Key updates to the Elzone, an electrical sensing zone particle characterization instrument, supports a growing range of applications.

Over the years, the electrical sensing zone (ESZ) method for particle size and count analyses has become a standard in industry. There has even been an ISO (International Organization for Standardization) standard developed for this analytical technique, ISO 13319:2007 (Determination of particle size distributions—Electrical sensing zone method). In addition, this method is used in many other global standards serving as established test methods for a broad spectrum of applications ranging from toner and catalytic material to blood cell analysis.

Since the development of this technology back in the early 1960s, the ESZ method has been used as a reference due to its unsurpassed ability to give high resolution particle size and particle count for samples suspended in an electrolytic solution. This technique under the “Elzone” name was commercialized in 1963, when the first Elzone was introduced, and it continues to be extensively used. However, advancements in electronics and software have allowed additional functionality to be incorporated into the recently released Elzone II ESZ instrument from Micromeritics Instrument Corp., Norcross, Ga.

Simplifying acquisition

The electrical sensing zone method is capable of accurately sizing samples of varying optical properties, densities, colors, and shapes. Micromeritics' Elzone II particle size analyzer utilizes this technique to quickly and accurately determine the size, number, concentration, and mass of a wide variety of organic and inorganic materials. The Elzone II simplifies acquisition while ensuring accuracy through innovative systems. Image: Micromeritics Instrument Corp.

One significant enhancement in the Elzone II is the ability to capture and store individual particle information as it is analyzed. The ability to do digital pulse capturing enables the Elzone II to display individual particle data as particle count vs pulse diameter vs particle duration as the individual particles pass through the sensing zone. This capability enables the discrimination of particles that may have similar equivalent spherical diameters but different morphology. A morphology disparity between two particles causes one particle to pass through the sensing zone in a different manner from the other, producing detector signals that differ in shape. The Elzone II sorts the signals by their shape characteristics and stores the data for display and analysis.

Since systems such as the Elzone II are applicable in a multitude of industries and suitable for use with so many sample types, there is a general need for such an instrument to be able to suspend particles of wide ranges of densities, fragility, and availability. In response to this demand, the Elzone II offers a choice of suspension methods. The traditional stir-rod is still available to suspend particles, however, the Elzone II also features a hydropulser suspension module. This unique suspension method eliminates the need for a larger sample vessel (beaker) and the use of the stir-rod and suspends the particles in smaller sample cuvettes by agitating the suspension with a gentle pulsing of the suspension fluid. The hydropulser (an industry first) is for use with fragile particles as well as with samples that are less abundant or very expensive; a common need in research applications.

Reducing errors

The heart of the ESZ technique is the use of a glass tube having an orifice of a known diameter and configured so that an electrical current flows through the orifice by way of the electrolyte in which it is immersed. The sensing zone is the volume of electrolyte in close proximity to and within the orifice. As particles pass through the orifice, the current flow is disrupted producing an electrical signal by which particles are measured. It is therefore important for accurate measurement that there is no obstruction to the orifice. The Elzone II has the ability to detect automatically when such an obstruction has occurred. In addition, the Elzone II has a new aperture viewing system that utilizes a CCD camera. This feature, unique to the Elzone II, allows users to see clearly any potential obstructions, zoom in on the aperture, pan in/out, and even capture digital pictures of the aperture for reference.

A significant new feature of the Elzone II that ensures accuracy of data is the manner in which the system is calibrated. All ESZ systems require the calibration of an orifice tube with a standard of known diameter. However, it has been shown that a single-point calibration, as done by competitive systems, may not always be desirable since there is a non-linear relationship between the signal characteristics and the particle size. This results in increasing error as the particle size deviates more and more from the calibration point. The accuracy of the system depends on ensuring the calibration across the entire range of the aperture (2% to 60% the size of an individual orifice). To ensure accuracy, the Elzone II offers the ability to perform multipoint calibration of orifice tubes. With this option, users have the ability of selecting more than one calibration standard and calibrate on their mean, mode, median, or any combination. These choices accommodate the various ways manufacturers of these standards certify size.

—Peter Bouza,
Market Development Manager, Micromeritics Instrument Corp.