Technical noteAn improved method for the quantification of SOA bound peroxides
Introduction
HO2 and organic peroxy radicals are important products in the oxidation of volatile organic compounds (VOCs). The ozonolysis of alkenes is one of the major sources for peroxy radicals in night-time (Simonaitis et al., 1991) and daytime chemistry. In the absence of NO, the fates of peroxy radicals are largely controlled by HO2–HO2, HO2–RO2 and RO2–RO2 radical recombination reactions (Atkinson, 2000). The HO2–HO2 reaction in the presence of water leads to the formation of hydrogen peroxide, and the HO2–RO2 reactions yield organic hydroperoxides. In addition, photochemical reactions in atmospheric aqueous phase can be an important source of hydrogen peroxide (Faust et al., 1993). These hydroperoxides are highly reactive, and known to play an important role in atmospheric chemistry such as the oxidation of SO2 to H2SO4 in atmospheric aqueous droplets (Penkett et al., 1979). So far the impact of atmospheric peroxides on human health is not well understood but an excess of particle-bound reactive organic species (ROS) is suggested to cause oxidative stress that influences human morbidity and mortality (Ayres et al., 2008). Furthermore, peroxides are suggested to account for a significant fraction of laboratory-generated secondary organic aerosol (SOA) (Docherty et al., 2005). It has been proposed that heterogeneous reactions of hydroperoxides and aldehydes can form higher molecular weight (HMW) compounds such as peroxyhemiacetals in SOA (Tobias and Ziemann, 2000). Recent results from smog chamber experiments have shown that peroxides account for up to 85% of the SOA mass produced from β-pinene ozonolysis and 47% from α-pinene ozonolysis (Docherty et al., 2005). There are a number of methods available to detect peroxides including fluorescence (e.g. Lazarus et al., 1985), chemiluminescence (e.g. Kok et al., 1978), electrochemical (e.g. Sanchez et al., 1990) and spectrophotometric (e.g. Kieber and Helz, 1986) detection. Among those, an iodometric-spectrophotometric method has been commonly used to quantify SOA-bound peroxides (Docherty et al., 2005; Nguyen et al., 2010; Mertes et al., 2012). This method is based on the reaction of the peroxide with iodide (I−) to form I2 molecules that react further producing triiodide (I3−). The absorbance of I3− can be measured at λ = 351 nm with an extinction coefficient ɛ = 26,400 L mol−1 cm−1 (Awtrey and Connic, 1951). This wavelength is chosen because this corresponds to the highest absorption for I3− and the lowest for I2 whereas the extinction coefficients of both I2 (ɛ = 745 L mol−1 cm−1) and I3− (ɛ = 975 L mol−1 cm−1) at λ = 460 nm used by Docherty et al. (2005) are much lower than that of I3− at λ = 351 nm. Within the present study, the iodometric method for SOA-bound peroxide determination was improved. First, a two-phase solvent was replaced by a less toxic and non-carcinogenic one-phase solvent system. Second, the method uses an orbital shaker instead of ultrasonic agitation to avoid artefact formation from acoustic cavitation. Consequently, the modified method presented in this study is simpler and more reproducible than the previously used method.
Section snippets
Chemicals and standards
Hydrogen peroxide (30% in water), sulphuric acid (98%), potassium iodide (≥99.5% purity) were obtained from Merck (Darmstadt, Germany) and acetic acid from Fluka (puriss, eluent additive for LC–MS, St. Louis, USA). Terebic acid, pinic acid and pinonic acid are purchased from Sigma–Aldrich (St. Louis, USA). Synthesised standards were available for terpenylic acid (Claeys et al., 2009), 3-methyl-1,2,3-butanetricarboxylic acid (Szmigielski et al., 2007) and diaterpenylic acid acetate (Iinuma
Improvement of the method
The method described by Docherty et al. (2005) was evaluated using a series of H2O2 standard solutions and α-pinene ozonolysis SOA samples. For the determination of SOA-bound peroxides it was approximated that the water-soluble peroxide fraction accounts for the whole peroxide content. This is consistent with the results reported by Nguyen et al. (2010). Since the target analytes were water-soluble, the extraction solvent (ethyl acetate) was substituted with ultrapure water to avoid side
Conclusion
Peroxides play an important role in the atmospheric gas and particle phase. Several techniques are available in the literature though the detection and quantification of such compounds are still challenging, and the methods require further refinement to determine this class of compounds reliably and reproducibly. The present study presents an improved iodometric-spectrophotometric method to determine the SOA-bound peroxides of laboratory-generated samples. The improved method is comprehensively
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