentrations of Fe(II) were produced upon irradiation of authentic rainwater with simulated sunlight. The magnitude of photoproduction was dependent on initial Fe(II), Fe(III), and hydrogen ion concentrations, with more Fe(II) photoproduction when initial Fe(III) and H+ concentrations were high and initial Fe(II) concentrations were low. An equation was developed that accurately predicts photoproduction of Fe(II) in rainwater based on initial Fe speciation values and pH. The quantum yield of Fe(II) photochemical production in rain decreased dramatically with increasing wavelength and decreasing energy of incoming radiation with the average quantum yield at 265 nm approximately an order of magnitude greater than at 546 nm. Probable photochemical precursors of Fe(II) in authentic rain include iron(III) oxalate, iron(III) hydroxide, and an undefined Fe(III) complex. The wavelength-dependent Fe(II) production was modeled using the average Fe(II) efficiency spectrum, an average rainwater absorption spectrum, and the modeled actinic flux for temperate latitudes in both summer and winter. The response spectrum has the highest photoproduction of Fe(II) in summer and winter at 325 and 330 nm, respectively, with greater production in summer rain due to increased actinic flux and longer hours of irradiation.