Typically, in real-time aerosol mass spectrometry (RTAMS), individual airborne particles are ablated and ionized a single focused laser pulse. This technique yields information that permits bulk characterization of the particle, information aboutthe particle's surface is often or diluted by the particle bulk. Here we show that it is possible to probe the surface composition of I airborne particles by separating the desorption and ionization steps using a two-laser real-time aerosol massspectrometry technique (L2RTAMS). First, a weak excimer laser pulse was used to desorb the semivolatile components particle surface when the particle was in the center ion trap. After a short delay, another excimer laser pulse was used to ionize thesemivolatile 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 aromatichydrocarbons (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.
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