The Electrical Sensing Zone Method

THE ELECTRICAL SENSING ZONE METHOD (ESZ, THE COULTER PRINCIPLE) - COUNTING AND SIZING (0.4-1200 

Since its invention in the early 1950 s [13], the Coulter principle has been so widely accepted in the field of medical technology that presently over 98% of automated cell counters incorporate the Coulter principle. Besides counting blood cells for which the Coulter Principle was originally invented, this method can be used to coimt and size any particulate material that can be suspended in an electrolyte solution. During the past fifty years, the method has been utilized to characterize thousands of different industrial particulate materials. Over 7000 references to the uses of various COULTER counter models have been documented [14]. 

and R are the amplitude of the voltage pulse, the particle volume, electrolyte resistivity, aperture current, particle shape factor, and aperture radius, respectively. If a constant particle density is assumed, the pulse height is proportional to the particle mass. The measured particle size can be channelyzed using the pulse height analyzer circuit, and a particle size distribution is thus obtained. The electrical response of the instrument is essentially independent of shape for particles of the same voliune, both in theory and in practice. A typical measurement takes less than a minute, as 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 [15]. 

Tlie advantages of the method are that it measures a particle s volume, and the result will hardly be biased due to the shape of 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 [15]. 

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Lines, R.W. 1996. The electrical sensing zone method (The Coulter Principle). In Liquid and Surface Borne Particle Measurement Handbook. (J.Z. Knapp, ed.) pp. 113-154 Marcel Dekker, New York. 

The electrical sensing zone method (the Coulter principle)... 

British Standard BS3405 Part 5,(1983), Determination of Particle Size Distributions, Recommendations for the Electrical Sensing Zone Method, The Coulter Principle, 450, 462, 463... 

FIG. 21-17 Multisizer 3 COULTER COUNTER from Beckman Coulter, Inc., uses the electrical sensing zone method. 

Photozone Counter A particle- or droplet-sizing technique, analogous to the electrical sensing-zone methods, that relies on visible light absorption in sample introduced into a small chamber. The particles or... 

Multiple techniques can be used to measure the particle size distribution, for example electrozone sensing, sedimentation, laser diffraction, and microscopy. With the exception of microscopy, they all require calibration and the results depend on the technique. For example, in a round-robin study reported in Reference 4, the commonly used electrical sensing zone technique (Coulter Counter) was compared to microscopy and sedimentation. The average particle size determined by the electrical sensing zone method was by about 25%... 

Introduced in 1983, this Part gives recommendations for the electrical sensing zone method (the Coulter principle). It is recommended that the primary calibration technique is that of "mass integration", where a known volume of particles under test is used to calibrate the volumetric size response directly. This allows the method to be self-calibrating and to approach being absolute. 

A method of particle size analysis of the drug substance that may be predictive of ultimate ointment quality has been developed. This is shown by the data in Tables 1 and 2. The laser diffraction method is able to establish which micronised batches are likely to give acceptable ointments. This method, which is carried out in liquid paraffin following dispersion with a high shear mixer, is much more representative of actual processing conditions than the electrical sensing zone method which is conducted in aqueous electrolyte. 

The Electrical Sensing Zone Method (The Coulter Principle)... 

Grover et al., Rachel,and others have examined the response of the electrical sensing zone method to blood cells and have proposed a "form factor" of 1 for an elongated object oriented along the axis of the aperture, 1.5 for a sphere, and 2.8 for a disc perpendicular to the flow. The analogy is that somehow the electrical path flows around a streamlined envelope, or outline, of the particle, thereby making the particle appear to be larger than it is. 

The electrical sensing zone method requires a conducting liquid within the sensor. Simensen74 and... 

The Coulter Counter, through its various models, has become an estabi ished and we I I evaluated method of counting and sizing particles. The Coulter principle, or better, the electrical sensing zone method, has been the subject of many publications, including textbooks and the British Standard 3406. The main advantage of this method is that it yields directly a number distribution of the equivalent volume diameter. 

The electrical sensing zone method, also known as Coulter Counter method, has since its development in 1953 [1] been subject to numerous studies and applications. As a result the technique is well described, as well in textbooks [2, 3] as in manuals accompanying the instrtunents [4], and accepted as a Standard Method [5, 6, 7]. A schematic drawing of the instrument is given in figure 1. 

As in many previous PSA conferences, the metrology of particle characterisation by the electrical sensing zone method was again prominent [Berge, Feder and Jfefssang]. The emphasis these days is however on the mass calibration of the instrument, sometimes also using certified or standard reference materials... 

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