Real-Time Surface Analysis of Individual Airborne Environmental Particles

Typically, in real-time aerosol mass spectrometry (RTAMS), individual airborne particles are ablated and ionized with a single focused laser pulse. This technique yields information that permits bulk characterization of the particle, but information about the particle's surface is often masked or diluted by the particle bulk. Here we show that it is possible to probe the surface composition of individual airborne particles by separating the desorption and ionization steps using a two-laser real-time aerosol mass spectrometry technique (L2RTAMS). First, a weak excimer laser pulse was used to desorb the semivolatile components of the particle surface when the particle was in the center of the ion trap. After a short delay, another excimer laser pulse was used to ionize the semivolatile surface components in the gas phase and subsequently mass analyzed. The results from the one- and two-laser techniques were compared and found to be complementary. The L2RTAMS technique was found very sensitive to polycyclic aromatic hydrocarbons (PAHs). PAHs, of the type emitted from diesel engines, were found on particle surfaces of National Institute of Standards and Technology (NIST) standard reference materials (SRMs) from Indiana Harbor Canal (1645) and urban particulate matter (1648). PAH partitioning on the environmental particles is discussed.