Application Example: Multisensor Analysis of Particulate Matter

Western Australian salt lakes exhibit formation of ultra-fine particles, which are supposed to precipitate rain. To study the diversity of organic and inorganic particles at these salt lakes, particles from 250 nm to several Ám have been collected by impaction. The sampling filters have been analyzed by high resolution electron-microscopy, energy dispersive X-ray (SEM-EDX), and Raman spectroscopy.

Selective images obtained by the analysis of fused Raman spectroscopy and EDX images. The results of the classifiers have been colored and blended with the SEM reference photo. Image (a) shows NaCl (violet), image (b) shows soot (red) and organic compounds (yellow), and image (c) displays calcium sulfate (blue) and silicates (green). An interesting particle can be seen in the center of the upper half of each image (see inset in the bottom row). This particle consists of a sodium chloride crystal, some silicate, and some calcium sulfate. The right half of the particle is covered by organic material.

The information obtained from these imaging techniques is complementary. SEM provides a high resolution image for studies of the morphology, EDX delivers the elemental composition, and finally the Raman spectroscopic imaging provides vibrational information of organic and inorganic species. While, e.g., NaCl is only visible in the EDX image, volatile organics on the other hand are only visible in the Raman image. Other species, e.g. CaSO4 or silicates, can only be verified unquestionably using both techniques, where the elemental composition supports the assignment of vibrational bands.