Automated Particle Analysis by SEM/EDS
Automated particle analysis by SEM/EDS provides a method to answer questions about particle populations that arise in a very wide range of industries. A few examples from our laboratory include: wear particle analysis, size distribution of pharmaceutical ingredients, source determination of airborne particulate, and nanoparticle characterization.
When it comes to particle analysis, common questions that can be addressed are:
- What kinds of particles are in this sample?
- What is the chemical composition of these particles?
- What is the particle size distribution of this powder mixture?
How do you answer questions about populations of particles when not all of the particles have the same composition and it may be necessary to obtain data from hundreds or thousands of individual particles to provide a complete picture? Manual analysis of such a sample could easily take days on a microscope. Fortunately, automated particle analysis by scanning electron microscopy combined with energy dispersive x-ray spectrometry (SEM-EDS) provides a powerful tool for solving these kinds of problems.
The first step in SEM/EDS automated particle analysis is acquisition of an image with sufficient contrast between the background and the particles that an image analysis algorithm is capable of differentiating between them (Figure 1). For automated image analysis systems, a “particle” is defined as a set of contiguous pixels all of which are brighter (or more rarely, darker) than the threshold brightness used to define the surrounding “background” pixels.
The next step is the recognition of particles by the analysis system, which is part of the SEM/EDS software. Figure 2 shows the same field of view as Figure 1, now indicating all of the particles that the system has found. The analysis system saves the location of each particle and two dimensional size and shape parameters for every particle are determined. Typical parameters include maximum, minimum and average diameters, perimeter and aspect ratio.
Once the particles in the field of view are recognized, the automation system of the microscope acquires a chemical analysis of each particle in the form of an EDS spectrum. A typical example appears as Figure 3. A peak in the EDS spectrum indicates the presence of the corresponding element in the particle and the particle can be classified based on its composition. In Figure 3, the spectrum shows the particle to be composed of iron (Fe) and oxygen (O), indicating an iron oxide particle.
When every particle in the field of view is recognized and its dimensions and composition saved, the microscope moves to a new field of view and the process is repeated until a set number of particles or a predetermined number of fields of view have been analyzed. In this way hundreds and even thousands of particles can be characterized in terms of size, shape and composition in a few hours without operator involvement beyond the initial setup.
Finally, the results are tabulated, giving a complete picture of the particle types, their size and their shapes. And this tabulation is entirely customizable since all of the data (size, shape and chemistry) is stored for each individual particle. Table 1 shows the results from one sample where the count, average size and average aspect ratio of each particle type have been calculated and tabulated.
Contact us to learn more about how MVA can use Scanning Electron Microscopy to handle your specific analysis and testing needs.