The sensitivity of thermal desorption-gas chromatography/mass spectrometry (TD-GC/MS) was greatly improved by replacing the thermal desorption unit and mass spectrometer and by applying the selected ion monitoring method. Quantification limits of 12 polycyclic aromatic hydrocarbons (PAHs) , 17α (H) , 21β (H) -hopane, and C18-C36 n-alkanes were 4-17, 13, and 13-39 pg, respectively. For extremely small amount (≈ 20 μg) of standard reference materials (1649a, 1650b, and 2975) , PAH concentrations determined by TD-GC/MS agreed reasonably well with the certified values. TD-GC/MS was applied to size-resolved particles in diesel exhaust, and in roadside and background atmospheres. For diesel exhaust particles emitted under no-load condition, 17α (H) , 21β (H) -hopane concentrations per particle mass were higher in smaller particles, suggesting that lubricating oil was the major component of the nuclei-mode particles (particles with a modal diameter of 0.01-0.03 μm) . For the roadside and the background particles, 17α (H) , 21β (H) -hopane concentrations per particle mass were also higher in smaller particles after correction for evaporation of 17α (H) , 21β (H) -hopane in the atmosphere. The results suggest that nanoparticles (≤0.03 μm) in the roadside and the background atmospheres originated mainly from oil-rich nanoparticles in diesel exhaust, although a large proportion of their original components had evaporated.
