34.4: Measuring Particle Size Using Image Analysis

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The chapter overview includes a photograph of an ion-exchange resin's beads. The photograph includes a scale and, in principle, we could use the photograph and scale to estimate the size of the resin's beads. Although the estimates likely are pretty crude, this still serves as an example of image analysis in which we equip an optical microscope or electron microscope with a digital camera that can capture images of the microscope's field of view. Software is used to differentiate the particles from the background, to establish the particle's boundaries, and to determine the particle's size. As shown in Figure \(\PageIndex{1}a\), the sample is dispersed on an optical platform and light is passed through the optical platform where it is magnified and focused before capturing the image using a camera. The optical platform can be manually or automatically moved in the xy-plane to capture more images, as in Figure \(\PageIndex{1}b\). The software then sorts the particles into groups based on size and reports a count of particles in each group, as in Figure \(\PageIndex{1}c\). Because the particles remained immobile, this is called a static image analysis.

Figure \(\PageIndex{1}\): Illustration of a static image analysis. The basic instrument is shown in (a) and consists of a light source, an movable optical platform, lens for magnifing and focusing the image, and a camera for capturing the image, an example of which appears in (b). The instrument's software returns, in (c), information about the distribution of particles sizes.

One limitation to static imaging analysis is that it generally samples a small number of particles as they must be sufficiently dispersed on the optical platform to allow the individual particles to be imaged, analyzed, and counted. In dynamic imaging analysis, the sample is placed in a flow cell set perpendicular to the camera and the light source. Images are collected by using a high-speed flash and shutter speed to capture a sequence of images that are analyzed. By essentially creating an infinite optical platform, dynamic imaging analysis can achieve analysis rates of 10000 particles per minute.