On dithiothreitol (DTT) as a measure of oxidative potential for ambient particles: evidence for the importance of soluble transition metals

Abstract

Abstract. The rate of consumption of dithiothreitol (DTT) is increasingly used to measure the oxidative potential of particulate matter (PM), which has been linked to the adverse health effects of PM. While several quinones are known to be very reactive in the DTT assay, it is unclear what other chemical species might contribute to the loss of DTT in PM extracts. To address this question, we quantify the rate of DTT loss from individual redox-active species that are common in ambient particulate matter. While most past research has indicated that the DTT assay is not sensitive to metals, our results show that seven out of the ten transition metals tested do oxidize DTT, as do three out of the five quinones tested. While metals are less efficient at oxidizing DTT compared to the most reactive quinones, concentrations of soluble transition metals in fine particulate matter are generally much higher than those of quinones. The net result is that metals appear to dominate the DTT response for typical ambient PM 2.5 samples. Based on particulate concentrations of quinones and soluble metals from the literature, and our measured DTT responses for these species, we estimate that for typical PM 2.5 samples approximately 80 % of DTT loss is from transition metals (especially copper and manganese), while quinones account for approximately 20 %. We find a similar result for DTT loss measured in a small set of PM 2.5 samples from the San Joaquin Valley of California. Because of the important contribution from metals, we also tested how the DTT assay is affected by EDTA, a chelator that is sometimes used in the assay. EDTA significantly suppresses the response from both metals and quinones; we therefore recommend that EDTA should not be included in the DTT assay.

Naphthalene SOA: redox activity and naphthoquinone gas–particle partitioning

“…4 assuming the redox activities are additive, as was reported for quinone species in the supporting information of Charrier and Anastasio (2012). There is a strong correlation between the predicted and observed DTT decay rates (r 2 = 0.87).…”

“…In one such study researchers found that hydrogen peroxide generation from the DTT assay performed on particles from Fresno, California, could be accounted for by measuring the aerosol mass loadings of 9,10-phenanthrenequinone, 1,2-naphthoquinone, and 1,4-naphthoquinone (Chung et al, 2006). In another study, by assigning the DTT loss rate to individual particulate species again from the Fresno area, Charrier and Anastasio were able to predict DTT decay as arising 80 % from transition metal species and 20 % from quinone species (Charrier and Anastasio, 2012).…”

“…In terms of importance, oxidation of phenanthrene to produce phenanthrenequinone (e.g., Wang et al, 2007;Lee and Lane, 2010), which is of much lower volatility than either 1,2-or 1,4-naphthoquinone and is more highly redox-active, may be a greater source of particle-phase redox-active organic species and has been shown to be an atmospherically relevant species (EigurenFernandez et al, 2008b). Certainly, previous apportioning of DTT activity to chemical species has shown phenanthrenequinone to be a much more substantial contributor to redox activity than 1,2-and 1,4-naphthoquinone (Chung et al, 2006;Charrier and Anastasio, 2012). The influence of naphthalene oxidation on redox activity in relation to other redox active species such as 9,10-phenanthrenequinone, transition metals, black carbon, and humic-like substances -particularly in warmer climates when partitioning favours the gas phase for small naphthoquinones -may ultimately depend on the volatility of the unknown redox-active species generated through its photooxidation.…”

“…For prediction of the SOA activity, we focussed on the 1,2-and 1,4-naphthoquinone isomers, which as mentioned in the Introduction, along with 9,10-phenanthrenequinone, are commonly believed to be the most important water soluble quinone electron transfer agents (e.g. Chung et al, 2006;Charrier and Anastasio, 2012). As mentioned in the Experimental section, the quinones were extracted directly from the filter using dichloromethane rather than measuring the water-soluble species, so it is assumed that the efficiency of quinone extraction was equal for phosphate buffer and dichloromethane.…”

“…The catalytic capacity of DTT to transfer these electrons to molecular oxygen can be observed by measuring the DTT rate of decay, which has been correlated to oxidative stress markers in cellular systems (Li et al, 2003b;Koike and Kobayashi, 2006). Constituents of PM that have been shown as active redox cycling catalysts include black carbon from diesel particles (Shinyashiki et al, 2009) transition metals (Netto and Stadtman, 1996;Charrier and Anastasio, 2012), humic-like substances (Lin and Yu, 2011;Verma et al, 2012), and quinones (Kumagai et al, 2002). Quinones are a class of organic molecules derived from aromatic species, containing two carbonyl functionalities on a larger conjugated hydrocarbon ring.…”