Real Time in Situ Chemical
Characterization of Submicrometer
Organic Particles Using Direct Analysis in Real Time-Mass Spectrometry
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Abstract
Direct analysis in real time mass spectrometry (DART-MS)
is used
to analyze the surface chemical composition of nanometer-sized organic
aerosol particles in real time at atmospheric pressure. By introducing
a stream of particles in between the DART ionization source and the
atmospheric pressure inlet of the mass spectrometer, the aerosol is
exposed to a thermal flow of helium or nitrogen gas containing some
fraction of metastable helium atoms or nitrogen molecules. In this
configuration, the molecular constituents of organic particles are
desorbed, ionized, and detected with reduced molecular ion fragmentation,
allowing for compositional identification. Aerosol particles detected
include alkanes, alkenes, acids, esters, alcohols, aldehydes, and
amino acids. The ion signal produced by DART-MS scales with the aerosol
surface area rather than volume, suggesting that DART-MS is a viable
technique to measure the chemical composition of the particle interface.
For oleic acid, particle size measurements of the aerosol stream exiting
the ionization region suggest that the probing depth depends upon
the desorption temperature, and the probing depth is estimated to
be on the order of 5 nm for a 185 nm diameter particle at a DART heater
temperature of 500 °C with nitrogen as the DART gas. The reaction
of ozone with submicrometer oleic acid particles is measured to demonstrate
the ability of this technique to identify products and quantify reaction
rates in a heterogeneous reaction