Effect of filter extraction solvents on the measurement of the oxidative potential of airborne PM.

Solvent extraction of PM samples collected on the filter is a preliminary step for assessing the PM oxidative potential (OP) using cell-free assays, as the dithiothreitol (DTT) and the ascorbic acid (AA) assays. In this study, we evaluated the effect of the solvent choice by extracting ambient PM samples with different solvents: methanol, as organic solvent, and two aqueous buffers, i.e., phosphate buffer (PB) and Gamble's solution (G), as a lung fluid surrogate solution. Both the measured volume-based OP and OP responses varied for the different extraction methods, since methanol extraction generated the lowest values and phosphate buffer the highest. Although all the tested solvents produced intercorrelated OP values, the phosphate buffer resulted the most useful for OP assessment, as it provided the most sensible measure (nearly double values) compared with other extractions. The association of the measured OP values with PM chemical composition suggested that oxidative properties of the investigated PM samples depend on both transition metals and quinones, as also supported by additional experimental measurements on standard solutions of redox-active species.