Fourier Transform Infrared Microscopy—a Tool for Speciation of Impactor-Sampled Single Particles or Particle Clusters

With the recent advent of Fourier transform infrared (FTIR) microscopy, a powerful new tool has been added to the methods available for integrated dust analysis (IDA). While the intrinsic molecular specificity of infrared spectroscopy was applied in the 1970s with the IDA strategy of impactor-sampled aerosol analysis, at that time only average information from at least two impactor spots (1–10 μg range) could be obtained after sample preparation by grinding with KBr. This preparation step for heavily loaded samples, which may induce secondary reactions within the particles and destroy part of the reactive (acidic) species originally present, is no longer required with the new technique. FTIR microscopy allows the direct observation of isolated particles (>5 μm diameter) or particle clusters (<5 μm diameter) impacted on KBr or ZnSe (by the transmission technique) after a typical sampling duration of 5 minutes in an urban environment. This technique not only shows significant superiority in the quality of the spectra obtained but also is the key to preservation of original particle acidity. By this technique inhomogeneities within one impactor spot could be shown clearly (compound mapping). Several source, city, and background aerosols have been analyzed in the micro domain for the first time for NH₄ ⁺ compounds, nitrates, sulfates, silicates, carbonates, and organics (aliphatic esters and aromatics). Acidic sulfates have been found in garage samples but not in city aerosols so far. By comparing samples at room temperature and heated up to 150°C, further information on the NH₄ ⁺ compounds and the volatility of the species could be obtained. The adaptation of millisecond-time resolution of the new rapid-scan FTIR spectrometer in our laboratory for molecular specific thermal analysis research (MOSTAR) has been started, extending the capabilities of previous work.