Infrared Analysis of Particulates by FT-IR Emission/Transmission Spectroscopy

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Abstract
In this paper we report on a new method for the on-line chemical characterization of gas-suspended particulate and liquid droplet streams. The method is a combination of emission and transmission (E/T) spectroscopy performed using a Fourier transform infrared (FT-IR) spectrometer and a cell which allows analysis of room-temperature particulates surrounded by hot walls. The method was based on the discovery that the spectrum of radiation coming from such particles contained structure corresponding to the infrared absorption spectrum of the particulates. It was determined that the observed energy was wall radiation which reaches the detector after refraction by, and transmission through the particles. The shape of the observed spectrum relative to that of the wall spectrum is reduced at the absorption bands of the particulates. The general concepts of the E/T method were validated by samples with sizes varying from a few to several hundred microns with varying optical properties. For solids, excellent results were achieved for composition measurements. For a sample of known particle size, quantitative absorbance spectra were obtained from the E/T spectra with the use of a model based on ray optics. The spectra were in good quantitative agreement with absorbance spectra derived by the KBr pellet method for the same sample thickness. The E/T derived spectra were somewhat noisier than the KBr pellet spectra, but were free of Christiansen effect band distortions and scattering contributions. The E/T method has the additional advantage of requiring little or no sample preparation.