Real-time surface analysis of individual airborne environmental particles

Typically, in real-time aerosol mass spectrometry (RTAMS), individual airbo rne particles are ablated and ionized with a single focused laser pulse. Th is technique yields information that permits bulk characterization of the p article, but information about the particle's surface is often masked or di luted by the particle bulk. Here we show that it is possible to probe the s urface composition of individual airborne particles by separating the desor ption and ionization steps using a two-laser real-time aerosol mass spectro metry technique (L2RTAMS). First, a weak excimer laser pulse was used to de sorb the semivolatile components of the particle surface when the particle was in the center of the ion trap. After a short delay, another excimer las er pulse was used to ionize the semivolatile surface components in the gas phase and subsequently mass analyzed. The results from the one- and two-las er techniques were compared and found to be complementary. The L2RTAMS tech nique was found very sensitive to polycyclic aromatic hydrocarbons (PAHs). PAHs, of the type emitted from diesel engines, were found on particle surfa ces of National Institute of Standards and Technology (NIST) standard refer ence materials (SRMs) from Indiana Harbor Canal (1645) and urban particulat e matter (1648). PAH partitioning on the environmental particles is discuss ed.