Infrared Spectral Model for Subwavelength Particles of Mixed Composition based on the Spectra of Individual Particles with Calibration Data for Airborne Dust

A Mie–Bruggeman
spectral model is presented which predicts the orientationally averaged,
infrared spectra of individual mixed-composition particles or the
average spectrum of collections of such particles. The model uses
parameters extracted from sets of individual particle spectra of pure
materials known to be in subject mixtures. The spectra of both calibrants
and subject particles were recorded by trapping size-selected particles
in the holes of plasmonic metal mesh. Calibrating data is presented
for quartz, calcite, dolomite, three clays, gypsum, polyethylene,
and living organic material (yeast cells). The individual particle
spectra of these calibrants are averaged to account for crystal orientation
effects, fit by a Mie theory model, and tabulated herein as dielectric
functions of each component. The component dielectric functions are
combined in this model with Bruggeman effective medium theory producing
a spectral prediction for mixed-composition particles. The Mie–Bruggeman
model was used to analyze the composition of dust from our lab air
[K. E. Cilwa et al. J. Phys. Chem. C 2011, 115, 16910] based on the average spectrum of the
dust particles. The model does a reasonable job of characterizing
the dust in our laboratory air exhibiting promise for future applications.
This work presents the model and illustrates potential; however, much
more work will be required before its accuracy as a quantitative analytical
method is established.