Effects of operating conditions on PM oxidative potential assays

Oxidative potential (OP) has been suggested as a biologically relevant exposure metric for estimating particulate matter (PM) capacity to induce oxidative stress in living organisms. However, standardized experimental procedures are not yet available. This study explores how a variety of operating conditions influences responses of several different assays for measuring OP: the 2 ',7 '-dichlorofluorescein (OPDCFH), the ascorbic acid (OPAA) and the dithiothreitol (OPDTT) assays. A recently optimized method for the evaluation of PM reducing properties, based on the 2,2-diphenyl-1-picrylhydrazyl assay (RPDPPH), was also included in the study. Two monitoring campaigns were carried out in Central Italy by using co-located PM10 samplers working in parallel, for comparing results obtained from different operating procedures simultaneously applied on equivalent samples. Extraction efficiency and repeatability of three different water-extraction methods (rotating agitator, ultrasonic bath, and vortex), and the influence of storage duration and conditions on OP results were examined. OPDCFH values were found to be significantly higher when ultrasonic bath (US) was used for extraction, probably due to the formation of free radicals induced by US; for all the OP assays, the highest repeatability was obtained by extracting samples with rotating agitator (RA). Sample storage was confirmed to be a very critical issue as all the assays, except OPDTT, showed a marked dependence on storage time and conditions. The influence of membrane filters used to collect PM was also assessed. No significant differences were observed between samples collected on quartz and polytetrafluoroethylene (PTFE) membrane filters, except for OPAA, that gave significantly higher results for samples collected on PTFE membranes. Lastly, the contribution of water-insoluble PM components to OP was examined and warrants further investigations.