Branching ratios for the reaction of selected carbonyl-containing peroxy radicals with hydroperoxy radicals.

An important chemical sink for organic peroxy radicals (RO(2)) in the troposphere is reaction with hydroperoxy radicals (HO(2)). Although this reaction is typically assumed to form hydroperoxides as the major products (R1a), acetyl peroxy radicals and acetonyl peroxy radicals have been shown to undergo other reactions (R1b) and (R1c) with substantial branching ratios: RO(2) + HO(2) → ROOH + O(2) (R1a), RO(2) + HO(2) → ROH + O(3) (R1b), RO(2) + HO(2) → RO + OH + O(2) (R1c). Theoretical work suggests that reactions (R1b) and (R1c) may be a general feature of acyl peroxy and α-carbonyl peroxy radicals. In this work, branching ratios for R1a-R1c were derived for six carbonyl-containing peroxy radicals: C(2)H(5)C(O)O(2), C(3)H(7)C(O)O(2), CH(3)C(O)CH(2)O(2), CH(3)C(O)CH(O(2))CH(3), CH(2)ClCH(O(2))C(O)CH(3), and CH(2)ClC(CH(3))(O(2))CHO. Branching ratios for reactions of Cl-atoms with butanal, butanone, methacrolein, and methyl vinyl ketone were also measured as a part of this work. Product yields were determined using a combination of long path Fourier transform infrared spectroscopy, high performance liquid chromatography with fluorescence detection, gas chromatography with flame ionization detection, and gas chromatography-mass spectrometry. The following branching ratios were determined: C(2)H(5)C(O)O(2), Y(R1a) = 0.35 ± 0.1, Y(R1b) = 0.25 ± 0.1, and Y(R1c) = 0.4 ± 0.1; C(3)H(7)C(O)O(2), Y(R1a) = 0.24 ± 0.15, Y(R1b) = 0.29 ± 0.1, and Y(R1c) = 0.47 ± 0.15; CH(3)C(O)CH(2)O(2), Y(R1a) = 0.75 ± 0.13, Y(R1b) = 0, and Y(R1c) = 0.25 ± 0.13; CH(3)C(O)CH(O(2))CH(3), Y(R1a) = 0.42 ± 0.1, Y(R1b) = 0, and Y(R1c) = 0.58 ± 0.1; CH(2)ClC(CH(3))(O(2))CHO, Y(R1a) = 0.2 ± 0.2, Y(R1b) = 0, and Y(R1c) = 0.8 ± 0.2; and CH(2)ClCH(O(2))C(O)CH(3), Y(R1a) = 0.2 ± 0.1, Y(R1b) = 0, and Y(R1c) = 0.8 ± 0.2. The results give insights into possible mechanisms for cycling of OH radicals in the atmosphere.

Method development for the measurement of quinone levels in urine.

A method was developed for the quantification of 1-4 ring quinones in urine samples using liquid-liquid extraction followed by analysis with gas chromatography-mass spectrometry. Detection limits for the ten quinones analyzed are in the range 1-2 nmol dm(-3). The potential use of this approach to monitor urinary quinone levels was then evaluated in urine samples from both Sprague-Dawley rats and human subjects. Rats were exposed to 9,10-phenanthraquinone (PQ) by both injection and ingestion (mixed with solid food and dissolved in drinking water). Urinary levels of PQ were found to increase by up to a factor of ten compared to control samples, and the levels were found to depend on both the dose and duration of exposure. Samples were also collected and analyzed periodically from human subjects over the course of six months. Eight quinones were detected in the samples, with levels varying from below the detection limit up to 3 µmol dm(-3).

(CH3)3COOH (tert-butyl hydroperoxide): OH reaction rate coefficients between 206 and 375 K and the OH photolysis quantum yield at 248 nm.

