How are particles sized and counted using the Coulter Principle?
With the Coulter Principle, particles or cells passing through an elongated glass tube with a finely-bored ruby disk (aperture) designed to constrict the flow of fluid through the tube while also constricting the current to a controlled value with little variation.
When an aperture is placed between two electrodes and a current path is introduced by a low-concentration electrolyte, the resistance between the electrodes can be measured. This is called the sensing zone. As a particle passes through the aperture, a volume of electrolyte-containing solution equivalent to the immersed volume of the particle is displaced from the sensing zone causing a short-term change in impedance across the aperture which can be measured as a voltage or current pulse. The voltage pulse is proportional to the volume of the detected particle.
Using counter and pulse analyzer circuits, the number and volume of particles passing through the sensing zone can be measured. The volume may be represented as the equivalent spherical diameter and measured particle sizes can be binned using a height analyzer circuit and a particle size distribution.
More advanced Coulter Counter systems, like the Multisizer 4e can obtain particle size distributions automatically in up to 400 size channels. Measurements are made in only a few seconds as counting and sizing rates of up to 10,000 particles per second is possible and the accuracy of size measurements are nearly perfect as particles are discretely counted and sized.