Differential Individual Particle Analysis (DIPA): applications in particulate matter characterization

Operator-controlled and computer-controlled scanning electron microscopy (CCSEM) are used extensively to characterize particulate matter in environmental media. Analysis in a scanning electron microscope (SEM) coupled with chemical extraction is a potentially powerful tool that is capable of determining how various sample components are associated at the individual particle level. This involves initial characterization in a SEM, after which the material is exposed to a liquid or gas phase reaction for a specified time, and once exposure is concluded, the particles are reanalyzed in the SEM. This particle analysis by difference, or differential individual particle analysis (DIPA), possesses considerable potential for describing the behavior of environmental particles under changing chemical conditions. Here we describe DIPA applications with illustrative examples drawn from the analysis of particulate matter modified by reactions in a fluid environment. In situ DIPA permits the same particles to be analyzed in the SEM before and after modification. Repeated exposure to the same, or different modifying conditions, provides information on the time dependence of specific reactions. Significant numbers of particles can be analyzed using CCSEM, and the same particles can be analyzed after the reaction by accurate sample relocation in the SEM. Ex situ DIPA, which involves a bulk sample modification, uses CCSEM to characterize significant numbers of particles pre- and postreaction. The CCSEM approach is extremely efficient; recent developments in silicon drift detectors have increased the speed of characteristic X-rays detection, and very large numbers of particles can be analyzed in a short period of time.