Particle Size and Shape Analysis

Image Analysis Diagram

Image Analysis is a powerful analytical technique which can provide additional information on a sample compared to just “particle size” and distribution. The majority of particle sizing techniques assume an equivalent spherical diameter of some measured property. This simplification is advantageous as a sphere is the only shape which can be described using a single number (i.e diameter). This assumption is acceptable in many applications where a comparison between samples is needed. In real world applications, particles are seldom spherical and cannot truly be described with a single number. The additional parameters provided by image analysis can help provide further insight into important properties which can affect dissolution, flowability, processing differences and material handling issues of a sample.

Image Analysis involves digitally capturing images of a sample using a CCD camera. The digital image is composed of pixels and each pixel is assigned a value. This array of pixels is then evaluated and a process referred to as thresholding occurs. Thresholding is performed within the image analysis software package and assigns a value where by each pixel measured is compared and thus treated as particle or considered as background (non-particle). Once thresholding has been performed and the system calibrated using NIST traceable standards, measurement of the digital image is performed. Upon completion of the measurement, results are available on numerous shape and size parameters; such as Aspect Ratio, Sphericity/Roundness, Solidity, Convexity, and Circular Area Diameter….along with numerous others. The numerous shape parameters and output formats available in modern Image Analysis software can provide exciting new ways for PTL to evaluate a client’s sample.

As in any analytical technique, sample preparation is extremely important. Image Analysis requires an understanding of the question at hand and proper treatment of the sample. Since Image Analysis is considered a true particle counting technique, the higher resolution/sensitivity can increase analysis time in order to count a statistically significant number of particles. Even with modern automated image analysis systems, which attempt to overcome these limitations, the analysis time for an image analysis measurement is greater than other particle sizing techniques. Coincidence (over lapping of discrete particles) or improper use of thresholding and imaging filters can adversely affect results and need to be considered during the measurement.

Image analysis can be divided into static and dynamic imaging methods. Both techniques are similar in their method of data acquisition, processing, and reporting. Both systems also have the ability to analyze shape, including the detection and enumeration of agglomerates, fibers, and contaminants. However, differences in the mode of sample presentation to the instrument create trade-offs between the analytical range, sampling rate, and sample preparation methods available.

In static imaging, the particulate is dispersed onto a stage where it remains fixed during analysis. Static imaging systems offer more versatility in that various sample states can be analyzed: dry powders, liquid suspensions, particulate captured on filters, in addition to creams and lotions. These systems also have the ability to reanalyze the same particles more than once, allowing for more in-depth investigation. However, the number of particles or the sample’s volume analyzed is generally not as high as in dynamic imaging.

PTL’s dynamic imaging systems offer advantages in sample volume and speed of measurement. In dynamic imaging, large amounts of sample are imaged by moving particulate past the microscope optics while using high speed flash illumination. The major advantage over the static method is its ability to acquire and measure particles at significantly higher speed. This means that statistically significant populations can be analyzed in far shorter time periods than static imaging particle analysis. However, these instruments generally have a narrower analytical range and can only measure particles in non-viscous liquid suspensions.

PTL has used Image Analysis since our company’s inception to solve client problems and provide innovative solutions to their projects. PTL’s years of experience with Image Analysis and stringent quality system allow our client’s to have the highest confidence in the results provided. Contact us to discuss how Image Analysis may be a valuable tool to evaluate your samples.