Enhancing the Detection of Sulfate Particles for Laser Ablation Aerosol Mass Spectrometry

A major limitation to the application of laser ablation aerosol mass spectrometry for the detection of particles less than 200 nm in diameter is a low ablation efficiency for sulfate particles. (Ablation efficiency is the probability that an ablated particle produces a detectable ion signal.) A method is described here to enhance the ablation efficiency of sulfate particles by coating them with a UVabsorbing compound. The method can be applied in-line with the aerosol mass spectrometer in a manner that does not significantly alter the aerosol size distribution. It is shown that a 12-nm coating of 1-naphthyl acetate increases the ablation efficiency of 136-nm ammonium sulfate particles by at least a factor of 20, while similar coatings on oleic acid and ammonium nitrate particles do not significantly alter the ablation efficiency. The results suggest that "undetected" particles, presumably sulfate, in ambient aerosol can be assessed.