Selective molecular characterization of particulate matter from gasoline cars using internal extractive electrospray ionization mass spectrometry

Particulate matter (PM) from on-road automobile exhaust remains analytically challenging as PM contains thousands of inorganic and organic species, despite its negative contribution to atmospheric air quality. Herein, a facile method based on internal extractive electrospray ionization mass spectrometry (iEESI-MS) has been established to focus on the less abundant polar substances of automobile PM, removing the dominating lubricating oil species out of the equation. Different sized PM (e.g., 0.22-0.45, 0.45-1, 1-2, 23, 3-5, and 5-10 mm) formed at a given engine speed and total PM at different engine speeds (e.g., 1000, 1500, 2000, 2500, and 3000 rpm) were collected on filter membranes and subjected to investigation. The aqueous methanol extraction solvent and the usage of electrospray-like ionization, characteristics of the iEESI technique, allow for the detection of species of moderate and high polarity. Major substances identified include alcohols, nitrogenous compounds, ketones, and esters. Direct infusion ESI measurements of the used lubricating oil sample inside the engine suggested that some of the observed substances originated from lubricating oil. Moreover, similar chemical fingerprints were observed in PM of different sizes, while more chemicals were observed in PM of 0.45-1.00 mm. The experimental data showed that iEESI-MS is practically useful for molecular characterization of exhaust PM, providing a mass spectrometry tool for advanced studies on PM.