What is Electrical Sensing Zone Method (ESZ)?

It is a widely accepted technology in the field of medical technology that presently over 98% of automated cell counters incorporate the ESZ. This method can be used to count and size any particulate material that can be suspended in an electrolyte solution.

In an ESZ experiment, a tube with an orifice is placed in an electrolyte solution containing the particles of interest in low concentration. The device has two electrodes, one inside and the other outside the orifice. The orifice creates what is called a “sensing zone.”  Particles pass through the orifice when the liquid is drawn from one side. As a particle passes through the sensing zone, a volume of the electrolyte equivalent to the immersed volume of the particle is displaced from the sensing zone. This causes a short-term change in the resistance across the aperture. This resistance change can be measured either as a voltage pulse or a current pulse.

By measuring the number of pulses and their amplitudes, one can obtain information about the number of particles and the volume of each individual particle. The number of pulses detected during a measurement is the number of particles measured, and the amplitude of the pulse is proportional to the volume of the particle.

If a constant particle density is assumed, the pulse height is proportional to the particle mass. The measured particle size can be channelyzed, and a particle size distribution is thus obtained. The electrical response of the instrument is essentially independent of shape for particles of the same volume, both in theory and in practice. A typical measurement takes less than a minute, like counting and sizing rates of up to 10,000 particles per second are possible. The accuracy of size measurements is usually within 1-2%. Calibration can be performed using known size standards or by the mass balance method.

The lower size limit of this method is defined by the ability to discriminate all kinds of noise from the signal generated from particles passing through the orifice. One source of interference is electronic noise generated mainly within the orifice itself, therefore, prevents small orifice from routine use in sizing small particles. The upper size limit is set by the ability to suspend particles uniformly in the sample beaker. The method is limited to those particles that can be suitably suspended in an electrolyte solution, either aqueous or non-aqueous. In order to suspend some large particles, it may be necessary to add a thickening agent such as glycerol or sucrose to raise the diluent viscosity. A thickening agent will also help reduce the noise generated by the turbulent flow of low viscosity electrolyte solutions as they pass through orifices. If particles to be measured cover a wider range than what any single aperture can measure, two or more apertures will have to be used, and the test results can be overlapped to provide a complete particle size distribution.

The advantages of the method are that it measures a particle’s volume, and the result will hardly be biased due to the shape of the particle, except in certain extreme cases and that it can also simultaneously count and size with very high resolution and reproducibility. However, the limitation or drawback of this method is that the particles that can be analyzed are restricted to those that can be dispersed in an electrolyte solution and still retain their original integrity.

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