The Coulter Principle

The setup of the Coulter counter is relatively simple. A sample beaker is placed under a calibrated aperture tube. Both are filled with a conductive liquid. The aperture tube is connected to the system's vacuum and internal liquid reservoir. Two electrodes provide a constant current across the aperture and create an electrical sensing zone. Using the vacuum, particles are drawn through the aperture. These particles displace the electrolyte, which results in a measurable increase in voltage. This response is proportional to the particle volume. The aperture tubes are calibrated upon installation, so the maximum value of the voltage pulse can be used to determine the particle volume. During sample analysis, many particles or cells traverse the aperture. Based on the determined volume, each particle will be categorized into a channel or volume bin. The range and granularity of this categorization can be set in the measurement parameters. It is important to avoid coincidence, which occurs when two particles traverse the aperture simultaneously. Two particles will displace a larger volume of electrolyte within the electrical sensing zone and thus could be recognized as a single larger particle. To avoid coincidence, the particle concentration in the sample should be kept low enough. Also, it is important to select the correct aperture size based on the size of the particles in the sample. Particles below the measurement range cannot be detected properly, and particles which are too big could clog or block the aperture. Of course, particle agglomeration needs to be considered as well. Once the measurement has been completed and all particles are sorted into their corresponding size bins, the result of the analysis is shown as a size distribution graph. The Multisizer 4e software offers users further analysis and visualization options.