Rate coefficients, k, for the gas-phase reaction of the OH radical with (CH(3))(3)COOH (tert-butyl hydroperoxide) were measured as a function of temperature (206-375 K) and pressure (25-200 Torr (He, N(2))). Rate coefficients were measured under pseudo-first-order conditions using pulsed laser photolysis to produce OH and laser induced fluorescence (PLP-LIF) to measure the OH temporal profile. Two independent methods were used to determine the gas-phase infrared cross sections of (CH(3))(3)COOH, absolute pressure and chemical titration, that were used to determine the (CH(3))(3)COOH concentration in the LIF reactor. The temperature dependence of the rate coefficients is described, within the measurement precision, by the Arrhenius expression k(1)(T) = (7.0 ± 1.0) × 10(-13) exp[(485 ± 20)/T] cm(3) molecule(-1) s(-1) where k(1)(296 K) was measured to be (3.58 ± 0.54) × 10(-12) cm(3) molecule(-1) s(-1). The uncertainties are 2σ (95% confidence level) and include estimated systematic errors. UV absorption cross sections of (CH(3))(3)COOH were determined at 185, 214, 228, and 254 nm and over the wavelength range 210-300 nm. The OH quantum yield following the 248 nm pulsed laser photolysis of (CH(3))(3)COOH was measured relative to the OH quantum yields of H(2)O(2) and HNO(3) using PLP-LIF and found to be near unity.

Monitoring acetaldehyde concentrations during micro-oxygenation of red wine by headspace solid-phase microextraction with on-fiber derivatization.

An analytical method was developed to quantify levels of acetaldehyde in wine samples. The method utilizes headspace solid-phase microextraction with on-fiber derivatization using O-(pentafluorobenzyl)hydroxylamine and quantification by gas chromatography with flame ionization detection. The technique showed good sensitivity and reproducibility in samples of Chardonnay, Petite Sirah, and Merlot wines containing acetaldehyde at levels below the sensory threshold (40-100 ppm). The method was used to monitor acetaldehyde concentrations during the micro-oxygenation of Merlot wine in a 141 L pilot-plant experiment and a 2400 L full-scale study. In both experiments, levels of acetaldehyde remained constant for several weeks before increasing at rates of the order of 1 ppm/day. Variations in the levels of acetaldehyde present are discussed within the context of the underlying chemical reactions.

Aerosol-borne quinones and reactive oxygen species generation by particulate matter extracts.

The mass loadings of quinones and their ability to generate reactive oxygen species (ROS) were investigated in total suspended particulate samples collected in Fresno, CA, over a 12-month period. Particles were collected on Teflon filters and were analyzed for the presence of 12 quinones containing one to four aromatic rings by gas chromatography with mass spectrometry. Measured levels are generally greater than mass loadings reported at other locations. The mass loadings were highest during winter months and were strongly anticorrelated with temperature. ROS generation was investigated by measuring the rate of hydrogen peroxide production from the reaction of laboratory standards and ambient samples with dithiothreitol (DTT). ROS generation from ambient samples shows a strong positive correlation with the mass loadings of the three most reactive quinones and may account for all of the ROS formed in the DTT test.

Quantum chemical and master equation simulations of the oxidation and isomerization of vinoxy radicals.

The vinoxy radical, a common intermediate in gas-phase alkene ozonolysis, reacts with O2 to form a chemically activated alpha-oxoperoxy species. We report CBS-QB3 energetics for O2 addition to the parent (*CH2CHO, 1a), 1-methylvinoxy (*CH2COCH3, 1b), and 2-methylvinoxy (CH3*CHCHO, 1c) radicals. CBS-QB3 predictions for peroxy radical formation agree with experimental data, while the G2 method systematically overestimates peroxy radical stability. RRKM/master equation simulations based on CBS-QB3 data are used to estimate the competition between prompt isomerization and thermalization for the peroxy radicals derived from 1a, 1b, and 1c. The lowest energy isomerization pathway for radicals 4a and 4c (derived from 1a and 1c, respectively) is a 1,4-shift of the acyl hydrogen requiring 19-20 kcal/mol. The resulting hydroperoxyacyl radical decomposes quantitatively to form *OH. The lowest energy isomerization pathway for radical 4b (derived from 1b) is a 1,5-shift of a methyl hydrogen requiring 26 kcal/mol. About 25% of 4a, but only approximately 5% of 4c, isomerizes promptly at 1 atm pressure. Isomerization of 4b is negligible at all pressures studied.