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.