Electrical sensing zone technique

II ASTM E 1772-95(1995) (1997), Particle size distribution of chromatography media by electrical sensing zone technique, 450... 

Electrical sensing zone technique commonly used to determine equivalent volume diameter, required in Eq. (19), might be problematic. The error associated with this technique is contributed by the breakup of aggregates and inclusion of pores in volume measurement. With this technique, an aggregate will have to be suspended in a liquid. The challenge is to preserve the structure of aggregates. Hence the first method is preferred to obtain the mass fractal dimension of aggregates in situ. 

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%... 

Electrical Sensing Zone Techniques (the Coulter Counter)... 

If the particle size distributions are characterized by the average diameters at the cumulative masses of 10, 50, and 90%, the data scattering measured by different techniques can be represented by the diameter ratios at the cumulative masses of 10, 50, and 90%, or DRio, DR50, and DR90. Since the particle diameter measured by the electrical sensing zone technique is to be the equivalent volume diameter and is independent of the particle shape, its particle diameter is defined to be one. The particle diameters measured by other techniques are then ratioed with this diameter. The experimental results of DRjo, DR50, and DR90 are summarized in Table 5. It can be seen that the results of particle analysis from different techniques can be quite different. 

A distinct advantage of the electrical sensing zone technique is that it allows setting up a mass balance for the measurement, since a volume related signal is measured with high resolution. The mass balance can be used for two purposes. The first and older one uses the mass balance to calculate appropriate calibration constants for those materials where latex calibration leads to systematic errors, such as porous and conducting particulates, the so-called mass calibration procedure. 

The work presented clearly indicates that the electrical sensing zone technique offers a good potential both for certification of reference materials and for analysis of materials such as pigments having a significant amount of sub-sized material, provided that a mass balance can be offered with high precision and accuracy. The target should be no systematic deviations in the mass balance with a precision in mass of 3X. 

One question which then arises is, What do the various particle analyzers measure From the electrical sensing zone technique, we get a number distribution, in spite of the fact that the method measures the volume of the individual particles. Prom the particle volume the instrument computes a volume equivalent particle diameter (see Chap. 2). The instrument then counts and reports the particles within a series of narrow (volume equivalent) diameter ranges. 

An example is the Coulter counter. The term sensing zone technique is also used with reference to similar techniques that use light absorption or scattering instead of electrical properties, see Photozone Counter. 

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... 

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. 

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. 

The electronic zone-sensing technique has been widely used as both a research and quality control tool. The major shortcoming of the technique is the requirement of an electrolyte solution in which to suspend the particles without causing dissolution. Other problems such as electrical noise and coincidence (two particles passing through the aperture at the same time) can usually be overcome by choice of appropriate experimental conditions. Example 4.2 shows data reduction using the Coulter-type data. 

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