Fourier Transform Infrared Spectrometric Analysis for Particle-Associated Ammonium Sulfate

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Abstract
A commercial Fourier transform infrared (FT-IR) spectrometer, modified for automated analysis of particulate-associated sulfate, was used to obtain the integrated absorbance (635 cm−1 to 595 cm−1) due to sulfate in ambient particulate matter collected onto Teflon® filters. An evaluation of this instrumentation was undertaken to determine its applicability for measuring sulfate and the correlation of results with those of ion chromatography and x-ray fluorescence. Particulate samples from six geographical areas were evaluated in this study. Results from different filter sets, using individual filter blanks, show values of the coefficient of determination of 0.90 or higher, with one notable exception. This exception was due to a skewed shift in the peak wavelength absorption (620 cm−1), and apparently indicates the effect of co-collected compounds in chemically shifting the spectra. For best analytical results, the spectrum of individual Teflon® filters instead of a generic blank should be used for subtracting the contribution of Teflon® absorption. Orientation of the filter in the FT-IR system must be the same during measurement of the blank and the sample to avoid spurious readings. The detection limit for ammonium sulfate is 1.2 μg/cm2, which corresponds to 0.36 μg/m3 sampled over 24 hours at 15.0 L/min for ammonium sulfate.