The potential of near-infrared photothermal spectroscopy for the analysis of aerosols collected on filter membranes is demonstrated. Laboratory-generated aerosols consisting of ammonium sulfate and soot are collected on Teflon® filters by the use of a dichotomous sampler. The N-H overtone/combination band at 2110 nm is used to determine ammonium in these mixtures. The presence of soot is shown to have a deleterious effect on the detectability of ammonium. The extent of this limitation is examined with the employment of well-characterized samples. An empirical expression, based on Beer's law, is developed which relates the photothermal signal to the relative soot and ammonium sulfate loadings.
The absorptivity of soot and methylene blue particles collected on Teflon filters is derived from photoacoustic measurements by least squares fitting a simple expression based on Beer's law to the experimental data. Refinements of the expression take into account the diffuse reflection of light by the filter substrate, yielding a base 10 absorptivity at 600 nm for soot of 3.00 +/- 0.37 m(2)/g. This value is in close agreement with the result of transmission measurements performed on the same samples (3.08 +/- 0.05 m(2)/g).
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