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LASER DESORPTION/IONIZATION OF SINGLE ULTRAFINE MULTICOMPONENT AEROSOLS. (R823980)
Citation:
Ge, Z., A. S. Wexler, M. V. Johnston, AND A. S. Wexler. LASER DESORPTION/IONIZATION OF SINGLE ULTRAFINE MULTICOMPONENT AEROSOLS. (R823980). ENVIRONMENTAL SCIENCE & TECHNOLOGY. American Chemical Society, Washington, DC, 32:3218-3223, (1998).
Description:
Laser desorption/ionization characteristics of single
ultrafine multicomponent aerosols have been investigated.
The results confirm earlier findings that (a) the negative
ion spectra are dominated by free electrons and (b) the ion
yield-to-mass ratio is higher for ultrafine particles than
that of larger ones. The smallest relative mass of KCl detected
in NaCl is about 0.06%, which corresponds to 10-20 g of
KCl in a 50-nm particle. Experimental results from mixtures
of KCl/NaCl and NaCl/NH4NO3 show that, by measuring
the peak area ratio of certain ions in the spectrum, the total
composition can be inferred. The experimental results
also show that ion yields vary with composition even for
ultrafine particles, making multicomponent analysis of complex
particles difficult. A simple model is developed to
quantify the relationship between ion yield and particle
composition. Using this model, a relative ion yield of 4.9
is found for K+ from KCl over Na+ from NaCl, while 4.4 is
found for Na+ from NaCl over NO+ from NH4NO3. It has
been shown that analyzing particles composed of chemicals
with common cations but different anions, such as NaCl/NaNO3 from positive ion spectra, is also possible. The ability
of laser desorption/ionization to detect trace metals is
studied. With relative mass of each element on the order
of 1%, which corresponds to the absolute mass of the
order of 10-17 g in a 60-nm particle, strong peaks of Na+,
Mg+, K+, Cr+, Fe+, Cu+, Zn+, Cd+, Cs+, La+, and Pb+
are observed in the spectrum. The smallest amount detectable
for some easily ionizable elements, such as K and Cs,
should be much lower than 1%. Although no ion yield is
observed for pure (NH4)2SO4 particles, peaks of S+ and SO+
associated with (NH4)2SO4 are shown in spectrum from
mixtures of (NH4)2SO4, NH4NO3, and several trace metals.
The application of laser desorption/ionization to measuring
atmospheric ultrafine aerosols is also discussed.